B-cell receptor (BCR)-mediated signals provide the basis for B-cell differentiation in the BM and subsequently into follicular, marginal zone, or B-1 B-cell subsets. We have previously shown that B-cell-specific expression of the constitutive active E41K mutant of the BCR-associated molecule Bruton's tyrosine kinase (Btk) leads to an almost complete deletion of immature B cells in the BM. Here, we report that low-level expression of the E41K or E41K-Y223F Btk mutants was associated with reduced follicular B-cell numbers and significantly increased proportions of B-1 cells in the spleen. Crosses with 3-83μδ and VH81X BCR Tg mice showed that constitutive active Btk expression did not change follicular, marginal zone, or B-1 B-cell fate choice, but resulted in selective expansion or survival of B-1 cells. Residual B cells were hyperresponsive and manifested sustained Ca2+mobilization. They were spontaneously driven into germinal center-independent plasma cell differentiation, as evidenced by increased numbers of IgM+plasma cells in spleen and BM and significantly elevated serum IgM. Because anti-nucleosome autoantibodies and glomerular IgM deposition were present, we conclude that constitutive Btk activation causes defective B-cell tolerance, emphasizing that Btk signals are essential for appropriate regulation of B-cell activation.

Antibodies, Autoimmunity, B cell-development, B cells, Signal transduction
dx.doi.org/10.1002/eji.201040521, hdl.handle.net/1765/27544
European Journal of Immunology
Erasmus MC: University Medical Center Rotterdam

Kersseboom, R, Kil, L-P, Flierman, R, van der Zee, M, Dingjan, G.M, Middendorp, S, … Hendriks, R.W. (2010). Constitutive activation of Bruton's tyrosine kinase induces the formation of autoreactive IgM plasma cells. European Journal of Immunology, 40(9), 2643–2654. doi:10.1002/eji.201040521