A role for TLR signaling during B cell activation in antiretroviral-treated HIV individuals

The mechanisms underlying B cell activation that persists during antiretroviral therapy (ART) are unknown. Toll-like receptor (TLR) signaling is a critical mediator of innate cell activation and though B cells express TLRs, few studies have investigated a role for TLR signaling in B cell activation during HIV infection. We addressed this question by assessing the activated phenotype and TLR expression/responsiveness of B cells from ART-treated HIV-infected subjects (HIV_ART⁺). We evaluated activation markers implicated in B cell-mediated T cell trans infection during HIV pathogenesis. We found no significant difference in TLR expression between B cells of HIV_ART⁺ and HIV^- subjects. However, B cells of HIV_ART⁺ subjects exhibited heightened endogenous expression levels of IL-6 (p=0.0051), T cell cognate ligands CD40 (p=0.0475), CD54 (p=0.0229), and phosphorylated p38 (p<0.0001), a marker of TLR signaling. In vitro, B cells of HIV_ART⁺ individuals were less responsive to TLR stimulation compared to B cells of HIV^- subjects. The activated phenotype of in vitro TLR-stimulated B cells of HIV ^- subjects was similar to ex vivo B cells from HIV_ART ⁺ individuals. TLR2 stimulation was a potent mediator of B cell activation, whereas B cells were least responsive to TLR4 stimulation. Compared to HIV^- subjects, the serum level of lipoteichoic acid (TLR2 ligand) in HIV_ART⁺ subjects was significantly higher (p=0.0207), correlating positively with viral load (p=0.0127, r=0.6453). Our data suggest that during HIV infection TLR-activated B cells may exert a pathogenic role and B cells from HIV_ART⁺ subjects respond to in vitro TLR stimulation, yet exhibit a TLR tolerant phenotype suggesting prior in vivo TLR stimulation.