Analyasis of Ig somatic hypermutaion by establishing assay system using in Vitro culture

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Pax5 activity is enhanced in activated B cells and is essential for class switch recombination (CSR). We show that Id2 suppresses CSR by repressing the gene expression of activation-induced cytidine deaminase (AID), which has been shown to be indispensable for CSR. Furthermore, a putative regulatory region of AID contains E2A-and Pax5-binding sites and the latter site is indispensable for AID gene expression. Moreover, the DNA binding activity of Pax5 is decreased in Id2-overexpressing B cells, and enhanced in Id2^<-/-> B cells. The kinetics of Pax5-occupancy, but not E2A, to AID locus is the same as AID expression in primary B cells. Finally, enforced-expression of Pax5 induces AID transcription in pro-B cell lines. Our results provide evidence that the balance between Pax5 and Id2 activities has a key role in AID gene expression. Recent studies demonstrated that AID is indispensable for somatic hypermutation (SHM) and CSR of immunoglobulin genes. These processes result in the product … More ion of high-affinity antibodies against the immunizing antigen and modification of effector functions of antibodies. On the, other hand, little is known about a possible inhibitory system for CSR that prevents an over-response of B cells. Here we show that enforced expression of Id2 reduced AID expression and inhibited CSR. By comparing human and murine sequences, we found a strikingly homologous region within 400bp upstream of the AID gene, in which there is a Pax5-binding site that is indispensable for AID gene expression. Moreover, a Pax5-containing DNA-binding complex at the AID promoter was reduced by Id2 over-expression, and enhanced in Id2^<-/-> B cells. Furthermore, we found that Pax5 was recruited to the AID locus after 2 days of stimulation of B cells in vitro, at which time AID transcription was initiated. Finally, we demonstrated. that enforced expression of Pax5, but not E2A, induced endogenous AID gene expression in a pro-B like cell lines. This effect was cancelled by addition alintroduction of Id2. Our results provide evidence that Id2 plays an important role to modulate Pax5 activity in regulation of AID expression that is functionally antagonized by Id2. This result further suggests a fundamental role in differentiation by antagonizing several sets of master-gene activities. In Id2^<-/-> B cells, though, there are no augmentation of SHM suggesting other factor(s) than AID limits the rate of SHM in B cells. Less