# Nasal mucus-derived KLK13 restricts SARS-CoV-2 infection via proteolytic cleavage of spike

## TL;DR

Nasal mucus contains KLK13, a protease that cuts the spike protein of SARS-CoV-2 and other coronaviruses, preventing infection and could be used as an antiviral treatment.

## Contribution

KLK13 is the first identified nasal mucus protease that specifically cleaves coronavirus spike proteins to inhibit infection.

## Key findings

- KLK13 cleaves SARS-CoV-2 spike protein and inhibits viral entry and cell fusion.
- Recombinant KLK13 and AAV-KLK13 reduce SARS-CoV-2 replication in vitro and in vivo.
- KLK13 expression increases in SARS-CoV-2-infected clinical samples and is stimulated by poly(I:C).

## Abstract

The epithelial cilia are the first line of defense against respiratory pathogens. For the first time, we found that Kallikrein-related peptidase 13 (KLK13), a serine protease expressed in airway ciliated epithelial cells with cell type specificity, was secreted into nasal mucus. KLK13 efficiently cleaved the spike of SARS-CoV-2, resulting in the inhibition of SARS-CoV-2 cell entry and spike protein-mediated cell-cell fusion. Recombinant KLK13 protease efficiently cleaved the spike protein as well as virus particles in vitro. Only KLK13, but not other members of the KLK family, specifically cleaved the spike proteins of SARS-CoV-2 as well as other coronaviruses. We also confirmed that endogenous KLK13 stimulated by CRISPR activation (CRISPRa) in A549 cells inhibited SARS-CoV-2 pseudovirus entry. The mRNA level of KLK13 was stimulated by poly (I:C) in both A549 and HeLa cells, and its expression level was also increased in SARS-CoV-2-infected clinical samples. Recombinant adeno-associated virus packaged KLK13 (AAV-KLK13) reduced SARS-CoV-2 replication in a K18-ACE2 mouse model. Collectively, the nasal mucus-derived KLK13 functions as a scissor of coronaviruses and holds the potential to be further developed as a broad-spectrum antiviral against coronaviruses.

Epithelial cilia directly come into contact with inhaled pathogens. The nasal mucus functions as a formidable barrier against penetration of viral particles. KLK13 is secreted into nasal mucus and efficiently cleaves the spike proteins across different coronavirus species. KLK13-mediated cleavage of spike inhibits SARS-CoV-2 entry and syncytium formation. Intranasally delivered KLK13 also restricts SARS-CoV-2 infection in vivo. The finding that KLK13 acts as a scissor of viral spike in nasal mucus paves the way for the development of new antivirals against respiratory viruses.