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NADC34-like porcine reproductive and respiratory syndrome virus (PRRSV) has become endemic in some provinces of China and caused huge economic losses to local pig industry. The increased reports of NADC34-like PRRSV outbreaks in vaccinated pig herds suggest the limited protection of immunization with commercial vaccines. In this study, we evaluated a commercial PRRSV vaccine that has been widely used in China against the challenge of JS2021NADC34 PRRSV, a highly pathogenic Chinese NADC34-like strain isolated in 2021. The vaccinated pigs developed PRRSV-specific antibody responses, as shown by IDEXX ELISA results. After JS2021NADC34 PRRSV challenge, the vaccinated pigs had low level of viremia but suffered pathological lesions in lungs and lymphoid tissues. The viral antigens were also detected in the above tissues of the vaccinated pigs by immunohistochemistry staining. One out of five pigs in vaccinated group died at 13 days postchallenge. The above results suggested that the commercial PRRSV vaccine could not provide complete protection to the NADC34-like PRRSV infection.

Neither inactivated nor attenuated vaccines can effectively prevent and control the infection and spread of porcine reproductive and respiratory syndrome virus (PRRSV). Therefore, it is necessary to broaden new horizons and to conceive effective preventive strategies. The main components of Tea polyphenol (TPP) are catechins and their derivatives. TPP has many physiological activities and has certain antiviral and antifungal effects. However, whether TPP shows anti-PRRSV activity remains unclear. We found that TPP effectively inhibited PRRSV infection in Marc-145 cells by suppressing the stages of viral attachment, internalization, replication, and release. TPP exhibited a potent anti-PRRSV effect regardless of pre-treatment or post-treatment. In addition, we demonstrated that TPP restrained PRRSV-induced p65 entry into the nucleus to suppress the activation of the NF-κB signaling pathway, which ultimately leads to the inhibition of the expression of inflammatory cytokines. Furthermore, TPP limited the synthesis of viral non-structural protein 2 (nsp2), the core component of viral replication transcription complexes, which may contribute to the inhibition of viral RNA replication. TPP has the potential to develop into an effective antiviral agent for PRRSV prevention and control in the future.

Triggering receptor expressed on myeloid cells 2 (TREM2) serves as an anti-inflammatory receptor, negatively regulating the innate immune response. TREM2 is mainly expressed on dendritic cells and macrophages, the target cells of porcine reproductive and respiratory syndrome virus (PRRSV). Thus, we investigated the potential role of TREM2 in PRRSV infection in porcine alveolar macrophages (PAMs). We found that there was an increased expression of TREM2 upon PRRSV infection in vitro. TREM2 silencing restrained the replication of PRRSV, whereas TREM2 overexpression facilitated viral replication. The cytoplasmic tail domain of TREM2 interacted with PRRSV Nsp2 to promote infection. TREM2 downregulation led to early activation of PI3K/NF-κB signaling, thus reinforcing the expression of proinflammatory cytokines and type I interferons. Due to the enhanced cytokine expression, a disintegrin and metalloproteinase 17 was activated to promote the cleavage of membrane CD163, which resulted in suppression of infection. Furthermore, exogenous soluble TREM2 (sTREM2)-mediated inhibition of PRRSV attachment might be attributed to its competitive binding to viral envelope proteins. In pigs, following PRRSV challenge in vivo, the expression of TREM2 in lungs and lymph nodes as well as the production of sTREM2 were significantly increased. These novel findings indicate that TREM2 plays a role in regulating PRRSV replication via the inflammatory response. Therefore, our work describes a novel antiviral mechanism against PRRSV infection and suggests that targeting TREM2 could be a new approach in the control of the PRRSV infection., Author summary Porcine reproductive and respiratory syndrome virus (PRRSV) is one of the most important pathogens in the global swine industry and is difficult to control. CD163 is a receptor that is required for PRRSV infection, and its expression is regulated by various inflammatory mediators. Triggering receptor expressed on myeloid cells 2 (TREM2) is a novel immunoregulatory receptor, which is involved in innate immunity and anti-inflammatory responses. Here we sought to elucidate whether TREM2 regulates CD163 expression to promote PRRSV infection via the inflammatory response. We found increased TREM2 levels upon PRRSV infection in vitro and in vivo in the lungs and lymph nodes of infected pigs. We demonstrated that the cytoplasmic tail of TREM2 interacts with PRRSV Nsp2 to promote infection. Additionally, TREM2 silencing triggers an early proinflammatory response via PI3K/NF-κB pathway, thus downregulating the expression of CD163, which leads to virus suppression. Our work provides a novel antiviral mechanism against PRRSV infection and suggests that targeting TREM2 could be a new approach for control of the disease.

Background Porcine reproductive and respiratory syndrome virus (PRRSV) is an economically important pathogen and causes significant economic losses to the swine industry worldwide each year. Current vaccination strategies do not effectively prevent and control the virus. Consequently, it is necessary to develop novel antiviral strategies. Carrageenan, extracted from marine red algae, exhibits anti-coagulant, anti-tumour, anti-virus and immunomodulatory activities. Methods We investigate the inhibitory effect of iota-carrageenan (CG) on PRRSV strain CH-1a via antiviral assay and viral binding, entry and release assays. Results We found that CG effectively inhibited CH-1a replication at mRNA and protein levels in both Marc-145 cells and porcine alveolar macrophages (PAMs). The antiviral activity of CG occurred during viral attachment and entry in virus life cycle. In addition, CG suppressed viral release in Marc-145 cells, as well as blocked CH-1a-induced apoptosis during the late period of infection. Furthermore, CG inhibited CH-1a-induced NF-κB activation, thus interfering with cytokine production in Marc-145 cells and PAMs, which contributes to its anti-PRRSV activity. Conclusions Taken together, our data imply that CG might be an ideal candidate that is worthwhile developing into a new anti-PRRSV prophylactic and therapeutic drug.

Porcine reproductive and respiratory syndrome virus (PRRSV) is a continuous threat to the pig industry, causing high economic losses worldwide. Current vaccination strategies provide only limited protection against PRRSV infection. Consequently, there is a need to develop new antiviral strategies. Pyrithione (PT), a zinc ionophore, is used as an antibacterial and antifungal agent, and evidence has shown that PT inhibits the replication of various RNA viruses. However, there is no data regarding its effects against PRRSV infection until now. In this study, we showed that PT strongly inhibited PRRSV replication in Marc-145 cells. Similar inhibitory effects were also found in porcine alveolar macrophages, the major target cell type of PRRSV infection in pigs in vivo. PT also attenuated virus-induced apoptosis during the late phase of infection. In addition, we provided evidence that PT caused a rapid import of extracellular zinc ions into cells, and imported Zn2+ was responsible for its antiviral property. We investigated the molecular mechanisms of PT against PRRSV and found that PT inhibited NF-κB and heparanase, producing the increased heparan sulfate expression to block the release of virus and cytokines, thus decreasing viral replication. These findings suggest that PT has the potential to the development of prophylactic and therapeutic strategies against PRRSV infection.

Porcine reproductive and respiratory syndrome virus (PRRSV) has been recognized to induce proinflammatory cytokine production and modulate the host interferon (IFN) system. Proinflammatory cytokines and type I IFNs contribute to the prevention of viral infection. Lipopolysaccharide (LPS), a specific agonist to Toll-like receptor 4 (TLR4), provokes signal transduction and activates immune response in vivo and in vitro. Here we identified LPS inhibited PRRSV infection in porcine alveolar macrophages (PAMs) and in Marc-145 cells. To investigate the possible mechanism, we found TLR4-NF-κB pathway was obviously activated in LPS-treated PAMs at the early stage of PRRSV infection. As a result, the expression of proinflammatory cytokines was strongly induced following LPS and PRRSV co-treatment. Due to the enhanced proinflammatory response, CD163 expression was significantly reduced and a disintegrin and metalloproteinase 17 was activated, which promotes the cleavage of membrane CD163. Ultimately, CD163 down-regulation led to the suppression of PRRSV replication. Our data demonstrate that LPS has an impact on PRRSV infection via inflammation response, which provides a new insight of inflammation-mediated antiviral immunity and a new strategy to control PRRSV infection.

Porcine alveolar macrophages without the CD163 SRCR5 domain are resistant to porcine reproductive and respiratory syndrome virus (PRRSV) infection. However, whether the deletion of CD163 SRCR5 in MARC-145 cells confers resistance to PRRSV and interaction of which of the host proteins with CD163 is involved in virus uncoating remain unclear. Here we deleted the SRCR5 domain of CD163 in MARC-145 cells using CRISPR/Cas9 to generate a CD163ΔSRCR5 MARC-145 cell line. The modification of CD163 had no impact on CD163 expression. CD163ΔSRCR5 cells were completely resistant to infection by PRRSV-2 strains Li11, CHR6, TJM, and VR2332. The modified cells showed no cytokine response to PRRSV-2 infection and maintained normal cell vitality comparable with the WT cells. The resistant phenotype of the cells was stably maintained through cell passages. There were no replication transcription complexes in the CD163ΔSRCR5 cells. SRCR5 deletion did not disturb the colocalization of CD163 and PRRSV-N in early endosomes (EE). However, the interaction of the viral proteins GP2a, GP3, or GP5 with CD163, which is involved in virus uncoating was affected. Furthermore, 77 CD163-binding cellular proteins affected by the SRCR5 deletion were identified by LC–MS/MS. Inhibition of calpain 1 trapped the virions in EE and forced then into late endosomes but did not block viral attachment and internalization, suggesting that calpain 1 is involved in the uncoating. Overall, CD163ΔSRCR5 MARC-145 cells are fully resistant to PRRSV-2 infection and calpain 1 is identified as a novel host protein that interacts with CD163 to facilitate PRRSV uncoating.

Porcine reproductive and respiratory syndrome virus (PRRSV) 1 and 2 differ in their recognition of CD163. Substitution of porcine CD163 SRCR5 domain with a human CD163-like SRCR8 confers resistance to PRRSV 1 but not PRRSV 2. The deletion of CD163 SRCR5 has been shown to confer resistance to PRRSV 1 in vivo and both PRRSV 1 and 2 in vitro. However, the anti-PRRSV 2 activity of modifying the CD163 SRCR5 domain has not yet been reported. Here, we describe the highly efficient generation of two pig breeds (Liang Guang Small Spotted and Large White pigs) lacking a short region of CD163 SRCR5, including the ligand-binding pocket. We generated a large number of gene-edited Large White pigs of the F0 generation for use in viral challenge studies. The results of this study show that these pigs are completely resistant to infection by species 2 PRRSV, JXA1, and MY strains. There were no clinical symptoms, pathological abnormalities, viremia, or anti-PRRSV antibodies in the CD163 SRCR5-edited pigs compared to wild-type controls after viral challenge. Porcine alveolar macrophages (PAMs) isolated from CD163 SRCR5-edited Large White pigs also displayed resistance to PRRSV in vitro. In addition, CD163 SRCR5-edited PAMs still exhibited a cytokine response to PRRSV infection, and no significant difference was observed in cytokine expression compared to wild-type PAMs. Taken together, these data suggest that CD163 SRCR5-edited pigs are resistant to PRRSV 2, providing a basis for the establishment of PRRSV-resistant pig lines for commercial application and further investigation of the essential region of SRCR5 involved in virus infection.

Porcine reproductive and respiratory syndrome virus (PRRSV) causes a great economic loss to the swine industry globally. Current prevention and treatment measures are not effective to control the outbreak and spread of porcine reproductive and respiratory syndrome (PRRS). In other words, new antiviral strategies are urgently needed. Chlorine dioxide (ClO2) is regarded as a broad-spectrum disinfectant with strong inhibitory effects on microbes and parasites. The purpose of this study was to evaluate the inhibitory effects and underlying molecular mechanisms of ClO2 against PRRSV infection in vitro. Here, we identified ClO2 (the purity is 99%) could inhibit the infection and replication of PRRSV in both Marc-145 cells and porcine alveolar macrophages (PAMs). ClO2 could block PRRSV binding to cells rather than internalization and release, suggesting that ClO2 blocks the first stage of the virus life cycle. We also demonstrated that the inhibition exerted by ClO2 was attributed to the degradation of PRRSV genome and proteins. Moreover, we confirmed that ClO2 could decrease the expression of inflammatory cytokines induced by PRRSV. In summary, ClO2 is an efficient agent and potently suppressed PRRSV infection in vitro.

Porcine reproductive and respiratory syndrome virus (PRRSV) continues to cause substantial economic losses to the pig industry worldwide. Heparan sulfate (HS) is used by PRRSV for initial attachment to target cells. However, the role of HS in the late phase of PRRSV infection and the mechanism of virus release from host cells remain largely unknown. In this study, we showed that PRRSV infection caused a decrease in HS expression and upregulated heparanase, the only known enzyme capable of degrading HS. We subsequently demonstrated that the NF-κB signaling pathway and cathepsin L protease were involved in regulation of PRRSV infection-induced heparanase. In addition, we found that ablation of heparanase expression using small interfering RNA duplexes increased cell surface expression of HS and suppressed PRRSV replication and release, whereas overexpression of heparanase reduced HS surface expression and enhanced PRRSV replication and release. These data suggest that PRRSV activates NF-κB and cathepsin L to upregulate and process heparanase, and then the active heparanase cleaves HS, resulting in viral release. Our findings provide new insight into the molecular mechanism of PRRSV egress from host cells, which might help us to further understand PRRSV pathogenesis. IMPORTANCE Porcine reproductive and respiratory syndrome virus (PRRSV) causes great economic losses each year to the pig industry worldwide. The molecular mechanism of PRRSV release from host cells largely remains a mystery. In this study, we demonstrate that PRRSV activates NF-κB and cathepsin L to upregulate and process heparanase, and then the active heparanase is released to the extracellular space and exerts enzymatic activity to cleave heparan sulfate, resulting in viral release. Our findings provide new insight into the molecular mechanism of PRRSV egress from host cells, which might help us to further understand PRRSV pathogenesis.