Pseudorabies virus VHS protein abrogates interferon responses by blocking NF-κB and IRF3 nuclear translocation.

Herpesviruses antagonize host antiviral responses through a myriad of molecular strategies culminating in the death of the host cells. Pseudorabies virus (PRV) is a significant veterinary pathogen in pigs, causing neurological sequalae that ultimately lead to the animal's demise. PRV is known to trigger apoptotic cell death during the late stages of infection. The virion host shutdown protein (VHS) encoded by UL41 plays a crucial role in the PRV infection process. In this study, we demonstrate that UL41 inhibits PRV-induced activation of inflammatory cytokine and negatively regulates the cGAS-STING-mediated antiviral activity by targeting IRF3, thereby inhibiting the translocation and phosphorylation of IRF3. Notably, mutating the conserved amino acid sites (E192, D194, and D195) in the RNase domain of UL41 or knocking down UL41 inhibits the immune evasion of PRV, suggesting that UL41 may play a crucial role in PRV's evasion of the host immune response during infection. These results enhance our understanding of how PRV structural proteins assist the virus in evading the host immune response.

Mammalian orthoreovirus capsid protein σ3 antagonizes RLR-mediated antiviral responses by degrading MAVS.

Mammalian orthoreovirus (MRV) outer capsid protein σ3 is a multifunctional protein containing a double-stranded RNA-binding domain, which facilitates viral entry and assembly. We reasoned that σ3 has an innate immune evasion function. Here, we show that σ3 protein localizes in the mitochondria and interacts with mitochondrial antiviral signaling protein (MAVS) to activate the intrinsic mitochondria-mediated apoptotic pathway. Consequently, σ3 protein promotes the degradation of MAVS through the intrinsic caspase-9/caspase-3 apoptotic pathway. Moreover, σ3 protein can also inhibit the expression of the components of the RNA-sensing retinoic acid-inducible gene (RIG)-like receptor (RLR) signaling pathway to block antiviral type I interferon responses. Mechanistically, σ3 inhibits RIG-I and melanoma differentiation-associated gene 5 expression is independent of its inhibitory effect on MAVS. Overall, we demonstrate that the MRV σ3 protein plays a vital role in negatively regulating the RLR signaling pathway to inhibit antiviral responses. This enables MRV to evade host defenses to facilitate its own replication providing a target for the development of effective antiviral drugs against MRV. IMPORTANCE: Mammalian orthoreovirus (MRV) is an important zoonotic pathogen, but the regulatory role of its viral proteins in retinoic acid-inducible gene-like receptor (RLR)-mediated antiviral responses is still poorly understood. Herein, we show that MRV σ3 protein co-localizes with mitochondrial antiviral signaling protein (MAVS) in the mitochondria and promotes the mitochondria-mediated intrinsic apoptotic pathway to cleave and consequently degrade MAVS. Furthermore, tryptophan at position 133 of σ3 protein plays a key role in the degradation of MAVS. Importantly, we show that MRV outer capsid protein σ3 is a key factor in antagonizing RLR-mediated antiviral responses, providing evidence to better unravel the infection and transmission mechanisms of MRV.

Pseudorabies virus UL38 attenuates the cGAS-STING signaling pathway by recruiting Tollip to promote STING for autophagy degradation.

Natural immunity is the first defense line of the host immune system, which plays a significant role in combating foreign pathogenic microorganisms. The IFN-ß (interferon-beta) signaling pathway, being a typical example of innate immunity, plays a vital function. This study aimed to elucidate the function of pseudorabies virus (PRV) UL38 protein (unique long region 38) in suppressing the activation of the IFN-ß signaling pathway. The findings from our study indicate that the PRV UL38 protein effectively hampers the activation of IFN-ß by poly (dA: dT) (poly(deoxyadenylic-deoxythymidylic)) and 2'3'-cGAMP (2'-3'-cyclic GMP-AMP). Furthermore, UL38 exhibits spatial co-localization with STING (stimulator of interferon genes) and effectively hinders STING dimerization. Subsequently, STING was downgraded to suppress the production of IFN-ß and ISGs (interferon stimulated genes). Immunoprecipitation analysis revealed that the interaction between UL38 and STING, which subsequently initiated the degradation of STING via selective autophagy mediated by TOLLIP (toll interacting protein). To summarize, this research elucidates the function of UL38 in counteracting the cGAS (cGAMP synthase)-STING-induced IFN-ß pathway. The PRV UL38 protein may attenuate the activation of IFN-ß as a means of regulating the virus's persistence in the host.

IFITM2 Presents Antiviral Response through Enhancing Type I IFN Signaling Pathway.

Interferon (IFN) helps cells fight viral infections by further inducing the expression of many downstream IFN-stimulated genes (ISGs). Human interferon-inducible transmembrane proteins (IFITM) are one of these ISGs. The antiviral function of human IFITM1, IFITM2, and IFITM3 are well known. In this study, we report that IFITM can significantly inhibit EMCV infectivity in HEK293 cells. Overexpression of IFITM proteins could promote IFN-ß production. Meanwhile, IFITMs facilitated type I IFN signaling pathway adaptor MDA5 expression. We detected the binding of IFITM2 to MDA5 in a co-immunoprecipitation assay. It was also found that the ability of IFITM2 to activate IFN-ß was significantly inhibited after interfering with MDA5 expression, suggesting that MDA5 may play an important role in the activation of the IFN-ß signaling pathway by IFITM2. Moreover, the N-terminal domain plays an active role in the antiviral activity and the activation of IFN-ß by IFITM2. These findings suggest that IFITM2 plays a vital role in antiviral signaling transduction. In addition, a positive feed-forward loop between IFITM2 and type I IFN establishes a key role for IFITM2 in enforcing innate immune responses.

DDX56 inhibits PRV replication through regulation of IFN-ß signaling pathway by targeting cGAS.

Pseudorabies virus (PRV) is an agent of Aujeszky's disease, and causes great economic losses to pig farming. Re-outburst of pseudorabies implies that new control measures are urgently needed. We show here that DDX56 possesses the ability to inhibit PRV replication in vitro, which may be an important factor for PRV infection. Overexpression of DDX56 inhibited PRV genomic DNA transcription and lower titers of PRV infection in PK15 cells, whereas down-regulation of the DDX56 expression had a promotion role on virus replication. Further study demonstrated that DDX56 exerted its proliferation-inhibitory effects of PRV through up-regulating cGAS-STING-induced IFN-ß expression. Moreover, we found that DDX56 could promote cGAS expression and direct interaction also existed between DDX56 and cGAS. Based on this, DDX56-regulated IFN-ß pathway may be targeted at cGAS. To verify this, down-regulated cGAS expression in DDX56 over-expression cells was performed. Results indicated that knockdown of cGAS expression could abrogate the inhibition role of DDX56 on PRV proliferation and weaken the effect of DDX56 on IFN-ß expression. In addition, DDX56 played a promotion role in IRF3 phosphorylation and nucleus translocation. Altogether, our results highlight DDX56's antiviral role in PRV infection, and our findings contribute to a better understanding of host factors controlling PRV replication.

HSP27 Attenuates cGAS-Mediated IFN-ß Signaling through Ubiquitination of cGAS and Promotes PRV Infection.

Pseudorabies (PR) is a domestic and wild animal infectious disease caused by the pseudorabies virus (PRV) and is one of the major infectious diseases that endanger the global swine industry. Studies have reported that PRV may achieve cross-species transmission from pigs to humans in recent years. Therefore, in-depth exploration of the relationship between PRV and host proteins is of great significance for elucidating the pathogenic mechanism of PRV and anti-PRV infection. Here, we report that heat shock protein 27 (HSP27) ubiquitinates and degrades cyclic GMP-AMP synthase (cGAS) and attenuates cGAS-mediated antiviral responses, thereby promoting PRV infection. Overexpression of HSP27 promoted PRV proliferation in vitro, while knockdown of HSP27 inhibited PRV infection. Importantly, we found that HSP27 inhibited PRV infection or poly(dA:dT)-activated IFN-ß expression. Further studies found that HSP27 may inhibit cGAS-STING-mediated IFN-ß expression through targeting cGAS. In addition, we found that HSP27 can suppress the expression of endogenous cGAS in different cells at both gene transcription and protein expression levels, and that HSP27 interacts with and ubiquitinates cGAS. In conclusion, we reveal for the first time that HSP27 is a novel negative regulator of the cGAS-STING signaling pathway induced by PRV infection or poly(dA:dT) activation and demonstrate that HSP27 plays a crucial role in PRV infection.

Pseudorabies Virus ICP0 Abolishes Tumor Necrosis Factor Alpha-Induced NF-κB Activation by Degrading P65.

Nuclear factor κB (NF-κB) is involved in a wide range of innate immune activities in host cells and serves as an important component of a host's immunity system. To survive in infected cells, viruses have evolved intricate strategies to evade the host immune response. Pseudorabies virus (PRV) is a member of the alpha herpesvirus family and is capable of causing reproductive and neurological dysfunction in pigs. PRV has a large DNA genome and therefore has the ability to encode numerous proteins that modulate host innate immune responses. In the present study, we demonstrated that the PRV-encoded immediate early protein ICP0 inhibits the tumor necrosis factor alpha (TNF-α)-mediated NF-κB signaling pathway. An in-depth study showed that ICP0 protein was able to limit NF-κB activation and decreased the expression of inflammatory cytokines interleukin-6 (IL-6) and interleukin 8 (IL-8). In addition, ICP0 blocked the activation of NF-κB through interacting with p65, degrading its protein expression and limiting its phosphorylation. PRV protein ICP0 is shown for the first time to enable escape from innate immune response through the regulation of NF-κB during PRV infection. These results illustrate that PRV ICP0 is able to block NF-κB activation. This mechanism may represent a critical role in the early events leading to PRV infection.