Immunology

The autoimmune regulator (Aire) serves an essential function for T cell tolerance by promoting the “promiscuous” expression of tissue antigens in thymic epithelial cells. Aire is also detected in rare cells in peripheral lymphoid organs, but the identity of these cells is poorly understood. Here, we report that Aire protein–expressing cells in lymph nodes exhibit typical group 3 innate lymphoid cell (ILC3) characteristics such as lymphoid morphology, absence of “classical” hematopoietic lineage markers, and dependence on RORγt. Aire+ cells are more frequent among lineage-negative RORγt+ cells of peripheral lymph nodes as compared with mucosa-draining lymph nodes, display a unique Aire-dependent transcriptional signature, express high surface levels of MHCII and costimulatory molecules, and efficiently present an endogenously expressed model antigen to CD4+ T cells. These findings define a novel type of ILC3-like cells with potent APC features, suggesting that these cells serve a function in the control of T cell responses.

Lymphatic endothelial cells (LECs) form the structure of the lymphatic vessels and the sinuses of the lymph nodes, positioning them to be key players in many different aspects of the immune response. Following an inflammatory stimulus, LECs produce chemokines that recruit immune cells to the lymph nodes. The recruitment of immune cells aids in the coordination of both LEC and lymph node expansion and contraction. More recent data has demonstrated that to coordinate LEC division and death, cell surface molecules, such as PD-L1 and interferon receptors, are required. During homeostasis, LECs use PD-L1 to maintain peripheral tolerance by presenting specific peripheral tissue antigens in order to eliminate tissue specific responses. LECs also have the capacity to acquire, present and exchange foreign antigens following viral infection or immunization. Here we will review how lymph node LECs require immune cells to expand and contract in response to an immune stimulus, the factors involved and how direct LEC-immune cell interactions are important for programming immunity.

Lymph node stromal cells support diverse processes, but bulk assessments obscure their
niche-specific functions. Rodda et al. identify transcriptional profiles for nine
lymph node stromal cell clusters using single-cell RNA sequencing, validate subset
markers in situ, and suggest niche-restricted functions.

Metastasis is the primary cause of cancer death. The inflammatory tumor microenvironment contributes to metastasis, for instance, by recruiting blood and lymph vessels. Among tumor-infiltrating immune cells, tumor-associated macrophages (TAMs) take a center stage in promoting both tumor angiogenesis and metastatic spread. We found that genetic deletion of the S1P receptor 1 ( S1pr1 ) alone in CD11bhi CD206+ TAMs infiltrating mouse breast tumors prevents pulmonary metastasis and tumor lymphangiogenesis. Reduced lymphangiogenesis was also observed in the nonrelated methylcholanthrene-induced fibrosarcoma model. Transcriptome analysis of isolated TAMs from both entities revealed reduced expression of the inflammasome component Nlrp3 in S1PR1-deficient TAMs. Macrophage-dependent lymphangiogenesis in vitro was triggered upon inflammasome activation and required both S1PR1 signaling and IL-1β production. Finally, NLRP3 expression in tumor-infiltrating macrophages correlated with survival, lymph node invasion, and metastasis of mammary carcinoma patients. Conceptually, our study indicates an unappreciated role of the NLRP3 inflammasome in promoting metastasis via the lymphatics downstream of S1PR1 signaling in macrophages.

During normal and pathologic immune responses, lymph nodes can swell considerably.
The lymph node vascular–stromal compartment supports and regulates the developing
immune responses and undergoes dynamic expansion and remodeling. Recent studies have
shown that dendritic cells (DCs), best known for their antigen presentation roles,
can directly regulate the vascular–stromal compartment, pointing to a new perspective
on DCs as facilitators of lymphoid tissue function. Here, we review the phases of
lymph node vascular–stromal growth and remodeling during immune responses, discuss
the roles of DCs, and discuss how this understanding can potentially be used for developing
novel therapeutic approaches.

B cells facilitate breast cancer metastasis to lymph nodes through production of antibodies targeting a protein on the surface of cancer cells that stimulates tumor dissemination.

Germinal center (GC) B cells are essential to generating protective antibody responses and are selected through a process of affinity maturation. Kwak et al . now define intrinsic properties of human GC B cells that are critical to antigen affinity discrimination. They identified BCR-containing actin-rich pod-like structures that facilitated formation of highly stable synapses and antigen internalization but only when GC B cells engaged high-affinity antigens. These findings reveal the importance of these structures in setting thresholds for affinity selection and driving GC B cell responses.

Protective antibody responses to vaccination or infection depend on affinity maturation, a process by which high-affinity germinal center (GC) B cells are selected on the basis of their ability to bind, gather, and present antigen to T follicular helper (Tfh) cells. Here, we show that human GC B cells have intrinsically higher-affinity thresholds for both B cell antigen receptor (BCR) signaling and antigen gathering as compared with naïve B cells and that these functions are mediated by distinct cellular structures and pathways that ultimately lead to antigen affinity– and Tfh cell–dependent differentiation to plasma cells. GC B cells bound antigen through highly dynamic, actin- and ezrin-rich pod-like structures that concentrated BCRs. The behavior of these structures was dictated by the intrinsic antigen affinity thresholds of GC B cells. Low-affinity antigens triggered continuous engagement and disengagement of membrane-associated antigens, whereas high-affinity antigens induced stable synapse formation. The pod-like structures also mediated affinity-dependent antigen internalization by unconventional pathways distinct from those of naïve B cells. Thus, intrinsic properties of human GC B cells set thresholds for affinity selection.

Ferrando-Martinez et al., showed that SIV-specific follicular (f)CD8 T cells are observed in both intact and disorganised lymph node follicles, and accumulate

Cellular cross-talk mediated by lymphotoxin αβ–lymphotoxin β receptor (LTβR) signaling plays a critical role in lymph node (LN) development. Although the major role of LTβR signaling has long been considered to occur in mesenchymal lymphoid tissue organizer cells, a recent study using a VE-cadherincreLtbrfl/fl mouse model suggested that endothelial LTβR signaling contributes to the formation of LNs. However, the detailed roles of LTβR in different endothelial cells (ECs) in LN development remain unknown. Using various cre transgenic mouse models ( Tekcre , a strain targeting ECs, and Lyve1cre , mainly targeting lymphatic ECs), we observed that specific LTβR ablation in Tekcre+ or Lyve1cre+ cells is not required for LN formation. Moreover, double- cre –mediated LTβR depletion does not interrupt LN formation. Nevertheless, TekcreLtbrfl/fl mice exhibit reduced lymphoid tissue inducer cell accumulation at the LN anlagen and impaired LN maturation. Interestingly, a subset of ECs ( VE-cadherin + Tekcre-low/neg ECs) was found to be enriched in transcripts related to hematopoietic cell recruitment and transendothelial migration, resembling LN high ECs in adult animals. Furthermore, endothelial Tek was observed to negatively regulate hematopoietic cell transmigration. Taken together, our data suggest that although Tekcre+ endothelial LTβR is required for the accumulation of hematopoietic cells and full LN maturation, LTβR in VE-cadherin+Tekcre-low/neg ECs in embryos might represent a critical portal-determining factor for LN formation.

Dendritic cell (DC) subsets with biased capacity for CD4+ and CD8+ T cell activation are asymmetrically distributed in lymph nodes (LNs), but how this affects adaptive responses has not been extensively studied. Here we used quantitative imaging to examine the relationships among antigen dispersal, DC positioning, and T cell activation after protein immunization. Antigens rapidly drained into LNs and formed gradients extending from the lymphatic sinuses, with reduced abundance in the deep LN paracortex. Differential localization of DCs specialized for major histocompatibility complex I (MHC I) and MHC II presentation resulted in preferential activation of CD8+ and CD4+ T cells within distinct LN regions. Because MHC I–specialized DCs are positioned in regions with limited antigen delivery, modest reductions in antigen dose led to a substantially greater decline in CD8+ compared with CD4+ T cell activation, expansion, and clonal diversity. Thus, the collective action of antigen dispersal and DC positioning regulates the extent and quality of T cell immunity, with important implications for vaccine design.