African swine fever (ASF), caused by African swine fever virus (ASFV), is a highly contagious and fatal disease found in swine. However, the viral proteins and mechanisms responsible for immune evasion are poorly understood, which has severely hindered the development of vaccines. This review mainly focuses on studies involving the innate antiviral immune response of the host and summarizes the latest studies on ASFV genes involved in interferon (IFN) signaling and inflammatory responses. We analyzed the effects of candidate viral proteins on ASFV infection, replication and pathogenicity and identified potential molecular targets for novel ASFV vaccines. These efforts will contribute to the construction of novel vaccines and wonder therapeutics for ASF.

Proinflammatory factors play important roles in the pathogenesis of African swine fever virus (ASFV), which is the causative agent of African swine fever (ASF), a highly contagious and severe hemorrhagic disease. Efforts in the prevention and treatment of ASF have been severely hindered by knowledge gaps in viral proteins responsible for modulating host antiviral responses. In this study, we identified the I10L protein (pI10L) of ASFV as a potential inhibitor of the TNF- α - and IL-1 β -triggered NF- κ B signaling pathway, the most canonical and important part of host inflammatory responses. The ectopically expressed pI10L remarkably suppressed the activation of NF- κ B signaling in HEK293T and PK-15 cells. The ASFV mutant lacking the I10L gene (ASFV _ΔI10L ) induced higher levels of proinflammatory cytokines production in primary porcine alveolar macrophages (PAMs) compared with its parental ASFV HLJ/2018 strain (ASFV _WT ). Mechanistic studies suggest that pI10L inhibits IKK β phosphorylation by reducing the K63-linked ubiquitination of NEMO, which is necessary for the activation of IKK β . Morever, pI10L interacts with the kinase domain of IKK β through its N-terminus, and consequently blocks the association of IKK β with its substrates I κ B α and p65, leading to reduced phosphorylation. In addition, the nuclear translocation efficiency of p65 was also altered by pI10L. Further biochemical evidence supported that the amino acids 1–102 on pI10L were essential for the pI10L-mediated suppression of the NF- κ B signaling pathway. The present study clarifies the immunosuppressive activity of pI10L, and provides novel insights into the understanding of ASFV pathobiology and the development of vaccines against ASF. IMPORTANCE: African swine fever (ASF), caused by the African swine fever virus (ASFV), is now widespread in many countries and severely affects the commercial rearing of swine. To date, few safe and effective vaccines or antiviral strategies have been marketed due to large gaps in knowledge regarding ASFV pathobiology and immune evasion mechanisms. In this study, we deciphered the important role of the ASFV-encoded I10L protein in the TNF- α -/IL-1 β -triggered NF- κ B signaling pathway. This study provides novel insights into the pathogenesis of ASFV and thus contributes to the development of vaccines against ASF.

The current view of nucleic acid–mediated innate immunity is that binding of intracellular sensors to nucleic acids is sufficient for their activation. Here, we report that endocytosis of virus or foreign DNA initiates a priming signal for the DNA sensor cyclic GMP-AMP synthase (cGAS)–mediated innate immune response. Mechanistically, viral infection or foreign DNA transfection triggers recruitment of the spleen tyrosine kinase (SYK) and cGAS to the endosomal vacuolar H ⁺ pump (V-ATPase), where SYK is activated and then phosphorylates human cGAS ^Y214/215 (mouse cGas ^Y200/201 ) to prime its activation. Upon binding to DNA, the primed cGAS initiates robust cGAMP production and mediator of IRF3 activation/stimulator of interferon genes-dependent innate immune response. Consistently, blocking the V-ATPase-SYK axis impairs DNA virus- and transfected DNA-induced cGAMP production and expression of antiviral genes. Our findings reveal that V-ATPase-SYK–mediated tyrosine phosphorylation of cGAS following endocytosis of virus or other cargos serves as a priming signal for cGAS activation and innate immune response.

The NS5A protein of classical swine fever virus (CSFV) is involved in the RNA synthesis and viral replication. However, the NS5A-interacting cellular proteins engaged in the CSFV replication are poorly defined. Using yeast two-hybrid screen, the eukaryotic elongation factor 1A (eEF1A) was identified to be an NS5A-binding partner. The NS5A–eEF1A interaction was confirmed by coimmunoprecipitation, glutathione S-transferase (GST) pulldown and laser confocal microscopy assays. The domain I of eEF1A was shown to be critical for the NS5A–eEF1A interaction. Overexpression of eEF1A suppressed the CSFV growth markedly, and conversely, knockdown of eEF1A enhanced the CSFV replication significantly. Furthermore, eEF1A, as well as NS5A, was found to reduce the translation efficiency of the internal ribosome entry site (IRES) of CSFV in a dose-dependent manner, as demonstrated by luciferase reporter assay. Streptavidin pulldown assay revealed that eEF1A could bind to the CSFV IRES. Collectively, our results suggest that eEF1A interacts with NS5A and negatively regulates the growth of CSFV.

The porcine reproductive and respiratory syndrome virus (PRRSV) can lead to severe reproductive problems in sows, pneumonia in weaned piglets, and increased mortality, significantly negatively impacting the economy. Post-translational changes are essential for the host-dependent replication and long-term infection of PRRSV. Uncertainty surrounds the function of the ubiquitin network in PRRSV infection. Here, we screened 10 deubiquitinating enzyme inhibitors and found that the ubiquitin-specific proteinase 1 (USP1) inhibitor ML323 significantly inhibited PRRSV replication in vitro . Importantly, we found that USP1 interacts with nonstructural protein 1β (Nsp1β) and deubiquitinates its K48 to increase protein stability, thereby improving PRRSV replication and viral titer. Among them, lysine at position 45 is essential for Nsp1β protein stability. In addition, deficiency of USP1 significantly reduced viral replication. Moreover, ML323 loses antagonism to PRRSV rSD16-K45R. This study reveals the mechanism by which PRRSV recruits the host factor USP1 to promote viral replication, providing a new target for PRRSV defense. IMPORTANCE: Deubiquitinating enzymes are critical factors in regulating host innate immunity. The porcine reproductive and respiratory syndrome virus (PRRSV) nonstructural protein 1β (Nsp1β) is essential for producing viral subgenomic mRNA and controlling the host immune system. The host inhibits PRRSV proliferation by ubiquitinating Nsp1β, and conversely, PRRSV recruits the host protein ubiquitin-specific proteinase 1 (USP1) to remove this restriction. Our results demonstrate the binding of USP1 to Nsp1β, revealing a balance of antagonism between PRRSV and the host. Our research identifies a brand-new PRRSV escape mechanism from the immune response.

African swine fever virus (ASFV) is the causative agent of African swine fever (ASF), a devastating disease epidemic in Asia and Europe. Large knowledge gaps regarding the biological characteristics of viral structural proteins have severely hindered the development of vaccines against ASF. The p22 protein, an internal envelope membrane protein of ASFV, is one such protein that is yet to be deciphered despite its significance. Here, our results indicated that p22 is not essential for the morphogenesis and replication of ASFV in porcine alveolar macrophages. The ASFV p22 negatively regulates the IFN-β-triggered activation of the Janus kinase-signal transducer and activator of transcription (JAK-STAT) signaling pathway. Mechanistically, the ASFV p22 promotes the association of the Tax1-binding protein 1 (TAX1BP1) with the type I IFN receptor 1 (IFNAR1) via its transmembrane region, thereby facilitating the autophagic degradation of IFNAR1 and impairing the host antiviral responses at the initial step of JAK-STAT signaling pathway. These findings clarify the biological functions of p22 in ASFV replication and uncover a novel autophagy degradation mechanism for IFNAR1, which provide a novel theoretical basis for understanding the biological characteristics of ASFV and may contribute to the development of vaccines and antiviral therapies against ASF.