Regulatory T cell adoptive transfer alters uterine macrophage populations, increasing a novel MHC-II-low macrophage associated with healthy pregnancy.

To investigate local immune mechanisms of intrauterine fetal demise (IUFD), we used the CBA mouse strain, which naturally has mid-late gestation fetal loss. We performed a Treg adoptive transfer and interrogated both pregnancy outcomes and the impact of systemic maternal Tregs on mucosal immune populations at the maternal-fetal interface. Treg transfer prevented fetal loss and increased an MHC-IIlow population of uterine macrophages. Single-cell RNA-sequencing was utilized to precisely evaluate the impact of systemic Tregs on uterine myeloid populations. A population of C1q+, Trem2+, MHC-IIlow uterine macrophages were increased in Treg-recipient mice. The transcriptional signature of this novel uterine macrophage subtype is enriched in multiple studies of human healthy decidual macrophages, suggesting a conserved role for these macrophages in preventing fetal loss. Overall design: Timed-pregnant CBA mice received Treg adoptive transfer or PBS injections on embryonic day 2 (E2) (N=3 mice/group). Uteri from these mice were harvested and processed at E15. Fluorescence activated cell sorting (FACS) was used to isolate myeloid cells gated on live, singlet, CD45+, CD11b+ cells from the uteri to use for single cell RNA-sequencing.