Human regulatory T cells at the maternal-fetal interface show functional site-specific adaptation with tumor-infiltrating-like features

Objectives Regulatory T cells (Tregs) are crucial for maintaining immune tolerance against the semi-allogeneic fetus during pregnancy. Since their functional profile at the human maternal-fetal interface is still elusive, we investigated the transcriptional profile and functional adaptation of human uterine Tregs (uTregs) during pregnancy. Methods Blood and uterine biopsies from the placental bed (=maternal-fetal interface) and incision site (=control), were obtained from women with uneventful pregnancies undergoing primary Caesarean section. Tregs and CD4 + non-Tregs (Tconv) were isolated for transcriptomic profiling by Cel-Seq2. Results were validated on protein and single cell level by flow cytometry. Results Placental bed uterine Tregs (uTregs) showed elevated expression of Treg signature markers compared to blood Tregs, including FOXP3, CTLA4 and TIGIT. The uTreg transcriptional profile was indicative of late-stage effector Treg differentiation and chronic activation with high expression of immune checkpoints GITR, TNFR2, OX-40, 4-1BB, genes associated with suppressive capacity (CTLA4, HAVCR2, IL10, IL2RA, LAYN, PDCD1), activation (HLA-DR, LRRC32), and transcription factors MAF, PRDM1, BATF, and VDR. uTregs mirrored uTconv Th1 polarization, and characteristics indicating tissue-residency, including high CD69, CCR1, and CXCR6. The particular transcriptional signature of placental bed uTregs overlapped strongly with the specialized profile of human tumor-infiltrating Tregs, and, remarkably, was more pronounced at the placental bed than uterine control site. Conclusion uTregs at the maternal-fetal interface acquire a highly differentiated effector Treg profile similar to tumor-infiltrating Tregs, which is locally enriched compared to a distant uterine site. This introduces the novel concept of site-specific transcriptional adaptation of human Tregs within one organ.