Effects of Early Conceptus Signals on Circulating Immune Cells: Lessons from Domestic Ruminants

While there are few similarities between mechanisms for extending corpus luteum (CL) function during early pregnancy in ruminants and primates, there is increasing evidence that conceptus-immune crosstalk in ruminants and primates affects the function of circulating immune cells at the very earliest stages of pregnancy. Most notable are changes in immune cell phenotypes with increased numbers of cells exhibiting the T regulatory phenotype and suppression of Th1 cytokines that promote tolerance to paternal alloantigens. Until recently, interferon tau produced by the ruminant trophectoderm was thought to act exclusively on the uterine endometrium; however, it is now clear that this unique embryonic interferon escapes the uterus and alters gene expression in the CL and in peripheral blood leukocytes (PBL). In fact, a large number of interferon-stimulated genes are now known to be increased during early pregnancy in PBL. What is not known is how this conceptus-immune system cross-talk affects maternal immune status outside the reproductive tract. It is attractive to hypothesize that some of these effects are designed to counter-balance progesterone-induced immunosuppression so as not to place the dam at a greater risk of infection on top of the tremendous stresses already induced by pregnancy. Furthermore, recent evidence suggests that pregnancy induced changes in peripheral immune cells may aid in orchestrating establishment of pregnancy. Existing evidence points toward a greater convergence of systemic immune responses to early pregnancy signaling between ruminants and primates.