Early T follicular helper cell responses and germinal center reactions are associated with viremia control in immunized Rhesus macaques.

T follicular helper (TFH ) cells are fundamental in germinal center (GC) maturation and selection of antigen-specific B cells within secondary lymphoid organs. GC-resident TFH cells have been fully characterized in human HIV infection. However, the role of GC TFH cells in GC B cell responses following various SIV vaccine regimens in Rhesus macaques (RMs) has not been fully investigated. We characterized GC TFH cells of RMs over the course of a mucosal/systemic vaccination regimen to elucidate GC formation and SIV humoral response generation. Animals were mucosally primed twice with replicating Ad5hr-SIV recombinants and systemically boosted with ALVAC/Env or DNA&Env including SIV gp120 proteins. Lymph nodes were biopsied in macaque subgroups pre-vaccination and at days 3, 7, or 14 after the 2nd Ad5hr-SIV prime and the 2nd vector/Env boost. Evaluations of GC TFH and GC B cell dynamics including correlation analyses supported a significant role of early GC TFH cells in providing B cell help during initial phases of GC formation. GC TFH responses at day 3 post-mucosal priming were consistent with generation of Env-specific memory B cells in GCs and elicitation of prolonged Env-specific humoral immunity in the rectal mucosa. GC Env-specific memory B cell responses elicited early post-systemic boosting correlated significantly with decreased viremia post-infection. Our results highlight the importance of early GC TFH cell responses for robust GC maturation and generation of long-lasting SIV specific humoral responses at mucosal and systemic sites. Further investigation of GC TFH cell dynamics should facilitate development of an efficacious HIV vaccine. IMPORTANCE The modest HIV protection observed in the human RV144 vaccine trial associated antibody responses with vaccine efficacy. T follicular helper (TFH ) cells are CD4+ T cells that select antibody secreting cells with high antigenic affinity in germinal centers (GCs) within secondary lymphoid organs. To evaluate the role of TFH cells in eliciting prolonged viral-specific humoral responses, we vaccinated Rhesus macaques with a combined mucosal prime/systemic boost regimen followed by repeated low-dose intrarectal challenges with SIV mimicking human exposure to HIV-1. Although the vaccine regimen did not prevent SIV infection, decreased viremia was observed in the immunized macaques. Importantly, vaccine-induced TFH responses elicited at day 3 post-immunization and robust GC maturation were strongly associated. Further, early TFH -dependent SIV-specific B cell responses were also correlated with decreased viremia. Our findings highlight the contribution of early vaccine-induced GC TFH responses to elicitation of SIV-specific humoral immunity and implicate their participation in SIV control.