Decreased TFR/TFH Ratio in SIV-Infected Rhesus Macaques

Background: T follicular helper cells (T_FH) are critical for the development and maintenance of germinal centers (GC) and the humoral immune response. During chronic stages of HIV/SIV infection T_FH cells accumulate, possibly as a result of antigenic persistence. The SIV/HIV-associated T_FH expansion may also reflect a lack of regulation by suppressive follicular regulatory CD4⁺ T cells (T_FR) . T_FR cells are natural regulatory T cells (T_REG) that migrate into the follicle and, similarly to T_FH cells, up-regulate CXCR5, Bcl6 and PD1.

Methods: Lymph node (LN) biopsies were obtained from SIV-infected and uninfected rhesus macaque (RM), as well as HIV-infected humans. T_FR cells were identified by flow cytometry and immunohistochemistry. Cell-associated viral DNA was measured by RT-PCR and next-generation RNA sequencing was performed on sorted populations from uninfected and SIV-infected RM by Illumina.

Results: We identified T_FR cells as CD4⁺, Foxp3⁺, CXCR5⁺, PD1⁺ and Bcl6⁺ within LN of both humans and RM and confirmed their localization within the GC by immunohistochemistry. RNA sequencing showed that T_FR cells share a T_FH and T_REG transcriptional profile with intermediate expression of FoxP3, Bcl6, PRDM1, IL-10 and IL-21. In healthy, SIV-uninfected RM, we observe a negative correlation between frequencies of T_FR cells and both T_FH (p=0.0358) and GC B cells (p=0.0140) as well as with levels of CD4⁺ T cell proliferation (p=0.0315). Following SIV infection, the ratio of T_FR to T_FH cells was reduced (p=0.0058) with no change in the frequency of T_REG cells or in the frequency of T_FR cells within the total CD4 T cell pool. Finally, we examined whether higher levels of direct virus infection of T_FR cells might be involved in their relative depletion post-SIV infection. We found that T_FH, T_FR and T_REG cells sorted from SIV- infected RM harbor comparable levels of cell-associated viral DNA.

Conclusions: Our data suggests that T_FR cells may contribute to the regulation and proliferation of T_FH and GC B cells in vivo and that a decreased ratio of T_FR/T_FH cells in chronic SIV infection of RM may lead to unchecked expansion of both T_FH and GC B cells.