The human mucus protease inhibitor and its mutants are novel defensive compounds against infection with influenza A and Sendai viruses.

Tryptase Clara, a trypsin-like protease localized exclusively in and secreted by Clara cells of the bronchial epithelium, is a prime host factor that processes viral envelope glycoproteins and determines the infectivity of influenza A and Sendai viruses (H. Kido, Y. Yokogoshi, K. Sakai, M. Tashiro, Y. Kishino, A. Fukutomi, and N. Katunuma, The Journal of Biological Chemistry, 1992, Vol. 267, pp. 13573-13579). We report here that human mucus protease inhibitor (MPI), a major inhibitor of granulocyte elastase in the lining fluid of the human respiratory tract, significantly inhibited induction of the infectivity of influenza A and Sendai viruses by tryptase Clara in vitro and multicycles of mouse-adapted influenza A virus replication in rat lungs in vivo. Recombinant MPI and the C- but not the N-terminal domain of MPI inhibited both the activity of tryptase Clara and the induction of virus infection by tryptase Clara. The 50% inhibitory concentrations of MPI and the C-terminal domain peptide (Pro50-Ala107) of MPI for tryptase Clara were 7.4 and 61.6 nM, respectively, with Sendai virus envelope glycoproteins as the substrate. Studies on deletion mutants of the C-terminal domain of MPI revealed that the minimal size of MPI required for the inhibition of tryptase Clara is the peptide Lys60-Ala107. Studies involving site-directed mutagenesis of the C-terminal domain of MPI indicated that the Leu72-Met73 site of MPI is the inhibitory site for tryptase Clara. Substitution of residue Leu72 with a basic amino acid significantly increased in the inhibitory activity of the C-terminal domain of MPI, but further substitution of residue Met73 with various amino acids in these mutants reduced the inhibitory activity. Since there is evidence suggesting that the concentration of MPI in respiratory fluid is insufficient for prevention of virus infection, the administration of MPI, the recombinant C-terminal domain of MPI, and their mutants, with residue Leu72 substituted with residues Arg72 and Lys72, may be useful for treatment of such pneumotropic virus infections.