Plant viruses are generally considered incapable of infecting vertebrates. Accordingly, they are not considered harmful for humans. However, a few studies questioned the certainty of this paradigm. Tobacco mosaic virus (TMV) RNA has been detected in human samples and TMV RNA translation has been described in animal cells. We sought to determine if TMV is detectable, persists, and remains viable in the lung tissues of mice following intratracheal inoculation, and we attempted to inoculate mouse macrophages with TMV. In the animal model, mice were intratracheally inoculated with 1011 viral particles and were sacrificed at different time points. The virus was detected in the mouse lungs using immunohistochemistry, electron microscopy, real-time RT-PCR and sequencing, and its viability was studied with an infectivity assay on plants. In the cellular model, the culture medium of murine bone marrow derived macrophages (BMDM) was inoculated with different concentrations of TMV, and the virus was detected with real-time RT-PCR and immunofluorescence. In addition, anti-TMV antibodies were detected in mouse sera with ELISA. We showed that infectious TMV could enter and persist in mouse lungs via the intratracheal route. Over 14 days, the TMV RNA level decreased by 5 log10 copies/ml in the mouse lungs and by 3.5 log10 in macrophages recovered from bronchoalveolar lavage. TMV was localized to lung tissue, and its infectivity was observed on plants until 3 days after inoculation. In addition, anti-TMV antibody seroconversions were observed in the sera from mice 7 days after inoculation. In the cellular model, we observed that TMV persisted over 15 days after inoculation and it was visualized in the cytoplasm of the BMDM. This work shows that a plant virus, Tobacco mosaic virus, could persist and enter in cells in mammals, which raises questions about the potential interactions between TMV and human hosts.
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Interesting paper! Which proves...which proves...which proves TMV is seriously resistant to degradation in animals and in mammalian cells; that it can enter macrophages; and that it...what? What, exactly, are the "...questions about the possible interactions..."? What would TMV do in mammalian cells? Yes, it might be incoated and be translated; it is far less likely that it MIGHT be able to replicate its RNA - and then? While it can apparently be taken up quite efficiently by macrophages - a property which, incidentally, has led to its being trialled as an RNA vaccine delivery system - this is a dead end, and one that is quite normal for particles of any kind being introduced into mammals.
Which is something that happens every day, as we and our cousin mammals eat: it has been shown elsewhere that animals are actually quite good spreaders of plant viruses, some of which - like TMV and the even tougher Cauliflower mosaic virus - pass right through at high survival rates, and remain infectious. We will all probably have eaten many grams of various viruses in our lives, and derived nothing more than nutition from them.
I also remember, even though it was very late at night, 31 years ago, and in a bar in Banff in Canada, a conversation with one Richard Zeyen, who told me they had used ELISA to test everyone in their lab for antibodies for TMV, seeing as they worked with it. And everyone was immune - presumably, to aerosolised TMV that had been breathed in or otherwise ingested. Proving...that oral vaccines based on TMV could work, and that most of us are probably immune to all sorts of viruses that don't replicate in us. Including, in the case of many people in the Eastern Cape Province of South Africa, sampled by one Don Hendry via the local blood bank, to a virus of Pine Emperor moths - because it multiples to such high levels in its host that anyone walking in the pine forests was bound to be exposed via the environment.
So this is an interesting paper - and no more. It will, of course, lead to alarmist articles ad blog posts, and people calling out for urgent surveillance of food, in which people will find many viruses. And so what?