In utero development of memory T cells.

Pathogen-specific immune memory develops subsequent to primary exposure to antigen, mainly in the context of infection or vaccination to provide protection. Although a safe fetal life requires a tolerogenic environment in order to circumvent unnecessary inflammatory responses, it needs to be prepared in utero to face the microbial environment outside the womb. The possibility of immune memory generation in the fetus would help such transition providing protection in early life. This requires fetal T cell exposure to foreign antigens presented by dendritic cells. There are evidences of fetal T cell priming in several cases of congenital infections or in uninfected children born of infected mothers. Fetal T cell memory seems to arise also without any reported infection during pregnancy. Such memory T cells display various effector functions, including Th1, Th2, or Th17 profiles, raising the issue of benefits and risks for postnatal life when considering maternal vaccination, susceptibility to infection, or environmental allergen sensitization.