Analysis of the rat major histocompatibility system by Southern blot hybridization

The organization of the rat major histocompatibility complex, RT1, was studied at the DNA level by Southern blot hybridization. Genomic DNA from eight different RT1 congenic rat strains was digested by various restriction enzymes and was hybridized under stringent conditions with probes of mouse class I and class II H-2 genes. Few cross-hybridizing DNA fragments, showing no polymorphism, were seen with class II A alpha and A beta probes. The class I probes allowed for the distinction of about 8 to 19 cross-hybridizing bands, which exhibited extensive polymorphism. With the use of five RT1 recombinants, about 20% of the DNA fragments could be mapped to the RT1.A region, which codes for the ubiquitously expressed class I antigens, and about 80% to the RT1.C region-determining class I-like antigens, which are different from RT1.A antigens with respect to tissue distribution, restriction function in immune responses, and allograft rejection. The number of class I genes present in the rat genome and the possible relationship of RT1.C to H-2Qa, Tla of the mouse are discussed.