Molecular basis of proteolytic activation of Sendai virus infection and the defensive compounds for infection.

It has been proposed that the pathogenicity of Sendai virus is primarily determined by a host cellular protease(s) that activates viral infectivity by proteolytic cleavage of envelope fusion glycoproteins. We isolated a trypsin-like serine protease, tryptase Clara, localized in and secreted from Clara cells of the bronchial epithelium of rats. The enzyme specifically cleaved the precursor of fusion glycoprotein F0 of Sendai virus at residue Arg116 in the consensus cleavage motif, Gln(Glu)-X-Arg, resulting in the presentation of the membrane fusion domain in the amino-terminus of the F1 subunit. Administration of an antibody against tryptase Clara in the airway significantly inhibited the activation of progeny virus and multiple cycles of viral replication, thus reducing the mortality rate. These findings indicate that tryptase Clara in the airway is a primary determinant of Sendai virus infection and that proteolytic activation occurs extracellularly. We identified two cellular inhibitory compounds against tryptase Clara in bronchial lavage. One was a mucus protease inhibitor, a major serine protease inhibitor of granulocyte elastase in the lining fluids of the human respiratory tract, and the other was a pulmonary surfactant which may adsorb the enzyme, resulting in its inactivation. These compounds inhibited virus activation by tryptase Clara in vitro and in vivo, but did not themselves affect the hemagglutination and the infectivity of the virus. The functional domain of the mucus protease inhibitor against the enzyme, which is organized in two homologous N- and C-terminal domains, is located in the C-terminal. Administration of these compounds in the airway may be useful for preventing infection with Sendai virus.