Interferon-stimulated genes as resilience factors for PRRSV infection

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Porcine Reproductive and Respiratory Syndrome (PRRS) is a panzootic viral disease of pigs that causes major economic losses. The causative agent of PRRS, PRRS virus (PRRSV) is a rapidly evolving small enveloped RNA virus. Whilst improvements have been effected with changes in husbandry and vaccination, PRRS still has major impacts on pig health and welfare. PRRS accounts for about a third of the cost of infectious disease to the US pig industry amounting to $650M per year. PRRS is the most costly disease to pig industries of Europe, China and North America and new PRRSV variants have the potential to be even more devastating as demonstrated by recent outbreaks of highly pathogenic variants of the virus in China and Southeast Asia. The presence of endemic PRRSV also supresses immunity and increases susceptibility to other infections, leading to complex disease and increased use of antimicrobials. PRRSV has a narrow host cell tropism, limited to cells of the monocyte/macrophage lineage. Like all viruses PRRSV relies on the host cell machinery to replicate itself and generate new viral particles.There is growing evidence from in vitro and in vivo challenge experiments and field studies that there is host genetic variation in responses to and outcomes of PRRSV infection. In particular genes involved in the cellular immune response, the interferon (IFN) response, have been found to be candidates for genetic resilience. However, a general issue with animal challenge experiments and even field studies is that they usually have insufficient power to dissect genetic effects as there are too few animals, which makes them miss resilience variants.IFNs are a crucial first line of defence against viral infection, acting by triggering the expression and activation of a suite of antiviral proteins. They shape the adaptive immune response by triggering release of cytokines and chemokines. The IFN response is fundamentally involved in the host response to virus infections. Developing a better understanding of the IFN response and how viruses counteract its effects has important implications for how we treat viral infections and the development of vaccines. Type I IFNs are the main cytokines for innate immunity against viral infections and include various subtypes depending on the animal species. IFN-gamma is the sole representative of type II IFN. While type I IFNs are produced in most cell types in response to different viruses, type II IFN is produced by limited types of cells including natural killer cells, activated T lymphocytes, macrophages, and neurons.While the antiviral nature of IFN has long been documented, the specific mechanisms of how IFN inhibits different viruses is poorly understood. Recently, more than 500 human genes induced by IFN have been individually cloned, allowing overexpression and investigation of their effects on viruses. This has proven to be a particularly powerful approach, resulting in significant discoveries on the action of IFN in human cells. In the BBSRC grant propoasal "Identification of interferon stimulated genes that restrict cross-species transmission of influenza A virus" (BB/S00114X/1) we generated an arrayed type I ISG library for the pig. In this proposal we will combine this unique tool with cutting-edge pluripotent stem cell technology and a newly generated pig type II ISG library to identify genes playing a role in restricting PRRSV infection. These genes will be further investigated to assess whether they are suitable candidates for genetic selection or genome editing in pigs or may be targeted by antiviral drugs to combat PRRSV infection. This proposal will not only identify valid PRRS resilience genes or antiviral strategy but also generate valuable technical resources and protocols that can be used for other porcine pathogens, in particular ones infecting immune cells, such as African swine fever virus, salmonella typhimurium, or toxoplasma gondii.