Genetic control of the expression of class I molecules on rat erythrocytes.

Class I major histocompatibility antigens of the rat and mouse are generally thought to be present on erythrocytes, and hemagglutination has frequently been used as a means of MHC typing in these species. Recent evidence suggests that there are genetic differences in the expression of class I molecules on the red blood cells of rats. We have confirmed and extended these findings with four monoclonal antibodies that define a minimum of three separate class I specificities. The lymphocytes of rats of the RT1b, RT1c, and RT1m haplotypes were killed by these antibodies, but their red blood cells were not agglutinated despite the fact that these antibodies had good hemagglutinating activity for the cells of other positive strains. By an indirect radioimmunoassay, it was shown that lymphocytes and erythrocytes of the hemagglutination-positive strains bound large amounts of antibody; lymphocytes of the RT1b, RT1c, and RT1m haplotypes also bound large amounts of antibody but their erythrocytes bound very little. Using the appropriate congenic strains and segregating populations, it was shown that there are a minimum of two genes that affect the red cell expression of these specificities. We have also used an anti-beta 2-microglobulin serum to quantitate independently the relative amounts of class I molecules on the cell surfaces, and have shown that the red cells of these three haplotypes express reduced amounts of available class I molecules, while their lymphocytes express quantities similar to those of other strains.