Hierarchy of signaling thresholds downstream of the T cell receptor and the Tec kinase ITK

The strength of peptide:MHC interactions with the T cell receptor (TCR) is correlated with the time to first cell division, the relative scale of the effector cell response, and the graded expression of activation-associated proteins like IRF4. To regulate T cell activation programming, the TCR and the TCR proximal interleukin-2-inducible T cell kinase (Itk) simultaneously trigger many biochemically separate signaling cascades. T cells lacking Itk exhibit selective impairments in effector T cell responses after activation, but under the strongest signaling conditions, Itk activity is dispensable. To gain insight into whether TCR signal strength and Itk activity tune observed graded gene expression through the unequal activation of distinct signaling pathways, we examined ERK1/2 phosphorylation or nuclear factor of activated T cells (NFAT) and nuclear factor (NF)-κB translocation in naïve OT-I CD8⁺ cell nuclei. We observed the consistent digital activation of NFAT1 and ERK1/2, but NF-κB displayed dynamic, graded activation in response to variation in TCR signal strength, tunable by treatment with an Itk Inhibitor. Inhibitor-treated cells showed the dampened induction of AP-1 factors Fos and Fosb, NF-κB response gene transcripts, and survival factor Il2 transcripts. ATAC Sequencing analysis also revealed that genomic regions most sensitive to Itk inhibition were enriched for NF-κB and AP-1 motifs. Specific inhibition of NF-κB during peptide stimulation tuned the expression of early gene products like c-Fos. Together, these data indicate a key role for Itk in orchestrating the optimal activation of separate TCR downstream pathways, specifically aiding NF-κB activation. More broadly, we revealed a mechanism by which variations in TCR signal strength can produce patterns of graded gene expression in activated T cells.

ITK; NF-κB; NFAT; T cell activation; T cell receptor signaling.