Lee, N.; Hong, B.-K.; You, S.; Kwon, R.; Kwon, J.; Choi, E.; Lee, K.-G.; Kim, Y.-M.; Li, Y.; Kim, J.; Park, Y.-J.; Chung, Y.; Im, S.-H.; Sabbagh, L.; Cho, C.-S.; Kim, W.-U.

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 PKC{beta}-CEBP{beta} 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.