Inflammation, NK cells and implantation: friend and foe (the good, the bad and the ugly?): replacing placental viviparity in an evolutionary perspective.

This review summarises an invited talk presented at the 2012 ESRI/ASRI meeting in Hamburg, concerning current views of inflammation in pregnancy, which is timely given that the effects of a local injury in the uterus acts to favour implantation. Recalling that inflammation can be good (it is useful and necessary for implantation), bad (in implantation failure, RSA) and ugly (at the extreme, endometriosis is associated with pain and infertility) leads to consideration of its status in pregnancy. Its role in implantation and the fact that pregnancy maintains some aspects of inflammation throughout, leads to revision of not only concepts of immunosuppression and the Th1/Th2 paradigm, but also the feto-maternal relationship as seen since Medawar's hypotheses were advanced. This is examined from an evolutionary perspective, which should lead to further review of our perception of uterine NK cells, and the emergence of Treg cells to control some aspects of adaptive immunity, which appeared long after placentation.