Clast5/Stra13 is a negative regulator of B lymphocyte activation

CD40 is a member of the tumor necrosis factor receptor family and mediates a variety of functions of B cells, including B cell survival, proliferation, immunoglobulin gene class switching, memory B cell formation, and regulation of Fas-mediated apoptosis. To begin to elucidate the molecular mechanism governing such diverse functions of CD40, we have isolated a gene from mouse splenic B cells, termed Clast5, whose expression is strongly repressed during B cell activation. Clast5 is identical with Stra13, a recently identified member of the basic helix-loop-helix family of transcription factors. Clast5/Stra13 is highly expressed in unstimulated, resting B cells and is rapidly downregulated by a variety of stimuli that activate B cells, including CD40 ligand, anti-IgM antibodies, lipopolysaccharides and interleukin-4. Forced expression of Clast5/Stra13 in B cells delayed the cell cycle progression into S phase and strongly suppressed Fas-mediated apoptosis. Moreover, Clast5/Stra13 inhibited the colony formation in fibroblasts. Our results suggest that Clast5/Stra13 functions as a negative regulator of B cell activation by inhibiting cell cycle progression and cell growth.