The mechanism of regulation of Foxp3+ regulatory

T cells remains elusive. The thymus supplies naturally occurring regulatory T cells constantly but the proportion of naturally occurring regulatory T cells seems to decrease by aging. Contrarily, in the periphery, inducible regulatory T cells increase in proportion in elderly people.[19] This finding suggests that inducible regulatory T cells may replenish depletion of the naturally occurring regulatory T cells or are induced as a response to chronic inflammation as a person ages. Regulatory T cells expand during pregnancy in mice and humans and play a key role in protection when maternal immune cells first contact fetal antigens associated with invading trophoblasts.[25] Draining lymph nodes from the uterus have been implicated as the predominant site of regulatory T-cell expansion Raf tumor and fetal alloantigen. Neither estrogen nor progesterone

was suggested as responsible for regulatory T-cell expansion in mice.[26] Mice seminal fluid was reported to contribute HM781-36B molecular weight to the accumulation of Foxp3+ regulatory T cells in the preimplantation uterus,[27] and insufficient expansion of regulatory T cells against paternal antigens may trigger spontaneous abortion in mice.[28] Additionally, the induction of paternal specific regulatory T cells has been demonstrated in pregnant women at 24–28 weeks of gestation.[29] Furthermore, fetus-specific CD4+ CD25bright regulatory T cells are selectively recruited from the peripheral blood into the deciduas in human pregnancy.[30] These findings suggest that paternal antigens in the seminal fluid and fetal allogeneic antigens may induce the Non-specific serine/threonine protein kinase expansion of maternal regulatory T cells in the periphery, which are preferentially recruited toward the fetomaternal interface so as to control

the maternal immune response to fetal antigens and lead to favorable pregnancy outcome. Recently, several reports have indicated that transient or low expression of Foxp3 did not confer regulatory function to the cells[31, 32] and those cells are converted into effector T cells producing pro-inflammatory cytokines such as IL-2, interferon-γ (IFN-γ) and IL-17.[31, 33-35] Intriguingly, IDO may play a role in T-cell differentiation. The presence of IDO is known to develop inducible regulatory T cells, but its absence reprograms regulatory T cells into the effectors such as Th17 cells.[36] About 25 years ago, the Th1 and Th2 hypothesis was first introduced.[37, 38] In this concept, type 1 CD4+ T helper cells (Th1 cells) that secrete IL-2 and IFN-γ induce cell-mediated immune reaction related to tissue damage, and type 2 CD4+ Th cells (Th2) lead to antibody-mediated immune responses, such as allergy. This theory of Th1/Th2 had been accepted as a solid dichotomy of effector T-cell immunity.