ELEVATED ENDOMETRIAL NATURAL-KILLER-CELL ACTIVITY DURING EARLY PORCINE PREGNANCY IS CONCEPTUS-MEDIATED

This study investigated an extended time course of endometrial NK cell activity during gestation and the mechanisms underlying changes in ut erine NK cell activity in pigs. Endometrial tissues were collected fro m cyclic, pseudopregnant and pregnant nulliparous pigs on various days post-estrus, and from pigs 10 days after insemination with seminal pl asma or killed spermatozoa. NK effector cells were isolated from each endometrial sample, size fractionated and tested for cytolytic activit y against NK target cells (K562) using chromium release assays and imm unocytochemically for the frequency of perforin-positive cells. Variou s cell fractions showed different levels of NK activity and had differ ent proportions of cells expressing perforin. Morphologically, cells i n the fraction with maximal NK activity almost all showed typical lymp hocyte size and shape. Substantially elevated NK cell activity was rec orded in pregnant pigs on days 10 and 20 of gestation. By day 30, the cytolytic activity declined dramatically to an almost undetectable lev el. Very little activity was found in uterine cells isolated from cycl ic, pseudopregnant, and seminal plasma or killed spermatozoa inseminat ed animals, and no differences were detected either between follicular and luteal phases of the estrous cycle or between different days of p seudopregnancy. These results indicate that elevated NK cell activity during early porcine pregnancy cannot be attributed to contributions f rom either the maternal systemic endocrine status or from components o f boar semen. The changes in NK cell activity observed in porcine endo metrial tissues during early pregnancy must therefore be associated wi th the actual presence of conceptuses.