Expression and characterization of activation-induced deaminase (AID) with dehydroaminase activity and its localization in IgM+ lymphocytes of spleen of common carp (Cyprinus carpio)

Activation-induced deaminase (AID) is a key enzyme that catalyzes the deamination of cytosine to uracil in DNA, driving somatic hypermutation (SHM) and class switch recombination (CSR) of antibody genes to promote antibody diversification in the adaptive immunity. The AID gene from Cyprinus carpio (CcAID) has a full-length open reading frame of 633 bp and encodes a protein of 210 amino acids. CcAID shares structural and functional similarities with apolipoprotein B mRNA-editing enzyme, catalytic polypeptide-like (APOBEC) family members and contains a conserved catalytic domain characteristic of cytidine deaminases. Notably, the absence of two key phosphorylation sites in CcAID (Ser38 and Tyr184) may be a significant factor contributing to the markedly lower levels of somatic hypermutation (SHM) in teleost fish. A high level of CcAID transcript expression was detected in carp larvae on the sixth day after fertilization. Intraperitoneal injection of T-independent and T-dependent antigens preferentially upregulated CcAID expression in head kidney and splenic lymphocytes. Immunofluorescence analysis confirmed its expression in IgM + spleen lymphocytes. Recombinant CcAID exhibited cytidine deaminase activity. These findings indicate that CcAID is specifically expressed in IgM + spleen lymphocytes, suggesting its potential role in antibody production.