# Liquid-liquid phase separation mediated immune evasion of respiratory syncytial virus against oligoadenylate synthetase-RNase L pathway

## TL;DR

This study shows how respiratory syncytial virus uses liquid-liquid phase separation to hide its RNA and avoid the host's immune defenses.

## Contribution

The study reveals a novel immune evasion mechanism by RSV through phase-separated inclusion bodies.

## Key findings

- RSV inclusion bodies protect viral RNA from the OAS-RNase L pathway.
- Perturbing phase separation releases dsRNA and activates the antiviral pathway.
- Sequestering dsRNA in IBs prevents immune detection by RSV-infected cells.

## Abstract

Respiratory syncytial virus (RSV) infection is the major cause of severe respiratory illnesses in infants and older adults. RSV forms phase-separated biomolecular condensates called inclusion bodies (IBs), which serve as hubs for viral replication. However, the contribution of IBs to host immune response evasion remains elusive. We report that RSV IBs protect viral RNA from the 2′-5′ oligoadenylate synthetase (OAS)-RNase L pathway, a critical antiviral defense mechanism that cleaves viral and cellular RNAs. RSV infection did not activate the OAS-RNase L pathway, and ectopically activated RNase L did not suppress viral replication. In RSV-infected cells, double-stranded RNA (dsRNA) was efficiently sequestered within liquid–liquid phase separation (LLPS)-mediated IBs, rendering its detection challenging. LLPS perturbation caused dsRNA release from IBs into the cytosol. dsRNA extracted from infected cells, which lacked LLPS shielding, triggered OAS-RNase L pathway activation. Thus, LLPS-driven IBs structurally sequester viral RNA, facilitating RSV to evade RNase-dependent genomic RNA degradation mediated by the OAS-RNase L antiviral pathway.

Biomolecular condensates are increasingly recognized as key mechanisms of subcellular organization. The role of viral infection-mediated phase-separated structures, called inclusion bodies (IBs), in replication has already been characterized in several viruses, including the human respiratory syncytial virus (RSV). Herein, we focused on the role of IBs in immune evasion and found that RSV uses inclusion bodies to compartmentalize viral RNA and escape detection via the OAS–RNase L antiviral defense pathway.