Activation of Interleukin-1β Release by the Classical Swine Fever Virus Is Dependent on the NLRP3 Inflammasome, Which Affects Virus Growth in Monocytes

Classical swine fever virus (CSFV) is a classic Flavivirus that causes the acute, febrile, and highly contagious disease known as classical swine fever (CSF). Inflammasomes are molecular platforms that trigger the maturation of proinflammatory cytokines to engage innate immune defenses that are induced upon cellular infection or stress. However, the relationship between the inflammasome and CSFV infection has not been thoroughly characterized. To understand the function of the inflammasome response to CSFV infection, we infected porcine peripheral blood monocytes (PBMCs) with CSFV. Our results indicated that CSFV infection induced both the generation of pro-interleukin-1β (pro-IL-1β) and its processing in monocytes, leading to the maturation and secretion of IL-1β through the activation of caspase 1. Moreover, CSFV infection in PBMCs induced the production and cleavage of gasdermin D (GSDMD), which is an inducer of pyroptosis. Additional studies showed that CSFV-induced IL-1β secretion was mediated by NLRP3 and that CSFV infection could sufficiently activate the assembly of the NLRP3 inflammasome in monocytes. These results revealed that CSFV infection inhibited the expression of NLRP3, and knockdown of NLRP3 enhanced the replication of CSFV. In conclusion, these findings demonstrate that the NLRP3 inflammasome plays an important role in the innate immune response to CSFV infection

Metabolic Profiles in Cell Lines Infected with Classical Swine Fever Virus

Viruses require energy and biosynthetic precursors from host cells for replication. An understanding of the metabolic interplay between classical swine fever virus (CSFV) and host cells is important for exploring the complex pathological mechanisms of classical swine fever (CSF). In the current study, and for the first time, we utilized an approach involving gas chromatography coupled with mass spectrometry (GC-MS) to examine the metabolic profiles within PK-15 and 3D4/2 cells infected with CSFV. The differential metabolites of PK-15 cells caused by CSFV infection mainly included the decreased levels of glucose 6-phosphate [fold change (FC) = −1.94)] and glyceraldehyde-3-phosphate (FC = −1.83) during glycolysis, ribulose 5-phosphate (FC = −1.51) in the pentose phosphate pathway, guanosine (FC = −1.24) and inosine (FC = −1.16) during purine biosynthesis, but the increased levels of 2-ketoisovaleric acid (FC = 0.63) during the citrate cycle, and ornithine (FC = 0.56) and proline (FC = 0.62) during arginine and proline metabolism. However, metabolite changes caused by CSFV infection in 3D4/2 cells included the reduced glyceraldehyde-3-phosphate (FC = −0.77) and pyruvic acid (FC = −1.42) during glycolysis, 2-ketoglutaric acid (FC = −1.52) in the citrate cycle, and the elevated cytosine (FC = 2.15) during pyrimidine metabolism. Our data showed that CSFV might rebuild cellular metabolic programs, thus aiding viral replication. These findings may be important in developing targets for new biomarkers for the diagnosis and identification of enzyme inhibitors or metabolites as antiviral drugs, or screening viral gene products as vaccines

CSFV Infection Up-Regulates the Unfolded Protein Response to Promote Its Replication

Classical swine fever (CSF) is an OIE-listed, highly contagious animal disease caused by classical swine fever virus (CSFV). The endoplasmic reticulum (ER) is an organelle in which the replication of many RNA viruses takes place. During viral infection, a series of events elicited in cells can destroy the ER homeostasis that cause ER stress and induce an unfolded protein response (UPR). In this study, we demonstrate that ER stress was induced during CSFV infection as several UPR-responsive elements such as XBP1(s), GRP78 and CHOP were up-regulated. Specifically, CSFV transiently activated IRE1 pathway at the initial stage of infection but rapidly switched off, likely due to the reduction in cytoplasm Ca2+ after viral incubation. Additionally, our data show that the ER stress induced by CSFV can promote CSFV production, which the IRE1 pathway play an important role in it. Evidence of ER stress in vivo was also confirmed by the marked elevation of GRP78 in CSFV-infected pig PBMC and tissues. Collectively, these data indicate that the ER stress was induced upon CSFV infection and that the activation of the IRE1 pathway benefits CSFV replication