The role of semen in induction of maternal immune tolerance to pregnancy

Successful pregnancy requires a state of maternal immune ‘tolerance’ to accommodate antigens expressed by the conceptus. Implantation failure and placental pathologies largely reflect insufficiencies in maternal immune adaptation, but progress in devising therapeutic strategies to treat these conditions is stalled because the mechanisms underlying the induction and maintenance of maternal tolerance are unknown. Increasingly, clinical and experimental data support the proposal that insemination has consequences for the reproductive process beyond delivery of male gametes. An emerging hypothesis, based mainly on clinical observations and experiments in mice, is that insemination is causally linked to the activation and expansion of populations of lymphocytes mediating forms of ‘active’ immune tolerance in the implantation site. This review examines existing evidence for a role for semen in the immunology of pregnancy, highlighting the limitations of our existing knowledge and the prospects for future research and its clinical application.