Bruton\u27s tyrosine kinase supports gut mucosal immunity and commensal microbiome recognition in autoimmune arthritis

Bruton\u27s tyrosine kinase

B cells, bugs, bowels, and bones: Bruton’s tyrosine kinase deficiency alters gut B lymphocyte populations and IgA recognition of commensal bacteria in autoimmune arthritis

Abstract IgA is the most abundantly produced isotype, much of which is secreted into the gut lumen. IgA interacts with bacteria, but the dialogue between intestinal B lymphocytes, their antibody products, and commensals is incompletely understood. Elimination of bacteria prevents spontaneous arthritis in the K/BxN model of rheumatoid arthritis, as does disrupted B cell receptor signaling via Bruton’s tyrosine kinase (Btk) deficiency. We hypothesized that altered B cell signaling impacts commensal bacteria recognition. We find that germinal center B cells and IgA class-switched cells are dramatically reduced by Btk deficiency in the in Peyer’s patches of K/BxN mice. Plasmablast and plasma cell numbers are not reduced by Btk deficiency in the small intestine lamina propria; however, the frequency of IgA-coated bacteria is decreased, and the IgA-coated community is shifted. Parabacteroides distasonis is reduced in Btk−/y mice. P. distasonis infection enhances autoimmune arthritis development in antibiotic-treated mice that are otherwise protected, suggesting this commensal plays an inciting role in disease development. In contrast, P. distasonis was protective in prior studies of a multiple sclerosis model. These data show impaired B cell receptor signaling alters the dynamic interplay between B lymphocytes and commensal microbes to impact autoimmune arthritis development. BTK inhibitors are used to treat lymphoma and clinical trials investigating their efficacy in several autoimmune diseases, including rheumatoid arthritis, are ongoing. Our work suggests that the impact of such regimens on the delicate balance between commensal bacteria and adaptive immunity should be considered in the specific autoimmune context.</jats:p

Mature anti-insulin B cells survive and present antigen in the absence of Bruton’s tyrosine kinase.

Abstract Bruton’s tyrosine kinase (Btk) is a tec-family kinase present in B lymphocytes and innate immune cells. Btk is an important regulator of autoreactive B cells. In the non-obese diabetic (NOD) mouse model of type 1 diabetes (T1D), Btk-deficiency is protective against disease development and results in significant loss of anti-insulin IgG, even as total IgG is preserved. Anti-insulin B cells drive T1D by presenting antigen to autoreactive T cells. In a transgenic model, conventional Btk-deficiency reduces anti-insulin B cells by 95%, effectively blocking their development. However, the ability of mature anti-insulin B cells to survive or present antigen without Btk was unknown. We induced deletion of Btk using a loxP-flanked Btk mouse model paired with tamoxifen-inducible Cre. Surprisingly, these anergic anti-insulin B cells survive without Btk, as normal numbers of mature B cells were maintained in transgenic Btkflox/Cre-ERT2 animals after tamoxifen treatment. Btk-negative anti-insulin B cells also remained able to internalize and present antigen to cognate T cells and to phosphorylate phospholipase C γ2 in response to anti-IgM. These findings show that though anti-insulin B cells require Btk for their development, it is not required for mature anti-insulin B cell survival. In addition, our finding that Btk-negative anti-insulin B cells can present antigen may have implications for the use of Btk-inhibition in autoimmunity driven by this mechanism.</jats:p

One size doesn’t fit all: SLAM-Associated Protein is dispensable for type 1 diabetes but required for autoantibody-mediated arthritis

Abstract Signaling lymphocytic activation molecule-associated protein (SAP), a critical intracellular signaling molecule for T-B interactions, drives Tfh development in germinal centers (GC) in the setting of protective immune responses. SAP promotes long-lived T cell-B cell interactions necessary to sustain germinal center reactions that lead to antibody production. High-affinity islet autoantibodies predict type 1 diabetes (T1D) but do not cause beta cell destruction. This paradox intimates T follicular helper cells (Tfh) as key pathologic effectors. To understand Tfh contribution to autoimmune processes, we investigated the role of SAP in T1D and autoantibody-mediated arthritis. Whereas spontaneous autoimmune arthritis depended on SAP, organized insulitis and diabetes onset were unabated, despite a blocked anti-insulin vaccine response. SAP-deficient T cell function was therefore investigated in the non-obese diabetic (NOD) T1D model. B lymphocyte antigen presentation drove SAP-deficient NOD T cell proliferation. Although germinal center B cell formation was markedly reduced, GC-Tfh were found at sites of autoimmune attack in SAP-deficient NOD mice. Thus, Tfh induced in germinal center reactions are dispensable for T1D and suggest that the autoimmune process in NOD retains pathogenic Tfh in the absence of prolonged B cell interactions. These findings demonstrate that SAP is essential for Tfh formation and autoimmunity when autoantibodies play a central role in autoimmune arthritis but not in cell-mediated T1D. SAP inhibition is thus a compelling strategy for autoantibody-mediated disease treatment, but additional cognate cellular interaction molecules must be targeted to treat cell-mediated autoimmune diseases.</jats:p

T–B Lymphocyte Interactions Promote Type 1 Diabetes Independently of SLAM-Associated Protein

Abstract Signaling lymphocytic activation molecule–associated protein (SAP), a critical intracellular signaling molecule for T–B lymphocyte interactions, drives T follicular helper (Tfh) cell development in germinal centers (GCs). High-affinity islet autoantibodies predict type 1 diabetes (T1D) but do not cause β cell destruction. This paradox intimates Tfh cells as key pathologic effectors, consistent with an observed Tfh signature in T1D. To understand how fully developed Tfh (GC Tfh) contribute to different autoimmune processes, we investigated the role of SAP in T1D and autoantibody-mediated arthritis. Whereas spontaneous arthritis depended on SAP in the autoantibody-mediated K/BxN model, organized insulitis and diabetes onset were unabated, despite a blocked anti-insulin vaccine response in SAP-deficient NOD mice. GC Tfh and GC B cell development were blocked by loss of SAP in K/BxN mice. In contrast, although GC B cell formation was markedly reduced in SAP-deficient NOD mice, T cells with a GC Tfh phenotype were found at disease sites. CXCR3+ CCR6− (Tfh1) subset bias was observed among GC Tfh cells infiltrating the pancreas of NOD mice, which was enhanced by loss of SAP. NOD T cells override SAP requirement to undergo activation and proliferation in response to Ag presentation, demonstrating the potential for productive cognate T–B lymphocyte interactions in T1D-prone mice. We find that SAP is essential when autoantibody-driven immune complexes promote inflammation but is not required for effective organ-specific autoimmune attack. Thus, Tfh induced in classic GC reactions are dispensable for T1D, but the autoimmune process in the NOD model retains pathogenic Tfh without SAP.</jats:p

Btk Supports Autoreactive B Cell Development and Protects against Apoptosis but Is Expendable for Antigen Presentation

Bruton's tyrosine kinase (Btk) propagates B cell signaling, and BTK inhibitors are in clinical trials for autoimmune disease. Although autoreactive B cells fail to develop in the absence of Btk, its role in mature cells is unknown. To address this issue, a model of conditional removal (Btkflox/Cre-ERT2) was used to excise Btk from mature transgenic B cells that recognize the pathophysiologic autoantigen insulin. Anti-insulin B cells escape central tolerance and promote autoimmune diabetes, mimicking human autoreactive cells. Lifelong Btk deficiency was previously shown to eliminate 95% of anti-insulin B cells, but in this model, mature anti-insulin B cells survived for weeks after targeted Btk deletion, even when competing with a polyclonal repertoire. BCR-stimulated cells could still signal via Syk, PLCy2, and CD22, but failed to upregulate the antiapoptotic protein Bcl-xL, and proliferation was impaired. Surprisingly, Btk-depleted anti-insulin B cells could still present Ag and activate T cells, a critical function in promoting T autoreactive cells, and preventing emergence of new ones. The Journal of Immunology, 2021, 207: 2922-2932

Bruton’s Tyrosine Kinase Supports Gut Mucosal Immunity and Commensal Microbiome Recognition in Autoimmune Arthritis

AbstractBruton’s tyrosine kinase (Btk) deficiency preferentially eliminates autoreactive B cells while sparing normal humoral responses, but has not been studied in mucosal immunity. Commensal microbes are essential for arthritis in K/BxN mice, used here to examine how BTK-mediated signaling interfaces with the microbiome. Btk-deficient K/BxN mice were found to have small Peyer’s Patches with reduced germinal center and IgA+ B cells. Although lamina propria IgA+ plasma cells were numerically normal, intestinal IgA was low and IgA coating of commensal bacteria was reduced. IgA-seq showed a shift in microbes that are normally IgA-coated into the uncoated fraction in Btk-deficient mice. In this altered microbial milieau, the proportion of Parabacteroides distasonis was reduced in Btk-deficient K/BxN mice. To determine whether P. distasonis contributes to arthritis, it was reintroduced into antibiotic-protected K/BxN mice, where it restored disease. This suggests that P. distasonis’ inability to thrive in Btk-deficient mice may be a factor in disease protection. Thus, BTK supports normal intestinal IgA development, with downstream effects on the microbiome that may contribute to autoimmunity.</jats:p