The natural response of pigs to porcine reproductive and respiratory syndrome virus (PRRSV) infections and vaccinations needs to be altered so that better protection is afforded against both homologous and heterologous challenges by this pathogen. To address this problem, real-time gene expression assays were coupled with cytokine Elispot and protein analyses to assess the nature of the anti-PRRSV response of pigs immunized with modified live virus (MLV) vaccine. Although T helper 1 (Th1) immunity was elicited in all vaccinated animals, as evidenced by the genesis of PRRSV-specific interferon-gamma secreting cells (IFNG SC), the overall extent of the memory response was variable and generally weak. Peripheral blood mononuclear cells (PBMC) isolated from these pigs responded to PRRSV exposure with a limited increase in their expression of the Th1 immune markers, IFNG, tumor necrosis factor-alpha and interleukin-15 (IL15), and a reduction in the quantity of mRNAs encoding the innate and inflammatory proteins, IL1B, IL8 and IFNA. Efforts to enhance Th1 immunity, by utilizing an expression plasmid encoding porcine IFNA (pINA) as an adjuvant, resulted in a temporary increase in the frequency of PRRSV-specific IFNG SC but only minor changes overall in the expression of Th1 associated cytokine or innate immune marker mRNA by virus-stimulated PBMC. Administration of pINA, however, did correlate with decreased IL1B secretion by cultured, unstimulated PBMC but had no effect on their ability to release IFNG. Thus, while exogenous addition of IFNA during PRRSV vaccination has an impact on the development of a Th1 immune response, other alterations will be required for substantial boosting of virus-specific protection.

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.

Plausible representatives of plasmacytoid dendritic cells (pDCs) in pigs have been characterized as being CD4(hi)CD172(lo). Due to their paucity in blood, we utilized novel fluorescent-activated cell sorting procedures to isolate them from PBMC. The resultant subset was greater than 98% homogeneous in regards to the selected phenotype and contained the preponderance of individuals secreting IFN-alpha after exposure to a known stimulant, transmissible gastroenteritis virus (TGEV). In addition to being a potent source of IFN-alpha, other properties of these porcine CD4(hi)CD172(lo) cells including their morphological transition from a plasma cell-like shape during quiescence to one resembling a dendritic cell (DC) after activation by TGEV and their relatively strong constitutive expression of interferon regulatory factor-7 (IRF-7) conformed to the expectations of genuine pDCs. While a substantial IFN-alpha response was also elicited from the porcine pDCs by pseudorabies virus (PrV), swine influenza virus (SIV), and TLR7 and 9 agonists, there was an agent-dependent induction of varying amounts of IL-2, IL-6, IL-8, IL-12, IFN-gamma, and TNF-alpha. Notably, porcine reproductive and respiratory syndrome virus (PRRSV) failed to provoke the pDCs to secrete any of the measured cytokines except IL-2. Moreover, whereas pDCs exposed to TGEV or the TLR9 agonist rapidly increased IRF-7 production and morphed into DCs with enhanced CD80/86 expression, similar alterations were not observed during incubation with PRRSV. This atypical response of pDCs to PRRSV may contribute to its pathogenesis, which unlike that associated with PrV, SIV or TGEV includes persistent infection and limited development of protective immunity.

Porcine reproductive and respiratory syndrome virus (PRRSV) infects alveolar macrophages (AMΦ) causing dysregulated interferon (IFN)-α and tumor necrosis factor (TNF)-α production through a mechanism(s) yet to be resolved. Here, we show that AMΦ infected with PRRSV secreted a reduced quantity of IFN-α following the cell exposure to synthetic dsRNA. This reduction did not correlate with reduced IFNA1 gene transcription. Rather, it coincided with two events that occurred late during infection and were indicative of translational attenuation, specifically, the activation of eukaryotic translation initiation factor 2α (eIF2α), and the appearance of stress granules. Notably, the typical rapid production of TNF-α by AMΦ exposed to lipopolysaccharide (LPS) was suppressed or enhanced by PRRSV depending on when the LPS exposure occurred after virus infection. If exposure was delayed until 6 h post-infection (hpi) so that the development of the cytokine response coincided with the time in which phosphorylation of eIF2α by the stress sensor PERK (protein kinase RNA (PKR)-like ER kinase) occurred, inhibition of TNF-α production was observed. However, if LPS exposure occurred at 2 hpi, prior to a detectable onset of eIF2α phosphorylation, a synergistic response was observed due to the earlier NF-κB activation via the stress sensor IRE1α (inositol-requiring kinase 1α). These results suggest that the asynchronous actions of two branches of the unfolded protein response (UPR), namely IRE1α, and PERK, activated by ER stress resulting from the virus infection, are associated with enhancement or suppression of TNF-α production, respectively. The activation of AMΦ is controlled by its microenvironment to deter excessive pro-inflammatory cytokine responses to microbes that could impair lung function. However, viral pneumonias frequently become complicated by secondary bacterial infections triggering severe inflammation, lung dysfunction, and death. Although dysregulated cytokine production is considered an integral component of the exacerbated inflammatory response in viral-bacterial co-infections, the mechanism responsible for this event is unknown. Here, we show that PRRSV replication in porcine AMΦ triggers activation of the IRE1α branch of the UPR, which causes a synergistic TNF-α response to LPS exposure. Thus, the severe pneumonias typically observed in pigs afflicted with PRRSV-bacterial co-infections could result from dysregulated, overly robust TNF-α production to opportunistic pathogens that is not commensurate with the typical restrained reaction by uninfected AMΦ. This notion could help design therapies to mitigate the severity of viral and bacterial co-infections.

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