Suppression of lymphocyte reactivity by culture supernatant from horse embryos and endometrium

Modulation of T-cell reactivity during equine pregnancy is antigen independent

PROBLEM

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.

RESULTS

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.

CONCLUSIONS

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.

Maternal immune responses to trophoblast: the contribution of the horse to pregnancy immunology

The horse has proven to be a distinctively informative species in the study of pregnancy immunology for several reasons. First, unique aspects of the anatomy and physiology of the equine conceptus facilitate approaches that are not possible in other model organisms, such as non-surgical recovery of early stage embryos and conceptuses and isolation of pure trophoblast cell populations. Second, pregnant mares make strong cytotoxic antibody responses to paternal major histocompatibility complex class I antigens expressed by the chorionic girdle cells, permitting detailed evaluation of the antigenicity of these invasive trophoblasts and how they affect the maternal immune system. Third, there is abundant evidence for local maternal cellular immune responses to the invading trophoblasts in the pregnant mare. The survival of the equine fetus in the face of strong maternal immune responses highlights the complex immunoregulatory mechanisms that result in materno-fetal tolerance. Finally, the parallels between human and horse trophoblast cell types, their gene expression, and function make the study of equine pregnancy highly relevant to human health. Here, we review the most pertinent aspects of equine reproductive immunology and how studies of the pregnant mare have contributed to our understanding of maternal acceptance of the allogeneic fetus.

Immunosuppressive macromolecules of endometrial and conceptus origins in livestock species

Since the mid- to late-1970s, intrauterine immunosuppressive macromolecules recovered from endometrial and conceptus secretions have been reported for livestock species. Using primarily in vitro assays, in conjunction with a limited number of techniques conducted in vivo, these macromolecules were shown to suppress various T-cell responses. Some macromolecules were also shown to suppress cytolytic activities of non T-cells. It remains unknown as to whether these macromolecules actually afford protection to the conceptus by suppressing cell-mediated immune responses directed toward conceptus tissues. Endometrial effector cells in the ewe respond to antigenic stimulation and preattachment trophoblastic cells of pigs and sheep can be lysed by effector cells. Consequently, these observations suggest a need for immunosuppression, either locally at placentation sites or within the entire uterus. This review describes the intrauterine macromolecules that have been shown to suppress lymphocyte responses. Additional information, although limited at this time, refers to their origin and possible mechanisms of action. As more reagents become available to complete the identification of the intrauterine immune cells in livestock animals, experiments (e.g. antibody-mediated depletion of cells) can be conducted to determine the precise functions of all these cells. Knowing their functions will help delineate whether or not immunosuppressive macromolecules have a role in the regulation and maintenance of conceptus tissues during pregnancy.

Regulation of mitogen- and TCGF-induced lymphocyte blastogenesis by prostaglandins and supernatant from equine embryos and endometrium

Immunosuppressive substances which interfere with lymphocyte blastogenesis are released in vitro by embryos and endometrium from mares in early pregnancy. Immunosuppression was not evident when tissues were cultured in the presence of indomethacin (a prostaglandin-synthesis inhibitor). Various prostaglandins (PGs) were added to equine lymphocytes and lymphocyte proliferation was measured after the addition of concanavalin A (Con A) or phytohaemagglutinin A (PHA). PGE2 and PGF2 alpha inhibited Con A-induced blastogenesis down to final concentrations of 1.8 x 10(-9) M and 1.3 x 10(-6) M, respectively. Other PGs tested (6-keto-PGF1 alpha and 13,14-dihydro-15-keto-PGF2 alpha) did not affect Con A-induced blastogenesis. PHA-induced blastogenesis was inhibited by concentrations down to 1.8 x 10(-9) M PGE2, 3.3 x 10(-7) M PGF2 alpha and 2.8 x 10(-4) M 6-keto-PGF1 alpha. At all concentrations, 13,14-dihydro-15-keto-PGF2 alpha only slightly reduced PHA-induced blastogenesis. Therefore, PGE2 was the only prostaglandin tested which, at physiological concentrations, significantly inhibited incorporation of [3H] thymidine. The mechanism of PGE2-mediated suppression was studied by adding PGE2 and T cell growth factors (TCGF) to TCGF-responsive lymphocytes. PGE2 reduced the TCGF-mediated blastogenic response in a dose-dependent manner. Furthermore, culture supernatant from embryos and endometrium from 14-day pregnant mares inhibited lymphocyte blastogenesis induced by TCGF. These results show that PGE2 interferes with lymphocyte blastogenesis by TCGF-dependent mechanisms. It is suggested that the PGE2 present in the uterus of the early pregnant mare may be one of the factors involved in immunosuppression at the time of maternal recognition of pregnancy.