Suppression of mixed lymphocyte reaction by cells of human first trimester pregnancy endometrium

In order to identify an immunological role for decidual tissue in pregnancy we have prepared single cell suspensions from the tissues of normal pregnant women and examined the effects of these cells on one-way mixed-lymphocyte reactions (MLR). The separated cells were heterogeneous, containing classical decidual cells, glandular epithelial cells, granular endometrial cells, macrophages and small lymphoid cells. [3H]Thymidine incorporation at day 6 of the MLR was suppressed by addition of the cells at the initiation of the cultures and the degree of suppression was inversely correlated to the gestational age of the decidual tissue, apparently through inhibition of the antigen recognition phase of the MLR. These findings support the view that the cells of the human first trimester pregnancy endometrium may play an important role in protecting the feto-placental unit from rejection, at least in the early phase of pregnancy.