Dynamics of neuropilin1 (Nrp1)-positive thymus-derived and Nrp1-negative peripherally induced paternal antigen specific regulatory T cells in the uterus and spleen during pregnancy in mice

Paternal antigen-specific regulatory T (PA-Treg) cells suppress the immune response against the fetus. Naturally occurring Treg (nTreg) cells derived from the thymus and peripherally induced Treg (iTreg) cells are functional for sustaining pregnancy. This study aimed to compare the variation in PA-Treg cells between the feto-maternal interface and the spleen and to elucidate the dynamics of nTreg and iTreg cells during the gestational period. PA-Treg cells, defined as Treg cells with paternally derived Mls-1a antigen-specific T cell receptors Vβ6, from allogeneic pregnant mice on days 3.5, 5.5, 11.5, and 18.5 post-coitum (pc) were evaluated by flow cytometry. The percentage of Vβ6⁺ Ki67⁺ PA-Treg cells activated by the paternal antigen increased on day 11.5 pc in the decidua (p < 0.05) compared to non-pregnant mice. On day 18.5 pc, this percentage in the decidua parietalis decreased to the level of the non-pregnant state but was significantly higher (p < 0.05) in the decidua basalis. No changes were observed in the spleens. We used two nTreg cell markers, neuropilin1 (Nrp1) and Helios, to distinguish between nTreg cells and iTreg cells. Nrp1⁺ PA-Treg cell levels decreased in late pregnancy compared to those observed in early pregnancy (day 3.5 pc: 57.14 ± 6.16% vs. day 18.5 pc: 30.43 ± 3.09%; p < 0.05), whereas Helios⁺ cell levels did not change. In conclusion, PA immune tolerance is induced by Nrp1⁺ nTreg cells in early pregnancy and Nrp1-negative Treg cells in late pregnancy.