T-Fh Cells Induced By Vaccination And Following Siv Challenge Support Env-Specific Humoral Immunity In The Rectal-Genital Tract And Circulation Of Female Rhesus Macaques

T follicular helper (T-FH) cells are pivotal in lymph node (LN) germinal center (GC) B cell affinity maturation. Circulating CXCR5(+) CD4(+) T (cT(FH)) cells have supported memory B cell activation and broadly neutralizing antibodies in HIV controllers. We investigated the contribution of LN SIV-specific T-FH and cT(FH) cells to Env-specific humoral immunity in female rhesus macaques following a mucosal Ad5hr-SIV recombinant priming and SIV gp120 intramuscular boosting vaccine regimen and following SIV vaginal challenge. T-FH and B cells were characterized by flow cytometry. B cell help was evaluated in T-FH-B cell co-cultures and by real-time PCR. Vaccination induced Env-specific T-FH and Env-specific memory (ESM) B cells in LNs. LN Env-specific T-FH cells post-priming and GC ESM B cells post-boosting correlated with rectal Env-specific IgA titers, and GC B cells at the same timepoints correlated with vaginal Env-specific IgG titers. Vaccination expanded cT(FH) cell responses, including CD25(+) Env-specific cT(FH) cells that correlated negatively with vaginal Env-specific IgG titers but positively with rectal Env-specific IgA titers. Although cT(FH) cells post-2(nd) boost positively correlated with viral-loads following SIV challenge, cT(FH) cells of SIV-infected and protected macaques supported maturation of circulating B cells into plasma cells and IgA release in co-culture. Additionally, cT(FH) cells of naive macaques promoted upregulation of genes associated with B cell proliferation, BCR engagement, plasma cell maturation, and antibody production, highlighting the role of cT(FH) cells in blood B cell maturation. Vaccine-induced LN T-FH and GC B cells supported anti-viral mucosal immunity while cT(FH) cells provided B cell help in the periphery during immunization and after SIV challenge. Induction of T-FH responses in blood and secondary lymphoid organs is likely desirable for protective efficacy of HIV vaccines.