Role of uterine immune cells in early pregnancy in pigs.

The immune system discriminates 'self' from 'non-self', and eliminates that which it determines to be non-self. Mammalian pregnancy appears to represent a failure of self-non-self discrimination, yet it is a highly successful reproductive strategy. We present evidence that the immune system of the female pig responds to the challenges of both mating and the presence of conceptuses. Mating induces an influx of inflammatory leukocytes into the endometrial stroma and uterine lumen. This response, while partially under endocrine control, is amplified by as yet unidentified factors in seminal plasma. In addition to preventing microbial infection, this mating-induced immune response may enhance reproductive performance. During the first month of pregnancy when intimate contact between maternal and fetal tissues is being established, the number of uterine lymphocytes decrease in the luminal epithelium and increase in the endometrial stroma at sites of conceptus attachment. The majority of these lymphocytes express the CD2 and CD8 surface markers, consistent with either T or natural killer (NK) cell lineage. Dispersed endometrial cells obtained during early gestation exhibit a pregnancy-specific increase in NK-like lytic activity. Our ongoing efforts to determine whether these NK-like lymphocytes are the ones that localize to sites of conceptus attachment are discussed. We speculate on how the reactions of pig uterine leukocytes to seminal plasma and conceptuses might contribute to successful pregnancy.