The development and composition of the mucosal immune system has previously been invoked to explain local damage by opportunistic pathogens, repair of normal damage to the mucosal barrier, and susceptibility to systemic autoimmune conditions. For instance, microbial colonization of mice triggers production of RegIIIγ, a bactericidal lectin that mediates the early protective effects of IL-22 against Citrobacter rodentium
, an attaching and effacing bacterial pathogen of mice (26
). More recently, Mazmanian et al.
showed that presence of the commensal organism, Bacteroides fragilis
, protects mice from experimental colitis induced by Helicobacter hepaticus
), and we have shown that Trichuris trichiura
colonization of the intestine can alleviate symptomatic ulcerative colitis (29
). Because of these examples, as well as published evidence of the role of Th17 depletion in SIV and HIV disease pathogenesis (9
), we wondered whether there might be substantial individual variability in intestinal immune cell populations at baseline that would, after acute lentiviral infection, affect the inflammatory stance of the host immune system and the subsequent course of disease.
Our data provide compelling evidence that the Th17 cell compartment varies widely between individual macaques and that, upon SIV infection, variation in the Th17 cell compartment may have an important influence on bacterial translocation and virus replication. Both observational ( and ) and prospective interventional (–) studies show that Th17 cells are correlated with protection against SIV virus replication, acutely and for months after infection. Additional cohort studies in humans will be needed to know if high levels of Th17 cell can reliably predict lower HIV viral loads. In addition to their effects on Th17 cells, IL-2 and G-CSF have effects on the proliferation and function of other immune cells. We attempted to control for these effects by discontinuing drug treatment before infection and by collecting data on cell populations of greatest interest (such as neutrophils and Tregs). However, it remains possible that lingering effects of these cytokines on the replication, activation, or migration of other cell populations contribute to higher viral loads. In the future, it will be of interest to identify drugs with more specific effects on Th17 cells.
The influence of Tregs on disease progression remains uncertain. In our observational study, Treg numbers were not associated with either viral load or the size of the Th17 cell population; however, after treatment with IL-2 and G-CSF, the Treg population was inversely correlated with the Th17 population and directly correlated with viral load (Figures S3E and S5A
). Furthermore, in the interventional study, there was an association between higher Treg levels and lower generalized T cell activation at early time points, although this did not translate into a viral load benefit or into lower T cell activation levels at later time points (Figures S5A–C
). Therefore, it is possible that the expanded or induced Tregs had a larger influence on course of disease than did Tregs present in untreated animals.
Th17 cells and MHC alleles were both influential in determining set-point viral loads. The importance of MHC alleles has been repeatedly demonstrated (21
), but immunologic correlates of protection that must mediate MHC effects have not been conclusively identified. We propose that the status of Th17 cells in the infected host may be a key determinant of whether certain MHC alleles can exert their protective effects. Similarly, an adequate Th17 cell population may be important for deriving maximal benefit from prophylactic vaccine strategies. A sufficient complement of Th17 cells might allow antiviral T cell responses to control virus by conferring either a kinetic or functional advantage to host immune responses. A kinetic advantage would consist of establishing conditions within the intestine that slow viral replication sufficiently to allow control before extensive spread and generalized immune activation occur, as suggested by lower peak viral loads (). A functional advantage would be conferred by establishment of conditions in which highly functional antiviral immune responses could be generated, perhaps because of reduced immune activation (). It will be interesting to determine whether this hypothesis can explain the impressive but variable protection afforded by new experimental vaccines (32
Considering the existing literature on Th17 cells, it is reasonable to ascribe their activity against SIV replication to effects on the mucosal epithelium of the gut (7
). It has become increasingly clear that (i) mucosal surfaces are important not only in the transmission but also in the pathogenesis of lentiviral infections and (ii) development of the immune system at mucosal surfaces is heavily shaped by interaction with the outside world. Our study synthesized these findings and tested directly how the status of the mucosal immune system at the time of infection may impact upon the pathogenesis of disease. We found that circulating and intestinal Th17 cell populations were associated with each other and with lower viral loads, whether Th17 cells were assessed as a fraction of CD4+
T cells or as a ratio with CD25+
Tregs. Furthermore, circulating 16S rDNA was reduced in high Th17:Treg animals, although 16S rDNA itself was not a superior predictor of viral load.
It is interesting that SIV has such dramatic effects on gut-resident lymphocytes, particularly Th17 cells, regardless of whether the route of infection is intravenous (as in this study and ref. 10
) or intrarectal (33
). Microbial translocation has also been demonstrated following infection by either route. Therefore, despite clear kinetic differences between intravenous and intrarectal infections (34
), it seems likely that lymphocytes of the intestinal mucosa provide a uniquely appropriate and perhaps required substrate for pathogenesis of lentiviral infection. It will be interesting to see whether the effects of altered Th17 cell compartments are different after intrarectal inoculation. One might imagine that the effects of small alterations in these cells, such as those reported here, would have greater impact in the face of a less overwhelming infection.
We also found that animals with high levels of Th17 cells generate more polyfunctional CD4+
T cells, including those expressing IL-2, than do animals with fewer Th17 cells. Because such polyfunctional cells have been shown to be more numerous in human controllers of HIV replication (35
), Th17 cells may protect against SIV replication by promoting an environment in which polyfunctional cells can be produced. Further analysis of these parameters at early time points after infection should be useful in defining where Th17 cells exert their effects and via which mechanisms.
We have demonstrated here that macaques with large pre-existing Th17 cell compartments experience lower viral loads after SIV infection and that the Th17:Treg ratio can be manipulated to produce higher viral loads than would be otherwise expected. These observations raise a number of important questions: is there sufficient variability in human Th17 cell numbers to account for inter-individual differences in HIV replication and disease progression? Can Th17 compartment expansion be achieved in vivo and, if so, will it have a measurable effect on HIV replication in acute or chronic infection? To this end, it will be important to establish practical interventions to achieve such expansion.