1. Background

The immune system is known to play a pivotal role in normal, healthy pregnancy in mammals. Much of this research has been in eutherian species, which have different reproductive strategies to that of marsupials. Marsupial pregnancy is short, followed by long periods of lactation, and it is unclear what role the immune system plays in this reproductive strategy. This study aims to identify the immune cell populations present in the gestational uterus of an Australian marsupial, the fat-tailed dunnart (Sminthopsis crassicaudata) from conception to parturition. Through immune cell localisation, morphology, and marker expression, this research provides new insight into the dynamics of inflammation in marsupial pregnancy.

2. Methods

Uterine biopsies were collected from pre-, peri-, and post-implantation stages in dunnart pregnancy. Tissues were either fixed in paraformaldehyde for histological studies or snap frozen for single-nuclei RNA sequencing using the PIPseq T20 platform. Tissue collected for histology was stained for immune cell biomarkers by histological staining or immunohistochemistry.

3. Results

Immune cells are present in the endometrial stroma during pregnancy in the dunnart, consisting primarily of lymphocytes, macrophages, dendritic cells, and granulocytic cells. These populations fluctuate in density and localisationduring pregnancy depending on stage, exposure to non-maternal antigen, and invasion by the fetal trophoblast.

4. Conclusion

The immune system is mobilised during gestation in the dunnart, infiltrating the endometrium during their short pregnancies. These populations are present from conception to late gestation and fluctuate depending on the uterine microenvironment and exposure to non-self antigen. While their specific roles remain to be discovered, these cells may control immune tolerance to paternal antigen and participate in the stromal, vasculature, and glandular remodelling observed at ovulation and pre-implantation. Despite divergence in the approach to reproduction, inflammatory responses to pregnancy appear to be normal parts of gestation in eutherians and marsupials alike.