North American Porcine Reproductive and Respiratory Syndrome Viruses Inhibit Type I Interferon Production by Plasmacytoid Dendritic Cells

Abstract

Although enveloped viruses typically trigger the prodigious secretion of alpha interferon (IFN-␣) by plasmacytoid dendritic cells (pDC), porcine pDC remain quiescent when exposed to porcine reproductive and respiratory syndrome virus (PRRSV). This inactivity is likely due to virus-mediated interference since the typical IFN-␣ response by either purified or nonsorted porcine pDC to transmissible gastroenteritis virus (TGEV) or the Toll-like receptor 9 agonist, oligodeoxynucleotide (ODN) D19, was markedly reduced in the presence of PRRSV. Suppression occurred independently of virus viability and acidification of pDC early endosomes but correlated with diminished levels of IFN-␣ mRNA. This change was attributed to an abrogation of transcription resulting from a decrease in the otherwise enhanced amounts of the requisite interferon regulatory factor 7 (IRF-7), whose gene expression in turn was limited as a consequence of a lessened availability of nuclear-localized signal transducer and activator of transcription 1 (STAT1). While PRRSV also inhibited tumor necrosis factor alpha (TNF-␣) synthesis by pDC responding to either agent, only the interleukin-2 (IL-2) and IL-6 production instigated by ODN D19 exposure was blocked. Likewise, PRRSV did not impact a specific TGEV-associated enhancement of IL-8 expression. Moreover, an augmented phosphorylation of NF-B seen in activated pDC was not only unaffected by PRRSV but actually occurred in its presence. Thus, as supported by a demonstrated resilience of pDC to PRRSV infection, this pathogen may interact with a cell surface protein(s) to selectively impede the completion of cascades involved in cytokine production by stimulated pDC.

Antiviral Regulation in Porcine Monocytic Cells at Different Activation States

“…Signature genes characteristic of the antiviral state include those involved in the activation of IRF-3/IRF-7 and stimulation of antiviral ISGs (2,7,10,12,15). Based on our studies and those of others, we know that induction of the antiviral state is common in all monocytic cells (2,10,17,18,21), but whether subsets of monocytes and Ms are preset for antiviral responses, such as pDCs showing a natural propensity of IFN-␣ production, is not known (11,24).…”

“…Because deviation of the IFN system has been well demonstrated in PRRSV infection and is a potential target for antiviral regulation (7,10,12,13,15,21), we first analyzed DEGs related to the IFN system, particularly type I IFN production and action in macrophages at differential activation statuses. In pigs, PRRSV strains exerted differential ability to inhibit the production and action of type I IFNs, particularly in IFN-␣ rather than IFN-␤ subtypes (21)(22)(23)(24)(25)34). We previously showed that the swine genome contains at least 39 functional genes encoding 7 subtypes of type I IFNs, including multiple members of the IFNs IFN-␣, Ϫ⌬CT from infected cells were plotted against that from control cells to indicate up-or downregulation and to reflect the relative expression.…”

“…These monocytotropic viruses include many of the world's most dreaded porcine viruses, such as African swine fever virus (ASFV), classical swine fever virus (CSFV), porcine circovirus 2 (PCV2), foot-and-mouth disease virus (FMDV), pseudorabies virus (PrV), swine influenza virus (SIV), and the focus of our studies, PRRSV (19,21,24,36). These viruses collectively cause the majority of swine viral infections and significant global economic losses.…”

“…For control cells, only BSA in DPBS was added to the cultures. Cells were then thoroughly washed and replenished with fresh medium for 16 h. Culture supernatants from an equal amount of cells from each treatment were collected, and secreted cytokines were measured using the SearchLight chemiluminescent multiplex assay (Aushon BioSystems, Billerica, MA) (24). Data were normalized as fold changes relative to the control cells and are presented as means of three independent replicates.…”

“…PRRSV is an enveloped positive-strand RNA virus that directly infects subsets of Ms and DCs and subverts immune responses in monocytic cells, making it ideal for deciphering how monocytic cell activation status interacts with antiviral immunity (21,(24)(25)(26)(27)(28). Monocytic cells are critical for providing early immune surveillance and bridging adaptive antiviral immunity (1-5, 7, 11).…”