KAPPA-CHAIN MONOALLELIC DEMETHYLATION AND THE ESTABLISHMENT OF ALLELIC EXCLUSION

Allelic exclusion in kappa light-chain synthesis is thought to result from a feedback mechanism by which the expression of a functional kapp a light chain on the surface of the B cell leads to an intracellular s ignal that down-regulates the V(D)J recombinase, thus precluding rearr angement of the other allele. Whereas such a feedback mechanism clearl y plays a role in the maintenance of allelic exclusion, here we provid e evidence suggesting that the initial establishment of allelic exclus ion involves differential availability of the two kappa alleles for re arrangement. Analysis of kappa(+) B-cell populations and of individual kappa(+) B cells that have rearranged only one allele demonstrates th at in these cells, critical sites on the rearranged allele are unmethy lated, whereas the nonrearranged allele remains methylated. This patte rn is apparently generated by demethylation that is initiated at the s mall pre-B cell stage, on a single allele, in a process that occurs pr ior to rearrangement and requires the presence in cis of both the intr onic and 3' kappa enhancers. Taken together with data demonstrating th at undermethylation is required for rearrangement, these results indic ate that demethylation may actually underly the process of allelic exc lusion by directing the initial choice of a single kappa allele for re arrangement.