DYRK1a mediates BAFF-induced noncanonical NF-kB activation to promote autoimmunity and B cell leukemogenesis.

B cell-activating factor (BAFF) mediates B cell survival and, when deregulated, also contributes to autoimmune diseases and B cell malignancies. The mechanism connecting BAFF receptor (BAFFR) signal to downstream pathways and pathophysiological functions is not well understood. Here we identified DYRK1a as a kinase that responds to BAFF stimulation and mediates BAFF-induced B cell survival. B cell-specific DYRK1a deficiency causes peripheral B cell reduction and ameliorates autoimmunity in a mouse model of lupus. An unbiased screen identified DYRK1a as a protein that interacts with TRAF3, a ubiquitin ligase component mediating degradation of the noncanonical NF-kB-inducing kinase (NIK). DYRK1a phosphorylates TRAF3 at serine-29 to interfere with its function in mediating NIK degradation, thereby facilitating BAFF-induced NIK accumulation and noncanonical NF-kB activation. Interestingly, B cell acute lymphoblastic leukemia (B-ALL) cells express high levels of BAFFR and respond to BAFF for noncanonical NF-kB activation and survival in a DYRK1a-dependent manner. Furthermore, DYRK1a promotes a mouse model of B-ALL through activation of the noncanonical NF-kB pathway. These results establish DYRK1a as a critical BAFFR signaling mediator and provide novel insight into B-ALL pathogenesis.