Can disabling a coronavirus protein expose it to immune defenses and weaken disease in mice?

Details

Coronaviruses are positive-sense RNA viruses that produce double-stranded RNA (dsRNA) intermediates as they replicate, yet evade detection by the host’s innate immune sensors. This study reports that nonstructural protein 15 (nsp15), an RNA-cutting enzyme called an endoribonuclease, is required for evading dsRNA sensors.

In laboratory experiments, researchers evaluated two independent nsp15 mutant mouse coronaviruses, N15m1 and N15m3, in mouse bone marrow-derived macrophages. Both viruses replicated poorly and caused rapid cell death. N15m1 produces an unstable nsp15 protein, while N15m3 produces an nsp15 protein deficient in catalytic activity. Infection with either mutant activated MDA5, PKR, and the OAS/RNase L system, resulting in early, strong induction of type I interferon (IFN), PKR-mediated apoptosis, and RNA degradation.

Using immunofluorescence imaging of infected macrophages, the researchers found significant dispersal of dsRNA early in infection with the nsp15 mutants. In contrast, in cells infected with wild-type (WT) virus, most dsRNA was associated with replication complexes.

Experiments in mice also showed that loss of nsp15 activity greatly attenuated disease and stimulated a protective immune response. Together, the findings demonstrate that coronavirus nsp15 is critical for evading host dsRNA sensors in macrophages and identify modulation of nsp15 stability and activity as a strategy for generating live-attenuated vaccines.