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The NF kappa B family of heterodimeric transcription factors is activated by many stimuli, and lead to the upregulation of countless genes. Not surprisingly, NF kappa B plays a critical role in many aspects of cellular function. In T and B lymphocytes, antigen receptor stimulation leads to the activation of NF kappa B through a signal transduction cascade involving the Bcl10-MALT1 complex. We hypothesized that this complex may be critical to signalling cascades other than those emanating from antigen receptors. B cell activation factor of the TNF family (BAFF) activates non-canonical NF kappa B heterodimers that promote B cell survival. Here, we show that MALT1 is required for BAFF-induced phosphorylation of NF kappa B2 (p100), p100 degradation and RelB nuclear translocation in B220+ B cells. TRAF3, a known negative regulator of BAFF-R mediated signaling, interacts with MALT1 in a manner which is negatively regulated by BAFF, and TRAF3 levels are enhanced in MALT1-/- B cells. MALT1-/- CD21highCD23low (MZ) B cells show a defect in BAFF-induced survival and MALT1-/- x BAFF-transgenic (Tg) mice have decreased MZ and B1 B cell levels compared to BAFF-Tg mice. In agreement with this in vitro data, phenotypes associated with over-expression of BAFF including increased serum immunoglobulin titres, spontaneous germinal center (GC) formation, and immune complex deposition in the kidney were found to be dependent on B cell-intrinsic MALT1 expression. Our results demonstrate a novel role for MALT1 in biological outcomes induced by BAFF-mediated signal transduction. The mechanism by which the Bcl10-MALT1 complex regulates antigen induced NF kappa B activation in T cells remains controversial. To shed light on this regulatory network, we conducted biochemical purification of Bcl10, and identified Uev1a, a known regulator of antigen receptor mediated NF kappa B activation. We hypothesized that mms2, and structurally similar molecule to Uev1a, may also impinge on NF kappa B activation. Mms2 overexpression in 293T cells inhibited the Bcl10-induced activation of an NF kappa B sensitive luciferase. Lymphocyte development and antigen receptor induced activation occurs normally mms2-/- mice. However, class switched serum immunoglobulins, and survival responses to DNA damage inducing gamma-irradiation, are decreased in mms2-/- mice. Therefore, mms2 is dispensible in vivo for lymphocyte function and development, but is required for DNA damage responses.