POS1399 BANK1 SIGNALLING SHAPES THE GUT MICROBIOTA COMPOSITION BY CONTROLLING THE GUT MUCOSAL B CELL RESPONSE IN A TLR7-MEDIATED LUPUS MODEL
Authors: M. Botía Sánchez, G. Galicia, D. Toro-Domínguez, A. López García, J. R. Valera, A. Margolles, N. Varela, M. Morell, M. Alarcon-Riquelme
Journal: Annals of the Rheumatic Diseases (2024)

<h3>Background:</h3> Systemic lupus erythematosus (SLE) is an autoimmune disease characterized by persistent inflammation affecting multiple organs, including the intestine. A new feature that seems decisive in autoimmune pathogenesis is the gut microbiota composition. Its role, however, remains elusive. The B-cell scaffold protein with ankyrin repeats (<i>Bank1</i>) regulates Toll-like receptor-7 (TLR7) signalling in B cells and it has been genetically associated with human lupus and its absence correlated with a less severe disease in mice. In the context of autoimmunity, IL-10+ B cells are being characterised in recent years for their immunosuppressive role and their feasible relationship with gut bacterial composition. <h3>Objectives:</h3> To investigate the involvement of BANK1 signalling in the gut mucosal B cell response to commensals during lupus pathogenesis. <h3>Methods:</h3> Lupus derived gut affectation was characterised by using two TLR7-mediated models of the disease by topical application of the TLR7 agonist Imiquimod (IMQ) or the transgenic expression of the <i>Tlr7</i> gene. We assessed splenomegaly and anti-dsDNA autoantibodies by ELISA and the IL-10+ B cell response by flow cytometry. IgA response in the gut and the faeces were assessed by flow cytometry and ELISA, and the IgA antigenic targets were evaluated by sorting and sequencing of IgA+ and IgA- faecal bacteria. Presence of <i>Parabacteroides distasonis</i> was assessed by qPCR. Bacterial composition was analysed by V4 16S rRNA sequencing. <h3>Results:</h3> In steady-state, <i>Bank1</i> deficiency (B1KO mice) resulted in reduced frequency of CD19<sup>+</sup>B220<sup>+</sup> B cells and IgA<sup>+</sup> plasma cells in the small intestine lamina propria, which correlated with reduced levels of faecal-free IgA antibodies. Additionally, B1KO mice presented differential microbial composition and bacterial specificity. While the TLR7-mediated intestinal inflammation induced enlarged germinal centers (GC) in the Peyer's patches and accumulation of GC B cells, these disease manifestations were reduced in the absence of <i>Bank1</i>. B1KO developed a milder form of the disease characterised by lower serum anti-dsDNA IgG2a levels, and reduced splenomegaly and gut inflammation. Concomitantly, upon TLR7-induced inflammation, levels of IgA+ plasma cells, concentration of faecal-free IgA antibodies and percentage of IgA-coated bacteria were comparable between B1KO and WT mice. However, analysis of IgA coating pattern of faecal bacteria showed that the specificities of gut-secreted IgA antibodies (antigenic targets) were significantly different between both genotypes, targeting different bacteria at genus and species level. Amelioration of lupus symptoms in mice lacking <i>Bank1</i> was associated with an increased proportion of <i>P. distasonis</i> that, when transferred onto <i>Bank1</i>-sufficient WT mice, reduced the autoimmune manifestations. The effect of these bacteria was evaluated through two independent experiments. First, <i>littermates</i> carrying the B1KO-deficient-associated microbiota were generated and grown in shared cages before and during lupus induction. In the second, <i>P. distasonis</i> was given orally to assess its effect as a probiotic. Both experiments showed reduced gut inflammation, germinal center formation, and splenomegaly and lower levels of autoantibodies, regardless of genotype. As a mechanistic insight, we evaluated the induction of IL-10-producung B cells. <i>Bank1</i> deficiency promoted the accumulation of IL-10-producing B cells in Peyer's patches of lupus mice upon lupus development. Moreover, colonization of gut microbiota with <i>P. distasonis</i> led to increased numbers of IL-10+ B cells in WT mice, which correlated with lower splenomegaly and reduced levels of serum anti-dsDNA IgG2a. <h3>Conclusion:</h3> The absence of <i>Bank1</i> predisposed to differential microbiota and differences in the numbers of IL-10+ B cells in the gut, together with reduced autoimmune inflammation. Changes in the humoral and cellular IgA response, may pave the way for the gut-activated IgA+ plasma cells to act peripherally and regulate autoimmunity. These results highlight the complex crosstalk between genes and environmental factors that modulate autoimmune inflammation. <h3>REFERENCES:</h3> <b>NIL.</b> <h3>Acknowledgements:</h3> <b>NIL.</b> <h3>Disclosure of Interests:</h3> <b>None declared.</b>