Seminal plasma exposure modulates peripheral T cell-associated transcription during early pregnancy in the mare.

Successful establishment of pregnancy requires maternal immune tolerance to paternal antigens introduced by insemination. Regulatory T cell (Treg)-associated responses are thought to play a central role in this process, whereas pro-inflammatory T helper 1 (Th1) responses may disrupt pregnancy success. In other species, seminal plasma (SP) has been shown to modulate maternal immune adaptation; however, this relationship remains poorly defined in mares, where SP is frequently reduced during routine breeding practices. The objectives of this study were to (1) characterize temporal changes in systemic Treg- and Th1-associated transcripts during early pregnancy, (2) determine the association between these immune mediated transcripts and pregnancy diagnosis, and (3) evaluate the influence of SP exposure on T cell development. Peripheral blood mononuclear cells (PBMCs) were collected from mares prior to insemination and during early pregnancy. Relative expression of Treg-associated (FoxP3, CD25, TGF-β) and Th1-associated (TBX21, IFN-γ) transcripts were quantified by qRT-PCR. Statistics were performed using SAS 9.4®, where the effect of gestational length, pregnancy diagnosis, and breeding method were assessed. FoxP3 expression increased temporally in pregnant mares (p = 0.04), while no changes were observed for other transcripts. Pregnancy outcome was associated with IFN-γ expression (p = 0.05), which was higher in mares that failed to establish pregnancy, particularly pre-breeding and day 7 post-ovulation. SP exposure influenced CD25 (p = 0.04) and trended for TGF-β (p = 0.06), with highest expression in mares bred with fresh semen at day 14. These findings suggest that SP exposure at insemination may promote Treg-associated signaling and contribute to maternal immunotolerance during early pregnancy in the mare.