Specialized natural killer (NK) lymphocytes are a feature of the pregnant uterus in humans and rodents. Conceptus-mediated recruitment of uterine (u)NK cells in the pig was proposed based on evidence that elevated uNK activity was temporally associated with increased leukocyte density in endometrium underlying conceptuses. The objective of this study was to determine whether uNK cells were more abundant at embryonic attachment sites during the early postattachment period. Mononuclear leukocytes were isolated from endometrium at attachment sites versus between attachment sites, and expression of CD16, a marker for NK cells, was assessed by flow cytometry. CD16 binding was normalized to leukocyte numbers in each sample. CD16 small lymphocytes were more frequent in uterus than in blood (41% ± 2% versus 26% ± 4%). Differences between pregnant and luteal phase uterus (43% ± 2% versus 31% ± 7%, respectively) were not statistically significant. In pregnant animals, CD16 lymphocytes were slightly but significantly more abundant in uterus at attachment sites versus between attachment sites at Days 15–17, 21–22, and 25–28. Before normalization, CD16 large, granular cells were more abundant at attachment sites versus between attachment sites; however, these differences were removed when data were normalized according to leukocyte numbers. Further characterization showed that the proportion of large granular leukocytes expressing CD8, reactive with NK cells and T cell subsets, was 2-fold higher in pregnant uterus than in maternal blood. These results raise the possibility that uNK cells resembling those in blood may be transformed into larger, more granulated forms in the uterine microenvironment.
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