Strong Basal/Tonic TCR Signals Are Associated with Negative Regulation of Naive CD4+ T Cells.

T cells experience varying intensities of tonic or basal TCR signaling in response to self-peptides presented by MHC (self-pMHC) in vivo. We analyzed four subpopulations of mouse naive CD4⁺ cells that express different levels of Nur77-GFP and Ly6C, surrogate markers that positively and inversely correlate with the strength of tonic TCR signaling, respectively. Adoptive transfer studies suggest that relatively weak or strong Nur77-GFP intensity in thymocytes tends to be maintained in mature T cells. Two-dimensional affinity measurements were lowest for Nur77-GFP^loLy6C⁺ cells and highest for Nur77-GFP^hiLy6C^- cells, highlighting a positive correlation between apparent TCR affinity and tonic TCR signal strength. Despite experiencing the strongest tonic TCR signaling, Nur77-GFP^hiLy6C^- cells were least responsive to multiple concentrations of a cognate or suboptimal pMHC. Gene expression analyses suggest that Nur77-GFP^hiLy6C^- cells induce a gene expression program that has similarities with that of acutely stimulated T cells. However, strong tonic TCR signaling also correlates with increased expression of genes with inhibitory functions, including coinhibitory receptors. Similarly, assay for transposase-accessible chromatin with sequencing analyses suggested that increased tonic TCR signal strength correlated with increased chromatin accessibility associated with genes that have positive and inhibitory roles in T cell activation. Strikingly, Nur77-GFP^hiLy6C^- cells exhibited differential accessibility within regions of Cd200r1 and Tox that were similar in location to differentially accessible regions previously identified in exhausted CD8⁺ T cells. We propose that constitutive strong tonic TCR signaling triggers adaptations detectable at both the transcriptional and epigenetic levels, ultimately contributing to the tuning of T cell responsiveness.