NS-segment-based reporter influenza A viruses: engineering strategy, applications, and limitations

Reporter-expressing recombinant influenza A viruses (IAVs) have emerged as powerful tools for studying viral infection, host-pathogen interactions, and the identification and characterization of prophylactic and/or therapeutic countermeasures for the treatment of IAV infections. By incorporating a reporter gene, such as fluorescent, recombinase, or luciferase proteins, into the viral genome, these genetically engineered recombinant IAVs enable real-time visualization and quantification of infection in vitro, ex vivo, and in vivo to detect the presence of IAV in infected cells or animal models of infection. Among the various approaches to express reporter genes from the viral genome, fusion of fluorescent or luciferase proteins to the C-terminus of the non-structural 1 (NS1) protein has been successfully used to generate replication-competent reporter-expressing IAVs. This minireview summarizes the strategies used to generate IAV-expressing reporter genes, particularly from the non-structural (NS) viral segment 8, and their use in real-time tracking of viral replication and pathogenesis. These replication-competent reporter-expressing IAVs also provide an efficient tool for high-throughput screening (HTS) of novel anti-IAV therapeutics. We also discuss the advantages and limitations of current fluorescent and luciferase reporter-expressing IAVs to track viral infections in cultured cells and/or animal models, and provide perspectives on future directions in this rapidly advancing field.