The severe acute respiratory syndrome (SARS) pandemic revealed that zoonotic transmission of animal coronaviruses (CoV) to humans poses a significant threat to public health and warrants surveillance and the development of countermeasures. The activity of host cell proteases, which cleave and activate the SARS-CoV spike (S) protein, is essential for viral infectivity and constitutes a target for intervention. However, the identities of the proteases involved have been unclear. Pioneer studies identified cathepsins and type II transmembrane serine proteases as cellular activators of SARS-CoV and demonstrated that several emerging viruses might exploit these enzymes to promote their spread. Here, we will review the proteolytic systems hijacked by SARS-CoV for S protein activation, we will discuss their contribution to viral spread in the host and we will outline antiviral strategies targeting these enzymes. This paper forms part of a series of invited articles in Antiviral Research on “From SARS to MERS: 10 years of research on highly pathogenic human coronaviruses.”

The type II transmembrane serine protease (TTSP) TMPRSS2 cleaves and activates the influenza virus and coronavirus surface proteins. Expression of TMPRSS2 is essential for the spread and pathogenesis of H1N1 influenza viruses in mice. In contrast, H3N2 viruses are less dependent on TMPRSS2 for viral amplification, suggesting that these viruses might employ other TTSPs for their activation. Here, we analyzed TTSPs, reported to be expressed in the respiratory system, for the ability to activate influenza viruses and coronaviruses. We found that MSPL and, to a lesser degree, DESC1 are expressed in human lung tissue and cleave and activate the spike proteins of the Middle East respiratory syndrome and severe acute respiratory syndrome coronaviruses for cell-cell and virus-cell fusion. In addition, we show that these proteases support the spread of all influenza virus subtypes previously pandemic in humans. In sum, we identified two host cell proteases that could promote the amplification of influenza viruses and emerging coronaviruses in humans and might constitute targets for antiviral intervention. IMPORTANCEActivation of influenza viruses by host cell proteases is essential for viral infectivity and the enzymes responsible are potential targets for antiviral intervention. The present study demonstrates that two cellular serine proteases, DESC1 and MSPL, activate influenza viruses and emerging coronaviruses in cell culture and, because of their expression in human lung tissue, might promote viral spread in the infected host. Antiviral strategies aiming to prevent viral activation might thus need to encompass inhibitors targeting MSPL and DESC1.

Defective interfering particles (DIPs) typically contain a highly deleted form of the viral genome, rendering them defective in virus replication. Yet upon complementation through coinfection with fully infectious standard virus (STV), interference with the viral life cycle can be observed, leading to suppressed STV replication and the release of mainly noninfectious DIPs. Interestingly, recent research indicates that DIPs may serve as an antiviral agent. Here we report the discovery of a yet-unknown type of influenza A virus-derived DIP (termed “OP7” virus) that contains numerous point mutations instead of large deletions in its genome. Furthermore, the underlying principles that render OP7 virions interfering and apparently defective seem to differ from those of conventional DIPs. In conclusion, we believe that OP7 virus might be a promising candidate for antiviral therapy. Moreover, it exerts strong effects, both on virus replication and on the host cell response, and may have been overlooked in other IAV preparations.

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The influenza virus hemagglutinin (HA) facilitates viral entry into target cells. Cleavage of HA by host cell proteases is essential for viral infectivity and the responsible enzymes are potential targets for antiviral intervention. The type II transmembrane serine protease (TTSP) TMPRSS2 has been identified as an HA activator in cell culture and in the infected host. However, it is less clear whether TMPRSS2-related enzymes can also activate HA for spread in target cells. Moreover, the activity of cellular serine protease inhibitors against HA-activating TTSPs is poorly understood. Here, we show that TMPRSS11A, another member of the TTSP family, cleaves and activates the influenza A virus (FLUAV) HA and the Middle East respiratory syndrome coronavirus spike protein (MERS-S). Moreover, we demonstrate that TMPRSS11A is expressed in murine tracheal epithelium, which is a target of FLUAV infection, and in human trachea, suggesting that the protease could support FLUAV spread in patients. Finally, we show that HA activation by the TMPRSS11A-related enzymes HAT and DESC1 but not TMPRSS11A itself is blocked by the cellular serine protease Influenza viruses, members of the family Orthomyxoviridae, are enveloped viruses with a negative sense, segmented RNA genome. Influenza viruses readily adapt to immune pressure and thus constantly circulate in the human population, causing annual influenza epidemics (1,2). Moreover, reassortment of genomic segments between different influenza A viruses can result in the emergence of antigenically new viruses, which may spread pandemically (1,2). The constantly changing nature of influenza viruses also compromises the efficacy of currently available antivirals, since the viruses can rapidly acquire resistance-conferring mutations (3). This may not be the case if host cell factors were targeted, which are required for viral spread but dispensable for cellular survival, and the identification of such factors is in the focus of current research efforts (4).The viral hemagglutinin protein (HA) is incorporated into the viral envelope and facilitates influenza virus entry into target cells (5,6). For this, the surface unit of HA, termed HA1, binds to sialic acids on cell surface receptors and stimulates viral uptake into host cell endosomes. Thereafter, the transmembrane unit HA2 fuses the viral and the endosomal membrane, allowing the delivery of the viral genomic information into the cellular cytoplasm (5,6). HA is synthesized as an inactive precursor protein, HA0, and cleavage of HA0 by host cell proteases primes HA for membrane fusion, which is triggered by endosomal low pH (7,8). The priming of HA, which, for historic reasons, will subsequently be referred to as HA activation, is essential for viral infectivity.As a consequence, the host cell proteases responsible for HA activation are potential targets for antiviral intervention.Evidence is constantly accumulating that the type II transmembrane serine protease TMPRSS2 plays an important role in HA activation. Thus, TMPRSS2 activates H...