Correlation of cell‐surface <scp>CD</scp> 8 levels with function, phenotype and transcriptome of naive <scp>CD</scp> 8 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.