Tanvi Arkatkar

B cell IFN-γ receptor signaling promotes autoimmune germinal centers via cell-intrinsic induction of BCL-6.

Jackson et al. propose a role for B cell–intrinsic IFN-γ receptor signaling in spontaneous germinal center activation and autoantibody production.

The A946T variant of the RNA sensor IFIH1 mediates an interferon program that limits viral infection but increases the risk for autoimmunity

The single-nucleotide polymorphism rs1990760 in the gene encoding the cytosolic viral sensor IFIH1 results in an amino-acid change (A946T; IFIH1T946) that is associated with multiple autoimmune diseases. The effect of this polymorphism on both viral sensing and autoimmune pathogenesis remains poorly understood. Here we found that human peripheral blood mononuclear cells (PBMCs) and cell lines expressing the risk variant IFIH1T946 exhibited heightened basal and ligand-triggered production of type I interferons. Consistent with those findings, mice with a knock-in mutation encoding IFIH1T946 displayed enhanced basal expression of type I interferons, survived a lethal viral challenge and exhibited increased penetrance in autoimmune models, including a combinatorial effect with other risk variants. Furthermore, IFIH1T946 mice manifested an embryonic survival defect consistent with enhanced responsiveness to RNA self ligands. Together our data support a model wherein the production of type I interferons driven by an autoimmune risk variant and triggered by ligand functions to protect against viral challenge, which probably accounts for its selection within human populations but provides this advantage at the cost of modestly promoting the risk of autoimmunity.

Cutting Edge: BAFF Promotes Autoantibody Production via TACI-Dependent Activation of Transitional B Cells

Mice overexpressing B cell activating factor of the TNF family (BAFF) develop systemic autoimmunity characterized by class-switched anti-nuclear Abs. Transmembrane activator and CAML interactor (TACI) signals are critical for BAFF-mediated autoimmunity, but the B cell developmental subsets undergoing TACI-dependent activation in settings of excess BAFF remain unclear. We report that, although surface TACI expression is usually limited to mature B cells, excess BAFF promotes the expansion of TACI-expressing transitional B cells. TACI+ transitional cells from BAFF-transgenic mice are characterized by an activated, cycling phenotype, and the TACI+ cell subset is specifically enriched for autoreactivity, expresses activation-induced cytidine deaminase and T-bet, and exhibits evidence of somatic hypermutation. Consistent with a potential contribution to BAFF-mediated humoral autoimmunity, TACI+ transitional B cells from BAFF-transgenic mice spontaneously produce class-switched autoantibodies ex vivo. These combined findings highlight a novel mechanism through which BAFF promotes humoral autoimmunity via direct, TACI-dependent activation of transitional B cells.

B cell–derived IL-6 initiates spontaneous germinal center formation during systemic autoimmunity

Recent studies have identified critical roles for B cells in triggering autoimmune germinal centers (GCs) in systemic lupus erythematosus (SLE) and other disorders. The mechanisms whereby B cells facilitate loss of T cell tolerance, however, remain incompletely defined. Activated B cells produce interleukin 6 (IL-6), a proinflammatory cytokine that promotes T follicular helper (TFH) cell differentiation. Although B cell IL-6 production correlates with disease severity in humoral autoimmunity, whether B cell–derived IL-6 is required to trigger autoimmune GCs has not, to our knowledge, been addressed. Here, we report the unexpected finding that a lack of B cell–derived IL-6 abrogates spontaneous GC formation in mouse SLE, resulting in loss of class-switched autoantibodies and protection from systemic autoimmunity. Mechanistically, B cell IL-6 production was enhanced by IFN-&ggr;, consistent with the critical roles for B cell–intrinsic IFN-&ggr; receptor signals in driving autoimmune GC formation. Together, these findings identify a key mechanism whereby B cells drive autoimmunity via local IL-6 production required for TFH differentiation and autoimmune GC formation.

DOCK8 regulates fitness and function of regulatory T cells through modulation of IL-2 signaling

Foxp3+ Tregs possess potent immunosuppressive activity, which is critical for maintaining immune homeostasis and self-tolerance. Defects in Treg development or function result in inadvertent immune activation and autoimmunity. Despite recent advances in Treg biology, we still do not completely understand the molecular and cellular mechanisms governing the development and suppressive function of these cells. Here, we have demonstrated an essential role of the dedicator of cytokinesis 8 (DOCK8), guanine nucleotide exchange factors required for cytoskeleton rearrangement, cell migration, and immune cell survival in controlling Treg fitness and their function. Treg-specific DOCK8 deletion led to spontaneous multiorgan inflammation in mice due to uncontrolled T cell activation and production of proinflammatory cytokines. In addition, we show that DOCK8-deficient Tregs are defective in competitive fitness and in vivo suppressive function. Furthermore, DOCK8 controls IL-2 signaling, crucial for maintenance and competitive fitness of Tregs, via a STAT5-dependent manner. Our study provides potentially novel insights into the essential function of DOCK8 in Tregs and immune regulation, and it explains the autoimmune manifestations associated with DOCK8 deficiency.

αv Integrins regulate germinal center B cell responses through noncanonical autophagy

Germinal centers (GCs) are major sites of clonal B cell expansion and generation of long-lived, high-affinity antibody responses to pathogens. Signaling through TLRs on B cells promotes many aspects of GC B cell responses, including affinity maturation, class switching, and differentiation into long-lived memory and plasma cells. A major challenge for effective vaccination is identifying strategies to specifically promote GC B cell responses. Here, we have identified a mechanism of regulation of GC B cell TLR signaling, mediated by &agr;v integrins and noncanonical autophagy. Using B cell–specific &agr;v-KO mice, we show that loss of &agr;v-mediated TLR regulation increased GC B cell expansion, somatic hypermutation, class switching, and generation of long-lived plasma cells after immunization with virus-like particles (VLPs) or antigens associated with TLR ligand adjuvants. Furthermore, targeting &agr;v-mediated regulation increased the magnitude and breadth of antibody responses to influenza virus vaccination. These data therefore identify a mechanism of regulation of GC B cells that can be targeted to enhance antibody responses to vaccination.

TACI deletion protects against progressive murine lupus nephritis induced by BAFF overexpression

B cells are known to promote the pathogenesis of systemic lupus erythematosus (SLE) via the production of pathogenic anti-nuclear antibodies. However, the signals required for autoreactive B cell activation and the immune mechanisms whereby B cells impact lupus nephritis pathology remain poorly understood. The B cell survival cytokine B cell activating factor of the TNF Family (BAFF) has been implicated in the pathogenesis of SLE and lupus nephritis in both animal models and human clinical studies. Although the BAFF receptor has been predicted to be the primary BAFF family receptor responsible for BAFF-driven humoral autoimmunity, in the current study we identify a critical role for signals downstream of Transmembrane Activator and CAML Interactor (TACI) in BAFF-dependent lupus nephritis. Whereas transgenic mice overexpressing BAFF develop progressive membranoproliferative glomerulonephritis, albuminuria and renal dysfunction, TACI deletion in BAFF-transgenic mice provided long-term (about 1 year) protection from renal disease. Surprisingly, disease protection in this context was not explained by complete loss of glomerular immune complex deposits. Rather, TACI deletion specifically reduced endocapillary, but not mesangial, immune deposits. Notably, although excess BAFF promoted widespread breaks in B cell tolerance, BAFF-transgenic antibodies were enriched for RNA- relative to DNA-associated autoantigen reactivity. These RNA-associated autoantibody specificities were specifically reduced by TACI or Toll-like receptor 7 deletion. Thus, our study provides important insights into the autoantibody specificities driving proliferative lupus nephritis, and suggests that TACI inhibition may be novel and effective treatment strategy in lupus nephritis.

Activated CARD11 accelerates germinal center kinetics, promoting mTORC1 and terminal differentiation

Activating mutations in the adapter protein CARD11 associated with diffuse large B cell lymphomas (DLBCLs) are predicted to arise during germinal center (GC) responses, leading to inappropriate activation of NF-&kgr;B signaling. Here, we modeled the B cell–intrinsic impact of the L251P activating mutation in CARD11 (aCARD11) on the GC response. Global B cell aCARD11 expression led to a modest increase in splenic B cells and a severe reduction in B1 B cell numbers, respectively. Following T cell–dependent immunization, aCARD11 cells exhibited increased rates of GC formation, resolution, and differentiation. Restriction of aCARD11 to GC B cells similarly altered the GC response and B cell differentiation. In this model, aCARD11 promoted dark zone skewing along with increased cycling, AID levels, and class switch recombination. Furthermore, aCard11 GC B cells displayed increased biomass and mTORC1 signaling, suggesting a novel strategy for targeting aCARD11-driven DLBCL. While aCARD11 potently impacts GC responses, the rapid GC contraction suggests it requires collaboration with events that limit terminal differentiation to promote lymphoma.

Generation of functional murine CD11c+ age-associated B cells in the absence of B cell T-bet expression

Age‐associated B cells (ABC), a novel subset of activated B cells defined by CD11b and CD11c expression, have been linked with both protective anti‐viral responses and the pathogenesis of systemic autoimmunity. Expression of the TH1 lineage transcription factor T‐bet has been identified as a defining feature of ABC biology, with B cell‐intrinsic expression of this transcription factor proposed to be required for ABC formation. In contrast to this model, we report that Tbx21 (encoding T‐bet)‐deficient B cells upregulate CD11b and CD11c surface expression in vitro in response to integrated TLR and cytokine signals. Moreover, B cell‐intrinsic T‐bet deletion in a murine lupus model exerted no impact of ABC generation in vivo, with Tbx21−/− ABCs exhibiting an identical surface phenotype to wild‐type (WT) ABCs. Importantly, WT and Tbx21−/− ABCs sorted from autoimmune mice produced equivalent amounts of IgM and IgG ex vivo following TLR stimulation, indicating that T‐bet‐deficient ABCs are likely functional in vivo. In summary, our data contradict the established literature by demonstrating that T‐bet expression is not uniformly required for ABC generation.

AI-18 B cell IFN-γ receptor signalling promotes autoimmune germinal centresvia cell-intrinsic induction of BCL-6

Background Dysregulated germinal centrecenter (GC) responses are implicated in the pathogenesis of human autoimmune diseases, including systemic lupus erythematosus (SLE). Although type 1 interferons (IFNs) are most frequently associated with lupus pathogenesis, type 2 interferon (IFN-γ) has also been shown to promote SLE. However, the respective impacts of these cytokines in promoting B cell activation during humoral autoimmunity have not been addressed. Materials and methods We recently developed a chimeric murine lupus model in which Wiskott-Aldrich syndrome protein (WAS)-deficient B cells promote spontaneous humoral autoimmunity (Jackson, et al. J Immunol 2014). An important advantage of the WAS chimaera model is that dysregulated immune responses are limited to the B cell compartment, allowing genetic manipulation in a B cell-intrinsic fashion. In the current study, we contrast the impact B cell-intrinsic type 1 IFN vs. IFN-γ signals on autoimmune GC formation and the pathogenesis of SLE. Results Although type 1 IFN prominently enhanced B cell responses in vitro, B cell-intrinsic IFNAR deletion exerted surprisingly minimal impacts on class-switched autoantibody titers and spontaneous GC formation in vivo. This finding suggested that other cytokines promote B cell activation in the WAS chimaera model. Notably, B cells directly initiated CD4+ T cell activation and T follicular helper cell formation via MHC Class II (MHC-II)-dependent antigen presentation. In addition, activated T cells exhibited prominent IFN-γ production that was lost following B cell-intrinsic MHC-II deletion, suggesting a direct role for IFN-γ in promoting autoimmune GC formation. Strikingly, B cell-intrinsic deletion of the IFN-γ receptor was sufficient to abrogate spontaneous GCs, class-switched autoantibodies and systemic autoimmunity. Mechanistically, although IFN-γ receptor signals increased B cell T-bet expression, B cell-intrinsic deletion of T-bet exerted an isolated impact on class-switch recombination to pathogenic IgG2c autoantibody subclasses without impacting GC development. Rather, in both murine and human B cells, IFN-γ synergized with BCR, TLR and/or CD40 activation signals to promote cell-intrinsic BCL-6 expression. Finally, IFN-γ driven BCL-6 expression in B cells was blocked using clinically-relevant Janus kinase inhibitors, ruxolitinib and tofacitinib. Conclusions Our study demonstrates that B cell-intrinsic IFN-γ receptor signals promote lupus pathogenesis via formation of spontaneous, autoimmune GCs. In addition, we have uncovered a novel cell-intrinsic program whereby IFN-γ, together with BCR-, TLR- and/or CD40 signals, orchestrates B cell expression of the GC master transcription regulator BCL-6. Our combined findings suggest that this IFN-γ signalling program may be a potential therapeutic target in SLE. Acknowledgements This work was supported by the National Institutes of Health under award numbers: R01HL075453 (DJR), R01AI084457 (DJR), R01AI071163 (DJR), DP3DK097672 (DJR) and K08AI112993 (SWJ). The content is solely the responsibility of the authors and does not necessarily represent the official views of the National Institutes of Health. Additional support provided by the Benaroya Family Gift Fund (DJR); by the ACR REF Rheumatology Scientist Development Award (SWJ); and by the Arnold Lee Smith Endowed Professorship for Research Faculty Development (SWJ).