A Review of the Immunological Mechanisms That Promote Materno-Foetal Tolerance

The foetus is a semiallograft. Why it is not rejected by the maternal immune system has been the focus of research for years. The earlier beliefs were that the foetus is not antigenically mature, there is a physical barrier between maternal and foetal tissues, and that the mother's immune system is inactive. These have however been proved wrong. This review discusses the placental and maternal factors which have been reported so far to promote tolerance. Some of the factors expressed by the placenta which promote maternalfoetal tolerance include the pattern of MHC expression (non-classical HLA-E, HLA-F and HLA-G); this induces tolerance in the maternal leukocytes. The B7 family molecules modulate the immune system. IDO suppresses Tcell levels and activity via tryptophan catabolism. Progesterone and human placenta growth hormone, LIF, Annexin II, HO-1, CD95/CD95R, CEACAMS and TRAIL are other factors expressed by the placenta that promote maternal tolerance of the foetus. From the maternal side, there is an increase in the number of T-regs which are immunosuppressive, the uterine NK cells are non-toxic and the dendritic cells are immature. The immune response is tilted towards the anti-inflammatory Th-2, and the anti-inflammatory cytokine TGF-β is increased in pregnancy.