Several mechanisms are thought to contribute to maternal-fetal
tolerance:
1.The presence of CD4+ nTreg cells are thought to carry out
immunosuppressive functions through direct intercellular contacts
with effector T cells and DCs in the uterus.
2. The hormonal changes associated with reproduction may
prepare the uterus to accept the “fetal graft” so that the fetus is not
seen as a foreign and cause damage.
3. The tissue forming the maternal-fetal interface are populated
with non-professional APCs that lack costimulatory molecule and
pregnancy does not usually generate the DAMPs that would induce
the production of inflammatory cytokines and stimulate
professional APCs.
4. If a professional APC becomes activated, local Tr1 and Th3 cells
at the maternal-fetal interface can suppress any incipient immune
response.
5. The placenta and tissues of the developing fetus are almost
devoid of MHC class I and II molecules. but instead have HLA-E and
-G genes which inhibit maternal uterine NK cells from attacking.
6. Some placental cells express FasL, so that activated T cells can be
killed before fetal tissues are attacked. Fetal cells also express the
RCA proteins DAF and MCP that can interfere with maternal
complement activation.