USP18 promotes innate immune responses and apoptosis in influenza A virus-infected A549 cells via cGAS-STING pathway

Influenza A virus (IAV) can infect respiratory epithelial cells where it replicates, triggers cellular innate immune responses, and even induces cell apoptosis. Ubiquitin-specific peptidase 18 (USP18) was reported to be associated with IAV replication and immune response homeostasis. Therefore, this study aimed to investigate the role of USP18 in IAV-infected lung epithelial cells. The cell viability was determined by the CCK-8 method. Viral titers were quantified by standard plaque assay. Innate immune response-associated cytokines were detected by RT-qPCR and ELISA and cell apoptosis was assessed by flow cytometry. The results showed that overexpression of USP18 promoted viral replication, innate immune factor secretion and apoptosis in IAV-infected A549 cells. Mechanistically, USP18 reduced cGAS degradation by decreasing its K48-linked ubiquitination to promote IAV-induced cGAS-STING pathway activation. In conclusion, USP18 is a pathological mediator of IAV in lung epithelial cells.

Ubiquitin-specific protease 18 in mallard (Anas platyrhynchos) interferes with type I interferon-mediated inhibition of high pathogenicity avian influenza virus replication

Ubiquitin-specific protease 18 (USP18) is a well-established innate immune factor in vertebrates. Although Anatidae birds rarely exhibit distinctive clinical signs during high pathogenicity avian influenza virus (HPAIV) infections, some virus strains cause deadly diseases. Here, we investigated the association between USP18 expression and pathogenicity during HPAIV infections in the Anatidae mallard Anas platyrhynchos. First, mallard USP18 gene (duUSP18) was cloned, and its transcriptional variants, with three different open reading frames, were characterized. Experimental infections with two different pathogenic strains, Miyazaki and Takeo, demonstrated an early induction of duUSP18 mRNA upon HPAIV infection in a bird's whole body in vivo and in primary duck cells in vitro, which was positively associated with pathogenicity in mallards. In addition, duUSP18 knockdown under interferon-β stimulation attenuated viral replication, regardless of pathogenicity. These results indicate a role for duUSP18 in favoring viral replication and virus resistance to type I interferon immunity in mallards.

USP18 mitigates lipopolysaccharide-induced oxidative stress and inflammation in human pulmonary microvascular endothelial cells through the TLR4/NF-κB/ROS signaling

As a type I interferon response gene, ubiquitin-specific protease 18 (USP18) has been shown to be widely involved in oxidative stress and immune regulation processes. However, the relationship between USP18 and acute lung injury (ALI) is unclear. This study aimed to analyze the role of USP18 in the pathogenesis of ALI. Lipopolysaccharide (LPS) treatment up-regulated the expression of USP18 mRNA and protein in human pulmonary microvascular endothelial cells (hPMVECs). USP18 overexpression increased the viability of LPS-induced hPMVECs, and reduced LPS-induced cell damage. Additionally, USP overexpression increased the activity of SOD and CAT, and reduced the production of NO and MDA in LPS-induced hPMVECs. Moreover, overexpression of USP18 inhibited the secretion of IL-1β, IL-6, TNF-α, and IL-18 in LPS-induced hPMVECs. USP18 overexpression restrained LPS-induced upregulation of TLR4 and the excessive phosphorylation of p65 and IκBα, as well as the production of reactive oxygen species (ROS). TLR4 agonist MPLA attenuated the inhibitory effect of USP18 overexpression on LPS-induced oxidative stress and inflammation in hPMVECs. In addition, USP18 ameliorated LPS induced ALI in vivo. In conclusion, USP18 may resist LPS-induced oxidative stress and inflammatory response in hPMVECs by inhibiting the TLR4/NF-κB/ROS signaling pathway, which may provide new and complementary strategies for ALI treatment.

Transcriptional response of USP18 predicts treatment outcomes of interferon-alpha in HBeAg-positive chronic hepatitis B patientsefere

Ubiquitin-specific protease 18 (USP18) is an important inhibitor of interferon (IFN) antiviral activity, and the aim of this study was to investigate the association between the USP18 mRNA level change in peripheral blood mononuclear cells (PBMCs) when stimulated with IFN in vitro before initiating treatment and the treatment outcomes in HBeAg-positive chronic hepatitis B (CHB) patients treated with IFN. A total of 44 patients who received standard IFN-based anti-HBV therapy and follow-up were enrolled in the study. The in vitro IFN-induced USP18 mRNA change (USP18_IFN-N ) was measured via comparison of quantitative PCR-determined USP18 transcription levels of BPMCs cultured with and without IFN stimulation. Either for virological (VR) or serological response (SR), the baseline USP18_IFN-N was significantly higher (P = 0.018 for VR, P = 0.008 for SR) among nonresponders (n = 23 for VR, n = 33 for SR) than that of responders (n = 21 for VR, n = 11 for SR). Multivariate analyses revealed baseline USP18_IFN-N was a novel independent predictor for either VR (OR = 0.292, 95% CI = 0.102-0.835, P = 0.022) or SR (OR = 0.173, 95% CI = 0.035-0.849, P = 0.031) in our cohort. In addition, baseline USP18_IFN-N in combination with HBV DNA loads or HBeAg levels showed improved accuracy of pretreatment prediction for VR or SR responders, respectively. Baseline USP18_IFN-N levels are associated with both virological and serological response, and have the potential to become a clinical predictor for treatment outcomes in HBeAg-positive CHB patients before initiating IFN-α therapy.

Innate Immune Responses to Avian Influenza Viruses in Ducks and Chickens

Mallard ducks are important natural hosts of low pathogenic avian influenza (LPAI) viruses and many strains circulate in this reservoir and cause little harm. Some strains can be transmitted to other hosts, including chickens, and cause respiratory and systemic disease. Rarely, these highly pathogenic avian influenza (HPAI) viruses cause disease in mallards, while chickens are highly susceptible. The long co-evolution of mallard ducks with influenza viruses has undoubtedly fine-tuned many immunological host–pathogen interactions to confer resistance to disease, which are poorly understood. Here, we compare innate responses to different avian influenza viruses in ducks and chickens to reveal differences that point to potential mechanisms of disease resistance. Mallard ducks are permissive to LPAI replication in their intestinal tissues without overtly compromising their fitness. In contrast, the mallard response to HPAI infection reflects an immediate and robust induction of type I interferon and antiviral interferon stimulated genes, highlighting the importance of the RIG-I pathway. Ducks also appear to limit the duration of the response, particularly of pro-inflammatory cytokine expression. Chickens lack RIG-I, and some modulators of the signaling pathway and may be compromised in initiating an early interferon response, allowing more viral replication and consequent damage. We review current knowledge about innate response mediators to influenza infection in mallard ducks compared to chickens to gain insight into protective immune responses, and open questions for future research.

A viral race for primacy: co-infection of a natural pair of low and highly pathogenic H7N7 avian influenza viruses in chickens and embryonated chicken eggs

Highly pathogenic avian influenza virus (HPAIV) infection in poultry caused devastating mortality and economic losses. HPAIV of subtypes H5 and H7 emerge from precursor viruses of low pathogenicity (LP) by spontaneous mutation associated with a shift in the susceptibility of the endoproteolytic cleavage site of the viral hemagglutinin protein from trypsin- to furin-like proteases. A recently described natural pair of LP/HP H7N7 viruses derived from two spatio-temporally linked outbreaks in layer chickens was used to study how a minority of mutated HP virions after de novo generation in a single host might gain primacy. Co-infection experiments in embryonated eggs and in chickens were conducted to investigate amplification, spread and transmissionof HPAIV within a poultry population that experiences concurrent infection by an antigenically identical LP precursor virus. Simultaneous LPAIV co-infection (inoculum dose of 106 egg-infectious dose 50% endpoint (EID50)/0.5 mL) withincreasing titers of HPAIV from 101 to 105.7 EID50/0.5 mL) had a significant impeding impact on HP H7 replication, viral excretion kinetics, clinical signs and histopathological lesions (in vivo) and on embryo mortality (in ovo). LP/HP co-infected chickens required a hundredfold higher virus dose (HPAIV inoculum of 105 EID50) compared to HPAIV mono-infection (HPAIV inoculum of 103 EID50) to develop overt clinical signs, mortality and virus spread to uninfected sentinels. Escape and spread of HP phenotypes after de novo generation in an index host may therefore be highly precarious due to significant competition with co-circulating LP precursor virus.