Virion stripping: A new role for bacterial proteases

We demonstrate that proteases produced by the oro-pharyngeal bacterial colonizer Porphyromonas gingivalis (Pg) reduce viral burden and modulate host interferon responses during respiratory syncytial virus (RSV) infection. Several oral bacteria, including Pg, have been shown to translocate to the upper airways through sub-clinical micro-aspiration. Our findings reveal that Pg, upon translocating to this new niche, significantly attenuated lung damage by reducing viral loads during respiratory viral infections in the lungs of wild-type mice. This protective effect was attributed to the activity of gingipains, cysteine endopeptidases produced by Pg, which cleaved envelope glycoproteins on RSV as well as on related murine-specific Sendai virus (SeV), thereby impairing their infectious capacity. Notably, the reduction in viral loads was independent of interferon lambda (IFN-{lambda}) signaling, which is actively suppressed by Pg in airway epithelial cells. However, the complete absence of IFN-{lambda} signaling resulted in a stronger inflammatory response despite a low viral load. Thus, we show a previously undescribed role for the oro-respiratory bacterial colonizer Pg in creating bottlenecks to viral infection by the activity of its proteases. SIGNIFICANCE STATEMENTReciprocal interactions between microbial colonizers and host epithelial cells are critical for providing initial defense against viral infections. However, our understanding of this phenomenon has been limited to microbiota-derived ligands that activate host pattern recognition receptors (PRRs), inducing basal interferon expression and downstream antiviral genes. Here, we present a novel mechanism that relies on microbial proteases to directly reduce viral load. Specifically, we discovered that the infectious capacity of the Respiratory Syncytial Virus (RSV) was significantly inhibited upon contact with the proteases (gingipains) produced by the oropharyngeal colonizer Porphyromonas gingivalis. Gingipains caused proteolytic degradation of the RSV envelope and attachment proteins, rendering them inactive. This preemptive reduction in viral infectious capacity consequently diminished the severity of respiratory viral infections in an IFN-independent manner.