Neutrophils and Influenza: A Thin Line between Helpful and Harmful

HBeAg induces neutrophils activation impairing NK cells function in patients with chronic hepatitis B

BACKGROUND

The role of neutrophils in hepatitis B virus (HBV) infection has been a subject of debate due to their involvement in antiviral responses and immune regulation. This study aimed to elucidate the neutrophil characteristics in patients with chronic hepatitis B (CHB).

METHODS

Through flow cytometry and ribonucleic acid-sequencing analysis, the phenotypes and counts of neutrophils were analyzed in patients with CHB. Moreover, the effects of HBeAg on neutrophils and the corresponding pattern recognition receptors were identified. Simultaneously, the cross-talk between neutrophils and natural killer (NK) cells was investigated.

RESULTS

Neutrophils were activated in patients with CHB, characterized by higher expression levels of programmed death-ligand 1 (PD-L1), cluster of differentiation 86, and interleukin-8, and lower levels of CXC motif chemokine receptor (CXCR) 1 and CXCR2. Hepatitis B e antigen (HBeAg) partially induces neutrophil activation through the Toll-like receptor 2 (TLR2). A consistent upregulation of the TLR2 and HBeAg expression was observed in patients with CHB. Notably, the genes encoding molecules pivotal for NK-cell function upon NK receptor engagement enriched in neutrophils after HBeAg activation. The HBeAg-activated neutrophils demonstrated the ability to decrease the production of interferon-gamma (IFN-γ) and tumor necrosis factor-alpha (TNF-α) in NK cells, while the PD-1 and PD-L1 pathways partially mediated the immunosuppression.

CONCLUSIONS

The immunosuppression of neutrophils induced by HBeAg suggests a novel pathogenic mechanism contributing to immune tolerance in patients with CHB.

Dynamic heterogeneity in COVID-19: Insights from a mathematical model

Critical illness, such as severe COVID-19, is heterogenous in presentation and treatment response. However, it remains possible that clinical course may be influenced by dynamic and/or random events such that similar patients subject to similar injuries may yet follow different trajectories. We deployed a mechanistic mathematical model of COVID-19 to determine the range of possible clinical courses after SARS-CoV-2 infection, which may follow from specific changes in viral properties, immune properties, treatment modality and random external factors such as initial viral load. We find that treatment efficacy and baseline patient or viral features are not the sole determinant of outcome. We found patients with enhanced innate or adaptive immune responses can experience poor viral control, resolution of infection or non-infectious inflammatory injury depending on treatment efficacy and initial viral load. Hypoxemia may result from poor viral control or ongoing inflammation despite effective viral control. Adaptive immune responses may be inhibited by very early effective therapy, resulting in viral load rebound after cessation of therapy. Our model suggests individual disease course may be influenced by the interaction between external and patient-intrinsic factors. These data have implications for the reproducibility of clinical trial cohorts and timing of optimal treatment.

Generation of Broad Protection against Influenza with Di-Tyrosine-Cross-Linked M2e Nanoclusters

Tyrosine cross-linking has recently been used to produce nanoclusters (NCs) from peptides to enhance their immunogenicity. In this study, NCs were generated using the ectodomain of the ion channel Matrix 2 (M2e) protein, a conserved influenza surface antigen. The NCs were administered via intranasal (IN) or intramuscular (IM) routes in a mouse model in a prime-boost regimen in the presence of the adjuvant CpG. After boost, a significant increase in anti-M2e IgG and its subtypes was observed in the serum and lungs of mice vaccinated through the IM and IN routes; however, significant enhancement in anti-M2e IgA in lungs was observed only in the IN group. Analysis of cytokine concentrations in stimulated splenocyte cultures indicated a Th1/Th17-biased response. Mice were challenged with a lethal dose of A/California/07/2009 (H1N1pdm), A/Puerto Rico/08/1934 (H1N1), or A/Hong Kong/08/1968 (H3N2) strains. Mice that received M2e NCs + CpG were significantly protected against these strains and showed decreased lung viral titers compared with the naive mice and M2e NC-alone groups. The IN-vaccinated group showed superior protection against the H3N2 strain as compared to the IM group. This research extends our earlier efforts involving the tyrosine-based cross-linking method and highlights the potential of this technology in enhancing the immunogenicity of short peptide immunogens.

Ex Pluribus Unum: The CD4 T Cell Response against Influenza A Virus

Current Influenza A virus (IAV) vaccines, which primarily aim to generate neutralizing antibodies against the major surface proteins of specific IAV strains predicted to circulate during the annual 'flu' season, are suboptimal and are characterized by relatively low annual vaccine efficacy. One approach to improve protection is for vaccines to also target the priming of virus-specific T cells that can protect against IAV even in the absence of preexisting neutralizing antibodies. CD4 T cells represent a particularly attractive target as they help to promote responses by other innate and adaptive lymphocyte populations and can also directly mediate potent effector functions. Studies in murine models of IAV infection have been instrumental in moving this goal forward. Here, we will review these findings, focusing on distinct subsets of CD4 T cell effectors that have been shown to impact outcomes. This body of work suggests that a major challenge for next-generation vaccines will be to prime a CD4 T cell population with the same spectrum of functional diversity generated by IAV infection. This goal is encapsulated well by the motto 'ex pluribus unum': that an optimal CD4 T cell response comprises many individual specialized subsets responding together.

Myeloid Zfhx3 deficiency protects against hypercapnia-induced suppression of host defense against influenza A virus

Hypercapnia, elevation of the partial pressure of CO2 in blood and tissues, is a risk factor for mortality in patients with severe acute and chronic lung diseases. We previously showed that hypercapnia inhibits multiple macrophage and neutrophil antimicrobial functions and that elevated CO2 increases the mortality of bacterial and viral pneumonia in mice. Here, we show that normoxic hypercapnia downregulates innate immune and antiviral gene programs in alveolar macrophages (AMØs). We also show that zinc finger homeobox 3 (Zfhx3) - a mammalian ortholog of zfh2, which mediates hypercapnic immune suppression in Drosophila - is expressed in mouse and human macrophages. Deletion of Zfhx3 in the myeloid lineage blocked the suppressive effect of hypercapnia on immune gene expression in AMØs and decreased viral replication, inflammatory lung injury, and mortality in hypercapnic mice infected with influenza A virus. To our knowledge, our results establish Zfhx3 as the first known mammalian mediator of CO2 effects on immune gene expression and lay the basis for future studies to identify therapeutic targets to interrupt hypercapnic immunosuppression in patients with advanced lung disease.

Bruton's tyrosine kinase is a possible therapeutic target in microscopic polyangiitis

BACKGROUND

Bruton's tyrosine kinase (Btk) is an enzyme expressed in leukocytes other than T lymphocytes and plasma cells and involved in B-cell receptor- and Fcγ receptor (FcγR)-mediated signal transduction. Btk inhibitors potentially suppress autoantibody production due to the expected inhibitory ability of B lymphocyte differentiation into antibody-producing plasma cells and reduce FcγR-mediated neutrophil activation, including the release of neutrophil extracellular traps (NETs). Microscopic polyangiitis (MPA) is a systemic small-vessel vasculitis characterized by the pathogenic autoantibody, antineutrophil cytoplasmic antibody (ANCA) that reacts with myeloperoxidase (MPO). MPO and MPO-ANCA immune complex (IC)-induced FcγR-mediated NETs are critically involved in MPA pathogenesis. This study aimed to demonstrate the therapeutic efficacy of the Btk inhibitor tirabrutinib on MPA.

METHODS

Various doses of tirabrutinib or vehicle were orally administered to Sprague-Dawley rats daily. Four weeks later, the number of peripheral B lymphocytes was counted, and Btk phosphorylation in B lymphocytes was evaluated by flow cytometry. Human peripheral blood neutrophils were stimulated by MPO and anti-MPO antibody ICs (MPO and anti-MPO-ICs), and Btk and its downstream Vav phosphorylation were assessed by western blotting. The effects of tirabrutinib on MPO and anti-MPO-IC-induced NET formation were examined in vitro. Wistar Kyoto rats were immunized with human MPO to induce experimental MPA and given drug-free or tirabrutinib-containing feed (0.0037% or 0.012%) from day 0 or 28. All rats were euthanized on day 42 for serological and histological evaluation.

RESULTS

Tirabrutinib inhibited Btk phosphorylation without decreasing B lymphocytes in vivo. Neutrophil Btk and Vav were phosphorylated when stimulated with MPO and anti-MPO-ICs. Tirabrutinib suppressed MPO and anti-MPO-IC-induced NET formation in vitro and ameliorated experimental MPA in a dose-dependent manner in vivo. Although MPO-ANCA production was not affected, NET-forming neutrophils in the blood were significantly reduced by tirabrutinib.

CONCLUSIONS

The Btk inhibitor tirabrutinib suppressed MPO and anti-MPO-IC-induced NET formation in vitro and ameliorated experimental MPA by reducing NET-forming neutrophils but not decreasing MPO-ANCA titer in vivo. This study suggests that Btk is a possible therapeutic target in MPA.

Prevention of severe lung immunopathology associated with influenza infection through adeno-associated virus vector administration

BACKGROUND

Influenza A viruses (IAVs) have long posed a threat to humans, occasionally causing significant morbidity and mortality. The initial immune response is triggered by infected epithelial cells, alveolar macrophages and dendritic cells. However, an exaggerated innate immune response can result in severe lung injury and even host mortality. One notable pathology observed in hosts succumbing to severe influenza is the excessive influx of neutrophils and monocytes into the lung. In this study, we investigated a strategy for controlling lung immunopathology following severe influenza infection.

RESULTS

To evaluate the impact of innate immunity on influenza-associated lung injury, we employed CB17.SCID and NOD.SCID mice. NOD.SCID mice exhibited slower weight loss and longer survival than CB17.SCID mice following influenza infection. Lung inflammation was reduced in NOD.SCID mice compared to CB17.SCID mice. Bulk RNA sequencing analysis of lung tissue showed significant downregulation of 827 genes, and differentially expressed gene analysis indicated that the cytokine-cytokine receptor interaction pathway was predominantly downregulated in NOD.SCID mice. Interestingly, the expression of the Cxcl14 gene was higher in the lungs of influenza-infected NOD.SCID mice than in CB17.SCID mice. Therefore, we induced overexpression of the Cxcl14 gene in the lung using the adeno-associated virus 9 (AAV9)-vector system for target gene delivery. However, when we administered the AAV9 vector carrying the Cxcl14 gene or a control AAV9 vector to BALB/c mice from both groups, the morbidity and mortality rates remained similar. Both groups exhibited lower morbidity and mortality than the naive group that did not receive the AAV9 vector prior to IAV infection, suggesting that the pre-administration of the AAV9 vector conferred protection against lethal influenza infection, irrespective of Cxcl14 overexpression. Furthermore, we found that pre-inoculation of BALB/c mice with AAV9 attenuated the infiltration of trans-macrophages, neutrophils and monocytes in the lungs following IAV infection. Although there was no difference in lung viral titers between the naive group and the AAV9 pre-inoculated group, pre-inoculation with AAV9 conferred lung injury protection against lethal influenza infection in mice.

CONCLUSIONS

Our study demonstrated that pre-inoculation with AAV9 prior to IAV infection protected mouse lungs from immunopathology by reducing the recruitment of inflammatory cells.

Interaction among inflammasome, PANoptosise, and innate immune cells in infection of influenza virus: Updated review

BACKGROUND

Influenza virus (IV) is a leading cause of respiratory tract infections, eliciting responses from key innate immune cells such as Macrophages (MQs), Neutrophils, and Dendritic Cells (DCs). These cells employ diverse mechanisms to combat IV, with Inflammasomes playing a pivotal role in viral infection control. Cellular death mechanisms, including Pyroptosis, Apoptosis, and Necroptosis (collectively called PANoptosis), significantly contribute to the innate immune response.

METHODS

In this updated review, we delve into the intricate relationship between PANoptosis and Inflammasomes within innate immune cells (MQs, Neutrophils, and DCs) during IV infections. We explore the strategies employed by IV to evade these immune defenses and the consequences of unchecked PANoptosis and inflammasome activation, including the potential development of severe complications such as cytokine storms and tissue damage.

RESULTS

Our analysis underscores the interplay between PANoptosis and Inflammasomes as a critical aspect of the innate immune response against IV. We provide insights into IV's various mechanisms to subvert these immune pathways and highlight the importance of understanding these interactions to develop effective antiviral medications.

CONCLUSION

A comprehensive understanding of the dynamic interactions between PANoptosis, Inflammasomes, and IV is essential for advancing our knowledge of innate immune responses to viral infections. This knowledge will be invaluable in developing targeted antiviral therapies to combat IV and mitigate potential complications, including cytokine storms and tissue damage.

Neutrophil diversity in inflammation and cancer

Neutrophils are the most abundant circulating leukocytes in humans and the first immune cells recruited at the site of inflammation. Classically perceived as short-lived effector cells with limited plasticity and diversity, neutrophils are now recognized as highly heterogenous immune cells, which can adapt to various environmental cues. In addition to playing a central role in the host defence, neutrophils are involved in pathological contexts such as inflammatory diseases and cancer. The prevalence of neutrophils in these conditions is usually associated with detrimental inflammatory responses and poor clinical outcomes. However, a beneficial role for neutrophils is emerging in several pathological contexts, including in cancer. Here we will review the current knowledge of neutrophil biology and heterogeneity in steady state and during inflammation, with a focus on the opposing roles of neutrophils in different pathological contexts.

DUOX2 regulates secreted factors in virus-infected respiratory epithelial cells that contribute to neutrophil attraction and activation

The first line of defense against respiratory viruses relies on the antiviral and proinflammatory cytokine response initiated in infected respiratory epithelial cells. The cytokine response not only restricts virus replication and spreading, but also orchestrates the subsequent immune response. The epithelial Dual Oxidase 2 (DUOX2) has recently emerged as a regulator of the interferon antiviral response. Here, we investigated the role of DUOX2 in the inflammatory cytokine response using a model of A549 cells deficient in DUOX2 generated using Crispr-Cas9 and infected by Sendai virus. We found that the absence of DUOX2 selectively reduced the induction of a restricted panel of 14 cytokines and chemokines secreted in response to Sendai virus by 20 to 89%. The secreted factors produced by epithelial cells upon virus infection promoted the migration, adhesion, and degranulation of primary human neutrophils, in part through the DUOX2-dependent secretion of TNF and chemokines. In contrast, DUOX2 expression did not impact neutrophil viability or NETosis, thereby highlighting a selective impact of DUOX2 in neutrophil functions. Overall, this study unveils previously unrecognized roles of epithelial DUOX2 in the epithelial-immune cells crosstalk during respiratory virus infection.

Susceptibility identification for seasonal influenza A/H3N2 based on baseline blood transcriptome

Introduction

Influenza susceptibility difference is a widely existing trait that has great practical significance for the accurate prevention and control of influenza.

Methods

Here, we focused on the human susceptibility to the seasonal influenza A/H3N2 of healthy adults at baseline level. Whole blood expression data for influenza A/H3N2 susceptibility from GEO were collected firstly (30 symptomatic and 19 asymptomatic). Then to explore the differences at baseline, a suite of systems biology approaches - the differential expression analysis, co-expression network analysis, and immune cell frequencies analysis were utilized.

Results

We found the baseline condition, especially immune condition between symptomatic and asymptomatic, was different. Co-expression module that is positively related to asymptomatic is also related to immune cell type of naïve B cell. Function enrichment analysis showed significantly correlation with "B cell receptor signaling pathway", "immune response-activating cell surface receptor signaling pathway" and so on. Also, modules that are positively related to symptomatic are also correlated to immune cell type of neutrophils, with function enrichment analysis showing significantly correlations with "response to bacterium", "inflammatory response", "cAMP-dependent protein kinase complex" and so on. Responses of symptomatic and asymptomatic hosts after virus exposure show differences on resisting the virus, with more effective frontline defense for asymptomatic hosts. A prediction model was also built based on only baseline transcription information to differentiate symptomatic and asymptomatic population with accuracy of 0.79.

Discussion

The results not only improve our understanding of the immune system and influenza susceptibility, but also provide a new direction for precise and targeted prevention and therapy of influenza.

Beyond neutralization: Fc-dependent antibody effector functions in SARS-CoV-2 infection

Neutralizing antibodies are known to have a crucial role in protecting against SARS-CoV-2 infection and have been suggested to be a useful correlate of protection for vaccine clinical trials and for population-level surveys. In addition to neutralizing virus directly, antibodies can also engage immune effectors through their Fc domains, including Fc receptor-expressing immune cells and complement. The outcome of these interactions depends on a range of factors, including antibody isotype-Fc receptor combinations, Fc receptor-bearing cell types and antibody post-translational modifications. A growing body of evidence has shown roles for these Fc-dependent antibody effector functions in determining the outcome of SARS-CoV-2 infection. However, measuring these functions is more complicated than assays that measure antibody binding and virus neutralization. Here, we examine recent data illuminating the roles of Fc-dependent antibody effector functions in the context of SARS-CoV-2 infection, and we discuss the implications of these data for the development of next-generation SARS-CoV-2 vaccines and therapeutics.

Comparison of H7N9 and H9N2 influenza infections in mouse model unravels the importance of early innate immune response in host protection

The outcome of infection with influenza A virus is determined by a complex virus-host interaction. A new H7N9 virus of avian origin crossed the species barrier to infect humans, causing high mortality and emerged as a potential pandemic threat. The mechanisms underlying the virulence and pathogenicity of H7N9 virus remains elusive. H7N9 virus originated from a genetic assortment that involved the avian H9N2 virus, which was the donor of the six internal genes. Unlike the H7N9 virus, the H9N2 virus caused only mild phenotype in infected mice. In this study, we used the mouse infection model to dissect the difference in the host response between the H7N9 and H9N2 viruses. Through analyzing transcriptomics of infected lungs, we surprisingly found that the H9N2 infection elicited an earlier induction of innate immunity than H7N9 infection. This finding was further corroborated by an immunohistochemical study demonstrating earlier recruitment of macrophage to the H9N2-infected lung than the H7N9-infected lung, which could occur as early as 6 hours post infection. In contrast, H7N9 infection was characterized by a late, strong lung CD8+ T cell response that is more robust than H9N2 infection. The different pattern of immune response may underlie more severe lung pathology caused by H7N9 infection compared to H9N2 infection. Finally, we could show that co-infection of the H9N2 virus protected mice from the challenge of both H7N9 and PR8 viruses, thereby strengthening the importance of the induction of an early innate immunity in the host’s defense against influenza infection. Collectively, our study unraveled a previously unidentified difference in host response between H7N9 and H9N2 infection and shed new insight on how virus-host interaction shapes the in vivo outcome of influenza infection.

Cigarette smoke augments CSF3 expression in neutrophils to compromise alveolar-capillary barrier function during influenza infection

BACKGROUND

Cigarette smokers are at increased risk of acquiring influenza, developing severe disease and requiring hospitalisation/intensive care unit admission following infection. However, immune mechanisms underlying this predisposition are incompletely understood, and therapeutic strategies for influenza are limited.

METHODS

We used a mouse model of concurrent cigarette smoke exposure and H1N1 influenza infection, colony-stimulating factor (CSF)3 supplementation/receptor (CSF3R) blockade and single-cell RNA sequencing (scRNAseq) to investigate this relationship.

RESULTS

Cigarette smoke exposure exacerbated features of viral pneumonia such as oedema, hypoxaemia and pulmonary neutrophilia. Smoke-exposed infected mice demonstrated an increase in viral (v)RNA, but not replication-competent viral particles, relative to infection-only controls. Interstitial rather than airspace neutrophilia positively predicted morbidity in smoke-exposed infected mice. Screening of pulmonary cytokines using a novel dysregulation score identified an exacerbated expression of CSF3 and interleukin-6 in the context of smoke exposure and influenza. Recombinant (r)CSF3 supplementation during influenza aggravated morbidity, hypothermia and oedema, while anti-CSF3R treatment of smoke-exposed infected mice improved alveolar-capillary barrier function. scRNAseq delineated a shift in the distribution of Csf3 + cells towards neutrophils in the context of cigarette smoke and influenza. However, although smoke-exposed lungs were enriched for infected, highly activated neutrophils, gene signatures of these cells largely reflected an exacerbated form of typical influenza with select unique regulatory features.

CONCLUSION

This work provides novel insight into the mechanisms by which cigarette smoke exacerbates influenza infection, unveiling potential therapeutic targets (e.g. excess vRNA accumulation, oedematous CSF3R signalling) for use in this context, and potential limitations for clinical rCSF3 therapy during viral infectious disease.

Reactive oxygen species associated immunoregulation post influenza virus infection

An appropriate level of reactive oxygen species (ROS) is necessary for cell proliferation, signaling transduction, and apoptosis due to their highly reactive character. ROS are generated through multiple metabolic pathways under a fine-tuned control between oxidant and antioxidant signaling. A growing number of evidence has proved their highly relevant role in modulating inflammation during influenza virus infection. As a network of biological process for protecting organism from invasion of pathogens, immune system can react and fight back through either innate immune system or adaptive immune system, or both. Herein, we provide a review about the mechanisms of ROS generation when encounter influenza virus infection, and how the imbalanced level of ROS influences the replication of virus. We also summarize the pathways used by both the innate and adaptive immune system to sense and attack the invaded virus and abnormal levels of ROS. We further review the limitation of current strategies and discuss the direction of future work.

Emerging Role of ZBP1 in Z-RNA Sensing, Influenza Virus-Induced Cell Death, and Pulmonary Inflammation

Influenza viruses cause respiratory tract infections, which lead to human disease outbreaks and pandemics. Influenza A virus (IAV) circulates in diverse animal species, predominantly aquatic birds. This often results in the emergence of novel viral strains causing severe human disease upon zoonotic transmission. Innate immune sensing of the IAV infection promotes host cell death and inflammatory responses to confer antiviral host defense. Dysregulated respiratory epithelial cell death and excessive proinflammatory responses drive immunopathology in highly pathogenic influenza infections. Here, we discuss the critical mechanisms regulating IAV-induced cell death and proinflammatory responses. We further describe the essential role of the Z-form nucleic acid sensor ZBP1/DAI and RIPK3 in triggering apoptosis, necroptosis, and pyroptosis during IAV infection and their impact on host defense and pathogenicity in vivo. We also discuss the functional importance of ZBP1-RIPK3 signaling in recent severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) and other viral infections. Understanding these mechanisms of RNA virus-induced cytopathic and pathogenic inflammatory responses is crucial for targeting pathogenic lung infections and human respiratory illness.

Neutrophil extracellular traps mediate severe lung injury induced by influenza A virus H1N1 in mice coinfected with Staphylococcus aureus

Influenza virus and bacterial infection contributed to massive morbidity and mortality. However, the underlying mechanisms were poorly understood. A coinfected model was generating by using sublethal doses of influenza A virus H1N1 A/FM/1/47(H1N1) and methicillin-resistant Staphylococcus aureus (MRSA). Further, the model was optimized to achieve the highest peak of mortality initiated by intranasal infection with 0.2LD₅₀ H1N1 and 0.16LD₅₀ MRSA at 3 days interval. Excessive neutrophil recruitment, accompanied by high levels of inflammatory cytokines and chemokines, and increased bacterial and viral load were observed in coinfected mice. Under the inflammatory environments triggered by H1N1 and MRSA, the excessive neutrophil recruitment led to the formation of neutrophil extracellular traps (NETs), associated with severe inflammation and vascular endothelial injury. Importantly, the severity of lung injury could be alleviated by treatment with DNase I or a selective neutrophil elastase inhibitor (NEi). Therefore, our data suggested that excessive neutrophil recruitment and NETs formation contributed to severe inflammation and acute lung injury in coinfected animals.

Inactivated and live-attenuated seasonal influenza vaccines boost broadly neutralizing antibodies in children

The induction of broadly neutralizing antibodies (bNAbs) that target the hemagglutinin stalk domain is a promising strategy for the development of “universal” influenza virus vaccines. bNAbs can be boosted in adults by sequential exposure to heterosubtypic viruses through natural infection or vaccination. However, little is known about if or how bNAbs are induced by vaccination in more immunologically naive children. Here, we describe the impact of repeated seasonal influenza vaccination and vaccine type on induction of bNAbs against group 1 influenza viruses in a pediatric cohort enrolled in randomized controlled trials of seasonal influenza vaccination. Repeated seasonal vaccination results in significant boosting of a durable bNAb response. Boosting of serological bNAb titers is comparable within inactivated and live attenuated (LAIV) vaccinees and declines with age. These data provide insights into vaccine-elicited bNAb induction in children, which have important implications for the design of universal influenza vaccine modalities in this critical population.

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Repeated inactivated influenza vaccination boosts bNAbs

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Inactivated and live attenuated vaccines are similarly efficient at boosting bNAbs

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The magnitude of IIV and LAIV vaccine-elicited bNAb boosting declines with age

Myeloid Protease-Activated Receptor-2 Contributes to Influenza A Virus Pathology in Mice

Background

Innate immune responses to influenza A virus (IAV) infection are initiated in part by toll-like receptor 3 (TLR3). TLR3-dependent signaling induces an antiviral immune response and an NFκB-dependent inflammatory response. Protease-activated receptor 2 (PAR2) inhibits the antiviral response and enhances the inflammatory response. PAR2 deficiency protected mice during IAV infection. However, the PAR2 expressing cell-types contributing to IAV pathology in mice and the mechanism by which PAR2 contributes to IAV infection is unknown.

Methods

IAV infection was analyzed in global (Par2-/-), myeloid (Par2fl/fl;LysMCre+) and lung epithelial cell (EpC) Par2 deficient (Par2fl/fl;SPCCre+) mice and their respective controls (Par2+/+ and Par2fl/fl). In addition, the effect of PAR2 activation on polyinosinic-polycytidylic acid (poly I:C) activation of TLR3 was analyzed in bone marrow-derived macrophages (BMDM). Lastly, we determined the effect of PAR2 inhibition in wild-type (WT) mice.

Results

After IAV infection, Par2-/- and mice with myeloid Par2 deficiency exhibited increased survival compared to infected controls. The improved survival was associated with reduced proinflammatory mediators and reduced cellular infiltration in bronchoalveolar lavage fluid (BALF) of Par2-/- and Par2fl/fl;LysMCre+ 3 days post infection (dpi) compared to infected control mice. Interestingly, Par2fl/fl;SPCCre+ mice showed no survival benefit compared to Par2fl/fl. In vitro studies showed that Par2-/- BMDM produced less IL6 and IL12p40 than Par2+/+ BMDM after poly I:C stimulation. In addition, activation of PAR2 on Par2+/+ BMDM increased poly I:C induction of IL6 and IL12p40 compared to poly I:C stimulation alone. Importantly, PAR2 inhibition prior to IAV infection protect WT mice.

Conclusion

Global Par2 or myeloid cell but not lung EpC Par2 deficiency was associated with reduced BALF inflammatory markers and reduced IAV-induced mortality. Our study suggests that PAR2 may be a therapeutic target to reduce IAV pathology.