1) Human host factors required for influenza virus entry
Influenza viruses are of high medical concern in humans and can cause devastating economic problems for the poultry and pig livestock industries. Currently, we have vaccines and antiviral drugs available, but both come with severe limitations. In particular, vaccines cannot protect against novel strains of influenza virus and must be continually updated. Furthermore, we experience increasing problems with drug-resistance of influenza viruses, and new antivirals with lower chances of resistance developing are urgently sought. A novel strategy currently being explored in the field is to target new antiviral drugs against host cell proteins that the virus requires for its replication. To identify such potential drug targets a detailed understanding of virus-host interactions at a molecular level is needed.
Our research focuses on the entry of influenza virus into its host cell, and the virus-host interactions required during this process. This is the first key stage of infection that all influenza viruses must accomplish, and is therefore an excellent target for antiviral drugs. With our work we aim to identify host factors involved in the entry process and characterize their mode of action with the overall goal of revealing novel drug targets. We employ RNAi screening methods to identify candidate factors that are then followed up in assays measuring the individual steps of viral entry. Moreover, we use proteomic approaches to unravel the phosphorylation-dependent regulatory changes that occur in the host cell early in influenza virus infection and determine their role for virus entry.
2) Potent restriction of influenza virus entry by IFITM proteins
Cells have elaborate strategies to defend themselves against invading viruses. Already more than 50 years ago, Isaacs and Lindenmann discovered that cells can produce a factor that can interfere with virus replication and named this soluble factor interferon (IFN). Meanwhile it has become clear that the IFN system is a crucial part of the innate immune response that controls virus infections until the adaptive immune response takes over. Binding of IFN to its receptor on the cell surface induces a signaling cascade that results in the upregulation of hundreds of so-called interferon-stimulated genes (ISGs). Surprisingly, we only have a few examples where we understand how such ISGs protect cells from virus infection.
Recently, the interferon-inducible transmembrane proteins (IFITMs) were identified as ISGs with potent activity against influenza A virus and it was found that the IFITMs inhibit entry of the virus into host cells. The antiviral mechanism of the IFITMs is not yet fully understood but it has become clear that viral fusion is targeted by these proteins. While we have a good understanding of the antiviral potential of human and mouse IFITMs, much less is known about the activity of IFITMs in other species. Given the important role of pigs and birds in the zoonotic cycle of influenza viruses we aim to elucidate the antiviral potential of swine and avian IFITM proteins.
3) Meta-analysis of RNAi screens for host factors of influenza A virus
In recent years several RNAi screens have been performed to unravel host factors necessary for influenza A virus replication. Surprisingly, there is only little overlap between the hit lists of the screens. Different set-ups of the screening assays with regard to virus strain, cell line or siRNA library can only partially explain the lack of broader overlap. It is assumed that the analysis of the raw data introduces bias and can therefore also contribute to the divergence of the results. In an attempt to analyze this problem and identify more overlapping hits we collaborate with Dr. Sumit Chanda (Sanford-Burnham Medical Research Institute, San Diego, U.S.) and Dr. Renate König (Paul Ehrlich-Institute, Langen, Germany). We obtained the primary data sets from five different RNAi screens and analyzed them with the same bioinformatic procedure. The resulting hit lists of the different screens suggested a much wider overlap between screens than seen previously. We are now validating these results experimentally and aim to identify host factors that influenza virus relies on across different experimental set-ups as these factors could represent promising drug targets.