Date Published: October 7, 2016
Publisher: Public Library of Science
Author(s): Brodie Miles, Shannon M. Miller, Joy M. Folkvord, David N. Levy, Eva G. Rakasz, Pamela J. Skinner, Elizabeth Connick, Guido Silvestri.
During chronic HIV infection, viral replication is concentrated in secondary lymphoid follicles. Cytotoxic CD8 T cells control HIV replication in extrafollicular regions, but not in the follicle. Here, we show CXCR5hiCD44hiCD8 T cells are a regulatory subset differing from conventional CD8 T cells, and constitute the majority of CD8 T cells in the follicle. This subset, CD8 follicular regulatory T cells (CD8 TFR), expand in chronic SIV infection, exhibit enhanced expression of Tim-3 and IL-10, and express less perforin compared to conventional CD8 T cells. CD8 TFR modestly limit HIV replication in follicular helper T cells (TFH), impair TFH IL-21 production via Tim-3, and inhibit IgG production by B cells during ex vivo HIV infection. CD8 TFR induce TFH apoptosis through HLA-E, but induce less apoptosis than conventional CD8 T cells. These data demonstrate that a unique regulatory CD8 population exists in follicles that impairs GC function in HIV infection.
In chronic HIV and SIV infection, viral replication is concentrated in B cell follicles in secondary lymphoid tissues [1–5], although factors that promote this are not fully understood. Follicular helper T cells (TFH), which reside in the secondary lymphoid follicles, are highly permissive to HIV  and exhibit anti-apoptotic properties [7, 8] which likely contributes to viral persistence. We have previously shown that virus-specific CD8 T cells are present at lower frequencies inside the follicle compared to outside the follicle in HIV and SIV infection [2, 9], which may contribute to impaired viral clearance in the follicle. While CD8 T cells are present in the follicle, little is known about the function of these cells. We have previously reported that CD4 follicular regulatory T cells (TFR) are increased in number, exhibit heighted regulatory capabilities, and impair TFH proliferation and function in ex vivo HIV and in vivo SIV infection . We hypothesized that follicular CD8 T cells may also have regulatory functions that further contribute to immune dysregulation in chronic HIV infection.
This is the first study to examine the role of CD8 TFR in the context of HIV and SIV infection. We determined that the majority of follicular CD8 T cells in human tonsils, as well as in secondary lymphoid tissues from rhesus macaques, exhibit a CD8 TFR phenotype. Of all the CD8 T cells expressing the follicular homing profile CXCR5+CCR7-, 90% or more of these cells are CD8 TFR in both human tonsils and rhesus macaque lymphoid tissues. Furthermore, their percentage is higher in lymph nodes and spleens of chronically SIV-infected rhesus macaques compared to uninfected macaques. A greater percentage of CD8 TFR express IL-10 and CD8 TFR express relatively lower levels of perforin compared to CD8 conv. In ex vivo HIV spinoculation and chronic SIV infection, CD8 TFR inhibit IL-21 production by TFH and IL-21 production is rescued by Tim-3 blockade. Further, CD8 TFR inhibit IgG production by B cells in ex vivo HIV infection. Additionally, CD8 TFR significantly, albeit modestly, reduce TFH permissivity to HIV ex vivo through contact-dependent mechanisms. Compared to CD8 conv, CD8 TFR induce a low level of TFH apoptosis. Blocking HLA-E interaction in CD8 TFR and TFH co-cultures further reduces TFH apoptosis rates. Collectively, these studies demonstrate that CD8 TFR constitute the majority of CD8 cells in B cell follicles and contribute to impaired GC function in lentiviral infections.