Impact of genotype 1 and 2 of porcine reproductive and respiratory syndrome viruses on interferon-α responses by plasmacytoid dendritic cells

Porcine reproductive and respiratory syndrome (PRRS) virus (PRRSV) infections are characterized by prolonged viremia and viral shedding consistent with incomplete immunity. Type I interferons (IFN) are essential for mounting efficient antiviral innate and adaptive immune responses, but in a recent study, North American PRRSV genotype 2 isolates did not induce, or even strongly inhibited, IFN-alpha in plasmacytoid dendritic cells (pDC), representing "professional IFN-alpha-producing cells". Since inhibition of IFN-alpha expression might initiate PRRSV pathogenesis, we further characterized PRRSV effects and host modifying factors on IFN-alpha responses of pDC. Surprisingly, a variety of type 1 and type 2 PRRSV directly stimulated IFN-alpha secretion by pDC. The effect did not require live virus and was mediated through the TLR7 pathway. Furthermore, both IFN-gamma and IL-4 significantly enhanced the pDC production of IFN-alpha in response to PRRSV exposure. PRRSV inhibition of IFN-alpha responses from enriched pDC stimulated by CpG oligodeoxynucleotides was weak or absent. VR-2332, the prototype genotype 2 PRRSV, only suppressed the responses by 34%, and the highest level of suppression (51%) was induced by a Chinese highly pathogenic PRRSV isolate. Taken together, these findings demonstrate that pDC respond to PRRSV and suggest that suppressive activities on pDC, if any, are moderate and strain-dependent. Thus, pDC may be a source of systemic IFN-alpha responses reported in PRRSV-infected animals, further contributing to the puzzling immunopathogenesis of PRRS.