MHC Class II-Restricted Interaction between Thymocytes Plays an Essential Role in the Production of Innate CD8+ T Cells

Abstract

We have recently shown that MHC class II-dependent thymocyte–thymocyte (T–T) interaction successfully generates CD4+ T cells (T–T CD4+ T cells), and that T–T CD4+ T cells expressing promyelocytic leukemia zinc finger protein (PLZF) show an innate property both in mice and humans. In this article, we report that the thymic T–T interaction is essential for the conversion of CD8+ T cells into innate phenotype in the physiological condition. CD8+ T cells developed in the presence of PLZF+ CD4+ T cells showed marked upregulation of eomesodermin (Eomes), activation/memory phenotype, and rapid production of IFN-γ on ex vivo stimulation. Their development was highly dependent on the PLZF expression in T–T CD4+ T cells and the IL-4 secreted by PLZF+ T–T CD4+ T cells. The same events may take place in humans, as a substantial number of Eomes expressing innate CD8+ T cells were found in human fetal thymi and spleens. It suggests that PLZF+ T–T CD4+ T cells in combination with Eomes+ CD8+ T cells might actively participate in the innate immune response against various pathogens, particularly in human perinatal period.