B lymphocytes acquire myeloid and autoimmune phenotypes via the downregulation of lymphocyte-specific protein-1

Actin-binding proteins (ABPs) have been established as important mediators of immune homeostasis, but their effects on lymphocytes are poorly understood. Here, we demonstrated that LSP1, an ABP, is a master regulator for innate immune responses in B lymphocytes. Lsp1 deficiency in B cells upregulated the expression of myeloid genes, including CD11b, CD11c, and myeloperoxidase, and bestowed myeloid morphology. Strikingly, Lsp1 -deficient B cells exhibited dual functions, namely, strong phagocytic activity and high antibody (Ab) production, like ‘chimera’. The PKCβ-CEBPβ pathway was found to be required for such functional chimerism. Moreover, Lsp1 deficiency induced the myeloid B cell phenotype and autoantibody production in B cells and consequently accelerated the progression of experimental lupus in mice. These changes were abrogated by retinoic acid, which upregulated LSP1 expression. In lupus patients, LSP1 expression in B cells was downregulated and inversely correlated with myeloperoxidase (MPO) expression. Overall, this study reveals a new role of the ABP LSP1 in B lymphocytes and emphasizes its critical involvement in promoting autoimmune responses, particularly by generating functionally chimeric B cells.