Conceptus attachment coincides with initiation of an anti-inflammatory cytokine profile in the pig endometrium

In eutherian mammals, blastocyst implantation is often associated with a quasi-inflammatory reaction in the endometrium, which is resolved with the establishment of the definitive placenta. This is understandable in the case of invasive placentation, since implantation entails a nidatory injury to the maternal tissue due to the invading blastocyst. Quasi-inflammatory processes have also been documented in pregnant pigs, even though the blastocyst only attaches to, rather than invades into, the endometrium of the uterus. In this study, we asked what processes in early porcine pregnancy lead to the resolution of attachment-associated inflammation. In generic wound healing the transition from a pro- to an anti-inflammatory state is caused by a corresponding transition from M1 to M2 polarized macrophages following efferocytosis by macrophages of apoptotic neutrophils. In order to determine whether this scenario applies to the pregnancy-related resolution of inflammation in the porcine uterus, we produced a series of bulk transcriptome samples spanning days (D) 13 to 25 of gestation. This time span corresponds to the transition from pre- to post-attachment stages of pregnancy. We found slower changes in the transcriptome between D20 and D25 than prior to D20, suggesting a turning point in pregnancy-related reprogramming. The turning point at D20 corresponds to the time of firm attachment of trophectoderm to uterine luminal epithelium and the cessation of IFNG signaling from the blastocyst. This transition coincides with increased expression of RNAs of genes implicated in resolution of inflammation and M2 polarization such as ARG1, MRC1/CD206, CD86, TGFb1 and IL10, as well as a significant increase in expression of HGPD, the enzyme that metabolizes prostaglandins. While immunoreactivity for ARG1 was found in putative macrophages in the sub-epithelial stratum compactum, other markers of M2 polarized macrophages were localized to non-immune cells: MRC1 was found on fibroblast-like stromal cells, CD86 on trophoblast cells, and IL10 in luminal and glandular epithelia. These results suggest that intrauterine immune regulation is decoupled from that of the rest of the body by engaging non-immune cell types as anti-inflammatory mediators during the peri-attachment period of pregnancy.