Immunosuppressive effect of pregnant mouse serum on allostimulatory activity of dendritic cells

In normal pregnancy, the maternal immune system is directed towards tolerance or suppression in order to prevent rejection of the semi-allogenic fetus. Antigen-presenting cells, especially dendritic cells (DCs), are key cells in initiation and regulation of immune responses. The presence of potent immunostimulatory DCs in the decidual tissue of pregnancy has been demonstrated. The aim of this study was to determine how allostimulatory activity of DCs could be affected during pregnancy. DCs were isolated from spleen of pregnant or non-pregnant Balb/c mice and co-cultured with allogenic T lymphocytes prepared from brachial lymph nodes of C57BL/6 mice. Some cultures of non-pregnant female DCs were treated by 2.5% serum obtained from pregnant mice at early, middle or late gestational periods, and were used in the same mixed lymphocyte reaction (MLR) settings. Cell proliferation was measured by 3H-thymidine incorporation, and cytokine production measured in supernatants of MLR cultures using ELISA. The effect of pregnant mouse serum on expression of DC surface markers was evaluated by flow cytometry. No significant difference was found between stimulatory potential of splenic DCs from pregnant and non-pregnant mice in induction of allogenic T cell proliferative response. Moreover, serum of early or late pregnancy did not have any effect on DC function in comparison with non-pregnant mouse serum, while mid-pregnancy serum significantly inhibited allostimulatory activity of DCs. IFNgamma production in co-culture of DCs treated with pregnant mouse serum was significantly lower than that of the control group; however, no significant difference in IL-10 production was observed. Treatment of DCs with pregnant mouse serum did not influence the percentage of cells expressing MHC-II, CD86, CD8alpha or CD11b. However, a marked reduction of the mean fluorescence intensity of MHC-II was observed. Collectively, our results concerning the diminished capacity of DCs to induce production of Th1 cytokines and allogenic T cell proliferation after treatment with pregnant mouse serum reveal a new way of immunologic tolerance against the semi-allogenic fetus.