Cross-reactions of anti-I-E^k monoclonal antibodies with subsets of I-A^b 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-E^k molecule with positive reactions against B10.A (A^kE^k) and B10.S(9R) (A^sE^k) but not B10.A(4R) (A^kE^b). 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-A^b molecule. These results showed that H9-15.4 and H39-459 recognize determinants shared by 1-E^k and I-A^b molecules, suggesting that I-A and I-E molecules may have a common evolutionary origin, possibly through gene duplication. Sequential immunoprecipitation analysis of l-A^b molecules precipitated by H9-15.4 and H39-459 also suggested that these monoclonal antibodies recognize subsets of I-A^b molecules. Pretreatment with the 17-227 monoclonal anti-1-A^b antibody (Ia.15) had no effect on I-A^b molecules precipitated by H9-15.4 and H39-459. Also, pretreatment with H9-15.4 and H39-459 had no effect on I-A^b 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 l-A^b 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_αA_α, A_β, and E_β polypeptide chains, further gene duplication occurred, forming multiple copies of genes encoding each Ia polypeptide chain.