At conception, the female reproductive tract encounters seminal fluid factors that prime T cells to initiate immune tolerance towards the ensuing genetically disparate fetus. Tolerance towards paternally-inherited major and minor histocompatibility antigens (MHC and MiHC) is critical. Differences between maternal and paternal histocompatibility antigens influence the maternal T cell response and impact pregnancy development, but the mechanisms are not clearly defined.
We hypothesised that disparate MHC and MiHC antigens in seminal fluid affect reproductive outcomes through effects on the female T cell repertoire stimulated after mating. To test this, we used C57BL/6 female mice mated with males of three genotypes: BALB/c (MHC/MiHC-disparate), BALB/b (MHC-matched, MiHC-disparate), or C57BL/6 (MHC/MiHC matched). Fetal and placental development were assessed during late gestation at 17.5 days post-coitum (dpc). We analysed the female T cell response using flow cytometry and bulk T cell receptor (TCR) sequencing from uterine-draining lymph nodes (udLNs) at 3.5 dpc, with unmated C57BL/6 females as a comparison. Finally, we performed single-cell RNA and TCR-sequencing on isolated udLN CD4+ T cells from BALB/c-mated dams at 4.5 dpc to comprehensively evaluate the female T cell landscape post-exposure to disparate MHC and MiHC antigens.
Pregnancies sired by BALB/c males showed a 16% increased fetal:placental weight ratio, indicating greater placental efficiency compared to pregnancies sired by other males (n=14-16/group). Bulk TCR-sequencing and flow cytometry revealed increased clonal T cell proliferation (n=3-5/group) and a higher ratio of permissive regulatory T cells (Treg) to pro-inflammatory conventional T cells (Tconv) after BALB/c mating (n=11-16/group), suggesting improved maternal immune tolerance. This enhanced Treg:Tconv ratio was validated by single-cell sequencing, which additionally identified distinct TCR repertoires for the Treg and Tconv cell populations.
These findings demonstrate that seminal fluid MHC antigens impact the quality and strength of female T cell-mediated tolerance in early pregnancy to facilitate optimal placentation and fetal development.