Maternal T Regulatory Cell Depletion Impairs Embryo Implantation Which Can Be Corrected With Adoptive T Regulatory Cell Transfer

Maternal immune tolerance of fetal engraftment is critical for the establishment and maintenance of pregnancy, but the exact mechanisms permitting this semi-allograft in the maternal host are not completely understood. Further, failure of the embryo to implant in the uterus accounts for at least 30% of the best prognosis in vitro fertilization cycles when a perfect embryo is transferred to a normal uterus. We hypothesized that T regulatory cells (Tregs), defined by CD4þCD25hi surface expression and the FoxP3þ transcription factor, play an important role in the initiation of the earliest stages of pregnancy, specifically implan- tation of the embryo. In this study, we evaluated the role of Tregs in the establishment of pregnancy using a conditional depletion of Treg transgenic mouse model. We found that embryo implantation in the syngeneic mating was defective as evidenced by smaller litter sizes after Treg depletion and that embryo implantation could be restored by adoptively transferring Tregs into the mating mice. In allogeneic mating, litter sizes were not different but breeding efficiency was significantly decreased. These data reveal that Tregs are important for the establishment of the earliest stages of pregnancy and may be a potential cause of infertility due to recurrent implantation failure, which may be amenable to cellular or pharmacologic therapy to improve maternal immune tol- erance of embryo implantation.