High-density ovine endometrial cells exhibit natural killer activity during early pregnancy

Natural killer (NK)-like activity was assessed for peripheral blood lymphocytes (PBL) and unfractionated and fractionated endometrial cells recovered from ewes during the estrus cycle (Days 12 to 14) and early pregnancy (Days 16 to 18). The PBL and endometrial cells (each designated as effector cells) were cocultured with chromium-51 (51Cr) labeled NK-sensitive K-562 target cells in effector:target cell ratios ranging from 25:1 to 200:1, respectively. Lytic activity (i.e., release of 51Cr into the medium) was assessed at 22 h of culture. A high-density (> or = 1.088 g/mL) population of endometrial cells from the pregnant ewes exhibited NK-like activity, whereas endometrial cells from the cyclic ewes failed to exhibit activity. Lytic activity of these cells was greater (P < 0.05) for pregnant than for cyclic ewes (12.0 and 2.1%, respectively) at the effector:target cell ratio of 100:1, respectively. For both groups of ewes, PBL exhibited NK-like activity. These data indicate that the ovine endometrium contains NK-like cells with lytic activity between Days 16 and 18 of pregnancy.