TRACING THE EXPRESSION OF CD7 AND OTHER ANTIGENS DURING T-CELL AND MYELOID-CELL DIFFERENTIATION IN THE HUMAN FETAL LIVER AND THYMUS

During the last decade, the function/s of the cell membrane CD7 antige n have been investigated in human mature T and NK cells, showing the d irect involvement of this molecule in multiple effector functions rela ted with activation, proliferation, production of cytokines and modifi cation of adhesion properties. The CD7 glycoprotein is not only expres sed by mature lymphoid cells, but also by early hematopoietic progenit ors and several types of leukemias, suggesting a role of CD7 during he matopoiesis. However, the function of CD7 in the early stages of hemat opoietic development has not yet been elucidated. CD7 has been classic ally considered the earliest T-cell specific marker. This assumption w as based on data indicating the presence of CD45(+)CD7(+)CD3(-)CD4(-)C D8(-) cells in the human embryonic/fetal liver at the gestational age at which the thymic rudiment is colonized by T-cell progenitors. In th e present article, we review recent results obtained by several groups concerning the expression of CD7 and various other cell surface antig ens by T-, B- and myeloid-cell progenitors generated in the adult bone marrow and fetal liver. In addition, we present an hypothetical model of hematopoiesis in the fetal liver and thymus.