The evolutionary conserved HoxC4 homeodomain protein induces AID expression and regulates immunoglobulin class switch DNA recombination and somatic hypermutation

The regulation of activation induced cytidine deaminase (AID) gene ( aicda ) expression and the modulation of immunoglobulin gene class switch DNA recombination (CSR) and somatic hypermutation (SHM) remain to be defined. We found that the HoxC4 homeodomain protein is preferentially expressed in germinal center B cells and upregulated by stimuli that induce AID expression and CSR. In HoxC4‐deficient mice, the mutation frequency in the intronic J H ‐iEμ region of IgH DNA and the serum IgG1 levels were decreased by about 60%. Accordingly, the high affinity anti‐NP response was significantly compromised in hoxC4 −/− mice, and CSR to IgG1, IgG2a, IgG2b, IgG3 and IgA in in vitro stimulated hoxC4 −/− B cells was impaired. This was not due to altered hoxC4 −/− B cell proliferation or germinal center formation in hoxC4 −/− mice. Rather, it resulted from a defective induction of AID expression. Indeed, by performing in vitro promoter bashing experiments using luciferase gene reporter assays and ChIP, we showed that HoxC4 activates aicda by binding to an evolutionarily conserved HoxC4/Oct‐binding ATTT(GCAT) motif in the promoter for this gene. Enforced expression of AID in HoxC4‐deficient B cells restored CSR. Thus, these findings show that by regulating AID expression through direct activation of the aicda promoter, HoxC4 plays a critical role in modulating CSR and SHM. Supported by NIH grants, AI 45011, AI 60573 and AR 40908.