FcRL4 expression identifies a pro-inflammatory B cell subset in viremic HIV-infected subjects

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^hiIL-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 FcRL4neg 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.