Single-cell RNA sequencing of leukocytes at the maternal-fetal interface in psyhiological and pathological Nodal-deficient pregnancies

Leukocytes at the maternal-fetal interface have critical functions during pregnancy to prevent maternal reactivity towards fetal alloantigens, suppress excess inflammation and promote placental angiogenesis. Failed maternal immune adaptations to pregnancy can lead to placental insufficiency and the development of severe reproductive complications. Recent evidence from mouse models has suggested that Nodal, a secreted morphogen of the TGFB superfamily, is an immunoregulator of pregnancy. The absence of Nodal expression in the female reproductive tract resulted in the loss of preimplantation regulatory T cells and a 50% implantation failure rate. By mid-gestation, Nodal Δ/Δ pregnancies showed placental dysfunction, fetal loss and intrauterine growth restriction. Therefore, the Nodal Δ/Δ model represents a unique system to study immune-mediated reproductive failure. In this study, single-cell RNA-sequencing was used to characterize leukocytes within the mid-gestational decidua and placenta during physiological and pathological Nodal Δ/Δ pregnancies. Eleven distinct immune cell clusters were identified, with an emphasis on myeloid populations (macrophages and neutrophils). In particular, the characterization of placenta-associated maternal macrophages revealed novel functions in the regulation of angiogenesis, immune suppression and endocytosis. In Nodal Δ/Δ females, the differential abundance and expression profile of these leukocytes reflected the immune dysfunction observed in human reproductive pathologies. Taken together, elucidating roles of leukocytes in placental development provides a basis in the understanding of mechanisms associated with physiological pregnancy and reproductive failure.