# PD-1 regulates CD4+ T cell-mediated CD8+ T cell responses in the brain to balance viral control and neuroinflammation

## TL;DR

This study shows how PD-1 in CD4+ T cells helps control brain virus levels while preventing excessive inflammation during a viral infection.

## Contribution

The study reveals that CD4+ T cell-intrinsic PD-1 signaling balances antiviral defense and neuroinflammation during polyomavirus infection.

## Key findings

- PD-1 loss increases brain-infiltrating CD4+ and CD8+ T cells and virus-specific CD8+ T cell function.
- Deleting PD-1 in CD4+ T cells alone reproduces the effects of global PD-1 loss.
- PD-1-deficient CD8+ T cells show increased effector function and upregulated proliferation transcripts.

## Abstract

Programmed cell death protein 1 (PD-1) is expressed by T cells during progressive multifocal leukoencephalopathy (PML), a life-threatening brain disease caused by the human-only JC polyomavirus. Why PD-1 blockade finds variable success in PML patients is unclear. Brain CD4+ and CD8+ T cells are PD-1high during mouse polyomavirus (MuPyV) encephalitis. Here, we show that PD-1 loss during MuPyV infection acts in a brain-autonomous manner to increase the magnitude of brain-infiltrating CD4+ and CD8+ T cells and the function of virus-specific CD8+ T cells; in concert, brain virus levels decline and neuroinflammation increases. Deletion of PD-1 in CD4+ T cells, but not CD8+ T cells, recapitulates effects of global PD-1 loss. Single-cell RNA sequencing shows that PD-1-deficient CD8+ T cells cluster as effectors while transcripts associated with proliferation and function are upregulated with loss of PD-1. Thus, CD4+ T cell-intrinsic PD-1 signaling balances antiviral defense against neural injury during polyomavirus CNS infection.