Co-Signaling Molecules in Maternal–Fetal Immunity

Physiologically, a successful pregnancy requires the maternal immune system to recognize and tolerate the semiallogeneic fetus, and allow for normal invasion of trophoblasts. Thus, pregnancy complications are considered to be associated with dysfunctional maternal–fetal crosstalk. Co-signaling molecules are a group of cell surface molecules that positively or negatively modulate the immune response. Well studied in the fields of oncology and transplantation, they are also suggested to be involved in maternal–fetal crosstalk. Here, we review the latest knowledge on the expression and function of such co-signaling molecules, highlighting their immunoregulatory roles in maternal–fetal tolerance and decidual vascular remodeling, and their involvement in pathological pregnancies. This review may instruct future basic research on, and clinical applications for, maternal–fetal immunity.