Interactions between the immune system and the bovine conceptus

Abstract

Immunological rejection of the bovine conceptus is blocked, in part, because of reduced expression of major histocompatibility (MHC) antigens on the surface of the conceptus. Also, an intrauterine environment is created during pregnancy in which maternal tissues and trophoblast secrete lymphocyte-inhibitory molecules that reduce immune reactivity. Data from other species suggest that there are also specific changes in functional activity of maternal lymphocyte populations during pregnancy that lead to 1) redirection of immune responses away from cellular immunity, 2) transient tolerance to conceptus antigens, and 3) activation of specific lymphocyte populations involved in immunosuppression or release of cytokines that stimulate placental function. Bovine trophoblast is potentially a target for maternal cytokines because placental cells express receptors for colony-stimulating factor-1 (CSF-1) and can be stimulated to proliferate in culture by lymphocyte-conditioned culture medium. Some degree of maternal recognition of conceptus antigens may be beneficial for the final stages of parturition because retained placenta occurs more frequently when dam and calf share MHC class I antigens. Methods to manipulate immunological function to enhance reproductive function remain speculative only — possibilities include alteration of uterine immune responses to reduce early embryonic mortality or to affect placental function, prevention of abortion caused by viral infection, enhancement of uterine antimicrobial defense mechanisms and reduction in frequency of retained placenta.