Proteolytic cleavage of the
hemagglutinin (HA) of influenza virus by host
trypsin-like
proteases is required for viral infectivity. Some
serine proteases are capable of cleaving influenza virus HA, whereas some
serine protease inhibitors (
serpins) inhibit the HA cleavage in various cell types.
Kallikrein-related
peptidase 1 (KLK1, also known as
tissue kallikrein) is a widely distributed
serine protease.
Kallistatin, a
serpin synthesized mainly in the liver and rapidly secreted into the circulation, forms complexes with KLK1 and inhibits its activity. Here, we investigated the roles of KLK1 and
kallistatin in influenza virus
infection. We show that the levels of KLK1 increased, whereas those of
kallistatin decreased, in the lungs of mice during influenza virus
infection. KLK1 cleaved H1, H2, and H3 HA molecules and consequently enhanced viral production. In contrast,
kallistatin inhibited KLK1-mediated HA cleavage and reduced viral production. Cells transduced with the
kallistatin gene secreted
kallistatin extracellularly, which rendered them more resistant to influenza virus
infection. Furthermore, lentivirus-mediated
kallistatin gene delivery protected mice against lethal influenza virus challenge by reducing the viral load,
inflammation, and injury in the lung. Taking the data together, we determined that KLK1 and
kallistatin contribute to the pathogenesis of influenza virus by affecting the cleavage of the HA
peptide and inflammatory responses. This study provides a proof of principle for the potential therapeutic application of
kallistatin or other KLK1 inhibitors for
influenza. Since proteolytic activation also enhances the infectivity of some other viruses,
kallistatin and other
kallikrein inhibitors may be explored as
antiviral agents against these viruses.