Have You Seen Your Mother, Baby…

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Science  17 Dec 2010:
Vol. 330, Issue 6011, pp. 1635-1636
DOI: 10.1126/science.1200406

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In 1953, immunologist (and later, Nobel Laureate) Peter Medawar posed the question, “How does the pregnant mother contrive to nourish within itself, for many weeks or months, a fetus that is an antigenically foreign body?” (1). Although none of his proposed mechanisms stood the test of time, they nevertheless provided the intellectual framework for years to come in the study of how a fetus evades rejection by the mother's immune system (maternal fetal tolerance). Regulatory T cells are now recognized as key mediators of immunological tolerance. Although they are best known for preventing autoimmunity (2), development of an embryo inside the body of the mother (invasive placental viviparity) would not be possible without these immune cells (3). On page 1695 of this issue, Mold et al. (4) show that fetal and adult T cells, including regulatory T cells, are distinct populations that arise from different hematopoietic stem-progenitor cells (HSPCs) present at different stages of development. Moreover, the early fetal immune system appears to be biased toward tolerance. This not only has important implications for understanding how a fetus avoids launching an immune response against the mother (fetal maternal tolerance), but it also raises questions about how the human immune system is set up during fetal development and in adulthood.