PRRSV Vaccine Strain-Induced Secretion of Extracellular ISG15 Stimulates Porcine Alveolar Macrophage Antiviral Response against PRRSV

Porcine reproductive and respiratory syndrome virus (PRRSV) has disrupted the global swine industry since the 1980s. PRRSV-host interactions are largely still unknown but may involve host ISG15 protein. In this study, we developed a monoclonal antibody (Mab-3D5E6) specific for swine ISG15 (sISG15) by immunizing mice with recombinant sISG15. A sandwich enzyme-linked immunosorbent assay (ELISA) incorporating this sISG15-specific Mab was developed to detect sISG15 and provided a lower limit of sISG15 detection of 200 pg/mL. ELISA results demonstrated that infection of porcine alveolar macrophages (PAMs) with low-virulence or attenuated PRRSV vaccine strains induced intracellular ISG15 expression that was independent of type I IFN production, while PAMs infection with a PRRSV vaccine strain promoted extracellular ISG15 secretion from infected PAMs. Conversely, the addition of recombinant sISG15 to PAMs mimicked natural extracellular ISG15 effects whereby sISG15 functioned as a cytokine by activating PAMs. Once activated, PAMs could inhibit PRRSV replication and resist infection with PRRSV vaccine strain TJM. In summary, a sandwich ELISA incorporating homemade anti-ISG15 Mab detected ISG15 secretion induced by PAMs infection with a PRRSV vaccine strain. Recombinant ISG15 added to cells exhibited cytokine-like activity that stimulated PAMs to assume an anti-viral state that enabled them to inhibit PRRSV replication and resist viral infection.