MHC class I-positive dendritic cells (DC) control CD8 T cell homeostasis in vivo: T cell lymphopenia as a prerequisite for DC-mediated homeostatic proliferation of naive CD8 T cells

The sizes of peripheral T cell pools are regulated by competition for environmental signals within a given ecological T cell niche. Cytokines and MHC molecules have been identified as resources for which naive T cells compete to proliferate homeostatically in lymphopenic hosts to fill up their respective compartments. However, it still remains unclear to what extent CD4 and CD8 T cells intercompete for these resources and which role dendritic cells (DC) play in this scenario. Using transgenic mice in which only DC express MHC class I, we demonstrate that this type of APC is sufficient to trigger complete homeostatic proliferation of CD8 T cells in vivo. However, normal numbers of endogenous naive CD4 T cells, but not CD25(+)CD4(+) T regulatory cells, efficiently suppress this expansion in vivo. These findings identify DC as a major resource and a possible target for homeostatic competition between naive CD4 and CD8 T cells.