SIV Vaccine Controllers Prevent Aberrant Accumulation of Follicular CD4 T cells in Rectal Mucosa

Background: Chronic SIV/HIV infection promotes an accumulation of virus-infected T follicular helper cells (T-FH cells, PD-1hi, CXCR5+, Bcl6+) in the lymph nodes, an important site of viral replication. However, the fate of these cells and their contribution to viral persistence in the rectal mucosa (rectum), another preferential site of virus replication, is not known. Here we studied the fate of PD-1hi CD4 T cells in the rectum following intrarectal SIV251 infection in a cohort of unvaccinated and DNA/MVA vaccinated rhesus macaques (RM). Methodology: Lymphocytes isolated from the rectum of SIV naive and SIV infected rhesus macaques (RM) were separated and characterized by multi-color flow cytometry. Sorted cells were used for measuring cell associated viral RNA by qRT-PCR. Cellular localization was determined by immunofluorescense staining. Results: In the SIV-naive RM, a small fraction of CD4 T cells in the rectum expressed high levels of PD-1 (PD-1hi cells), the majority co-expressed CXCR5 and Bcl-6 (T-FH), and localized to germinal centers (GC) of lymphoid follicles. Following a pathogenic SIV infection, despite a global depletion of CD4 T cells, T-FH cells increased dramatically in animals that failed to control SIV infection below 104 viral RNA copies/ml of plasma at set-point. Similar to uninfected animals, the PD-1hi cells in the non-controller RM expressed CXCR5 and Bcl-6 (T-FH), did not express CCR5, and localized to GC of B cell follicles. Moreover, these PD-1hi CD4 T cells were enriched in viral RNA, indicating that these cells exist as an active site of viral production. In contrast, in vaccine-controllers that maintained viral loads below 104 viral RNA copies/ml of plasma at set-point, the frequency of T-FH cells did not increase, and lower frequencies of CXCR5+ PD-1hi CD4 T cells and higher frequencies of CCR5+ CD4 T cells were observed. This CXCR5lo CCR5+ phenotype suggests a possible altered distribution of these cells away from the GC region of B cell zones towards the T cell zone where anti-viral CD8 T cells predominantly reside. Interestingly, higher frequencies of functional SIV specific CD8 T cells (Granzyme B+ CXCR5+ Gag CM9+ CD8 T cells) were observed in the LN follicles of vaccine-controllers compared to non-controllers. Conclusions: These data demonstrate that SIV-infected T-FH cells accumulate in B cell follicles of the rectum during uncontrolled chronic SIV infection and thus may evade killing by anti-viral CD8 T cells. They also demonstrate that vaccine-mediated SIV control is associated with lack of T-FH cell accumulation. These data warrant the exploration of therapeutic interventions that limit aberrant T-FH accumulation as a means to enhance control of chronic SIV/HIV replication.