Correlation of cell-surface CD8 levels with function, phenotype and transcriptome of naive CD8 T cells

We have previously demonstrated co-receptor level-associated functional heterogeneity in apparently homogeneous naive peripheral CD4 T cells, dependent on MHC-mediated tonic signals. Maturation pathways can differ between naive CD4 and naive CD8 cells, so we tested whether the latter showed similar co-receptor level-associated functional heterogeneity. We report that, when either polyclonal and T-cell receptor (TCR)-transgenic monoclonal peripheral naive CD8 T cells from young mice were separated into CD8^hi and CD8^lo subsets, CD8^lo cells responded poorly, but CD8^hi and CD8^lo subsets of CD8 single-positive (SP) thymocytes responded similarly. CD8^lo naive CD8 T cells were smaller and showed lower levels of some cell-surface molecules, but higher levels of the negative regulator CD5. In addition to the expected peripheral decline in CD8 levels on transferred naive CD8 T cells in wild-type (WT) but not in MHC class I-deficient recipient mice, short-duration naive T-cell-dendritic cell (DC) co-cultures in vitro also caused co-receptor down-modulation in CD8 T cells but not in CD4 T cells. Constitutive pZAP70/pSyk and pERK levels ex vivo were lower in CD8^lo naive CD8 T cells and dual-specific phosphatase inhibition partially rescued their hypo-responsiveness. Bulk mRNA sequencing showed major differences in the transcriptional landscapes of CD8^hi and CD8^lo naive CD8 T cells. CD8^hi naive CD8 T cells showed enrichment of genes involved in positive regulation of cell cycle and survival. Our data show that naive CD8 T cells show major differences in their signaling, transcriptional and functional landscapes associated with subtly altered CD8 levels, consistent with the possibility of peripheral cellular aging.