Studies in humans and mice have shown that leukemia inhibitory factor (LIF), IL-1, IL-6, IL-11, vascular endothelial growth factor, placenta growth factor, fibroblast growth factor-2, heparin-binding epidermal growth factor (HB-EGF), and TGF-β play important roles for successful implantation by modulating angiogenesis processes, trophoblast differentiation, or the immune system.12 Recent data show that uterine DCs play a central role for successful implantation.13,14 The number of uterine DCs increases at the implantation period, and depletion of DCs results in severe implantation failure.13,14 DC depletion impairs uterine NK cell maturation, tissue remodeling, and angiogenesis.13,14 Surprisingly, depletion of DCs also causes embryo resorption in syngeneic and T-cell-deficient pregnancy, suggesting that DCs appear to govern uterine receptivity independent of the immunological tolerance.14 Although tolerogenic DCs take a part in inducing materno-fetal tolerance, DCs may play a principal role in implantation. But depletion of Treg cell by anti-CD25 monoclonal antibody on the day or 2.5 days after mating results in severe impairment of implantation in allogeneic mice, but this was not observed in syngeneic mice, suggesting that Treg cells are essential for inducing immunological tolerance.15,16 Treg cells are already increased in the lymph nodes draining the uterus 2 or 3 days after mating, suggesting that Treg cells accumulate into the lymph nodes draining the uterus before the implantation.13,14 Robertson et al. reported seminal plasma, but not sperm, plays an essential role for the induction of paternal antigen-specific tolerance.19 The maternal immune system prepares for the semiallograftic embryo to come into the uterus. Alvihare et al. reported that expansion of Treg cells in the lymph nodes draining uterus was also observed in allogeneic mice and syngeneic mice.17 They proposed that pregnancy hormones such as estrogen might induce Treg cells numbers and Arruvito et al.18 showed that Treg cells increased during the follicular phase of the menstrual cycle, suggesting that estrogen plays a part for the expansion of Treg. But our recent data showed paternal antigen-specific Treg cells proliferate in the lymph nodes draining the uterus 3 days after coitus. When BALB/c female mice were mated to DBA/2 male mice, fetuses express DBA/2-derived Mls Ia antigen on the cell surface. As Mls Ia antigen is recognized by T-cell receptor Vβ6, CD4+ CD25+ Foxp3+ Vβ6+ cells are paternal antigen-specific Treg cells. Ki67 is a marker for cell-proliferation; therefore, Ki67+ CD4+ CD25+ Foxp3+ Vβ6+ cells are proliferating fetal antigen-specific Treg cells. Interestingly, Ki67+ Vβ6+ Treg cells increase in the uterine draining lymph node on 3.5 days post-coitus in BALB/c x DBA/2 mating but not in BALB/c x BALB/c mating, suggesting that paternal antigen-specific Treg cells proliferate in the uterine draining lymph node before the implantation.19 After implantation, on day 5.5 post-coitus, Ki67+ paternal antigen-specific Treg cells increase in the uterus in BALB/c × DBA/2 mating.20 These findings demonstrate that paternal antigen-specific Treg cell in draining lymph nodes quickly move to the pregnant uterus and proliferate in the uterus, resulting in the induction of paternal antigen-specific tolerance at the early stage of pregnancy (Fig. 2).20 The ligands such as CCR2, CCR4, CCR5, CCR6, and CD62L are known as molecules for Treg cell migration. CCL2, CCL22, CCL17, CCL4, CCL20, or L-selection in the uterus might selectively accumulate Treg cells from peripheral tissues.21 Recent data demonstrate that some Treg cells express LH/CG receptor, and human chorionic gonadotropin produced by human trophoblast has an ability to attract Treg cells to the uterus.22 Further studies are needed to clarify whether paternal antigen-specific Treg cells express these chemokine receptors or LH/CG receptors on their surface. Th1/Th2/Th17 balance at implantation in such Treg cell-deficient or depleted mice has not been reported, and further studies are needed.
In humans, augmented Th1-type immunity or suppressed Th1-type immunity in endometrium is observed in repeated implantation failure.23 Deceased number or function of Treg cells might be a cause of augmented Th1-type immunity in such cases. Jasper et al.24 reported that primary unexplained infertility is associated with reduced expression of Foxp3 mRNA in endometrial tissue. It can be speculated that decreased Treg cells might induce implantation failure, resulting in unexplained infertility.