Apoptosis is a hallmark event observed upon infection with many viral pathogens, including in¯uenza A virus. The apoptotic process is executed by a proteolytic system consisting of a family of cysteinyl proteases, termed caspases. Since the consequences of apoptosis induction and caspase activation for the outcome of an in¯uenza virus infection are not clear, we have addressed this issue by interfering with expression or function of a major virus-induced apoptosis effector, caspase 3. Surprisingly, in¯uenza virus propagation was strongly impaired in the presence of an inhibitor that blocks caspase 3 and in cells where caspase 3 was partially knocked down by small interfering RNAs. Consistent with these ®ndings, poor replication ef®-ciencies of in¯uenza A viruses in cells de®cient for caspase 3 could be boosted 30-fold by ectopic expression of the protein. Mechanistically, the block in virus propagation appeared to be due to retention of the viral RNP complexes in the nucleus, preventing formation of progeny virus particles. Our ®ndings indicate that caspase 3 activation during the onset of apoptosis is a crucial event for ef®cient in¯uenza virus propagation.

Activation of the transcription factor NF-B is a hallmark of infections by viral pathogens including influenza viruses. Because gene expression of many proinflammatory and antiviral cytokines is controlled by this factor, the concept emerged that NF-B and its upstream regulator IB kinase are essential components of the innate antiviral immune response to infectious pathogens. In contrast to this common view we report here that NF-B activity promotes efficient influenza virus production. On a molecular level this is due to NF-B-dependent viral induction of the proapoptotic factors tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) and FasL, which enhance virus propagation in an autocrine and paracrine fashion. Thus, NF-B acts both proapoptotically and provirally in the context of an influenza virus infection.

SummaryInfluenza is still one of the major plagues worldwide. The statistical likeliness of a new pandemic outbreak highlights the urgent need for new and amply available antiviral drugs. We and others have shown that influenza virus misuses the cellular IKK/NF-kB signalling pathway for efficient replication suggesting that this module may be a suitable target for antiviral intervention. Here we examined acetylsalicylic acid (ASA), also known as aspirin, a widely used drug with a well-known capacity to inhibit NF-kB. We show that the drug efficiently blocks influenza virus replication in vitro and in vivo in a mechanism involving impaired expression of proapoptotic factors, subsequent inhibition of caspase activation as well as block of caspase-mediated nuclear export of viral ribonucleoproteins. As ASA showed no toxic side-effects or the tendency to induce resistant virus variants, existing salicylate-based aerosolic drugs may be suitable as anti-influenza agents. This is the first demonstration that specific targeting of a cellular factor is a suitable approach for anti-influenza virus intervention.

Measles virus (MV) interacts