Abstract
Porcine reproductive and respiratory syndrome viruses (PRRSV) cause the most economically significant disease of swine. The emergence of highly pathogenic strains and the limitations of current vaccines pose significant challenges to PRRS control. An improved understanding of pathogenesis is needed to support the development of improved vaccines. Experimental infection with virulent PRRSV-1 subtype 3 strains is associated with enhanced immune responses, which have been attributed to the more effective viral clearance compared to ‘typical’ PRRSV-1 subtype 1 strains. Previous ex vivo data has also shown that this increased virulence correlates with a broader cell tropism for cells of the myeloid lineage. The causation of this correlation was investigated by assessing whether European PRRSV-1 strains can infect dendritic cells (DC), which are not canonically a target cell. A staining strategy was optimised to enrich and phenotype blood-derived DC (BDC) subsets. Infection experiments demonstrated that BDC were not susceptible, and that cytokine responses in enriched BDC culture were suppressed by inoculation of subtype 3 strain Lena. Further characterisations of host responses instead used monocyte-derived macrophages (MΦ) and DC. Functional assays determined that pro-inflammatory responses were increased by subtype 3 strain SU1-Bel more than subtype 1 strains, including modulation of surface activation markers, ability to prime allogeneic T cell proliferation, and secretion of inflammatory cytokines; Lena, however, did not induce pro-inflammatory cytokine responses but decreased endocytic and phagocytic capacity. This trend was validated by transcriptomic analysis of infected monocyte-derived MΦ, which identified putative genes of interest for future studies, including pattern recognition receptors. Inflammasome treatments suggested that SU1-Bel primed inflammasome activation and that subtype 3 strain replication was decreased by inflammasome inhibition, proportionately increasing MΦ viability. Finally, preliminary trials of a novel macrophage cell line demonstrated its utility for further investigation of the mechanistic basis for the enhanced virulence of PRRSV-1 subtype 3 strains.