Background B cells play a pivotal role in pathological processes of autoimmune diseases. Btk, which mediates BCR signaling, is considered as a suitable target for the treatment of autoimmune diseases. However, the underlying mechanisms of B cell activation mediated by Btk remain elusive.
Objectives The aim of this study is to investigate the mechanism underlying human B cell activation and to assess the role of Btk in this process.
Methods Purified B cells from healthy donors were stimulated with BCR, sCD40L/BAFF, and sIL-21 with/without a Btk inhibitor. B cell line, BJAB was utilized to assess the molecular mechanism.
Results The expression of AICDA, encoding a RNA-editing deaminase and involved in class-switch recombination of Ig, was slightly induced by IL-21 stimulation alone in human primary B cells. Likewise, under BCR and/or CD40/BAFF stimulation, very little AICDA expression was induced. However, combinatorial stimulation with BCR, CD40/BAFF and IL-21 induced robust expression of AICDA, BCL6 and XBP1 and IgG production. A Btk inhibitor (ONO-A) significantly abrogated the expression to the extent of IL-21 alone in a dose-dependent manner. To assess the mechanism by which Btk regulates the genes induced by IL-21 in human B cells, we set up the Btk knock-down (KD) cell line by using BJAB. The transient exposure of IL-21 induced both p-STAT1 and p-STAT3 in BJAB. However, in Btk KD BJAB, p-STAT1 was not induced in the nucleus, while it was induced in the cytoplasm. Furthermore, IL-21-induced expression of T-bet significantly decreased in Btk-KD cells, compared to mock transferred BJAB, indicating that phosphorylation of STAT-1 by IL-21 affected its target gene T-bet, resulting in robust activation of B cells through Btk.
Conclusions Our results suggest that Btk activation induced by BCR and CD40/BAFF amplify IL-21 signaling to induce robust expression of differentiation- and class-switch DNA recombination (CSR)-related genes and IgG production in human B cells. Btk-mediated signal directly regulate p-STAT1 translocation from the cytoplasm to the nucleus, thereby allowing efficient propagation of IL-21-signal critical for human B cell differentiation and CSR. These results underscore the potential role of Btk in B cell-mediated pathological processes in autoimmune diseases.
Disclosure of Interest S.-P. Wang: None Declared, S. Iwata: None Declared, K. Yamaoka: None Declared, H. Niiro: None Declared, S. Nakayamada: None Declared, S. Tabrizi: None Declared, S. Kubo: None Declared, M. Kondo: None Declared, K. Akashi: None Declared, Y. Tanaka Grant/research support from: Bristol-Myers Squibb, MSD K.K., Chugai Pharma Co., Ltd., Mitsubishi-Tanabe Pharma Co., Ltd., Astellas Pharma Inc., Abbott Japan Co., Ltd., Eisai Co., Ltd. and Janssen Pharmaceutical K.K., Speakers bureau: Mitsubishi-Tanabe Pharma Co., Ltd., Abbott Japan Co., Ltd., Eisai Co., Ltd., Chugai Pharma Co., Ltd., Janssen Pharma K.K., Santen Pharma Co., Ltd., Pfizer Japan Inc., Astellas Pharma Inc., Daiichi-Sankyo Co., Ltd., GlaxoSmithKline K.K., Astra-Zeneca, Otsuka Pharma Co., Ltd., Actelion Pharma Japan Ltd., Eli Lilly Japan K.K., Nippon Kayaku Co., Ltd., UCB Japan Co., Ltd., Quintiles Transnational Japan Co. Ltd., Ono Pharma Co., Ltd.,