Pregnancy-associated effect on mouse thymocytes in vitro.

Thymic involution during pregnancy is a phenomenon which has been described for a long time in various mammalian species but its significance for the success of pregnancy in not yet known. Consequently, we have studied the effect of placental cells on functional activity and cell surface antigen expression of thymocytes in combination with the thymic stroma. Trophoblast cells inhibited almost completely the proliferative response of thymocytes whether or not combined with the thymic stroma. Decidual cells were also found to have an inhibitory effect on thymocytes proliferation while their combination with the thymic stroma decreased the thymocyte proliferation rate almost completely. Both decidual and trophoblast explants in combination with the thymic stroma increased CSF production by the thymocytes while inhibiting IL-6 production. Also, both trophoblast and decidual cells caused no significant changes in the expression of Thy-1, CD5, CD4, CD8, CD3, CD25, CD44, and L-selectin by the thymocytes. Comparable results were obtained for lymph node cells in terms of both proliferation and cell surface antigen expression, except for a trophoblast-dependent decrease in L-selectin expression. These results suggest a possible role for placental cells in immunoregulation of functional activity of thymocytes, which might represent one of the mechanisms mediating pregnancy-associated thymic involution in vivo. However, no correlation between thymocytes functional activities and changes in their cell surface antigen expression could be established.