Host cell proteases that cleave the hemagglutinin (HA) of influenza viruses in the human respiratory tract are still not identified. Here we cloned two human type II transmembrane serine proteases with known airway localization, TMPRSS2 and HAT, into mammalian expression vector. Cotransfection of mammalian cells with plasmids encoding HA and either protease resulted in HA cleavage in situ. Transient expression of either protease in MDCK cells enabled multicycle replication of influenza viruses in these cells in the absence of exogenous trypsin. These data suggest that TMPRSS2 and HAT are candidates for proteolytic activation of influenza viruses in vivo.The ability of the hemagglutinin protein (HA) of influenza viruses to mediate fusion between viral and endosomal membranes during virus entry into the cell depends on cleavage of fusion-incompetent precursor HA0 into disulfide-linked subunits HA1 and HA2 by a host endoprotease. Cleavage of HA is essential for infection and determines viral pathogenicity and tissue tropism (reviewed in references 8, 10, 11, and 22). Thus, the highly pathogenic avian influenza viruses of subtypes H5 and H7 are cleaved at the multibasic motif R-X-R/K-R by ubiquitous subtilisin-like cellular proteases (11, 23) and cause lethal systemic infection in birds. All other influenza A viruses, including human epidemic and pandemic strains, have a single arginine at the HA cleavage site; these viruses can only be cleaved in a limited number of tissues, such as the intestinal tract in birds and the respiratory tract in birds and mammals (11,22).Early studies demonstrated that influenza viruses with monobasic cleavage site can be proteolytically activated in cell culture by the addition of trypsin (12, 13). Less is known about proteases that cleave influenza viruses under conditions of natural infection. Several trypsin-like proteases isolated from rat and swine lung were shown to support replication of influenza viruses in vitro (3,9,18,25). However, it remains unclear whether these proteases play a role in in vivo infection. In the case of human influenza, specific proteases responsible for HA cleavage in the human respiratory tract have not been identified thus far.In search of such proteases, we use a new approach. Instead of isolating and characterizing influenza virus-activating enzymes from respiratory tissues, we clone and express genes of trypsin-like proteases known to be present in the human airway epithelium and test them for cleavage of HA. We have analyzed here two such proteases, TMPRSS2 (5, 14, 21) and HAT (human airway trypsin-like protease) (4, 24, 27, 28), given their previous detection in the human airways and the availability of their full-length coding sequences. As the source of human genetic material for cloning, we used differentiated cultures of human airway epithelial cells grown at the air-liquid interface in serum-free, hormone-and growth factor-supplemented medium as previously described (6,15). These cultures support multicycle replication of human influenza virus...

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Host cellular proteases induce influenza virus entry into cells by cleaving the viral surface envelope glycoprotein hemagglutinin (HA). However, details on the cellular proteases involved in this event are not fully available. We report here that ubiquitous type II transmembrane serine proteases, MSPL and its splice variant TMPRSS13, are novel candidates for proteases processing HA proteins of highly pathogenic avian influenza (HPAI) viruses, apart from the previously identified furin and proprotein convertases 5 and 6. HAs from all HPAI virus H5 and H7 strains have one of two cleavage site motifs, the R-X-K/R-R motif with R at position P4 and the K-K/R-K/T-R motif with K at position P4. In studies of synthetic 14-residue HPAI virus HA peptides with these cleavage site motifs, furin preferentially cleaved only HA peptides with the R-K-K-R motif in the presence of calcium and not peptides with the other motif, whereas MSPL and TMPRSS13 cleaved both types of HA peptides (those with the R/K-K-K-R motif) efficiently in the absence of calcium. Full-length recombinant HPAI virus HA with the K-K-K-R cleavage motif exhibited poor susceptibility to cleavage in the absence of MSPL or TMPRSS13 and the presence of furin in infected cells, but it was converted to mature HA subunits in transfected cells expressing MSPL or TMPRSS13, with membrane-fused giant-cell formation. This conversion and membrane fusion were suppressed by inhibitors of MSPL and TMPRSS13. Furthermore, infection with and multiplication of genetically modified live HPAI virus A/Crow/Kyoto/53/2004 (H5N1) with the K-K-K-R cleavage site motif were detected only in MSPL- and TMPRSS13-expressing cells.

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