Bystander activation in CD8 memory T cells is an innate program that is standalone from, yet influences TCR and TCR-driven responses 2259216

Abstract Introduction T cell receptors (TCRs) bestow CD8 memory T cells with their conventional function: highly-effective and -specific target killing. While TCR is critical to unlock these effector functions, CD8 memory T cells can become innate-like effectors when activated by proinflammatory cytokines in combination (a phenomenon referred to as bystander activation). Given the parallels between TCR-mediated and bystander activation, including IFNg secretion, cytolytic granule expression, and upregulation of co-inhibitory molecules and transcripts suggestive of TCR signals, there has been significant controversy as to whether bystander activation utilizes TCR or downstream machinery. Methods We use a combination of TCR signaling inhibitors, targeting of CD3e using CRISPR/Cas9 at priming, and inducible TCR deletion at T cell memory to dissect the TCR-dependent and -independent consequences of bystander activation. We use in vitro stimulations and in vivo heterologous infections to bystander activate CD8 memory T cells with defined specificity. Results We demonstrate that bystander activation induces cytotoxicity completely independent of TCR, downstream signaling components like NFAT, and regulators of TCR signaling, demonstrating that bystander activation is a standalone innate program conserved in adaptive cells. On the other hand, we report that the expression of co-inhibitory molecules (like PD-1) during bystander activation requires intact TCR expression. This suggests inflammation may be able to lower TCR activation thresholds needed to unlock co-inhibitory/-stimulatory molecule expression. Conclusion We show that CD8 memory T cells are capable of conventional adaptive (TCR-driven) responses as well as those that are entirely innate. Though this innate activation does not require TCR/TCR signaling machinery, it can influence how a T cell interprets sub-agonist and agonist signals through its TCR. Future work will be necessary to dissect the interplay between the innate and adaptive programs of T cells. Funding Source NCI K00 CA245735 (NJM), NCI K99 CA296729 (NJM), DRG-2427-21 (NNJ), NIAID K22 AI177360 (NNJ), NIAID R01 AI038903 (SCJ) Topic Categories Immune Response Regulation: Cellular Mechanisms (IRC)