At least four MHC class I genes are transcribed in the horse: phylogenetic analysis suggests an unusual evolutionary history for the MHC in this species.

Nineteen horse MHC class I specificities have been serologically identified previously at a single locus (ELA-A), and two other specificities appear to be coded at other loci. Biochemical studies indicate that there are at least two expressed loci. In order to establish the number of transcribed horse MHC class I genes, we made a cDNA library from a heterozygous animal (ELA-A3/A7), and screened for positive clones using a bovine class I probe. More than 200 class I clones were isolated in this way, and so far seven unique full length sequences have been identified. All of the sequences are predicted to code for surface expressed, functional molecules. The number of different sequences identified demonstrate that at least four genes are transcribed, although variations in transmembrane length (which is generally conserved in class I loci) suggest that five genes could be represented. Evolutionary analysis of these sequences (and two additional sequences known to represent different horse class I loci) reveals no firm relationships, such that the division between the different loci cannot be discerned. These results suggest an unusual evolutionary history for the horse MHC, the precise nature of which may be revealed only following further cross-species comparisons.