Inflammation in the endometrium of the tammar wallaby (Macropus eugenii) at the time of birth.

During mammalian pregnancy, the mother's immune system could treat the embryo as foreign given it expresses antigens inherited from the father. In eutherian mammals (such as humans), which have longer pregnancies than marsupials, this immune response is suppressed until the onset of birth. Marsupials, such as the tammar wallaby, have a restricted period of embryo-maternal contact. We compared the amount of different immune cells in the pregnant and non-pregnant uteri of tammar wallabies at the time of birth. We found that neutrophils, which are part of the non-specific immune system, were the predominant cell type in the uterus carrying the embryo. Together with the degeneration of placental cells we observed, these findings suggest that marsupial birth is preceded by a non-specific inflammatory response triggered by the ageing placenta rather than an adverse specific reaction against the fetus. Our study contributes to the understanding of the evolution of the immune environment that supports live birth.