Periodic Accumulation of Regulatory T Cells in the Uterus: Preparation for the Implantation of a Semi-Allogeneic Fetus?

Background Naturally occurring Foxp3 + regulatory T cells play an important role in the inhibition of an immunological attack of the fetus. As implantation of the fetus poses an immediate antigenic challenge, the immune system has to prepare itself for this event prior to implantation. Methodology and Principal Findings Here, we show using quantitative RT-PCR and flow cytometry that regulatory T cells accumulate in the uterus not only during pregnancy, but also every time the female becomes fertile. Their periodic accumulation is accompanied by matching fluctuations in uterine expression of several chemokines, which have been shown to play a role in the recruitment and retention of regulatory T cells. Conclusions/Significance The data lead us to propose that every time a female approaches estrus, regulatory T cells start to accumulate in the uterus in preparation for a possible implantation event. Once pregnancy is established, those regulatory T cells that have seen alloantigen need to be retained at their site of action. Whilst several chemokines appear to be involved in the recruitment and/or retention of regulatory T cells during estrus, in pregnancy this role appears to be taken over by CCL4.