NeuroImmunology

In addition to central hyperexcitability and impaired top–down modulation, chronic inflammation probably plays a role in the pathophysiology of fibromyalgia (FM). Indeed, on the basis of both animal experiments and human studies involving the ...

Summary The blood‐brain barrier (BBB) is a highly regulated interface that separates the peripheral circulation and the brain. It plays a vital role in regulating the trafficking of solutes, fluid, and cells at the blood‐brain interface and maintaining the homeostasis of brain microenvironment for normal neuronal activity. Growing evidence has led to the realization that ischemic stroke elicits profound immune responses in the circulation and the activation of multiple subsets of immune cells, which in turn affect both the early disruption and the later repair of the BBB after stroke. Distinct phenotypes or subsets of peripheral immune cells along with diverse intracellular mechanisms contribute to the dynamic changes of BBB integrity after stroke. This review focuses on the interaction between the peripheral immune cells and the BBB after ischemic stroke. Understanding their reciprocal interaction may generate new directions for stroke research and may also drive the innovation of easy accessible immune modulatory treatment strategies targeting BBB in the pursuit of better stroke recovery. 1 INTRODUCTION Ischemic stroke is one of the most common neurological disorders and a major cause of disability and death with limited transitional success of mounting stroke researches, posing an economic and societal burden.1, 2 Under normal condition, the brain is under continuous immune surveillance and regulation. The neurovascular unit (NVU) regulates the homeostasis of brain microenvironment for normal neuronal activity. It is composed of neurons, glial cells (oligodendrocytes, microglia, and astrocytes) and vascular cells (endothelial cells, pericytes, and smooth muscle cells as well as the basal lamina matrix within brain vasculature).3, 4 Compared to the concept of NVU, the notion of blood‐brain barrier (BBB), which traditionally includes endothelia cells (ECs), astrocytes, pericytes, and the basal lamina matrix, tight junction proteins within the vasculature is more straight forward and more intensively studied.5-12 BBB damage is a common pathological feature shared by stroke and a variety of neurological diseases.13-18 Notably, it is closely associated with poor neurological outcomes.19-23 In response to cerebral ischemic stroke, the brain can release a variety of “danger” signals or “help me” signals, such as ATP, high‐mobility group box 1 (HMGB1), hypoxia‐inducible factor 1α (HIF‐1α), S100B, brain‐derived antigens and et al, all of which activate multiple subsets of peripheral immune cells24, 25.Once activated, these cells can migrate to the ischemic brain through detection of chemoattractant gradients.26-28 Upon BBB disruption, the components in blood including immune cells can enter into brain sequentially and interaction of neuro‐immune interaction can be initiated.29-32 During their penetration through the injured BBB, these immune cells become a double edge sword, which could either exacerbate the BBB disruption or protect the integrity of BBB.36, 37 Pleiotropic intracellular mechanisms and diverse secretory factors, including cytokines, proteolytic enzymes, exosomes, micro vesicles, and miRNAs, have been implicated in their interaction both in early disruption and later repair phase of the BBB.33-37 Importantly, distinct phenotypes and subsets of immune cells exhibit diverse impact on the poststroke BBB. This review will discuss the dynamic changes of BBB integrity after stroke followed by a discussion of the double‐faced roles of peripheral immune activation on the BBB integrity after stroke. Finally, the newly emerged mechanisms by which the peripheral immune cells impact the BBB integrity have also been covered at the end, including exosomes and micro vesicles. 2 DYNAMIC CHANGES OF BLOOD‐BRAIN BARRIER INTEGRITY (BBB) AFTER STROKE In response to ischemic stroke, the integrity of BBB compromises promptly and changes dynamically, which can be observed by magnetic resonance imaging (MRI) and other imaging techniques.38-42 During the early phase, cytoskeletal alterations in brain ECs can be initiated by actin polymerization followed by translocation of tight junctions (TJs) within 30‐60 minutes of reperfusion through activation of the Rho‐associated protein kinase (ROCK)/myosin.43 The activation of ROCK pathway may further promote the cross‐linking of F‐actin into force‐generating linear stress fibers through the phosphorylation/activation of myosin light chains (MLC) and increase cytoskeletal tension thus lead to the disassembly of TJs. These changes can widen the paracellular space between ECs, eventually resulting in hyperpermeability.43 In addition, opening of sodium and calcium channels, endothelial connexin‐43 hemichannels, the alterations in endocytotic vesicles, endothelial endocytosis/transcytosis, and transcellular vesicular trafficking, which could be mediated by caveolin‐1, endothelial growth factor, or exocytotic machinery, may also account for BBB hyperpermeability as early as 6 hours after cerebral stroke.44-47 Increased expression of aquaporin 4 (AQP4) in astrocytes of the ischemic brain is also associated with the initial cerebral edema after stroke.48-50 Additionally, pericytes may separate from basement membrane and detach from endothelial cells through paracellular pathway or transcellular routes, which contributes to increased microvascular permeability via disruption of pericyte‐tight junction interactions.51-53 The expression of TJ proteins may be decreased due to increased oxidative stress or matrix metalloproteinases 9 (MMP9)‐mediated protein degradation after stroke, leading to a “second wave” of increased BBB permeability.40 Disruption of BBB integrity is not only a common consequence of stroke, but also contributes to the progression of stroke.54, 55 Infarct size can progress with time after cerebral ischemia reperfusion, with more resident immune cells activated and more peripheral immune cells recruited to the brain and thereby further aggravates the injury of BBB.56 Cerebral ischemia upregulates the expression of adhesion molecules, such as intercellular adhesion molecule 1 (ICAM‐1), vascular cell adhesion molecule 1 (VCAM‐1), integrin and E‐selectin in the brain, which in turn facilitate a massive “second wave” of immune cell entry into the brain parenchyma through the BBB, leading to exacerbated neuroinflammation. The inflammation in the lesioned brain also contributes to the “second wave” of BBB disruption.57, 58 At 1‐3 days after stroke, BBB breakdown is featured by TJ degradation, basement membrane disruption, and eventually loss of endothelial cells.59 Chronic BBB opening caused by the loss of pericytes or chronic stress can lead to neuronal uptake of multiple blood‐derived neurotoxic products as well as reductions in microcirculation that in turn results in a chronic neuronal dysfunction and degenerative changes.60, 61 During the late phase of stroke, the BBB dysfunction becomes less severe, in which BBB restoration possibly plays an important role.62 Multiple processes may be involved in the restoration of BBB permeability after stroke at the late phase of stroke. Neovascularization begins in the peri‐infarct region, which involves the proliferation of endothelial cells and sprouting of the vessels that eventually increase vascular density.24 Astrocytes act to maintain endothelial permeability and survival after stroke through improving tight junction constituents.52, 63 Pericytes can stable actin filaments in endothelial cells and preventing their death, guiding the correct location of endothelial cells, and clearing up neuronal debris during injury.52, 64 The illustration of structural changes of ECs, TJs, astrocytes, and pericytes of the BBB after ischemic stroke is shown in Figure 1. 3 THE DOUBLE‐FACED ROLES OF PERIPHERAL IMMUNE ACTIVATION ON THE BBB INTEGRITY AFTER STROKE The injured ischemic brain can release ATP, high‐mobility group box 1 (HMGB1), HIF‐1α, S100B, brain‐derived antigens and et al as alarm signals to activate the peripheral immune system through purinergic receptors, TLRs, and the receptor for advanced glycation endproducts (RAGE).61 Once activated, these cells can migrate to areas of injury through detection of chemoattractant gradients.24, 26 During their penetration through the injured BBB, these immune cells become a double edge sword, which could either exacerbate the BBB disruption or protect the integrity of BBB. 3.1 Activated PMNS and BBB disruption after ischemic stroke Polymorphonuclear neutrophils (PMNs) are the most abundant cell population present at the site of injury with a peak influx between 1 and 3 days in the ischemic brain.56, 65, 66 Infiltration of PMNs is closely related to BBB disruption by a series of biological processes, such as releasing proteases, MMP, elastase, cathepsin G, and proteinase, producing reactive oxygen species (ROS), causing endothelial dysfunction and disorganization of junctional proteins, all of which are known to damage the BBB.67, 68 MMP9 is a subtype of matrix metalloproteinase. It may completely degrade the basal lamina and TJ components through attacking type IV collagen, laminin, and fibronectin and result in gross barrier disruption,69, 70 brain edema, leukocyte infiltration, and hemorrhage.71-73 Under normal condition, the expression of MMPs in the brain is very low. Ischemic stroke may induce increased expression of MMPs, especially MMP9.74 Both brain vascular ECs and infiltrating neutrophils can produce MMP9 after focal cerebral ischemia, which can be used to predict stroke patient outcome.74-77 In ischemic stroke patients, MMP9 increases in the plasma within the first 2 to 6 hours and the MMP9 mRNA in leukocytes increased within 3 hours78 Activated neutrophils may be an important source of pre‐existing intracellular MMP9 pool, which can be secreted in response to middle cerebral artery occlusion (MCAO) and oxygen‐glucose deprivation (OGD) insults69 In addition, activated MMPs may also be triggered by increased TNF‐α, IL‐6, and α2‐antiplasmin in the blood79, 80 The granulocyte‐colony stimulating factor (G‐CSF) can stimulate the proliferation of peripheral neutrophils and thus increase the level of MMP9, leading to exacerbated BBB disruption and increased ischemic infarction.81 In addition to MMP9,the elastase secreted by neutrophils also degrades basal lamina and extracellular matrix.82 In mice lacking elastase, ischemia‐induced BBB disruption is reduced, as is infarct volume and cerebral edema. Inhibiting elastase can further decrease infarct volume and BBB disruption in MMP9‐null mice, suggesting an MMP9 independent mechanism of elastase on the BBB disruption.83 Besides MMP9 and elastase, neutrophils are important source of ROS in cerebral ischemia and reperfusion injury. ROS itself can disrupt the BBB through direct damage to endothelial cells, pericytes, smooth muscle cells, and astrocytes.84 In addition, activated PMNs stimulate inflammatory cytokine production, which attracts more leukocytes from the periphery and aggravates the adhesion molecule expression on ECs, thus further propagates the postischemic inflammation cascade that exacerbates BBB disruption85 and increases the risk of secondary bleeding within the ischemic focus.46, 71, 86 3.2 The bright side of PMNS in poststroke BBB disruption Given the well‐established negative impact of poststroke BBB disruption, PMNs may also have beneficial effects for the BBB to repair in the late phase of stroke.87 MMP9, which induces BBB disruption in the early phase, is suggested to promote BBB remodeling in the late phase of stroke by enhancing degradation of proinflammatory DAMPs and vascular remodeling.87-90 In addition, PMNs can release antiinflammatory molecules, such as annexin‐1, lipoxin A4, resolvins, and protectins to alleviate the poststroke inflammatory reaction.91 PMNs also attract monocytes that clear apoptotic neutrophils and cellular debris through phagocytosis.36 Neutrophil‐derived MMP9 is also involved in the regulation of the proangiogenic and release hematopoietic progenitor cells from the bone marrow.89 Like monocytes, PMNs are divided into two phenotypes, N1 and N2 phenotype. The N2 phenotype is encompassed with antiinflammatory properties that may have protective effects in stroke.92 Thus, under certain conditions neutrophils are not detrimental and may be beneficial in the progression of stroke. 3.3 Pleiotropic effects of microglia/macrophages on the BBB integrity after stroke The function of microglia/macrophages function is complex and largely depends on the existence of varied, plastic, and multilayered macrophage phenotypes.93 The impact of microglia/macrophages activation on the ischemic BBB largely depends on the phenotype or status of these cells which can be affected by micro‐environmental cues.94-96 According to distinct cues, the microglia/macrophages can differentiate into two subtypes—inflammatory and antiinflammatory phenotypes.97 In their proinflammatory phenotypes, they can aggravate the BBB injury while in their phagocytosis or antiinflammatory phenotypes they may play distinct roles toward BBB repair and regeneration.98-101 However, recent studies suggest that the terminology of microglia/macrophage polarization may be limited. Transcriptomic and proteomic profiles, regional heterogeneity, sexual dimorphism, and age could all take into account while determining the functions of microglia/macrophages.102 The destructive effects on BBB are mainly mediated by M1 phenotypes which are characterized by the production of proinflammatory mediators including IL‐1β, TNF‐α, IL‐6, and IL‐12 and MHC II,103, 104 MCP1/CCL2.105, 106 These effects mainly fall into several categories: (a) increased expression of inducible nitric oxide synthase (iNOS); (b) increased production of ROS; (c) synthesis of proteolytic enzymes (MMP9, MMP3)107; (d) upregulated expression of macrophage migration inhibitory factor (MIF); (v) phagocytosis of endothelial cells; and (vi) recruit other proinflammatory cells to further exacerbate the inflammatory cascade. It has been reported that proinflammatory cytokines together with nitric oxide (NO) and proteolytic enzymes can induce the increase of BBB permeability108 by downregulating TJ proteins expression in ECs and modulate the expression of adhesion molecules.109 ROS, generated by NADPH oxidase, provokes EC contractions and consequently increases permeability of the BBB.110 Both inflammatory microglia and macrophages have been suggested to produce ROS after neurological diseases, including stroke.111-114 MIF, also known as glycosylation‐inhibiting factor (GIF) is an important regulator of innate immunity. It has been shown to promote leukocyte‐endothelial cell interactions through promoting endothelial adhesion molecule expression.115, 116 It has recently been suggested to directly degrade the BBB after ischemic brain.117 Perivascular microglia/macrophages migrate toward the disrupted blood vessels and further damage them by phagocytizing ECs.118 Simultaneously, they also attract more proinflammatory cells, such as Th1 cells by secreting CXCL9, CXCL10 and IL‐6 and IL‐23.119-123 The interaction between microglia/macrophages and monocytes and lymphocytes may further exacerbate the BBB damage and immune cascades.110 Unlike the M1 phenotype, microglia/macrophages have a distinct M2 phenotype, which is mainly protective in cerebral ischemic injury.124, 125 The impact of M2 microglia/macrophage on the BBB after stroke may include (a) immunosuppressive functions; (b) phagocytosis of ischemic debris; and (c) pro‐angiogenesis. Initially, M2 are the dominant cell type in the ipsilateral penumbra after stroke.126 They can not only express antiinflammatory cytokines, such as IL‐4, IL‐10, and TGF‐β by themselves to maintain the integrity of the neurovascular unit in murine stroke models99, 127-129 but also stimulate Th2 cells, which produce high levels of IL10 and IL13130 and drive Treg polarization by IL‐10 and TGF‐β.131 Both microglia and infiltrated macrophages can migrate into the infarction area and elicited phagocytic response, which contribute to the clearance of cell debris or hematoma in the context of ischemia and intracerebral hemorrhage.118, 132 Osteopontin (OPN), an adhesive glycoprotein,133 has been suggested as a cell surface receptor associated with their phagocytosis function after ischemic stroke.133 Phagocytosis by microglia/macrophages can also exert favorable effects through engulfment of infiltrated PMN.36 3.4 Dualistic roles of T lymphocytes on the BBB integrity after stroke Similar to neutrophils and microglia/macrophages, peripheral T lymphocytes infiltrated to the ischemic brain also exert dualistic roles on the evolution of BBB damage based on their different subtypes. T‐helper 1(Th1), Th17, γδT cells, and CD8+ T cells mainly play detrimental roles on the BBB disruption after several neurological diseases, including stroke.84, 99, 134-136 Th1 cells promote BBB permeability by secreting proinflammatory cytokines (IL‐2, IFN‐γ, and TNF‐α) and mediating a cellular immune response.137 IFN‐γ activates the small GTPase RhoA and increases the expression of Rho‐associated kinase (ROCK), which in turn phosphorylates and activates MLC.138 TNF‐α stimulates NF‐kβ to increase myosin light chain kinase (MLCK) transcription, which further correlates with increased MLCK protein levels, MLC hyper‐phosphorylation, and paracellular permeability. Activated MLCK phosphorylates MLC and decreases TJ protein amounts, leading to cytoskeletal rearrangement and impairment of TJ integrity.139 In addition, Th1 cells can interact with M1 phenotype through releasing soluble cytokines, which transform the microglia to M1 type and thereby increase secondary ischemic damage.140 Th17 cells release IL‐17, IL‐21, and IL‐2234 and clear pathogens by inflammatory immunity,141 playing a proinflammatory role distinguished from Th1 cells. Th17 cells are demonstrated to disrupt the BBB by the activation of IL‐17A and promote the recruitment of additional CD4+ lymphocytes.84 IL‐17A induced NADPH oxidase‐dependent ROS production. The resulting oxidative stress activated the endothelial contractile machinery, which was accompanied by a downregulation of the TJ molecule occluding.84, 142 As unconventional T lymphocytes, γδT cells respond swiftly to ischemia and are regarded as detrimental to the BBB, largely through their production of cytotoxic cytokines, including IL‐17.136, 143 Depletion of γδT cells reduces brain injury secondary to experimental stroke with reperfusion.144 CD8+ T cells mainly promote BBB damage and play proinflammatory roles by killing target cells directly or indirectly.144 They initiate BBB breakdown through perforin‐mediated disruption of TJs. In turn, leakage from the vasculature into the parenchyma causes brain swelling and edema.145, 146 The subtype of Th2 cells mainly exerts antiinflammatory function thus maintain the BBB integrity after stroke by releasing antiinflammatory cytokines, such as IL‐4, IL‐5, IL‐10, and IL‐13, which can promote the M2 polarization.120, 147, 148 Regulatory T cells (Tregs) are one of the most important subtypes of Th2 cells in protecting ischemic brain injury. They release TGF‐β and IL‐10149 to maintain immune tolerance and counteract tissue damage.150 They inhibit the activation of neutrophils, lymphocytes, and microglia, thus function as key endogenous modulators of postischemic neuroinflammation.151 The inhibition of neutrophils of Tregs through the inhibitory molecule programmed death‐ligand 1 significantly reduces the level of MMP9, thus protects against BBB disruption after stroke and attenuates tPA‐induced hemorrhagic transformation.152-154 Importantly, adoptive transfer of Tregs does not exacerbate poststroke immunosuppression but improves immune status after focal cerebral ischemia155 and is beneficial for protection/repair following stroke.131 However, there are also conflicting data showing that depletion of Treg in a depletion of regulatory T cell(DEREG) mouse model protects brain from acute ischemic stroke while adoptive transfer of Tregs worsens outcome after ischemic stroke.144, 150, 156 The conflict finding may be attributed to discrepancies in Treg delivery protocols used in different studies.157 3.5 The function of other immune responses after ischemic stroke In addition to the above‐mentioned immune cells that have been intensively investigated after cerebral ischemic stroke, there are also some special subsets of immune cells that gained relatively less attention in the field of poststroke BBB integrity, including mast cells, dendritic cells, B lymphocytes, and NK cells.158-160 B cells are important adaptive immune cells that have been suggested to have beneficial effects on the ischemic brain as early as 24‐48 hours after MCAO.161 Lack of B cells substantially increases infiltration of various leukocyte subpopulations into the brain and exacerbates the BBB disruption.162 On the other hand, B cells may also have detrimental effects to the ischemic brain injury by eliciting antibody‐mediated immune response. Since brain proteins are detected in the cerebral‐spinal fluid and the peripheral blood of stroke patients, these proteins could elicit the activation of antigen‐presenting cells, such as dendritic cells after stroke and later on even induce antibody production from B cells, just like what have been seen in multiple sclerosis lesions.159, 160 NK cells are key members of the innate immune system, accumulating in the ischemic hemisphere.163, 164 NK cells can function as very early responders to pathogen invasion through their cytolytic activity.163, 164 In mice with large infarcts induced by MCAO, NK cells promote local inflammation and exacerbated brain infarction and BBB damage and determine the size of the brain infarct.164, 165 In addition, the activation of the complement system, which is part of the innate immune response, has been described in clinical and experimental stroke.166 The complement system also has dual roles during the injury and recovery of ischemic stroke.167 It contributes to the recruitment and activation of immune cells, especially microglia, which may worsen BBB damage.168 It also plays a role in stroke recovery by promoting the resolution of inflammation and regeneration.169, 170 Despite the above findings, the relationship between these immune cells and the poststroke BBB integrity remains largely unknown and investigation in this regard would be of interesting and worthwhile. In conclusion, the peripheral immune response is a double edge sword for the poststroke BBB. Distinct subtypes or phenotypes of immune cells may have diverse effects on the BBB disruption or repair at distinct phases after stroke (Figure 2). Understanding the roles of immune cells and their underlying mechanisms in BBB damage may help the development of promising BBB protective strategies for ischemic stroke patients. 4 OTHER RELEASED SIGNALING FROM PERIPHERAL IMMUNE CELLS THAT MEDIATE BBB DISRUPTION OR PROTECTION AFTER STROKE Over the past decades, enormous efforts have been put in exploring the mechanisms underlying the BBB disruption or protection afforded by peripheral immune cells after stroke. The immune cells may impact the integrity of the BBB by direct contact of the endothelial cells, through their cell surface molecules. For example, the regulatory T cells (Tregs) are able to be recruited to the ischemic BBB through the chemokine receptor, CCR5,153 and inhibit neutrophil‐derived MMP9 production through the programmed death‐ligand 1 (PD‐L1) molecule, and meanwhile inhibit CCL2 expression in endothelial cells, thereby exert protective effect on BBB.152, 154, 155 Releasing proteins, proteinases, cytokines, and chemokines constitutes as another important mechanism to impact the BBB, in which some of the cytokines and chemokines propagate the inflammatory cascade and degrade the BBB structure as discussed above while others promote the BBB recovery (Table 1). In addition, the peripheral immune cells may also affect the BBB disruption after stroke through releasing exosomes, microvesicles, and miRNAs. Name Source cells Mechanisms Effects References IL‐1 Mononuclear cells TJ disruption; upregulation of ICAM‐1; activation of MMPs Disruption 171, 172 IL‐6 Macrophages, T cells, endothelia cells TJ protein loss; PKC‐dependent cytoskeletal rearrangement; Disruption 173, 174 IL‐9 Mononuclear cells and T cells Induce eNOS production; downregulation phosphorylated pkβ/pp3k signaling; TJ protein loss Disruption 175, 176 IL‐17 Th17 cells and γδT cells Induce ROS production Disruption 34, 144 IFN‐γ T cells Activate the small GTPase RhoA and activate myosin light chains Disruption 138 MIF Endothelia cells and macrophages Disruption TJs Disruption 117 TNF‐α CD4+ T cells, NK cells, neutrophils, astrocytes, and neurons Downregulation of TJ proteins Disruption 174, 177 CCR5 Microglia and astrocytes Enhance MMP9 activity, regulate the migration and activity of T cells, monocytes, and dendricytes Disruption 24, 178 CCL2 Astrocytes, microglia, EC, and macrophages Redistribute the TJs and AJs and reorganization of actin cytoskeleton Disruption 179, 180 HMGB1 Neurons Induces a contractile response in pericytes and vascular ECs Disruption 181, 182, 187 TGF‐β Microglia/macrophages Inhibit MMPs Recovery 184, 185 IL‐1α Macrophages Induce angiogenic mediator expression and promote formation of tube‐like structures Recovery 186, 187 IL‐10 Th2 cells Promote the M2 polarization and maintain immune tolerance Recovery 27, 148, 150 LCN‐2 Neutrophils and neurons Enhance angiogenesis and induce tube formation and migration Recovery 188, 189 HIF‐1α Lymphocytes Regulate VEGF and control MMPs induce BBB damage or promote angioneurogenesis Disruption/recovery 190-192 MMP9 Neutrophils and ECs Degrad the TJ proteins and basal lamina proteins and active proinflammatory agents: CXCL‐8, IL‐1β or TNF‐α Disruption/recovery 36, 69, 89, 193, 194 Facilitate tissue remodeling, activate bound growth factors: VEGF‐A, regulate pro‐angiogenesis TJ, tight junction; ICAM‐1, intercellular adhesion molecule; MMP, matrix metalloproteinase enzymes; PKC, protein kinase C; eNOS, endothelial nitric oxide synthase; ROS, reactive oxygen species; GTPase, guanosine triphosphatase; RhoA, Ras homolog gene family, member A; MIF, macrophage migration inhibitory factor; IFN‐γ, interferon gamma; TNF‐α, tumor necrosis factor alpha; NK cell, natural killer cell; CCR5, C‐C chemokine receptor type 5; CCL2, chemokine (C‐C motif) ligand 2; HMGB1, high‐mobility group box‐1 protein; TGF‐β, transforming growth factor beta; EC, endothelial cell; LCN‐2, lipocalin‐2; VEGF, vascular endothelial growth factor; HIF‐1α, hypoxia‐inducible factor‐1 4.1 Exosomes Emerging evidence is showing that activation of peripheral immune cells may release exosomes and microvesicles, both of which have been implicated in the evolving of BBB damage after stroke.195 Exosomes are endosome‐derived small membrane vesicles. They carry proteins, lipids, and genetic materials and play essential roles in intercellular communication between source and target cells under physiological and pathophysiological conditions.196, 197 Both immune cells and nonimmune cells can secret exosomes. It is recently suggested that exosomes released from activated immune cells are responsible for carrying proinflammatory contents including miRNAs to the brain via the brain endothelium.195 These exosomes alone can activate human brain microvascular endothelial cells to increase the expression of adhesion molecules such as CCL2, ICAM1, VCAM1, and cytokines such as IL‐1β and IL‐6.195, 198 Preventing exosome release from activated monocytes could completely inhibit the expression of inflammatory molecules on brain endothelial cells and therefore regulate the BBB function under different diseases.195, 198 Exosomes from different cell types may have diverse functions on the BBB integrity. It has been shown that exosomes from circulating endothelia progenitor cells and stem cells may transfer miRNAs into cerebral endothelial cells and pericytes, thus activate PI3K/Akt signaling pathway and notch signaling pathway to mediate angiogenesis and to maintain BBB integrity.199-201 Thus, it is highly possible that there are specific subtypes of peripheral immune cells may release exosomes carrying BBB protective properties. However, studies in this regard are still warranted. 4.2 Microvesicles Microvesicles (MVs) are small membranous vesicles released from various cells in response to diverse biochemical agents or mechanical stresses.202 Leukocyte‐derived microvesicles (LMVs) are one of microvesicles, which act as proinflammatory mediators implicated in some diseases.203, 204 LMVs originate from mature leukocytes, including monocyte, lymphocyte, and granulocytes.205 It is suggested that LMVs are involved in the vascular inflammation in cardiovascular diseases and cerebrovascular diseases including stroke.206, 207 LMVs can increase the production of TNF‐α, IL‐6, IF‐8, activated protein C, and IF‐1β206 and induce the translocation of NF‐kβ into the nucleus, leading to increased production of IL‐8 and monocyte chemoattractant protein 1(MCP1),208 both of which can promote the inflammatory response, leading to vascular endothelial cell dysfunction and vascular permeability. During cerebral ischemia, circulating MVs increase significantly and cause a large increase in barrier permeability and reduce trans‐epithelial electrical resistance (TEER) in in vitro endothelial barriers.209 MVs themselves contain pro‐TNF‐α, RhoA, and Rho‐associated protein kinase (ROCK), increasing the permeability of barriers in rat brain microvascular endothelial cells (RBMVECs) by activating caspase 3 and Rho/ROCK signaling pathways.209 4.3 MicroRNAs MicroRNAs are small noncoding RNAs that broadly affect cellular and physiological function in all multicellular organisms. More than 5000 miRNAs likely exist in humans and each miRNA binds an average of 200 RNAs.210 MicroRNAs are divided into three categories, for example, proinflammatory, antiinflammatory, and mixed immunomodulatory. All of these regulate neuroinflammation in various pathologies, including spinal cord injury, multiple sclerosis, and ischemic stroke.211 After ischemic stroke, miRNAs can also mediate BBB disruption by regulating gene expression at transcriptional and posttranscriptional levels.212, 213 MiR‐130a aggravates BBB leakage and brain edema via various ways.214 It executes its damaging effects on BBB by downregulating HoxA5 and thereby reducing occludin expressions.213 Besides HoxA5, microRNA‐130a might act as a suppressor of aquaporin 4 by targeting its transcripts.215 MiR‐130a can also reduce the expression of caveolin‐1 and increase the level of MMP‐2/9, which contributes to the increased permeability of BBB and increased perihematomal edema after intracerebral hemorrhage.214 MiRNA‐15a (miR‐15a) has recently been shown to contribute to the pathogenesis of ischemic vascular injury through direct inhibition of the antiapoptotic gene bcl‐2.216 Of particular interest, miR‐15a itself was found to be transcriptionally regulated by peroxisome proliferator‐activated receptor (PPARδ). Administration of PPARδ agonist significantly reduced ischemia‐induced miR‐15a expression, increased bcl‐2 protein levels, and attenuated caspase‐3 activity, leading to decreased BBB disruption and reduced cerebral infarction in mice after transient focal cerebral ischemia.216 In addition, miR‐15a can suppress the angiogenesis in the peri‐infarct region by decreasing FGF2 and VEGF levels,217 thus downregulation miR‐15a can promote angiogenesis and maintain BBB integrity.201 Overexpression of let‐7 and miR‐98 in vitro and in vivo resulted in reduced leukocyte adhesion to and migration across endothelium, diminished expression of proinflammatory cytokines, and increased BBB tightness, attenuating barrier “leakiness” in neuroinflammation conditions.212 Therefore, a variety miRNAs could be used as a therapeutic tool to prevent neuroinflammation and BBB dysfunction. Recent findings in exosomes, microvesicles, and miRNAs have evidenced that their releases from peripheral immune cells play critical roles in the evolution of BBB pathology after stroke. Notably, exosomes, microvesicles, and miRNAs released from distinct immune cells under distinct contexts may exert divergent roles on the BBB integrity after stroke. 5 SUMMARY AND CONCLUSION Targeting the highly dynamic events that occur during stroke in the relatively inaccessible brain microenvironment is challenging. Emerging evidence suggests that peripheral immune cells could provide promising therapeutic targets to rescuing BBB after stroke. In clinical, some drugs with translational potential to target the peripheral immune response in order to preserve the BBB integrity after stroke are being tested in clinical settings, such as minocycline, adjudin, and curcumin.218-222 Further understanding of the interactions between the immune system and the BBB disruption and repair process could move the translation of promising preclinical results forward. Recent studies suggest that the peripheral immune response is a double edge sword both for the disruption and repair of BBB after stroke. Distinct subtypes or phenotypes of immune cells may have diverse impacts on the BBB integrity at distinct phases after stroke. Considering the double facet roles of immune cells and their pleiotropic underlying mechanisms in BBB damage and repair, we envision that researches regarding the interaction between peripheral immune cells and BBB may gain increasing attention in the pursuit of developing effective and easy accessible therapeutic targets of stroke. ACKNOWLEDGMENTS P.L is supported by the National Natural Science Foundation of China (NSFC) grant 81400956 and 81722017, the Shanghai Rising‐Star Program (16QA1402600), the Shanghai Natural Science Program (13ZR1452200) by the Science and Technology Commission of Shanghai Municipality and the Shanghai Municipal Education Commission—Gaofeng Clinical Medicine Grant Support (20181805). W. Y is supported by the NSFC (81370513, 81571048). This work is also supported by the Shanghai Municipal commission of Health and Family Planning Funding for Key Developing Disciplines (2015ZB0101). CONFLICT OF INTEREST None. REFERENCES Notes : The first two authors contributed equally to this work. TJ, tight junction; ICAM‐1, intercellular adhesion molecule; MMP, matrix metalloproteinase enzymes; PKC, protein kinase C; eNOS, endothelial nitric oxide synthase; ROS, reactive oxygen species; GTPase, guanosine triphosphatase; RhoA, Ras homolog gene family, member A; MIF, macrophage migration inhibitory factor; IFN‐γ, interferon gamma; TNF‐α, tumor necrosis factor alpha; NK cell, natural killer cell; CCR5, C‐C chemokine receptor type 5; CCL2, chemokine (C‐C motif) ligand 2; HMGB1, high‐mobility group box‐1 protein; TGF‐β, transforming growth factor beta; EC, endothelial cell; LCN‐2, lipocalin‐2; VEGF, vascular endothelial growth factor; HIF‐1α, hypoxia‐inducible factor‐1

In this Review, Handel and colleagues examine the contribution of thymic T cell selection to CNS autoimmune conditions and consider how a better understanding of this contribution could lead to novel therapeutic strategies for these conditions.

Abstract Substance use disorders are global health problems with few effective treatment options. Unfortunately, most potential pharmacological treatments are hindered by abuse potential of their own, limited efficacy, or adverse side effects. As a consequence, there is a pressing need for the development of addiction treatments with limited abuse potential and fewer off target effects. Given the difficulties in developing new pharmacotherapies for substance use disorders, there has been growing interest in medications that act on non‐traditional targets. Recent evidence suggests a role for dysregulated immune signaling in the pathophysiology of multiple psychiatric diseases. While there is evidence that immune responses in the periphery and the central nervous system are altered by exposure to drugs of abuse, the contributions of neuroimmune interactions to addictive behaviors are just beginning to be appreciated. In this review, we discuss the data on immunological changes seen in clinical populations with substance use disorders, as well as in translational animal models of addiction. Importantly, we highlight those mechanistic findings showing causal roles for central or peripheral immune mediators in substance use disorder and appropriate animal models. Based on the literature reviewed here, it is clear that brain‐immune system interactions in substance use disorders are much more complex and important than previously understood. While much work remains to be done, there are tremendous potential therapeutic implications for immunomodulatory treatments in substance use disorders. Introduction Substance use disorders affect millions of individuals in the United States and around the world. The consequences of illicit drug use, including death from overdose (Rudd et al., 2016), the spread of infectious disease (Alcabes & Friedland, 1995), and shorter life expectancies (Smyth et al., 2006) all contribute to the tremendous cost of addiction and create significant social and economic burden for users, their families, and society at large (Patel et al., 2016; Jiang et al., 2017). While overall illicit drug use rates have remained relatively steady, use of certain drugs of abuse, including opioids, has been on the rise and rates of pathological drug use remain dangerously high (Substance Abuse and Mental Health Services Administration, 2008). Despite the dangers associated with addiction, few pharmacological treatments are available for substance use disorders and these treatments often have limited efficacy. Varenicline and bupropion have been approved for nicotine use disorder (Gómez‐Coronado et al., 2018), while naltrexone, acamprosate, and disulfiram (Soyka & Müller, 2017) are approved for alcohol use disorder. For opioid use disorder the primary means of treatment is with opioid replacement therapies such as methadone or buprenorphine. While opioid replacement therapy is often effective as a means of risk reduction, there are myriad associated adverse effects and potential complications (Mattick et al., 2014; Schuckit, 2016). For psychostimulant use disorders there are currently no FDA‐approved pharmacotherapies. Agonist therapies targeting dopaminergic systems have been heavily investigated for treatment of psychostimulant use disorders, however, the potential for significant abuse potential and side effects have limited their use (Castells et al., 2016). Similarly, antipsychotic and/or antidepressant medications have been tested for multiple substance use disorders, but these trials have been hampered by limited efficacy and high rates of undesirable side‐effects including sexual dysfunction and weight gain (Tollefson, 1991; Solmi et al., 2017). Given the negative health consequences and economic burden ascribed to addiction, there is clearly a pressing need for more effective treatment options with fewer side effects. In recent years, there has been a growing understanding that interactions of the immune system and the central nervous system are likely playing causal roles in the pathophysiology of multiple psychiatric illnesses. As a result of observational clinical studies and translational animal studies, we are moving closer to understanding the mechanistic links between immune system dysregulation and psychiatric pathology. These findings have led to a recent uptick in clinical trials investigating immune or inflammatory treatment targets in psychiatric disease. While the role of inflammatory processes is better understood in other psychiatric conditions, such as depression and schizophrenia (Hodes et al., 2015; Prata et al., 2017), recent work has identified neuroimmune mechanisms as possibly important in addictive disorders as well. Here, we will present a brief overview of the literature delineating the role of neuroimmune processes in psychiatric pathology and will then discuss clinical and translational evidence for these processes in illicit substance use disorders. For the purposes of this review we will focus primarily on psychostimulants and opioids as these are the illicit substances that account for the greatest levels of morbidity and mortality. The role of the immune system in alcoholism has been reviewed thoroughly elsewhere (Mayfield et al., 2013; Montesinos et al., 2016; Crews et al., 2017). Thus, this review will provide a perspective on the clinical and basic science literature of inflammation in addictive disorders with the goal of highlighting promising translational strategies that have potential for rapid clinical translation. Inflammation and the brain The immune system is the major regulator of inflammatory processes and is composed of the innate and adaptive immune systems. The innate system contains cells of the myeloid lineage, including granulocytes, monocytes, macrophages and natural killer cells (Galli et al., 2011; Montaldo et al., 2014) while the adaptive immune system is largely composed of T and B lymphocytes (den Haan et al., 2014). The innate immune system is available to mount rapid inflammatory response to either sterile or microbial injuries. This occurs primarily via pattern recognition receptors (PRR) that detect the presence of foreign entities via pathogen‐associated molecular patterns (PAMPs) and endogenous danger‐associated molecular patterns (DAMPs) that come from damaged cells (Frank et al., 2015; Portou et al., 2015). The response to DAMPs and PAMPs in the central nervous system (CNS) is primarily mediated by tissue resident macrophages called microglia (Salter & Stevens, 2017), although astrocytes have also been shown to contribute to immune responses (Bylicky et al., 2018). Upon activation of PRRs, these cells produce and release cytokines, which have myriad effects. Pro‐inflammatory cytokines (e.g., IL‐1β & tumor necrosis factor‐alpha, TNF‐α) are necessary to launch appropriate immune responses to pathogens or injury and aid in the proliferation and recruitment of phagocytic cells at the site of insult (Shi & Pamer, 2011; Zhang & Wang, 2014). Conversely, anti‐inflammatory cytokines (e.g., IL‐10) are associated with tissue repair and help maintain normal tissue homeostasis (Khanna et al., 2010; Ferrante & Leibovich, 2012). To properly respond to pathogen exposure and tissue damage, organisms must have a balanced pro‐ to anti‐inflammatory profile. The inability to mobilize and recruit macrophages to a site of injury or infection, present when pro‐inflammatory cytokines are low, can impair an organism's ability to fight invading pathogens. However, prolonged inflammation, present when pro‐inflammatory cytokines are high, can cause tissue damage and this pro‐inflammatory state significantly contributes to several chronic diseases, including psychiatric pathologies (Dinarello, 2011; Hodes et al., 2015). Inflammation and addiction The concept of immune system derangements in psychiatric conditions has been around for many years, but recently has begun to gain more attention both for its role in the pathophysiology of mental illness and also as a potential therapeutic strategy (Hodes et al., 2015; Miller & Goldsmith, 2017). Indeed, inflammatory changes have been seen in multiple neuropsychiatric conditions such as autism spectrum disorder (Vargas et al., 2005), bipolar disorder (Sayana et al., 2017), Alzheimer's disease (Salter & Stevens, 2017), and schizophrenia (Ripke et al., 2014; Wray & Sullivan, 2018). Research into the role of neuroimmune interactions in substance use disorders has been more limited than in the psychiatric conditions described above. Despite this, it has been known for some time that neuroimmune interactions can lead to important changes in brain systems that are key to the development of substance use disorders. In particular, there is a wealth of literature demonstrating that inflammatory changes can affect dopamine signaling within the brain. Healthy controls given a typhoid vaccination showed increased activation of their substantia nigra by fMRI, a change that correlated with increased serum levels of IL‐6 seen after the vaccine (Brydon et al., 2008). Similar effects have been reported in patients receiving infusions of the interferon‐α (IFN‐α) pro‐inflammatory cytokine (Capuron et al., 2012). It was previously shown that inflammatory conditions are capable of decreasing dopamine synthesis by affecting important cofactors and may even affect dopamine transporter function. This literature linking inflammation with altered dopaminergic signaling is reviewed in much more detail elsewhere (Felger & Miller, 2012; Felger & Treadway, 2017). Given that drugs of abuse all work via increasing mesolimbic dopamine release (Chao & Nestler, 2004), the possibility of inflammation and drug interactions to heighten these effects is a real possibility. Inflammation has also been found to affect glutamatergic signaling, which is another key neurotransmitter system in drug addiction and relapse (Dong et al., 2017). High levels of peripheral inflammation in patients with major depressive disorder are correlated with decreased functional connectivity of the glutamatergic projection neurons between the frontal cortex and ventral striatum (Felger et al., 2016). Also, pro‐inflammatory cytokines downregulate glutamate transporters at the synapse thus disrupting the homeostasis of glutamate signaling in the circuit (Tilleux & Hermans, 2007; Felger & Treadway, 2017). Work in animal models of addiction has also shown that glutamate transporters are downregulated in cocaine treated animals, and that this increase in synaptic glutamate can contribute to relapse behaviors (Moussawi et al., 2009; Scofield & Kalivas, 2014). Electrophysiological studies have demonstrated that the TNF‐⍺ cytokine is an important regulator of synaptic strength in striatal medium spiny neurons (Lewitus et al., 2014). Given the importance of corticostriatal signaling in both drug reward and relapse to drug seeking, the effects of inflammation on this circuit could have major implications for the development and persistence of substance use disorders. Inflammation and psychostimulants Psychostimulants such as cocaine and amphetamines are highly addictive substances that increase levels of dopamine in the CNS and the periphery via inhibition or reversal of the dopamine transporter (Kalivas et al., 2005). Many populations of leukocytes including B cells, T cells, and monocytes are known to express different subtypes of dopamine receptor, and stimulation with dopamine can affect their production of cytokines and other inflammatory mediators (Arreola et al., 2016). In recent years, there is additional evidence that cocaine itself may engage PRRs and create an inflammatory response independent of its effects on dopamine (Northcutt et al., 2015). Cocaine is well established as a factor that increases HIV replication rates in infected microglia and macrophages (Webber et al., 1999; Tyagi et al., 2016). Stimulants also lead to activation of the hypothalamic‐pituitary‐adrenal axis leading to the release of cortisol which has acutely anti‐inflammatory effects and chronically pro‐inflammatory effects (Manetti et al., 2014). While stimulants can affect immune function via multiple mechanisms, the role of these inflammatory changes in the development and persistence of addictive behaviors is a matter of ongoing research. Clinical findings There are a number of studies that have examined the effects of cocaine on peripheral expression of inflammatory mediators (Summarized in Table 1), including studies showing that acute cocaine alters cytokine expression in serum or in isolated peripheral leukocytes. In abstinent users that there are decreases in serum levels of the chemokine monocyte chemoattractant protein 1 (MCP‐1) and a number of pro‐inflammatory cytokines including IL‐6, IL‐17, and TNF‐α among others (Pedraz et al., 2015; Maza‐Quiroga et al., 2017; Gupta et al., 2018). Of note, chemokines are frequently discussed using two different nomenclatures, which can lead to confusion. In an attempt to simplify, we have provided a side‐by‐side comparison of the naming structure of all chemokines in this review in Table 2. Intravenous infusion of cocaine suppressed expression of serum IL‐6 for four hours in healthy controls (Halpern et al., 2003). This is corroborated by a finding that isolated monocytes from patients who are actively using cocaine have a decreased IL‐6 and TNF‐α pro‐inflammatory response to a bacterial antigen challenge, an effect that was made more pronounced by acute infusion of cocaine (Irwin et al., 2007). However, not all studies show this same pattern of decreased inflammatory markers. More recently studies have shown that active cocaine users express higher levels of IL‐6 and decreased levels of the anti‐inflammatory cytokine IL‐10 (Levandowski et al., 2016; Moreira et al., 2016). Of particular interest, recent studies have shown that patients with psychostimulant use disorders exhibit increased serum expression of pro‐inflammatory markers in response to drug cues (Fox et al., 2012) or in response to unpleasant images (Ersche et al., 2014) – an effect not seen in healthy controls. These studies suggest that while the immune system of drug users may be inhibited in certain situations it may also be primed to respond to certain environmental stimuli in a pro‐inflammatory manner. While far from conclusive, it is possible that these changes in peripheral immune function may be playing a role in the development or persistence of psychostimulant use disorders. Drug Species Target Direction Location Administration Citation Cocaine Animal G‐CSF ↑ Peripheral/Central Repeated Calipari et al. (2018) Cocaine Human IL‐10 ↓ Peripheral Active users Moreira et al. (2016) Cocaine Human IL‐17 ↓ Peripheral Abstinence Maza‐Quiroga et al., (2017) Cocaine Human IL‐6 ↓ Peripheral Acute Halpern et al., (2003) Abstinence Pedraz et al., (2015) Cocaine Human IL‐6 ↑ Peripheral Active users Levandowski et al., (2016) Moreira et al., (2016) Cocaine Human MCP‐1 ↓ Peripheral Acute Gupta et al., (2018) Abstinence Pedraz et al., (2015) Cocaine Human MIP‐1α ↓ Peripheral Abstinence Maza‐Quiroga et al., (2017) Cocaine Animal SDF‐1 ↑ Peripheral Repeated Araos et al., (2015) Cocaine Human TGF‐α ↓ Peripheral Abstinence Maza‐Quiroga et al., 20170 Cocaine Animal TNF‐α ↑ Central Repeated Lewitus et al., (2016) Cocaine Human TNF‐α ↓ Peripheral Abstinence Pedraz et al., (2015) Opioid Animal ECP ↑ Central Acute Schwarz et al., (2011) Opioid Animal Fractalkine ↑ Central Repeated Hutchinson et al., (2008a) Opioid Animal GRO/KC ↑ Central Repeated Hutchinson et al., (2008a) Opioid Animal IFN‐γ ↑ Peripheral/Central Acute Pacifici et al., (2000) Schwarz et al., (2011) Opioid Animal IL‐10 ↑ Peripheral/Central Repeated Hutchinson et al., (2008a) Acute Schwarz et al., (2011) Opioid Animal IL‐17A ↑ Peripheral Chronic Meng et al., (2015) Opioid Animal IL‐1β ↑ Peripheral/Central Acute Eidson et al., (2016) Repeated Hutchinson et al., (2008a) Chronic Johnston et al., (2004) Chronic Kang et al., (2017) Repeated Raghavendra et al., (2002) Opioid Animal IL‐2 ↓ Peripheral Acute Lysle et al., (1993) Opioid Animal IL‐2 ↑ Peripheral Acute Pacifici et al., (2000) Opioid Animal IL‐6 Peripheral/Central Repeated Hutchinson et al., (2008a) Chronic Johnston et al., (2004) Chronic Meng et al., (2015) Repeated Raghavendra et al., (2002) Chronic Zhang et al., (2011) Opioid Animal MCP‐1 ↑ Central Repeated Hutchinson et al., (2008a) Opioid Animal MCP‐5 ↑ Central Acute Schwarz et al., (2011) Opioid Animal MIG ↑ Central Acute Schwarz et al., (2011) Opioid Animal MIP‐1α ↑ Central Repeated Hutchinson et al., (2008a,b) Opioid Animal MIP‐1β ↑ Central Acute Schwarz et al., (2011) Opioid Animal RANTES ↑ Central Repeated Campbell et al., (2013) Opioid Animal TARC ↑ Central Acute Schwarz et al., (2011) Opioid Animal TECK ↑ Central Acute Schwarz et al., (2011) Opioid Animal TNF‐α ↑ Peripheral/Central Chronic Johnston et al., (2004) Repeated Niwa et al., (2007) Acute Pacifici et al., (2000) Repeated Raghavendra et al., (2002) Chronic Zhang et al., (2011) A number of cytokines and chemokines have been shown to be differentially regulated by stimulants or opioids in the periphery or in the CNS of human subjects and animal models. This table outlines the known changes in these factors by substance, location, direction and subjects. Traditional Modern MCP‐1 (monocyte chemoattractant protein 1) CCL2 MIP‐1α (macrophage inflammatory protein‐1 alpha) CCL3 MIP‐1β (macrophage inflammatory protein‐1 beta) CCL4 RANTES (regulated on activation, normal T cell expressed and secreted) CCL5 MCP‐2 (monocyte chemoattractant protein 2) CCL8 ECP (eosinophil chemotactic protein) CCL11 MCP‐5 (monocyte chemoattractant protein 5) CCL12 TARC (thymus and activation‐regulated chemokine) CCL17 TECK (thymus‐expressed chemokine) CCL25 GRO/KC (growth‐related oncogene/keratinocyte chemoattractant) CXCL1 MIG (monokine induced by gamma interferon) CXCL9 SDF‐1 (stromal cell‐derived factor 1) CXCL12 Fractalkine CX3CL1 CCL2, chemokine (C‐C motif) ligand 2; CCL3, chemokine (C‐C motif) ligand 3; CCL4, chemokine (C‐C motif) ligand 4; CCL5, chemokine (C‐C motif) ligand 5; CCL8, chemokine (C‐C motif) ligand 8; CCL11, chemokine (C‐C motif) ligand 11; CCL12, chemokine (C‐C motif) ligand 12; CCL17, chemokine (C‐C motif) ligand 17; CCL25, chemokine (C‐C motif) ligand 25; CXCL1, chemokine (C‐X‐C motif) ligand 1; CXCL9, chemokine (C‐X‐C motif) ligand 9; CXCL12, chemokine (C‐X‐C motif) ligand 12; CX3CL1, chemokine (C‐X3‐C motif) ligand 1. Given that diverse populations of leukocytes express dopamine receptors, it is entirely possible that changes in peripheral inflammation seen after psychostimulants will not necessarily correlate with central inflammatory processes. However, it is important to note that populations of peripheral monocytes and T cells have been shown to have marked effects on brain and behavior and may even be able to extravasate into the CNS parenchyma (Hodes et al., 2015; Menard et al., 2017). Evidence for central inflammatory mechanisms in human populations with psychostimulant use disorder is somewhat sparse. Postmortem examination of the midbrain of cocaine addicts demonstrated an increase in both activated microglia, CD68 +  activated macrophages, and decreased number of dopamine cell bodies (Little et al., 2009). This is suggestive of a central pro‐inflammatory mechanism engaged by cocaine use, but this finding is subject to confounds from both the lifestyle of the patients as well as the myriad issues with histological analysis of postmortem tissue. More recently there have been two positron emission tomography (PET) studies performed in human subjects with substance use disorder using tracers believed to bind to activated glial cells (Banati, 2002). Subjects with methamphetamine use disorder with 1‐12 years of reported abstinence were found to have increased markers of microglial activation, with a negative correlation between time of abstinence and microglial activity (Sekine et al., 2008). A similar study looked at patients with long‐term cocaine use disorder who had at least two weeks of abstinence by urine toxicology and found no differences in binding of microglial radiotracers between controls and patients with cocaine use disorder (Narendran et al., 2014). While important, these studies should be viewed with some caution given the very different lengths of abstinence, and that these PET tracers are known to not be fully specific for activated microglia (Cosenza‐Nashat et al., 2009). Animal models In addition to the clinical studies outline above, there has been a growing body of research investigating the role of neuroinflammatory changes in animal models of psychostimulant use disorder. In particular there has been interest in the role of psychostimulants and activating microglia. There is also considerable evidence for psychostimulant activation of astrocytes and other glial cells reviewed more extensively by Lacagnina et al., (2017b). Methamphetamine has been repeatedly shown to induce microglial activation throughout the brain in a dose‐dependent manner (Thanos et al., 2016). Studies have found that inhibition of microglial activation via the anti‐inflammatory antibiotic minocycline, or via the phosphodiesterase inhibitor and antagonist of the toll‐like receptor 4 (TLR‐4) ibudilast reduce the rewarding properties of stimulants (Chen et al., 2009; Snider et al., 2013; Attarzadeh‐Yazdi et al., 2014). While the exact mechanism of microglial activation by psychostimulants is not fully clear, work is ongoing to make this determination. Recently, there has been growing evidence that cocaine in particular may bind to TLR4, a PRR expressed on microglia in the CNS (Zhang et al., 2014). Northcutt et al., (2015) did in silico and in vitro modeling to determine that cocaine binds the TLR4 receptor. Additionally, they found that signaling through the TLR4 receptor was important for cocaine‐induced dopamine release as well as conditioned place preference (CPP) and self‐administration of cocaine (Fig. 1). Subsequent studies have shown that TLR4 is necessary for basal synaptic transmission and NMDA‐dependent synaptic plasticity in the nucleus accumbens (NAc; Kashima & Grueter, 2017). Another study found that TLR4 activity in the ventral tegmental area (VTA) affects reinstatement of cocaine seeking in a manner dependent on IL‐1 signaling (Brown et al., 2018). However, another study from Tanda et al., (2016) disputes the effects of the TLR4 antagonists (+)‐naloxone on dopamine release and reports the effects on behavior to be non‐specific (Tanda et al., 2016). While the role of TLR4‐mediated signaling in models of substance use remains a subject of ongoing research, it does represent a potentially important line of translational research in the neurobiology of addiction. In addition to the studies on TLR signaling, there has also been considerable interest in the role of cytokine and chemokine signaling in models of psychostimulant use disorders. These immune mediators are expressed by, and have receptors on, myriad cell types in the brain and can have diverse and complex effects on plasticity and behavior that are only beginning to be understood (Wohleb et al., 2016; Peineau et al., 2018). Recently, our group identified granulocyte colony‐stimulating factor (G‐CSF) as a cytokine upregulated in both serum and brain after prolonged cocaine administration (Calipari et al., 2018). Interestingly, G‐CSF treatment lead to increased neuronal activation in the NAc and prefrontal cortex after an acute cocaine injection. Behaviorally, G‐CSF enhanced cocaine CPP, locomotor sensitization and self‐administration (Fig. 1) while inhibition of G‐CSF signaling in the NAc abolished the formation of cocaine CPP. Others have found a role for canonical pro‐inflammatory cytokines in the neuronal and behavioral response to cocaine. Lewitus et al. (2016) demonstrated that microglial production of TNF‐α is increased in NAc after cocaine where it inhibits AMPA receptor function specifically in dopamine D1 receptor containing medium spiny neurons (Fig. 1). Additionally, TNF‐α knockouts showed increased locomotor sensitization to cocaine and inhibition of TNF‐α signaling reduced cocaine sensitization. While the paper from Lewitus and colleagues found that TNF‐α seemed to be inhibitory to the behavioral and neurophysiologic sequelae of cocaine, the above mentioned paper found that signaling via TLR‐4 via IL‐1β enhanced behavioral response to cocaine and was necessary for cocaine‐induced dopamine release into the NAc (Northcutt et al., 2015). Given that both TNF‐α and IL‐1β are known to be downstream of the TLR4 receptor, these findings appear somewhat contradictory. While the Lewitus group looked at locomotor sensitization and glutamatergic plasticity in the NAc, the Northcutt group focused primarily on the rewarding effects of cocaine and dopamine release from VTA neurons. This raises the possibility that these two closely‐related signaling pathways may have differing effects on various aspects of the response to drugs of abuse, and certainly suggests that a more nuanced understanding of the molecular pathways will be crucial. In addition to these cytokines, chemokine signaling has also been implicated. Infusions of MCP‐1 into the midbrain result in increased locomotor activity and increased striatal dopamine release (Guyon et al., 2009), whereas knockout of MCP‐1 receptor C‐C chemokine receptor 2 (CCR2) leads to decreased locomotor sensitization to cocaine and decreased activation of the ERK signaling cascade in the striatum (Trocello et al., 2011). Additionally, levels of stromal cell‐derived factor 1 (SDF‐1) are increased in both humans and mice after prolonged exposure to cocaine (Araos et al., 2015), and intraventricular or VTA infusion of SDF‐1 protein potentiates the locomotor response to cocaine (Trecki & Unterwald, 2009). The microbiome and psychostimulants A topic of growing interest in the field is in the interactions between the gut microbiome, the immune system and the brain – the so‐called gut‐immune‐brain axis (Fung et al., 2017). While the majority of work looking at the role of the microbiome in neuropsychiatric diseases has focused on affective (Foster et al., 2017), neurodevelopmental (Vuong & Hsiao, 2017), and neurodegenerative (Sampson et al., 2016) disorders, there is growing evidence that alterations in the gut microbiome might be playing a role in the development of addictive disorders via their interactions with the immune system. Recently, Volpe et al. (2014) examined patients with cocaine use disorder and healthy controls with and without HIV and found that cocaine use disorder was an independent predictor of an altered gut microbiome – with users having markedly increased representation of the Bacteroidetes phylum. Additionally, cocaine users had a strong trend toward higher levels of bacterial DNA in their serum, which is suggestive of increased bacterial translocation from the gut that may result in a pro‐inflammatory state. While evidence of interactions between the gut microbiome and psychostimulants has been relatively limited, there is growing evidence from translational studies that the gut microbiome can affect the behavioral response to drugs of abuse, and in turn the microbiome can be altered by psychostimulant treatment. Recently, our group found that knockdown of the gut microbiome with broad spectrum antibiotics altered CPP and locomotor sensitization to cocaine, finding that microbiome depleted animals were more sensitive to the rewarding and sensitizing effects of cocaine at low doses (Kiraly et al., 2016). We also found that depletion of the microbiome affected gene expression in the NAc, resulting in increased levels of brain derived neurotrophic factor (BDNF), D1 and D2 dopamine receptors, and GluR2 AMPA receptors. This corroborated a number of previous reports suggesting that alterations in the gut microbiome affect brain expression of BDNF and dopamine receptors (Bercik et al., 2011; Heijtz et al., 2011; Savignac et al., 2013; Desbonnet et al., 2015). Given the importance of these pathways in the pathophysiology of addictive behaviors, further investigation is needed to establish causal links between the microbiome and BDNF and dopamine signaling in the brain. While a number of groups are currently investigating the effect of psychostimulant drugs on the composition of the gut microbiome, the only currently published study is one in which rats were injected every other day with methamphetamine. This study finds modest increases in bacterial diversity after methamphetamine with small shifts in families of bacteria (Ning et al., 2017). Additionally, they found a decrease in caecal content of the short chain fatty acid propionate. The production of short chain fatty acids by the gut microbiota has been implicated in the integrity of the blood–brain barrier, and was found to play a key role in the behavioral effects of the microbiota and cocaine in the (2016) Kiraly et al. paper. While there is clearly more work to be done, evidence for gut‐immune‐brain connection in addiction is mounting. Inflammation and opioids Clinical findings Opioids are used as first‐line treatment for many types of acute and chronic pain (Zuckerman & Harris, 1994; Roberts et al., 2012; Mesgarpour et al., 2014) and are often prescribed to individuals suffering from other chronic illnesses with weakened immune systems. Despite their abundant use in pain management, recent evidence suggests that opioids might be impairing recovery due to their interactions with inflammatory pathways. Several clinical studies have noted that opioid abusers have worse overall health and are more susceptible to infectious disease complications including HIV infection (Donahoe & Vlahov, 1998; Risdahl et al., 1998). The potentiated negative health outcomes, resulting from opioid exposure are likely due, in part, to general immunosuppression (Budd, 2006; Sacerdote, 2008). As it pertains to HIV, one study demonstrated that heroin users have lower levels of the retrovirus restriction factors tripartite motif‐containing 22 (TRIM22), tripartite motif‐containing 5α (TRIM5α), apolipoprotein B mRNA editing enzyme catalytic polypeptide‐like 3G (APOBEC3G), and IFN‐α, which leads to decreased the ability to combat retroviral infection (Zhu et al., 2017). Additionally, leukocytes and granulocytes exposed to morphine show decreased levels of activation and have impaired chemotactic ability (Pérez‐Castrillón et al., 1992; Makman et al., 1995; Grimm et al., 1998). Studies measuring cytokine release in human cells have generated mixed results (Table 1). First, heroin users have higher levels of circulatory inflammatory cytokines such as TNF‐α, transforming growth factor α (TGF‐α), and IL‐8 (Piepenbrink et al., 2016). Conversely, giving morphine to healthy volunteers results in natural killer cells that produce less cytotoxic response compared to placebo‐treated volunteers (Yeager et al., 1995). When administered alone, a mu opioid receptor selective agonist increased release of several cytokines including TGF‐β1 and regulated on activation, normal T cell expressed and secreted (RANTES) from lymphocytes (Happel et al., 2008) and morphine increased MCP‐1 expression in cultured brain cells (Rock et al., 2006). Yet, when opioids are administered concurrently or before toxin exposure, they act to dampen immune cell activation. Cells from heroin abusers have lower levels of pro‐inflammatory cytokines such as IL‐1β, IL‐6, TNF‐α, and IFN‐γ after in vitro stimulation with the bacterial endotoxin lipopolysaccharide (LPS; Meijerink et al., 2015). Similarly, serum from chronic heroin users produced less IFN‐γ and more IL‐10 than healthy controls when stimulated with LPS (Azarang et al., 2007). Cultured human lymphocytes pretreated with morphine decreased production of IFN‐γ in response to the toxin Concanavilin A (Con‐A; Peterson et al., 1987) and dendritic cells produced less IL‐10 but more IL‐12 in response to LPS after morphine compared to LPS alone (Messmer et al., 2006). In addition to the possible immunosuppressive effects of opioids and increased risk of chronic illness, chronic use of opioids comes with other undesirable side effects, including tolerance, dependence, and addiction. Opioids produce their rewarding effects centrally by binding to opioid receptors and inhibiting GABA neurons in the VTA, resulting in disinhibition of dopamine neurons (Johnson & North, 1992). Opioids also directly bind to opioid receptors in spinal cord to reduce pain (Yaksh, 1981). While the data discussed above focused on peripheral cytokine release or cytokine production in vitro, changes in cytokine expression within the nervous system might more accurately reflect changes in complex behavior. In the CNS, microglia are known to play important roles in regulating both analgesia and addiction (Miguel‐Hidalgo, 2009). These cells, along with astrocytes, are likely to be a source of cytokine upregulation in the brain after opioid exposure due to their general role in modulating inflammation (Sofroniew, 2013; Salter & Stevens, 2017). To this end, glial inhibitors are being investigated for their potential to decrease opioid‐related side effects, namely: addiction and physical dependence (Bachtell et al., 2017). Ibudilast, a glial inhibitor known to decrease TNF‐α release (Suzumura et al., 1999), attenuated subjective morphine withdrawal (Cooper et al., 2016) and motivation for low‐dose oxycodone in a progressive ratio task conducted on heroin‐dependent individuals (Metz et al., 2017). Animal models In rodents, morphine produces an upregulation of peripheral IL‐1β acute or chronic administration (Pacifici et al., 2000; Wang et al., 2012; Kang et al., 2017), IL‐6 after chronic administration (Meng et al., 2015), IL‐17A after chronic administration (Meng et al., 2015), IL‐10 after acute administration (Schwarz et al., 2011), TNF‐α, IL‐2, and IFN‐γ after acute administration (Pacifici et al., 2000). However, two groups have observed decreased IL‐2 release from peripheral immune cells after acute and chronic morphine (Lysle et al., 1993; Wang et al., 2007). As seen in humans, opioids impair immune response to infection or injury in rodent models. Animals injected with morphine and infected with pneumonia (Wang et al., 2005), Candida (Szabo et al., 1993), or peripherally injured by caecal ligation (Meng et al., 2015) all have decreased survival rates compared to controls. Rodent peripheral blood mononuclear cells or splenic leukocytes treated with opioids also produce less IFN‐γ in response to LPS or Con‐A applied in vitro (Peterson et al., 1987; Lysle et al., 1993; Fecho et al., 2000; Carrigan et al., 2004). However, morphine in combination with HIV protein Tat‐1 potentiates Tat‐1‐induced IL‐6, IL‐10, MCP‐1, MCP‐5, and RANTES (El‐Hage et al., 2005). While only a few studies have measured cytokine expression after opioid exposure using cells from the human nervous system (Rock et al., 2006), considerable work has been done to measure these in rodents. The majority of published work demonstrates upregulation and/or enhanced release of cytokines in CNS preparations, including whole brain sections from areas that regulate reward and analgesic functions such as: periaqueductal grey (Eidson et al., 2016), NAc (Niwa et al., 2007; Schwarz et al., 2011; Zhang et al., 2011), hippocampus (Liu et al., 2011), and spinal cord (Hutchinson et al., 2008a). The majority of upregulated cytokines are pro‐inflammatory and occur after acute, repeated, and chronic opioid administration, including IL‐1β (Raghavendra et al., 2002; Johnston et al., 2004; Hutchinson et al., 2008a; Liu et al., 2011; Eidson et al., 2016), IL‐6 (Raghavendra et al., 2002; Johnston et al., 2004; Hutchinson et al., 2008a; Zhang et al., 2011), and TNF‐α (Raghavendra et al., 2002; Johnston et al., 2004; Niwa et al., 2007; Hutchinson et al., 2008a; Zhang et al., 2011). However, the anti‐inflammatory cytokine IL‐10 is also upregulated after acute and repeated opioid exposure (Hutchinson et al., 2008a; Schwarz et al., 2011). In addition to the traditional cytokines, several chemokines also show differential expression after opioids. Campbell et al. (2013) measured enhanced RANTES release in cortex and striatum after repeated morphine, while Hutchinson et al. (2008a,b) measured several differentially regulated chemokines in spinal cord: monokine induced by gamma interferon (MIG), eosinophil chemotactic protein (ECP), MCP‐1, MCP‐5, thymus‐expressed chemokine (TECK), thymus and activation regulated chemokine (TARC), macrophage inflammatory protein‐1α and β (MIP‐1α and β), growth‐related oncogene/keratinocyte chemoattractant (GRO/KC), and fractalkine (Hutchinson et al., 2008a; Schwarz et al., 2011). Finally, some studies have observed conflicting results, suggesting that some pro‐inflammatory cytokines are downregulated after morphine. One such study measured a reduction in prefrontal cortex IL‐1β in BALB/c mice only (Liu et al., 2011). Some discrepancies can be explained by altered timing of cytokine measurements (Pacifici et al., 2000), whether animals went through a withdrawal period (Campbell et al., 2013), and the strain of mouse utilized (Liu et al., 2011), as all these factors affected experimental outcome when they were the subject of study. As mentioned above, inhibition of microglia or astrocytes has been shown to decrease a variety of opioid side effects, including the development of tolerance (Song & Zhao, 2001; Raghavendra et al., 2003; Hutchinson et al., 2008a; Eidson & Murphy, 2013; Fukagawa et al., 2013), physical dependence (Bland et al., 2009; Hutchinson et al., 2009), morphine reward (Hutchinson et al., 2008b; Zhang et al., 2012), and reinstatement of morphine CPP (Schwarz et al., 2011). IL‐1β seems to be a crucial mediator of morphine's analgesic effects. Inhibition of IL‐1 enhanced morphine's analgesic effect and attenuated tolerance (Johnston et al., 2004; Shavit et al., 2005). Additionally, inhibition of fractalkine and IL‐6 also produced a reduction in morphine tolerance (Hutchinson et al., 2008a). The importance of TNF‐α is not as clear. Some studies demonstrate that inhibition of TNF‐α reduced or reversed morphine tolerance (Hutchinson et al., 2008a; Eidson et al., 2016). However, another study suggested that knockout of TNF‐α did not affect acute nociceptive responses or tolerance (Niwa et al., 2007). Enhanced pro‐inflammatory signaling seems to potentiate opioid reward mechanisms. When given IFN‐α alone, rats showed potentiated reinstatement of morphine CPP (Lang et al., 2009). However, upregulation of anti‐inflammatory processes may counteract inflammatory actions and reduce the rewarding effects of opioids. Overexpression of the anti‐inflammatory cytokine IL‐10 decreased activation of microglia, reinstatement of morphine CPP, and acquisition of self‐administration of the short‐acting opioid remifentanil (Schwarz et al., 2011; Lacagnina et al., 2017a). Morphine and other opioids are known to bind opioid receptors to produce reward, dependence, and analgesia (Matthes et al., 1996); however; there is no consensus about which receptor morphine binds in order to disrupt inflammatory signaling. Recent evidence suggests that (+)‐ and (−)‐isomers of opioids bind TLR4 and opioid receptors (Hutchinson et al., 2010), respectively, and that this stereo‐specificity might contribute to conflicting reports on the role of inflammation in morphine‐induced behaviors. (+)‐Naloxone and (+)‐naltrexone are putative antagonists at TLR4 that do not bind opioid receptors (Wang et al., 2016). In behavioral studies both compounds have been shown to reverse morphine tolerance (Eidson & Murphy, 2013) and enhance morphine's analgesic effect (Wang et al., 2012). More selective pharmacological disruption of TLR4 signaling impaired the development of morphine CPP, self‐administration of remifentanil (Hutchinson et al., 2012), and incubation of heroin craving (Theberge et al., 2013). However, TLR4 knockout mice still show enhanced microglial activation in response to morphine (Fukagawa et al., 2013), as well as normal morphine tolerance (Fukagawa et al., 2013; Mattioli et al., 2014) and dependence (Mattioli et al., 2014). Additionally, Tanda et al. (2016) have recently noted that (+)‐naloxone and (+)‐naltrexone do not affect opioid reward at clinically relevant doses. The microbiome and opioids Alterations in the gastrointestinal tract have been noted in human opioid users. In patients with hepatic encephalopathy (HE), opioid use, not degree of liver disease, predicted hospital readmission (Acharya et al., 2017). Opioid users in this cohort displayed significantly different patterns of bacterial populations compared to non‐opioid users with HE (Acharya et al., 2017). Recent studies in rodents have confirmed these findings and have identified opioid‐induced alterations in the microbiome that impact the behavioral effects of opioids. For example, experimenter‐administered morphine increased systemic spread of bacteria through compromised intestinal epithelium (Meng et al., 2015; Banerjee et al., 2016; Kang et al., 2017) and altered the proportion of gram‐negative to gram‐positive bacteria present in the gastrointestinal tract (Meng et al., 2015; Wang et al., 2018). This disruption in bacterial balance directly contributes to the development of morphine analgesic tolerance. Administration of the gram‐positive bacteria Enterococcus faecalus to mice accelerated the development of morphine tolerance (Wang et al., 2018) while oral antibiotics, which reduced systemic bacterial spread, attenuated morphine tolerance (Kang et al., 2017). The effect of morphine on bacterial spread and intestinal compromise are dependent on IL‐17A. Treatment with IL‐17A antibody increased survival after caecal ligation, decreased bacterial spread, decreased IL‐6 production, and restored the intestinal epithelial barrier (Meng et al., 2015). In conclusion, opioids influence inflammation and immune reactivity in a variety of ways that are purported to affect opioid‐related behaviors. Some of these factors include upregulation of pro‐inflammatory cytokines after short‐ or prolonged opioid exposure (Hutchinson et al., 2008a), enhanced activation of microglia (Zhang et al., 2011; Fukagawa et al., 2013), reduction in the immune response to pathogens (Azarang et al., 2007; Meijerink et al., 2015), and disruption of intestinal permeability leading to systemic bacterial spread (Meng et al., 2015). The increase in pro‐inflammatory cytokines and enhanced activation of microglia resulting from opioid intake most likely contribute to the negative consequences of opioid use, such as tolerance (Johnston et al., 2004; Shavit et al., 2005), dependence (Bland et al., 2009; Hutchinson et al., 2009), and reward (Hutchinson et al., 2008b; Zhang et al., 2012), although the exact cellular mechanisms have yet to be elucidated. Conclusions Based on our review of the literature it is clear that psychostimulants and opioids have robust and complex effects on immune system function. As demonstrated in Table 1, the effects of drugs of abuse on immune system function is highly dependent on timing after administration, chronicity of use, and a number of other factors. In fact, a number of inflammatory markers differ depending on whether they are being assessed peripherally or centrally. Despite the complexity of these interactions, we are beginning to develop more of a mechanistic understanding of the role that inflammatory and immune‐related mechanisms may be playing in the development and persistence of substance use disorders. As diagrammed in Fig. 1, there are now multiple different mechanisms that have begun to be outlined for how drugs of abuse affect specific synaptic functions in animal models of substance use disorders. While there is evidence that all of these pathways can be activated by treatment with drugs of abuse, there is clearly a subtlety to how they affect the neuronal and behavioral response. At this time the evidence suggests that the distinction is not as simples as pro‐ vs. anti‐inflammatory; pro‐inflammatory IL‐1β drives increased behavioral response to cocaine and opioids (Hutchinson et al., 2012; Northcutt et al., 2015) while pro‐inflammatory TNF‐⍺ inhibits response to cocaine (Lewitus et al., 2016). Additionally, anti‐inflammatory IL‐10 reduces behavioral response for opioids (Lacagnina et al., 2017a) while G‐CSF, which is primarily anti‐inflammatory, enhances behavioral responses to cocaine (Calipari et al., 2018). The findings outlined in the figure stem primarily from studies during the development of addiction‐like behaviors or shortly after drug administration. This raises the interesting discrepancy between the clinical and animal literature in this field, in that the majority of studies on patients are on patients with chronic substance use disorders, often with some period of abstinence, while the majority of animal studies are performed after relatively shorter periods of drug exposure (Table 2). Of the pathways outlined in animals it seems that only the TLR‐4 signaling pathway has been examined after abstinence and during reinstatement of drug seeking (Theberge et al., 2013; Brown et al., 2018). Further animal studies that more closely mirror what is seen in the clinical literature will be crucial for identification of potential biomarkers and translational research strategies. At this time there have been relatively few neuroimmune pathways that have been examined in animal models of both psychostimulant and opioid use (Table 1). However, there may be commonalities in inflammatory cascades that affect behavioral response to different drugs of abuse. Increases in TNF‐⍺ inhibit behavioral responding for both psychostimulants (Lewitus et al., 2016) and opioids (Niwa et al., 2007). Conversely, signaling through the IL‐1β pathway enhances behavioral response to both cocaine (Northcutt et al., 2015) as well as opioids (Hutchinson et al., 2012). Given the shared changes in neurocircuitry seen in models of drug abuse this is not surprising but will be important for possible clinical translation. While far from complete, our understanding of how neuroimmune interactions are affecting the pathophysiology of substance use disorders is expanding rapidly. Further study is required to clarify which specific signaling pathways are affected and affect neuronal and behavioral response to drugs at different phases of the addiction cycle – including during relapse/reinstatement. Unraveling the intricacies of this signaling will be crucial for our understanding of the neurobiology of addiction and has great potential to help us understand the possible translational potential of immune‐targeted therapies for substance use disorders in the clinical population. Acknowledgements This work was supported by NIH grants to SJR (MH104559) and DDK (DA044308) as well as by funds from the Brain and Behavior Research Foundation, the Leon Levy Foundation, and the Seaver Family Foundation all to DDK. The figure was made using BioRender software with full license to publish. Conflict of interests The authors declare no competing interests. Data accessibility There are no original data contained in this manuscript. Author contributions RSH, SJR, and DDK wrote and edited the manuscript. Abbreviations AMPA α‐amino‐3‐hydroxy‐5‐methyl‐4‐isoxazolepropionic acid APOBEC3G Apolipoprotein B mRNA editing enzyme catalytic polypeptide‐like 3G BDNF Brain‐derived neurotrophic factor CCR2 C‐C chemokine receptor type 2 CNS Central nervous system Con‐A Concanavilin A CPP Conditioned place preference DAMP Danger‐associated molecular pattern ECP Eosinophil chemotactic protein ERK Extracellular‐regulated kinase fMRI Functional magnetic resonance imaging G‐CSF Granulocyte colony‐stimulating factor GRO/KC Growth‐related oncogene/keratinocyte chemoattractant HE Hepatic encephalopathy IFN‐α Interferon alpha IFN‐γ Interferon gamma IL‐10 Interleukin‐10 IL‐12 Interleukin‐12 IL‐17 Interleukin‐17 IL‐1β Interleukin‐1 beta IL‐2 Interleukin‐2 IL‐6 Interleukin‐6 IL‐8 Interleukin‐8 LPS Lipopolysaccharide MCP‐1 Monocyte chemoattractant protein 1 MCP‐5 Monocyte chemoattractant protein 5 MDD Major depressive disorder MIG‐1 Monokine induced by gamma interferon MIP‐1α Macrophage inflammatory protein 1 alpha MIP‐1β Macrophage inflammatory protein 1 beta NAc Nucleus accumbens NMDA N‐methyl‐d‐aspartate PAMP Pathogen‐associated molecular pattern PET Positron emission tomography PRR Pattern recognition receptor RANTES Regulated on activation, normal T cell expressed and secreted SDF‐1 Stromal cell‐derived factor 1 TARC Thymus and activation‐regulated chemokine TECK Thymus‐expressed chemokine TGF‐α Transforming growth factor alpha TGF‐β1 Transforming growth factor beta TLR4 Toll‐like receptor 4 TNF‐α Tumor necrosis factor alpha TRIM22 Tripartite motif‐containing 22 TRIM5α Tripartite motif‐containing 5α VTA Ventral tegmental area Supporting Information References Notes : Edited by Antonello Bonci. Reviewed by Margaret McCarthy and David Stellwagen. All peer review communications can be found with the online version of the article. A number of cytokines and chemokines have been shown to be differentially regulated by stimulants or opioids in the periphery or in the CNS of human subjects and animal models. This table outlines the known changes in these factors by substance, location, direction and subjects. CCL2, chemokine (C‐C motif) ligand 2; CCL3, chemokine (C‐C motif) ligand 3; CCL4, chemokine (C‐C motif) ligand 4; CCL5, chemokine (C‐C motif) ligand 5; CCL8, chemokine (C‐C motif) ligand 8; CCL11, chemokine (C‐C motif) ligand 11; CCL12, chemokine (C‐C motif) ligand 12; CCL17, chemokine (C‐C motif) ligand 17; CCL25, chemokine (C‐C motif) ligand 25; CXCL1, chemokine (C‐X‐C motif) ligand 1; CXCL9, chemokine (C‐X‐C motif) ligand 9; CXCL12, chemokine (C‐X‐C motif) ligand 12; CX3CL1, chemokine (C‐X3‐C motif) ligand 1.