Generation of PLZF(+) CD4(+) T cells via MHC class II–dependent thymocyte–thymocyte interaction is a physiological process in humans

Human thymocytes, unlike mouse thymocytes, express major histocompatibility complex (MHC) class II molecules on their surface, especially during the fetal and perinatal stages. Based on this observation, we previously identified a novel developmental pathway for the generation of CD4(+) T cells via interactions between MHC class II–expressing thymocytes (thymocyte–thymocyte [T–T] interactions) with a transgenic mouse system. However, the developmental dissection of this T–T interaction in humans has not been possible because of the lack of known cellular molecules specific for T–T CD4(+) T cells. We show that promyelocytic leukemia zinc finger protein (PLZF) is a useful marker for the identification of T–T CD4(+) T cells. With this analysis, we determined that a substantial number of fetal thymocytes and splenocytes express PLZF and acquire innate characteristics during their development in humans. Although these characteristics are quite similar to invariant NKT (iNKT) cells, they clearly differ from iNKT cells in that they have a diverse T cell receptor repertoire and are restricted by MHC class II molecules. These findings define a novel human CD4(+) T cell subset that develops via an MHC class II–dependent T–T interaction.