Gamma delta T cells expressing CD8 or CD4low appear early in murine foetal 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.