Regulatory T Cell Transfer Increases Trem2+Uterine Macrophages and Decreases Fetal Loss

Abstract Intrauterine fetal demise (IUFD) – fetal loss after 20 weeks – affects 6 pregnancies per 1,000 live births in the United States, and the majority of IUFDs are of unknown etiology. We investigated immunological mechanisms of IUFD using the CBA mouse model, which has mid-late gestation fetal loss. Regulatory T cells (Tregs) are critical to establishing maternal-fetal tolerance, thus we hypothesized they may be protective from fetal loss in this model. Adoptive transfer of 2×105 Tregs decreased fetal loss 2.7-fold (p=0.04) and increased pup viability by 69% (p=0.003) (N=7–9/group). To examine how systemic Treg transfer influenced local uterine immune cells, we used single-cell RNA-sequencing of uterine CD11b+ cells from pregnant dams that had received Tregs vs. PBS (N=3/group). Sequencing identified 15 uterine CD11b+ populations, with Trem2+ macrophages increased (p=2.2e–12) and CCL3+ neutrophils decreased (p=3.6e-12) by Treg transfer. Compared to other uterine macrophages, this Trem2+ population was characterized by low expression of MHC-II genes and high expression of Trem2, Apoe, Hexb, cathepsin genes, and C1q genes. The uterine Trem2+ macrophages are enriched for gene sets from Trem2+ macrophages found in other tissues, including those in atherosclerotic plaques and skin. We identified this Trem2+, MHC-II-low macrophage population in the uterus of normal and CBA pregnant mice by flow cytometry (N=6–8/group); and found that CBA mice had 2-fold fewer of this macrophage subtype than normal mice (p=0.0004). These data identify a novel uterine macrophage population that may be critical for healthy pregnancy and demonstrate systemic Treg transfer altering tissue immune populations, which has broad implications for immunotherapy.