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In autoimmune diseases, toll-like receptor (TLR)-stimulated pro-inflammatory IL-6-secreting B cells exert pathogenic roles. Similarly, B cell Fc receptor-like 4 (FcRL4) expression amplifies TLR stimulation, and in rheumatoid arthritis patients, FcRL4 expression identifies a pro-inflammatory B cell subset. B cells from HIV-infected subjects also express heightened levels of FcRL4 and secrete high levels of IL-6: a critical mediator of HIV disease progression. In this study, we sought to determine if FcRL4 identifies a pro-inflammatory B cell subset in HIV-infected subjects and further elucidate the mechanisms underlying FcRL4 amplification of TLR stimulation. We determine that tissue-like memory B cells express the highest endogenous levels of FcRL4 positively correlating with IL-6 expression ( p  = 0.0022,  r  = 0.8667), but activated memory B cells exhibit the highest frequency of FcRL4 hi IL-6 hi  cells. FcRL4 hi  B cells exhibit an activated TLR-signaling pathway identified by elevated phosphorylation levels of: pERK ( p  = 0.0373), p38 ( p  = 0.0337), p65 ( p  = 0.1097), and cJUN ( p  = 0.0239), concomitant with significantly elevated expression of the TLR-signaling modulator hematopoietic cell kinase (HcK,  p  = 0.0414). Compared to FcRL4 neg  B cells from healthy controls, TLR9-stimulated FcRL4 pos  B cells express significantly higher levels of lL-6 ( p  = 0.0179). Further, TLR9-stimulated B cells also upregulate the expression of FcRL4 ( p  = 0.0415) and HcK ( p  = 0.0386). In B-cell lines, siRNA-mediated HcK knockdown downmodulates TLR9-induced FcRL4-mediated activation quantified by CD23 upregulation ( p  = 0.0553). We present data suggesting that, in viremic HIV-infected individuals, FcRL4 expression identifies unique IL-6 producing pro-inflammatory B-cell subsets. Further, TLR stimulation likely modulates FcRL4 expression and FcRL4 expression is associated with Hck, potentially enhancing the activation of TLR-signaling associated transcription factors. Pathogenic B-cells have been identified in other disease settings, and this study represents a novel report describing a pro-inflammatory B cell subset in HIV-infected patients.

FcRL4 Expression Identifies a Pro-inflammatory B Cell Subset in Viremic HIV-Infected Subjects