# bTRM Control of Murine Cytomegalovirus CNS Reactivation

## TL;DR

This study explores how brain-resident memory T-cells control a virus in the brain and influence immune cell behavior.

## Contribution

The study reveals that brain tissue-resident memory T-cells control viral reactivation and influence microglial phenotypes in the CNS.

## Key findings

- Depletion of bTRMs leads to viral reactivation and reduced viral recovery in explants.
- bTRM depletion triggers antiviral microglia with disease-associated or neurodegenerative phenotypes.
- Lgals3, Gpnmb, and Hmox1 are upregulated in bTRM-depleted groups.

## Abstract

T lymphocytes infiltrate the CNS in response to murine cytomegalovirus (MCMV) infection and form a pool of long-lived brain tissue-resident memory T-cells (bTRMs), which display markers of residency (i.e., CD103, CD69, CD49a). However, the functional role of these bTRMs is still unknown. By 30 days postinfection, a latent viral brain infection was established, as indicated by absence of viral transcripts (IE1, E1, and gB) produced during productive infection. Following intracerebroventricular injection of either depleting α-CD8 Ab (clone YTS169.4) or α-CD103-sap (clone IT50) into the brain, 90–95% T-cell depletion was achieved. Using luciferase-expressing mice, we observed recommenced imaging signals indicative of de novo MCMV IE promoter activity in depleted animals. Surprisingly, using an explant assay, we efficiently recovered reactivatable, infectious virus from untreated, latent animals, but not from those depleted of bTRMs (viral recovery in explants was reduced from 100% to 50% by day 21). We identified Lgals3 (galectin 3), Gpnmb (glycoprotein nonmetastatic melanoma protein B) and Hmox1 (heme oxygenase 1) as genes that were most upregulated in bTRM-depleted groups. When bTRMs were depleted, there was transient expression of viral IE genes which resulted in antiviral microglia with a phagocytic, disease-associated (DAM) or neurodegenerative (MGnD) phenotype. These data provide new insights into the role of bTRMs in controlling both CNS reactivation and driving microglial phenotypes.