Mechanisms controlling ABC differentiation and function in SLE

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ABSTRACT Abnormalities in B cell subsets play a key role in SLE, a disease that, in addition to AutoAb production and multi-organ involvement, often includes upregulation of interferon stimulated genes (ISGs). One of the hallmarks of SLE is that it preferentially affects women. Both sex hormones and the X chromosome (where TLR7 is located) have been implicated in the heightened susceptibility of women to SLE and to other autoimmune disorders. Understanding the molecular mechanisms that underlie the sex-bias that accompanies SLE pathogenesis will thus provide critical information into the development of autoimmunity and help uncover novel therapeutic targets. While expansion of germinal center (GC) B cells and plasmablasts/plasma cells (PB/PC) has long been associated with SLE, recent studies have implicated a novel B cell subset, termed Age/Autoimmune-associated B cells (ABCs), in lupus pathogenesis. In addition to classical B cell markers, ABCs also express CD11c and the transcription factor T-bet. Formation of ABCs is promoted by a combination of signals that includes TLR7 or TLR9 engagement and cytokines like IFN-g and IL-21. Aberrant accumulation of ABCs is observed both in murine lupus and in SLE patients where they are major producers of autoAbs and correlate with disease activity and clinical manifestations. Our lab has had a long-standing interest in dissecting the regulation and function of IRF family members, which have emerged as key controllers of B cell responses. While identifying IRF-interacting proteins, we isolated a protein termed Def6. Def6 and its only other homologue, SWAP-70, play an important immunoregulatory role in humans and mice. Def6 is a genetic risk factor for human SLE and biallelic mutations in Def6 result in early-onset systemic autoimmunity. Furthermore, in C57BL/6 mice, the concomitant lack of Def6 and SWAP-70 (Double-knockout mice=DKOs) leads to the spontaneous development of SLE, which, similarly to humans, preferentially affects female mice. Lupus development in DKO mice is accompanied by a marked accumulation of ABCs, which is controlled by IRF5. We have recently found that, as compared to ABCs from DKO males, ABCs from DKO females expand to a greater extent, express an ISG signature, and readily produce autoAbs upon TLR7 stimulation. Furthermore, in comparison with DKO males, DKO females accumulate greater numbers of GC B cells and PB/PCs that contain CD11c+ subsets. Dysregulating TLR7 expression in DKO males results in a marked expansion of ABCs and other B cell effector lineages including CD11c-expressing B cell subsets and promotes autoAb production and disease development in DKO males. Taken together these data suggest that sexual dimorphism underlies several aspects of ABC biology in autoimmune settings. In this proposal we will investigate the hypothesis that sex- specific mechanisms control the function and differentiation of ABCs as well as characterize the developmental relationships between ABCs and other effector B cell lineages.