bTRM Control of Murine Cytomegalovirus CNS Reactivation

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 (bT_RMs), which display markers of residency (i.e., CD103, CD69, CD49a). However, the functional role of these bT_RMs 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 bT_RMs (viral recovery in explants was reduced from 100% to 50% by day 21). We identified <i>Lgals3</i> (galectin 3), <i>Gpnmb</i> (glycoprotein nonmetastatic melanoma protein B) and <i>Hmox1</i> (heme oxygenase 1) as genes that were most upregulated in bT_RM-depleted groups. When bT_RMs 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 bT_RMs in controlling both CNS reactivation and driving microglial phenotypes.