Equine leucocyte antigen system. IV. Recombination within the major histocompatibility complex (MHC).

A case of recombination between the putative class I ELA antigen series and the structure(s) governing mixed lymphocyte reactivity in an informative horse family is described. The results of serological typing, 'lysostripping' and mixed lymphocyte culture tests strongly suggest that the recombination took place between two loci and is not intragenic. An alloantigenic membrane structure, provisionally called B1, which does not belong to the known ELA series, was also involved in the cross-over. The B1 antigen resembles the class II gene products of other species in two respects: it is not present on platelets, and alloantiserum with specificity for B1 inhibits the stimulatory effect of B1-carrying cells in mixed lymphocyte cultures. The B1 antigen does not follow the classical distribution, however, being expressed on both B and T lymphocytes. The finding of separate loci for the first series of ELA antigens and the MLR governing structure(s) demonstrates the similarity of the genetic organization of the horse MHC to that in other species.