The control mechanism of B cells activation due to pKnox1 homeodomain transcription factor

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Conditional B cell-specific knockout of pKnox1, a homeodomain transcription factor, in mice leads to age-associated development of autoimmune phenotypes. In order to understand the molecular basis, we analyzed B cell development and immune responses. The mice showed abnormal marginal zone B cell development, decreased IgG level, delayed affinity maturation caused by decreased frequency of somatic hypermutation and delayed germinal center formation. The expression level of AID (Activation-induced cytidine deaminase) was decreased in the mice compared with control mice. The expression level of HoxC4 that directly regulates AID expression also decreased. We found the sequence that is similar to pKonox1-Pbx1 complex binding domain in HoxC4 promoter, too. These findings indicate that the autoimmune phenotypes in B cell–specific knockout of pKnox1 mice might be related to the failure of AID expression regulatory mechanisms.