Paternal antigen-specific proliferating regulatory T cells are increased in uterine-draining lymph nodes just before implantation and in pregnant uterus just after implantation by seminal plasma-priming in allogeneic mouse pregnancy

• Paternal antigen-specific regulatory T (PA-Treg) cells were detected as VB6+Foxp3 cells.
• Proliferated PA-Treg cells increased in uterine draining lymph nodes before the implantation.
• PA-Treg cells express CCR4 and CCR5.
• Seminal plasma is important for the induction of PA-Treg cells in uterine-draining lymph nodes and pregnant uterus.

Paternal antigen-specific regulatory T (PA-specific Treg) cells play an important role in feto-maternal tolerance. To detect the PA-specific Tregs, female BALB/c mice were mated with male DBA/2 mice. Mls Ia antigen on DBA/2 mice is recognized by the T-cell receptor Vβ6; thus, CD4+Foxp3+Vβ6+ cells are recognized as PA-specific Treg cells. CD4+CD25+Vβ6+ cells effectively suppressed the allo-reactive proliferation of lymphocytes compared with that of CD4+CD25+Vβ6− cells. Vβ6+ PA-specific Treg cells expressed CCR4 and CCR5 on their surface. The frequency of Ki67+ PA-specific Treg cells among Treg cells was significantly increased in draining lymph nodes on day 3.5 post-coitus (pc; 6.8 ± 1.1%, p < 0.05) and day 5.5 pc (7.2 ± 1.1%, p < 0.05) in allogeneic pregnant mice compared with that in nonpregnant mice (2.7 ± 0.2%). The frequency of Ki67+ PA-specific Treg cells in the uterus increased significantly after day 5.5 pc in allogeneic pregnant mice compared with that in nonpregnant mice (8.8 ± 2.8% vs. 1.2 ± 1.3%, p < 0.05). However, Ki67-PA-specific Tregs did not change during pregnancy. To analyze the role of seminal fluid or sperm in Treg expansion, female BALB/c mice were mated with vasectomized DBA/2 male mice (VAS) or seminal vesicle-excised DBA/2 male mice (SVX). The frequency of Ki67+ PA-specific Treg cells did not increase in draining lymph nodes or uterus in BALB/c × DBA/2 (SVX) allogeneic mating mice. These findings suggest that the priming by seminal fluid is important for the induction of proliferating PA-specific Tregs in uterine-draining lymph nodes just before implantation and pregnant uterus after implantation, resulting in successful implantation and the maintenance of allogeneic pregnancy.