Potential role of CARMA1 in CD40‐induced splenic B cell proliferation and marginal zone B cell maturation

NF‐κB activation through B cell receptor (BCR) ligation is critical for B cell development, survival and antigen‐mediated activation of B cells. CARD domain and MAGUK‐domain containing protein‐1 (CARMA1), recently identified adaptor molecule, has been shown to play an essential role in BCR‐induced NF‐κB activation. CARMA1‐deficient B cells fail to proliferate upon BCR stimulation, leading to defective humoral responses. Surprisingly, CARMA1‐deficient B cells are also defective in CD40‐induced proliferation. The mechanisms responsible for CD40‐induced proliferation defect have not yet been characterized. In this study, we show that signaling cascades activated by CD40 stimulation are largely unaffected in CARMA1‐deficient B cells. Instead, we have found that the defective proliferation of CARMA1‐deficient B cells is due to two events. First, CARMA1‐deficient B cells show defective cell‐cycle progression. Secondly, the numbers of marginal zone (MZ) B cells, which are the main responders upon CD40 stimulation, are greatly diminished in CARMA1‐deficient mice. Since B cell maturation requires basal signaling through BCR and NF‐κB activation, we propose that impaired BCR signaling in CARMA1‐deficient mice leads to defective maturation of MZ B cell population, which in turn, contributes to impaired proliferation upon CD40 stimulation.