Porcine Reproductive and Respiratory Syndrome Virus Induces Interleukin-15 through the NF-κB Signaling Pathway

Porcine reproductive and respiratory syndrome virus (PRRSV) mainly infects macrophages/dendritic cells and modulates cytokine expression in these cells. Interleukin-15 (IL-15) is a pleiotropic cytokine involved in wide range of biological activities. It has been shown to be essential for the generation, activation, and proliferation of NK and NKT cells and for the survival and activation of CD8(+) effector and memory T cells. In this study, we discovered that PRRSV infection upregulated IL-15 production at both the mRNA and protein levels in porcine alveolar macrophages (PAMs), blood monocyte-derived macrophages (BMo), and monocyte-derived dendritic cells (DCs). We subsequently demonstrated that the NF-kappa B signaling pathway was essential for PRRSV infection-induced IL-15 production. First, addition of an NF-kappa B inhibitor drastically reduced PRRSV infection-induced IL-15 production. We then found that NF-kappa B was indeed activated upon PRRSV infection, as evidenced by I kappa B phosphorylation and degradation. Moreover, we revealed an NF-kappa B binding motif in the cloned porcine IL-15 (pIL-15) promoter, deletion of which abrogated the pIL-15 promoter activity in PRRSV-infected alveolar macrophages. In addition, we demonstrated that PRRSV nucleocapsid (N) protein had the ability to induce IL-15 production in porcine alveolar macrophage cell line CRL2843 by transient transfection, which was mediated by its multiple motifs, and it also activated NF-kappa B. These data indicated that PRRSV infection-induced IL-15 production was likely through PRRSV N protein-mediated NF-kappa B activation. Our findings provide new insights into the molecular mechanisms underling the IL-15 production induced by PRRSV infection.