Host-virus interactions in porcine reproductive and respiratory syndrome virus infection
Sorensen, George Edwin Peter
Porcine reproductive and respiratory syndrome virus (PRRSV) is a rapidly evolving virus that has significant economic and welfare implications for the pig industry. Vaccination strategies have proved largely ineffective in controlling PRRSV, in some cases even reverting to virulence. An increasing body of evidence suggests a host genetic basis for PRRSV resistance so there is a need to examine the role of host genetics in a biologically relevant in vitro cell culture system. However, PRRSV research is inhibited by the current scarcity of suitable in vitro culture systems. With the aim of developing a convenient in vitro model, porcine bone marrow-derived macrophages (BMDM) were evaluated as a PRRSV cell culture system. BMDM were found to be highly permissive to Type I PRRSV and amenable to genetic manipulation. BMDM proved to be excellent cells for virus production, producing significantly higher titres of PRRSV than commonly used alternative cell types. Surprisingly, PRRSV entry into BMDM was found to be independent of both the prototypic PRRSV receptors, CD163 and CD169, providing further evidence for the existence of alternate PRRSV entry mechanisms in primary cell types. To explore the genetics of pig susceptibility to PRRSV, network-based analysis of host transcriptional datasets, following PRRSV challenge, revealed important differences in co-regulated gene pathways between samples from pigs with different PRRSV-permissiveness. These pathways included genes with important, recently characterised, anti-pathogen activities. The incorporation of network-based transcriptional analysis and published genetic variation data led to the identification of a member of the guanlyate binding protein family, GBP-1, as a candidate host gene involved in controlling PRRSV replication. Overexpression of GBP-1 in BMDM revealed a significant anti-PRRSV function for this protein. Further investigation of published genetic variation in GBP-1 suggested a potential role of this gene in PRRSV tolerance. The results presented in this thesis provide evidence for an alternate PRRSV entry pathway in a biologically relevant cell type. The discovery of a highly PRRSV-infectable cell type with potential for genetic manipulation adds a useful new tool to the area of PRRSV research. The identification of GBP-1 as a novel anti-viral protein with a significant inhibitory effect on PRRSV infection, together with genetic variation in this gene, prompts further research into the genetic basis for PRRSV resistance.