• cytotoxic T lymphocytes;
  • histocompatibility;
  • horse;
  • maternal tolerance;
  • paternal alloantigen


Pregnant mares demonstrate a reduction in cytotoxic T lymphocyte (CTL) reactivity against cells from the breeding stallion. We investigated whether this effect is limited to activity against paternal major histocompatibility complex (MHC) antigens, and whether it occurs during MHC-compatible pregnancy.

Method of Study

Mares were mated to carry MHC-compatible or MHC-incompatible pregnancies. CTL activity of these mares when pregnant and non-pregnant was measured against cells from horses with MHC haplotypes unrelated to the mare or breeding stallion.


While carrying MHC-incompatible pregnancies, mares demonstrated reduced CTL activity against lymphocytes from third-party horses in addition to those from the breeding stallion. This effect was also observed in mares carrying MHC-compatible pregnancies.


The decrease in maternal T-cell reactivity characteristic of normal equine pregnancy is not restricted to paternal alloantigen, and occurs during MHC-matched matings. This suggests that antigen-independent mechanisms may be responsible for this reduction in cell-mediated immune activity.