INTERACTIONS BETWEEN THE IMMUNE-SYSTEM AND THE RUMINANT CONCEPTUS

Interactions of the conceptus with the immune system can involve eithe r anti-sperm or anti-conceptus immune responses that limit the success of pregnancy or beneficial effects of cytokines released from lymphoi d cells on embryonic growth and gene expression. The immune system is functional in the uterus and therefore there is the potential for anti -conceptus immune responses. However, endometrial lymphocytes are dist inct in many respects from lymphoid cells at peripheral sites; one maj or subpopulation expresses the gamma delta T-cell receptor and may not recognize major histocompatibility antigens. There are also several c ontrol systems to limit anti-conceptus immune responses. In particular , expression of major histocompatibility antigens on the trophoblast i s either absent or of limited distribution. In addition, activation of anti-conceptus immune responses leading to cytolytic responses is fur ther limited by the presence of molecules that can inhibit lymphocyte transformation. The most well-characterized of these are prostaglandin E(2) from placental and endometrial tissues, interferon-tau from the trophoblast during early pregnancy, and two endometrial proteins calle d the uterine milk proteins (UTMP). Progesterone plays a central role in inhibition of immune responses in actions that are mediated at leas t in part through endometrial secretion of UTMP. Cytokines play import ant roles as autocrine and paracrine regulators in many tissues includ ing the reproductive tract. In ruminants, the best described example i s interferon-tau. Other cytokines found in the reproductive tract or p roduced by the conceptus include interleukin-1, leukaemia inhibitory f actor, granulocyte-macrophage colony stimulating factor and interleuki n-6. It is possible that the major source of cytokines in the reproduc tive tract is non-lymphoid cells of the endometrium and trophoblast. I t is not known to what extent endometrial lymphocytes contribute to th e cytokine milieu because no cytokine has been identified as a product of endometrial lymphocytes. However, there is a population of granula ted lymphocytes that increase in number and granularity in the luminal epithelium of the late-pregnant ewe that is a potential source of cyt okines.