Correlation of cell‐surface CD 8 levels with function, phenotype and transcriptome of naive CD 8 T cells

We have previously demonstrated co‐receptor level‐associated functional heterogeneity in apparently homogeneous naive peripheral CD 4 T cells, dependent on MHC ‐mediated tonic signals. Maturation pathways can differ between naive CD 4 and naive CD 8 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 CD 8 T cells from young mice were separated into CD 8 hi and CD 8 lo subsets, CD 8 lo cells responded poorly, but CD 8 hi and CD 8 lo subsets of CD 8 single‐positive ( SP ) thymocytes responded similarly. CD 8 lo naive CD 8 T cells were smaller and showed lower levels of some cell‐surface molecules, but higher levels of the negative regulator CD 5. In addition to the expected peripheral decline in CD 8 levels on transferred naive CD 8 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 CD 8 T cells but not in CD 4 T cells. Constitutive pZAP 70/ pS yk and pERK levels ex vivo were lower in CD 8 lo naive CD 8 T cells and dual‐specific phosphatase inhibition partially rescued their hypo‐responsiveness. Bulk mRNA sequencing showed major differences in the transcriptional landscapes of CD 8 hi and CD 8 lo naive CD 8 T cells. CD 8 hi naive CD 8 T cells showed enrichment of genes involved in positive regulation of cell cycle and survival. Our data show that naive CD 8 T cells show major differences in their signaling, transcriptional and functional landscapes associated with subtly altered CD 8 levels, consistent with the possibility of peripheral cellular aging.