Cross-reactions of anti-I-Ek monoclonal antibodies with subsets of I-Ab molecules.

Monoclonal antibodies H9-15.4 and H39-459 were derived from an A.TH anti-A.TL immunization. Antibodies H9-15.4 and H39-459 were found to be directed against the I-Ek molecule with positive reactions against B10.A (AkEk) and B10.S(9R) (AsEk) but not B10.A(4R) (AkEb). The monoclonal antibodies were also found to react with the b and q haplotypes, which do not express an I-E molecule. Sequential precipitation analysis showed that in the b haplotype, H9-15.4 and H39-459 react with the I-Ab molecule. These results showed that H9-15.4 and H39-459 recognize determinants shared by I-Ek and I-Ab molecules, suggesting that I-A and I-E molecules may have a common evolutionary origin, possibly through gene duplication. Sequential immunoprecipitation analysis of I-Ab molecules precipitated by H9-15.4 and H39-459 also suggested that these monoclonal antibodies recognize subsets of I-Ab molecules. Pretreatment with the 17-227 monoclonal anti-I-Ab antibody (Ia.15) had no effect on I-Ab molecules precipitated by H9-15.4 and H39-459. Also, pretreatment with H9-15.4 and H39-459 had no effect on I-Ab molecules immunoprecipitated by 17-227. Also H9-15.4 and H39-459 only partially cleared Ia molecules immunoprecipitated by each other. These results suggest that 17-227, H9-15.4, and H39-459 detect a minimum of four subsets of I-Ab molecules. To account for these observations, it is proposed that during evolution of the mouse H-2 complex, in addition to gene duplication of ancestral gene(s) to yield the genes encoding E alpha A alpha, A beta, and E beta polypeptide chains, further gene duplication occurred, forming multiple copies of genes encoding each Ia polypeptide chain.