Epstein-Barr virus causes most cases of infectious mononucleosis and posttransplant lymphoproliferative disease. It contributes to several types of cancer, including Hodgkin's lymphoma, Burkitt's lymphoma, diffuse large B cell lymphoma, nasopharyngeal carcinoma, and gastric carcinoma. EBV genome variation is important because some of the diseases associated with EBV have very different incidences in different populations and geographic regions, and differences in the EBV genome might contribute to these diseases. Some specific EBV genome alterations that appear to be significant in EBV-associated cancers are already known, and current efforts to make an EBV vaccine and antiviral drugs should also take account of sequence differences in the proteins used as targets.

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Cellular intrinsic immunity, mediated by the expression of an array of interferon-stimulated antiviral genes, is a vital part of host defence. We have previously used a bioinformatic screen to identify two interferon stimulated genes (ISG) with poorly characterised function, Interferon-induced protein 44 (IFI44) and interferon-induced protein 44-like (IFI44L), as potentially being important in Respiratory Syncytial Virus (RSV) infection. Using overexpression systems, CRISPR-Cas9-mediated knockout, and a knockout mouse model we investigated the antiviral capability of these genes in the control of RSV replication. Over-expression of IFI44 or IFI44L was sufficient to restrict RSV infection at an early time post infection. Knocking out these genes in mammalian airway epithelial cells increased levels of infection. Both genes express antiproliferative factors that have no effect on RSV attachment but reduce RSV replication in a minigenome assay. The loss of Ifi44 was associated with a more severe infection phenotype in a mouse model of infection. These studies demonstrate a function for IFI44 and IFI44L in controlling RSV infection. IMPORTANCE RSV infects all children under two years of age, but only a subset of children get severe disease. We hypothesize that susceptibility to severe RSV necessitating hospitalization in children without pre-defined risk factors is in part mediated at the anti-viral gene level. But there is a large array of anti-viral genes, particularly in the ISG family about which the mechanism is poorly understood. Having previously identified IFI44 and IFI44L as possible genes of interest in a bioinformatic screen, we dissected the function of these two genes in the control of RSV. Through a range of over-expression and knockout studies we show that the genes are anti-viral and anti-proliferative. This study is important because IFI44 and IFI44L are upregulated after a wide range of viral infections and IFI44L can serve as a diagnostic bio-marker of viral infection.

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