Generation of MHC Class II Diversity by Intra- and Intergenic Recombination

The occurrence and significance of recombination events in the generation of MHC class II diversity were discussed. Evidence that intragenic recombination has contributed to the generation of allelic diversity at a DRB locus in the moose was presented. Intergenic recombination (i.e. exchange of sequence information between nonallelic genes) is expected to be rare and only to play a minor role in the generation of class II diversity. Exchange of sequence information between the major isotypic forms of class II genes (DQ, DR and DP) is restricted to a segment encoding a major part of the a-helical region of polymorphic class II beta-chains. In this segment there is no or only weak locus divergence and the frequency of synonymous substitutions between nonallelic genes (DQB vs. DRB) within species is remarkably low, implying that exchange of sequence information has occurred repeatedly during the course of evolution.