Interleukin-21 Reduces Residual Immune Activation in ART-treated SIV-infected Rhesus Macaques

Background: Current HIV antiretroviral therapy (ART) successfully inhibits virus replication, improves health, and prolongs life expectancy in people living with HIV. Although HIV replication is suppressed, residual immune activation persists and critically contributes to non-AIDS-related morbidity and mortality in ART-treated individuals. Interleukin (IL)-21 regulates three immunological functions – Th17 cell homeostasis, differentiation of memory B cells, and longterm maintenance of functional CD8+ T cells – that are compromised following HIV infection. Previously, we showed that when administered during acute infection, Interleukin (IL)-21 reduces microbial translocation and systemic immune activation in SIV-infected rhesus macaques (RMs). In this new study we investigated if administration of IL-21 to chronically SIV-infected, ART-treated RMs is effective in limiting the extent of on-ART residual immune activation. Methodology: Sixteen RMs were infected with SIVmac239 i.v. and, starting at day 60 post-infection, were treated for seven months with PMPA, FTC, Raltegravir, Darunavir and Ritonavir. Eight RMs received IL-21-Fc (100 μg/kg, s.c., weekly for six weeks) at the beginning and at the end of ART, with the other eight serving as ART-treated controls. Longitudinal collections of blood, lymph nodes and rectum were performed. IL-21 effects on immune activation, T cell subset levels and functions were assessed by flow cytometry. The Mann-Whitney test was used for statistical analyses. Results: ART was very effective, with fully suppressed plasma viremia (<60 SIV-RNA copies/ml) in all RMs. Compared to ART-controls, ART+IL-21 treated RMs showed better restoration of mucosal immunity, with higher levels of intestinal IL-17 (Th17), IL-22 (Th22), as well as IL-17 and IL-22 (Th17/Th22) producing CD4+ T cells (P<0.01 for all three subsets). Furthermore, while ART alone was sufficient to restore Th22 functions, the ability of Th17 cells to produce multiple cytokines, as assessed by co-staining of IL-17, IL-2, IL-22, TNF-α, and IFN-γ, was restored at levels significantly higher in IL-21 treated RMs as compared to controls. Remarkably, the combined ART+IL-21 treatment reduced immune activation significantly more than ART alone, with IL-21 treated RMs showing lower levels (P<0.01) of activated (HLA-DR+CD38+), proliferating (Ki-67+) and PD-1+ T cells in rectum and blood. Conclusions: IL-21 administration in ART-suppressed SIV-infected RMs more rapidly restores intestinal IL-17 and/or IL-22 producing CD4+ T cells while limiting residual immune activation. Thus, IL-21 may provide important therapeutic benefits when used as an immunomodulator in ART-treated HIV-infected individuals.