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

Introduction/Rationale: 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.