Constitutive activation of Bruton's tyrosine kinase induces the formation of autoreactive IgM plasma cells

Abstract

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 3-83md 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 Ca 21 mobilization. They were spontaneously driven into germinal center-independent plasma cell differentiation, as evidenced by increased numbers of IgM 1 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. IntroductionSignals transmitted by the B-cell receptor (BCR) control the antigen response of B cells and are also essential regulators of survival, tolerance and differentiation (reviewed in [1,2]). Inducible and stage-specific targeting experiments demonstrated that mature B cells undergo apoptosis upon in vivo BCR ablation or mutation of one of its signaling units, Ig-a, and consequently disappear from the circulation [3,4]. A critical survival signal is provided by PI3K [5], but how this signaling is initiated in resting mature B cells is not fully understood. BCR signal strength is also a key factor in deciding between the three functionally distinct mature B-cell compartments of follicular, marginal zone (MZ) and B-1 B cells. Increases in BCR signaling strength, induced by low-dose selfantigen, direct maturation of naive immature B cells from the follicular into the B-1 or MZ B-cell fate [6,7]. Eur. J. Immunol. 2010. 40: 2643-2654 DOI 10.1002 Leukocyte signaling 2643In mature B cells, BCR engagement induces phosphorylation of Ig-a and Ig-b and the formation of a lipid raft-associated calcium-signaling module. In this complex Syk phosphorylates the adapter molecule Slp65, thereby providing docking sites for Bruton's tyrosine kinase (Btk) and phospholipase Cg2 (Plcg2). Activation of Plcg2 by Btk results in the generation of the Ca 21 -releasing factors inositol-1,4,5-trisphosphate and diacylglycerol (reviewed in [8,9]). During these events various co-receptors modulate BCR signaling either positively or negatively [10].Deficiencies of BCR signaling molecules, such as Btk, Slp65 or Plcg2 or the excitatory co-receptor CD19 result in a hyporesponsive phenotype, mainly characterize...

A Balance between B Cell Receptor and Inhibitory Receptor Signaling Controls Plasma Cell Differentiation by Maintaining Optimal Ets1 Levels

“…BCR signaling pathways can promote B cell differentiation to antibody-secreting plasma cells (69, 70) and are normally counterbalanced by inhibitory signaling pathways such that B cell differentiation is limited (4). The crucial targets whose expression is regulated by the BCR pathway and inhibitory signaling pathways have remained ill-defined.…”

Btk levels set the threshold for B-cell activation and negative selection of autoreactive B cells in mice

“…We proposed that strong Btk E41K mediated signaling may mimic self-antigen binding to the BCR, thereby instructing massive B-cell deletion. [26][27][28] Nevertheless, residual B cells were hyperresponsive and spontaneously driven into GC-independent plasma cell differentiation. The critical difference between Btk E41K transgenic and Btk overexpressing mice lies in the mode of increase in Btk kinase activity, which is constitutive versus conditional, respectively.…”

“…26 Even low levels of E41K-Btk hampered B-cell differentiation, characterized by reduced numbers of follicular B cells and selective survival or expansion of B-1 B cells. 28 Here we show that overexpression of WT Btk restricted to the B-cell lineage induced spontaneous germinal center (GC) and plasma cell formation, and eventually antinuclear autoantibody production and SLE-like autoimmune disease. This autoimmune phenotype was fully dependent on Btk kinase activity.…”

The role of Bruton’s tyrosine kinase in autoimmunity and implications for therapy

“…Because tolerance induction and maintenance require nuanced responses to signaling, these proteins are good targets for fine-tuning and improving tolerance, and Btk regulation in mouse models has been shown to be important for B cell tolerance. Transgenic Btk overexpression causes a systemic lupus erythematosus-like disease, propagated by spontaneous germinal centers and autoantibody formation (25), and a model featuring constitutively activated Btk results in autoreactive IgM plasma cells (57). Conversely, Btk expression at 25% normal levels abrogates autoantibody production and an autoimmune syndrome produced in lyn-deficient mice (23).…”