Recent studies have shown that HIV/SIV can use high levels of the α4β7 receptor to infect CD4 T cells in the mucosa suggesting that mucosal CD4 T cells are selectively targeted by HIV/SIV upon viral entry. Interestingly, not all mucosal CD4 T cells expressed high levels of the α4β7 receptor; a minor population of mucosal CD4 T cells was α4+β7hiCD4 T cells, whereas a majority of them expressed intermediate levels of α4β7 receptor suggesting that α4+β7hiCD4 T cells were likely the early targets for viral infection. In fact, we found higher levels of SIV DNA in α4+β7hiCD4+ T cells at peak viral infection as compared to the α4+β7intCD4 T cell subsets.
Most of the mucosal CD4 T cells expressed CD28, indicating that they were central memory T cells. Previous studies have shown that central memory CD4 T cells were infected during acute SIV infection(24
). However, unlike previous reports by Li et al(20
), nearly all α4+
CD4 T cells in the mucosa were CD69+
suggesting that they were activated T cells. The absence of any major difference in the either the phenotypic or activated nature between the two subsets would suggest that the high level of α4β7 receptor expression likely plays a role in the increased infection of α4+
CD4 T cells in the mucosa. Lending support to this argument is our recent observation that α4+
CD4 T cells in peripheral blood that displayed a predominantly central memory (CD45RA−
) and resting (Ki-67−
) phenotype were preferentially infected very early during the course of infection(15
). It is difficult to determine the exact reasons for the differences in CD69 expression observed in our study, and the lack of CD69 expression by SIV infected CD4 T cells reported by Li et al. It is possible that the different techniques used to obtain tissues/cells may have contributed to these apparent differences; Li et al(20
) used in situ hybridization along immunohistochemistry to identify SIV infected and CD69+CD4+ T cells, whereas we used purified cells from tissues that were subjected to rigorous enzymatic digestion followed by percoll gradient centrifugation.
Interestingly, early viral infection was associated with ~15 fold increase in the endogenous expression of IL-17 in the mucosa. This increase was observed prior to the loss of mucosal CD4 T cells. Studies(2
) have shown that mucosal CD4 T cells were a primary source of Th-17 cells, and it is likely that these cells significantly upregulate the expression of IL-17 in response to viral infection. We have previously shown that mucosal homing CD4 T cells harbored most Th-17 cells as compared to non-mucosal CD4 T cell subsets(15
The loss of mucosal CD4 T cells by 2 weeks pi was accompanied by an increase in the expression of IL-17 in the mucosa suggesting that non-Th-17 cells, most likely CD8 T cells, were upregulating the expression of IL-17 in the mucosa following the depletion of CD4 T cells in the mucosa. Previous studies(13
) have shown that loss of Th-17 cells was accompanied by an increase in IL-17 production by mucosal homing CD8 T cells. Others have shown that CD8+
T cells were capable of producing IL-17(11
IL-17 is a pro-inflammatory cytokine that has been shown to play a role in the recruitment of neutrophils to mucosal tissues. It is likely that the high levels of mucosal inflammation observed during HIV infection is associated with the dysregulation of the IL-17 network in mucosal tissues.
High level of IL-17 expression was associated with an increase in the expression of IL-17 promoting cytokines, namely IL-21, IL-23 and TGFβ, suggesting that the dysregulation of the mucosal IL-17 network is likely aided by these cytokines. Though TGFβ is an anti-inflammatory cytokine, and plays a critical role in maintaining homeostasis in the normal mucosa, increased levels of TGFβ following infection likely contributes to the pro-inflammatory environment in the mucosa by promoting IL-17 production by non-Th-17 cells such as CD8 T cells. TGFβ is produced by a variety of cells(18
), and increased levels of TGFβ production has been reported in acute SIV infected rhesus macaques(6
). Kekow et al(16
) showed that TGFβ is upregulated in plasma during acute HIV infection, and that it remains elevated throughout chronic infection. Likewise, Estes et al(8
) reported high levels of TGFβ in mucosal tissues very early during SIV infection. Despite elevated expression of this anti-inflammatory cytokine, high levels of immune activation, commonly seen in HIV and SIV infection, still persisted(28
Likewise, other studies have shown that the dysregulated IL-23/IL-17 axis plays a role in chronic intestinal inflammation(23
). Production of IL-23 by antigen presenting cells, including dendritic cells and macrophages, is not required for initial differentiation of Th-17 cells, but is believed to be necessary for terminal differentiation and maintenance of the Th-17 phenotype. Other studies have shown that IL-23 drives pathogenic IL-17 producing CD8 T cells.(5
Taken together, our data suggests that mucosal CD4 T cells that express high levels of the α4β7 receptor harbor greater levels of viral DNA than other subsets. The loss of these cells along with other mucosal CD4 T cells is associated with a significant dysregulation of the IL-17 network and IL-17 promoting cytokines that likely contributes to the pro-inflammatory environment in the mucosa of SIV infected rhesus macaques.