Viral Determinants in H5N1 Influenza A Virus Enable Productive Infection of HeLa Cells.

Influenza A virus (IAV) is a human respiratory pathogen that causes yearly global epidemics, as well as sporadic pandemics due to human adaptation of pathogenic strains. Efficient replication of IAV in different species is, in part, dictated by its ability to exploit the genetic environment of the host cell. To investigate IAV tropism in human cells, we evaluated the replication of IAV strains in a diverse subset of epithelial cell lines. HeLa cells were refractory to the growth of human H1N1 and H3N2 viruses and low-pathogenic avian influenza (LPAI) viruses. Interestingly, a human isolate of the highly pathogenic avian influenza (HPAI) H5N1 virus successfully propagated in HeLa cells to levels comparable to those in a human lung cell line. Heterokaryon cells generated by fusion of HeLa and permissive cells supported H1N1 virus growth, suggesting the absence of a host factor(s) required for the replication of H1N1, but not H5N1, viruses in HeLa cells. The absence of this factor(s) was mapped to reduced nuclear import, replication, and translation, as well as deficient viral budding. Using reassortant H1N1:H5N1 viruses, we found that the combined introduction of nucleoprotein (NP) and hemagglutinin (HA) from an H5N1 virus was necessary and sufficient to enable H1N1 virus growth. Overall, this study suggests that the absence of one or more cellular factors in HeLa cells results in abortive replication of H1N1, H3N2, and LPAI viruses, which can be circumvented upon the introduction of H5N1 virus NP and HA. Further understanding of the molecular basis of this restriction will provide important insights into the virus-host interactions that underlie IAV pathogenesis and tropism.IMPORTANCE Many zoonotic avian influenza A viruses have successfully crossed the species barrier and caused mild to life-threatening disease in humans. While human-to-human transmission is limited, there is a risk that these zoonotic viruses may acquire adaptive mutations enabling them to propagate efficiently and cause devastating human pandemics. Therefore, it is important to identify viral determinants that provide these viruses with a replicative advantage in human cells. Here, we tested the growth of influenza A virus in a subset of human cell lines and found that abortive replication of H1N1 viruses in HeLa cells can be circumvented upon the introduction of H5N1 virus HA and NP. Overall, this work leverages the genetic diversity of multiple human cell lines to highlight viral determinants that could contribute to H5N1 virus pathogenesis and tropism.

Identification of Polo-like kinases as potential novel drug targets for influenza A virus.

In recent years genome-wide RNAi screens have revealed hundreds of cellular factors required for influenza virus infections in human cells. The long-term goal is to establish some of them as drug targets for the development of the next generation of antivirals against influenza. We found that several members of the polo-like kinases (PLK), a family of serine/threonine kinases with well-known roles in cell cycle regulation, were identified as hits in four different RNAi screens and we therefore studied their potential as drug target for influenza. We show that knockdown of PLK1, PLK3, and PLK4, as well as inhibition of PLK kinase activity by four different compounds, leads to reduced influenza virus replication, and we map the requirement of PLK activity to early stages of the viral replication cycle. We also tested the impact of the PLK inhibitor BI2536 on influenza virus replication in a human lung tissue culture model and observed strong inhibition of virus replication with no measurable toxicity. This study establishes the PLKs as potential drug targets for influenza and contributes to a more detailed understanding of the intricate interactions between influenza viruses and their host cells.

Fluorescence-Activated Cell Sorting-Based Analysis Reveals an Asymmetric Induction of Interferon-Stimulated Genes in Response to Seasonal Influenza A Virus.

UNLABELLED: Influenza A virus (IAV) infection provokes an antiviral response involving the expression of type I and III interferons (IFN) and IFN-stimulated genes (ISGs) in infected cell cultures. However, the spatiotemporal dynamics of the IFN reaction are incompletely understood, as previous studies investigated mainly the population responses of virus-infected cultures, although substantial cell-to-cell variability has been documented. We devised a fluorescence-activated cell sorting-based assay to simultaneously quantify expression of viral antigens and ISGs, such as ISG15, MxA, and IFIT1, in IAV-infected cell cultures at the single-cell level. This approach revealed that seasonal IAV triggers an unexpected asymmetric response, as the major cell populations expressed either viral antigen or ISG, but rarely both. Further investigations identified a role of the viral NS1 protein in blocking ISG expression in infected cells, which surprisingly did not reduce paracrine IFN signaling to noninfected cells. Interestingly, viral ISG control was impaired in cultures infected with avian-origin IAV, including the H7N9 virus from eastern China. This phenotype was traced back to polymorphic NS1 amino acids known to be important for stable binding of the polyadenylation factor CPSF30 and concomitant suppression of host cell gene expression. Most significantly, mutation of two amino acids within the CPSF30 attachment site of NS1 from seasonal IAV diminished the strict control of ISG expression in infected cells and substantially attenuated virus replication. In conclusion, our approach revealed an asymmetric, NS1-dependent ISG induction in cultures infected with seasonal IAV, which appears to be essential for efficient virus propagation. IMPORTANCE: Interferons are expressed by infected cells in response to IAV infection and play important roles in the antiviral immune response by inducing hundreds of interferon-stimulated genes (ISGs). Unlike many previous studies, we investigated the ISG response at the single-cell level, enabling novel insights into this virus-host interaction. Hence, cell cultures infected with seasonal IAV displayed an asymmetric ISG induction that was confined almost exclusively to noninfected cells. In comparison, ISG expression was observed in larger cell populations infected with avian-origin IAV, suggesting a more resolute antiviral response to these strains. Strict control of ISG expression by seasonal IAV was explained by the binding of the viral NS1 protein to the polyadenylation factor CPSF30, which reduces host cell gene expression. Mutational disruption of CPSF30 binding within NS1 concomitantly attenuated ISG control and replication of seasonal IAV, illustrating the importance of maintaining an asymmetric ISG response for efficient virus propagation.