# Memory-like CD8+ T cells lacking PD-1 adapt to persistent stimulation by reducing TCR signal transduction rather than increasing exhaustion

## TL;DR

Memory-like CD8+ T cells adapt to chronic stimulation by reducing TCR signaling instead of becoming exhausted, even when they lack PD-1.

## Contribution

The study reveals a novel adaptation mechanism in memory-like CD8+ T cells that reduces TCR signaling instead of relying on PD-1 to manage chronic stimulation.

## Key findings

- TML cells lacking PD-1 persist during chronic infection but have reduced regenerative capacity upon restimulation.
- Absence of PD-1 leads to reduced TCR activation and lower expression of stemness genes like Myb and Klf4.
- Anti-PD-L1 treatment also weakly reduces TCR signaling in TML cells, mimicking PD-1 absence.

## Abstract

CD8+ T cells respond to persistent stimulation during chronic viral infection by stably expressing co-inhibitory receptors and other exhaustion-related molecules. Here we addressed how memory-like CD8+ T (TML) cells, which sustain the immune response to chronic infection thanks to their stem-like properties, adapt to chronic stimulation when they cannot express the co-inhibitory receptor PD-1. We found an increased initial generation and stable long-term persistence of TML cells lacking PD-1 during chronic viral infection. However, these cells had a reduced ability regenerate upon acute restimulation in the context of a recall response. Mechanistically, the lack of PD-1-mediated inhibition was not compensated by an increased expression of other co-inhibitory receptors or exhaustion related molecules. Rather, the absence of PD-1 resulted in a reduced capacity of the TCR to activate TML cells and to express stemness genes including Myb and Klf4. Similar albeit weaker effects on TML cells were noted when PD-1 engagement was transiently interrupted due to anti-PD-L1 treatment. Thus, stem-like CD8+ T cells responding to chronic viral infection adapt to the absence of PD-1-dependent co-inhibitory signals by further reducing TCR-mediated activation signaling, likely to prevent excessive or prolonged stimulation of these cells.