PHENOTYPIC CHARACTERISTICS OF LYMPHOID POPULATIONS OF MIDDLE GESTATION HUMAN FETAL LIVER, SPLEEN AND THYMUS

Mononuclear cells isolated from liver spleen and thymus of fetuses bet ween 18 and 24 weeks gestational age were stained for a number of lymp hoid cell markers by indirect immunofluorescence and analyzed by flow cytometry. Studies were carried out on freshly isolated mononuclear ce ll preparations and on cultured cells after selective expansion in int erleukin 2 (IL2). Many mononuclear cells in fresh isolates of liver an d spleen could not be identified with antibodies to mature T- and B-ce ll markers. An average of 3%, of isolated liver cells and 34% of isola ted spleen cells stained positively for CD3, and 19% of liver cells an d 37% of spleen cells stained positively for CD20. Lymphoid cells of t he fetal thymus were an average 67% CD3+, 76% CD4+, 84% CD8+, and show ed greater CD45RO staining (93%) than mononuclear cells of other tissu es. Propagation of liver and spleen cell populations in culture favore d CD3 phenotypes and CD8 phenotypes. Propagated T cell populations of liver and spleen were primarily TCR alpha/beta+ (81% in liver, 85% in spleen), suggesting a selective advantage in IL2 expansion of alpha/be ta T cells over gamma/delta T cells. Propagated gamma/delta T cells of liver and spleen were predominantly TCR gamma/delta2+ Whereas propaga ted cells of liver and spleen consisted of approximately 10% gamma/del ta+ cells thymus-derived cells expanded in culture were only an averag e of 2% TCR gamma/delta+, demonstrating a rarity of IL2-responsive gam ma/delta T cells in middle gestation fetal thymus.