Germinal center follicular T helper cells (GC Tfh) provide B cell help to support high affinity antibody responses. HIV infection leads to impairment of GC Tfh and their circulating counterpart (circulating Tfh, cTfh) contributing to inefficient humoral response to HIV. Tfh impairment persists in HIV-infected subjects on antiretroviral therapy (ART) despite viral control. Due to technical limitations in assessing antigen (Ag)-specific Tfh, previous studies on mechanisms of Tfh impairment have been restricted to bulk Tfh. Therefore, it remains unclear if this Tfh dysfunction is HIV-specific or reflects a global dysregulation of the Tfh compartment.
Here, we used co-expression of the surface activation-induced markers CD40L and CD69 after Ag stimulation to identify HIV-specific CD4 independently of their functional profile. We studied blood of HIV+ subjects on ART and compared phenotype and function of the CD4 responses specific to HIV Ags (Gag, Env, Nef) to those specific for another chronic but controlled virus (CMV) and to responses to a resolved infection or vaccine (HBV) within the same donors.
We observed a preferential expansion of cTfh within HIV-specific CD4 responses when compared to CMV and HBV. In contrast to non-cTfh cells and to all other Ags tested, Gag-specific cTfh exhibited an increased CXCR3+ Th1-like polarization. Gag-specific CD4, but particularly cTfh, were characterized by high expression of multiple co-inhibitory receptors. Longitudinal samples collected before and after ART initiation revealed that this dysregulated phenotype was established during the untreated phase of infection and persisted despite subsequent viral control. More Gag-specific cTfh produced Tfh- (CXCL13, IL-21) and, surprisingly, Th1-cytokines (IL-2, IFNγ, TNFα) after Ag stimulation compared to CMV, suggesting a functionally dysregulated profile. Importantly, this increased production of cytokines by cTfh was mirrored by the functional profile of GC Tfh, as observed in surgical lymph node (LN) biopsies.
Our results thus reveal that in contrast to cTfh responses to other viruses in the same subjects, HIV-specific cTfh show a persistently expanded and activated phenotype in HIV+ subjects on ART. This might reflect a functional hyperactivity of GC Tfh in the LN that has been associated with aberrant B cell responses. These permanent alterations may contribute to lack of restoration of effective HIV-specific immunity despite prolonged ART, and complicate HIV cure efforts.