Dysfunction of CCR1 decidual macrophages is a potential risk factor in the occurrence of unexplained recurrent pregnancy loss.

Recurrent pregnancy loss (RPL) puzzles 1-3% of women of childbearing age worldwide. Immunological factors account for more than 60% of cases of unexplained RPL (URPL); however, the underlying mechanism remains unclear. Here, using single-cell sequencing data and functional experiments with clinical samples, we identified a distinct population of CCR1 decidual macrophages (dMφ) that were preferentially enriched in the decidua from normal early pregnancies but were substantially decreased in patients with URPL. Specific gene signatures endowed CCR1 dMφ with immunosuppressive and migration-regulatory properties, which were attenuated in URPL. Additionally, CCR1 dMφ promoted epithelial-to-mesenchymal transition (EMT) to promote trophoblast migration and invasion by activating the ERK1/2 signaling pathway. Decidual stromal cell (DSC)-derived CCL8 was the key regulator of CCR1 dMφ as CCL8 recruited peripheral CCR1 monocytes, induced a CCR1 dMφ-like phenotype, and reinforced the CCR1 dMφ-exerted modulation of trophoblasts. In patients with URPL, CCL8 expression in DSCs was decreased and trophoblast EMT was defective. Our findings revealed that CCR1 dMφ play an important role in immune tolerance and trophoblast functions at the maternal-fetal interface. Additionally, decreased quantity and dysregulated function of CCR1 dMφ result in URPL. In conclusion, we provide insights into the crosstalk between CCR1 dMφ, trophoblasts, and DSCs at the maternal-fetal interface and macrophage-targeted interventions of URPL.