Transcription factors PU.1 and IRF4 regulate activation induced cytidine deaminase in chicken B cells

The chicken immunoglobulin (Ig) variable region is preceded by multiple pseudo genes. During B cell maturation, the Ig variable region is converted into various pseudo gene sequences, and the diversity of antigen binding site is expanded as a consequence. Ig gene conversion is initiated by a B cell-specific enzyme, activation induced cytidine deaminase (AID). The mechanisms that regulate AID expression and function during Ig gene conversion in chicken B cells are not fully elucidated. In this report, we show that the transcription factor PU.1 and its functional partner, IRF4, are involved in the expression of AID gene in chicken B cells, and the regulation may be mediated in part through a composite PU.1/IRF4 binding site near the AID promoter. Relative to IRF4, PU.1 appears to be more important for AID expression, implying additional IRF4 independent roles for PU.1. Besides regulating AID expression, PU.1 and IRF4 also bind to two composite PU.1/IRF4 binding sites in the Iglambda locus, and mutations in these binding sites reduce AID-mediated gene conversion. Moreover, AID-mediated gene conversion is impaired in IRF4 deficient cells, and the defect may be a consequence of both reduced AID expression and lack of IRF4 interaction at the Iglambda locus. Based on these observations, we propose that PU.1 and IRF4 regulate AID at two levels in chicken B cells: the expression of AID as well as its gene conversion function at the Iglambda locus.