The regulation of AID function by transcription factors PU.1 and IRF4 in chicken B cells

B cells are capable of producing antibodies of diverse antigen specificities and effector functions to counter infection by a wide range of pathogens. The diversification of immunoglobulin (Ig) is achieved through a series of programmed DNA recombination and mutagenic events during B cell maturation. A key factor involved in the Ig diversification process is Activation Induced Cytidine Deaminase (AID). AID is a B cell specific enzyme that is critical for three distinct pathways of Ig diversification: class switch recombination, somatic hypermutation and Ig gene conversion. AID functions by deaminating cytosine to uracil in target DNA at the Ig loci. Although essential for effective immunity, the mutagenic activity of AID needs to be confined to the Ig loci in order to protect genomic integrity, but the underlying mechanism is not fully understood. In this study, I show that two lymphoid specific transcription factors, PU.1 and IRF4, play important roles in regulating AID function in chicken B cells. PU.1 and IRF4 have been implicated in many aspects of B cell development and function. The two factors could form a heterodimer and regulate target gene expression cooperatively. However, we found that PU.1 and IRF4 appear to have different impacts on AID function. We show that PU.1 is important for the expression of AID gene in chicken B cells, and the regulation appears to involve direct interaction of PU.1 with the AID gene. By comparison, IRF4 plays a minor role in AID expression. On the other hand, both PU.1 and IRF4 are required for efficient gene conversion that is mediated by AID at the Igλ locus. Moreover, the gene dosage of PU.1 is critical for AID function, since a severe gene conversion defect is observed in PU.1+/- cells. The function of PU.1 and IRF4 in AID-mediated gene conversion involves binding sites for the PU.1/IRF4 complex within a regulatory element at the Igλ locus. Future studies will be directed at understanding how PU.1 and IRF4 regulate AID-mediated gene conversion.