GAMMA-DELTA-T-CELLS EXPRESSING CD8 OR CD4LOW APPEAR EARLY IN MURINE FETAL THYMUS DEVELOPMENT

Three-colour flow cytometry was used to study the distribution of TCR-gamma-delta+ cells among CD4+CD8-, CD4-CD8+, CD4+CD8+, and CD4-CD8- cell populations during thymic development. Thymocytes were obtained either directly from embryos at different stages of gestation (ex vivo) or from organ cultures maintained in vitro. In both cases, TCR-gamma-delta+ cells were found predominantly among the double negative (CD4-CD8-) and CD8 single positive subsets. These cells were actively dividing as demonstrated by 7 amino actinomycin D (7AAD) labelling. A small population of TCR-gamma-delta+ cells expressing low levels of CD4 was identified early and transiently (days 15-18) during development, but this subset was rare in the adult thymus. In newborn mice, adult mice, and late during organ culture, TCR-gamma-delta+ cells were found mainly within the CD4-CD8- compartment of thymocytes, although a minor population of CD8+ cells (5-10%) bearing gamma-delta-receptor was routinely observed. In contrast, few gamma-delta-cells were contained among the CD4+CD8+ subset at any timepoint studied. These data highlight differences between the ontogeny of alpha-beta and gamma-delta-cells in the thymus, and suggest that a CD4+CD8+ intermediate may not be a requisite for the intrathymic differentiation of murine gamma-delta-T cells.