Development of cellular immunity in the human fetus: dichotomy of proliferative and cytotoxic responses of lymphoid cells to phytohemagglutinin.

The reactivity of cells in vitro was investigated with specimens from various lymphoid organs of seven human fetuses. Thymocytes responded to stimulation by phytohemagglutinin with significant increases in synthesis of DNA, but failed to produce destruction of xenogeneic target cells. In cells from bone marrow, precisely the converse pattern of reactivity to the mitogen was detected. Lymphocytes from spleen and peripheral blood demonstrated both phytohemagglutinin-dependent functions, while hepatic cells did not respond to phytohemagglutinin. Based on the striking dichotomy of phytohemagglutinin-dependent responses in fetal thymocytes and bone-marrow lymphoid cells, we conclude that phytohemagglutinin-dependent cytotoxicity and DNA synthesis are functions of different populations of lymphoid cells during human embryonic development.