Lymphocyte alloantigens of the horse. II. Antibodies to ELA antigens produced during equine pregnancy.

Evidence is presented for a reproducible maternal immune response to histocompatibility antigens during equine pregnancy. Mares were stimulated as a result of pregnancy to produce cytotoxic antibodies to paternal lymphocyte alloantigens. The majority of these antibodies were directed against antigens of the equine lymphocyte antigen (ELA) system, which is the major histocompatibility complex (MHC) of the horse. In 16 experimental pregnancies produced using 12 mares and 4 stallions which had been typed for ELA antigens, there was correlation between ELA incompatibility between sire and dam and the appearance of antibody in the serum of the dam as a result of pregnancy. In ELA incompatible pregnancies, antibody was first detected very early in pregnancy, usually by day 60 in a 336-day gestation. The appearance of antibody following shortly the development of the endometrial cups, which can be seen macroscopically for the first time between days 38 and 40. The strength (titre) of the cytotoxic antibody response appeared to vary with the paternal ELA antigens expressed by the fetus. Circulating antibody to 'third party' alloantigens generated during prior pregnancies did not alter the kinetics of the appearance of antibody in a current pregnancy. Evidence for an anamnestic response was obtained in second pregnancies in which the histocompatibility difference between mare and fetus was the same as in the first pregnancy. However, this accelerated response did not occur before the time of formation of the endometrial cups. Serum samples from approximately 90% of parturient mares on horse farms in New York State contained antibody reactive with paternal lymphocyte alloantigens.