Characterisation of Porcine Monocytes, Macrophages and Dendritic Cells and Their Susceptibility to Porcine Reproductive and Respiratory Syndrome Virus (PRRSV).

The effective control of porcine reproductive and respiratory syndrome virus (PRRSV), which causes substantial economic losses to the pig industry worldwide, is challenged by complicated host-pathogen interactions and delayed immunity. This study aimed to characterise myeloid immune cells associated with PRRSV (monocytes, macrophages, dendritic cells), and to assess their susceptibility to infection, in order to help identify underlying mechanisms that might facilitate PRRSV success. Cellular expression of putative PRRSV receptors CD163 (scavenger receptor) and CD169 (sialic-acid binding lectin), was central to investigations. Porcine monocytes isolated from peripheral blood were treated with various cytokines and macrophage activating factors before infection with Eastern European PRRSV strain Lena. IL-10 and dexamethasone (dexa) significantly up-regulated PRRSV replication, which correlated with increased CD163/CD169. M-CSF differentiated monocyte derived macrophages (MoM0s) were stimulated with activators for classical (LPS/IFN-γ) or alternative (IL-4) activation. GM-CSF and IL-4 generated monocyte derived dendritic cells (MoDCs) were activated with a maturation cocktail containing LPS, IFN-γ, IL-1β, IL-6, TNF-α and PGE2. Dexa and IL-10 were added to MoM0s and MoDCs to further assess their significance. Cells were characterised by morphology, phenotype and function, and PRRSV replication measured using flow cytometry and RT-qPCR. Analysis of porcine macrophage subsets highlighted some divergence from described human counterparts. MoDCs, however, appeared similar to mouse and human DCs, showing a MHCII hiCD80/86hiCD14low phenotype upon maturation. Infection studies revealed similar replication across activation states, and dexa and IL-10 significantly increased MoM0 susceptibility. MoDCs showed low replication, which was independent of CD163/CD169. These novel findings demonstrate the high variability of porcine myeloid cells. PRRSV tropism was not restricted to macrophages, and not always dependent on CD163/CD169. Cortisol appeared not to be associated with immunosuppression of myeloid cells, but supported PRRSV replication in monocytes and macrophages, a finding significant for future PRRSV control strategies and perhaps relevant to other porcine infectious diseases