In Vivo Infection Model of Severe Influenza A Virus

The lung is constantly exposed to both environmental and microbial challenge. As a "contained" organ, it also constitutes an excellent "self-contained" tissue to examine inflammatory responses and cellular infiltration into a diseased organ. Influenza A virus (IAV) causes both mild and severe inflammation that is strain specific following infection of the lung epithelium that spreads to other cells of the lung environment. Here, we describe a method of intranasal inoculation of the lung with IAV that can be used as a preclinical model of infection. Mice can be monitored for clinical signs of infection and tissue and lung fluid collected for further analysis to dissect the immunological consequences of IAV infection. Importantly, this method can be modified to introduce other pathogens, therapies and environmental stimuli to examine immune responses in the lung.

Lung Diseases

Inflammasomes are large innate cytoplasmic complexes that play a major role in promoting inflammation in the lung in response to a range of environmental and infectious stimuli. Inflammasomes are critical for driving acute innate immune responses that resolve infection and maintain tissue homeostasis. However, dysregulated or excessive inflammasome activation can be detrimental. Here, we discuss the plethora of recent data from clinical studies and small animal disease models that implicate excessive inflammasome responses in the pathogenesis of a number of acute and chronic respiratory inflammatory diseases. Understanding of the role of inflammasomes in lung disease is of great therapeutic interest.

Defining the distinct, intrinsic properties of the novel type I interferon, IFNϵ

The type I interferons (IFNs) are a family of cytokines with diverse biological activities, including antiviral, antiproliferative, and immunoregulatory functions. The discovery of the hormonally regulated, constitutively expressed IFNϵ has suggested a function for IFNs in reproductive tract homeostasis and protection from infections, but its intrinsic activities are untested. We report here the expression, purification, and functional characterization of murine IFNϵ (mIFNϵ). Recombinant mIFNϵ (rmIFNϵ) exhibited an α-helical fold characteristic of type I IFNs and bound to IFNα/β receptor 1 (IFNAR1) and IFNAR2, but, unusually, it had a preference for IFNAR1. Nevertheless, rmIFNϵ induced typical type I IFN signaling activity, including STAT1 phosphorylation and activation of canonical type I IFN signaling reporters, demonstrating that it uses the JAK-STAT signaling pathway. We also found that rmIFNϵ induces the activation of T, B, and NK cells and exhibits antiviral, antiproliferative, and antibacterial activities typical of type I IFNs, albeit with 100-1000-fold reduced potency compared with rmIFNα1 and rmIFNβ. Surprisingly, although the type I IFNs generally do not display cross-species activities, rmIFNϵ exhibited high antiviral activity on human cells, suppressing HIV replication and inducing the expression of known HIV restriction factors in human lymphocytes. Our findings define the intrinsic properties of murine IFNϵ, indicating that it distinctly interacts with IFNAR and elicits pathogen-suppressing activity with a potency enabling host defense but with limited toxicity, appropriate for a protein expressed constitutively in a sensitive mucosal site, such as the reproductive tract.

Highly pathogenic avian H5N8 influenza viruses: should we be concerned?

Avian influenza A viruses pose a constant threat to global human health as sporadic infections continue to occur with associated high mortality rates. To date, a number of avian influenza virus subtypes have infected humans, including H5N1, H7N9, H9N2 and H7N7. The majority of ‘bird flu’ cases are thought to have arisen from direct contact with infected poultry, particularly in live markets in Asia.¹ While human cases of the H5N8 subtype have not been documented as yet, there is the potential that H5N8 viruses could acquire mutations which favour infection of human cells. There is also the possibility that novel viruses with a tropism for human cells could be generated if H5N8 should reassasort with other circulating avian viruses, such as those of the H5N1 subtype. The emergence of a novel H5N8 virus with the capability of infecting humans could have drastic consequences to global health.

SIDT2 Transports Extracellular dsRNA into the Cytoplasm for Innate Immune Recognition

Double-stranded RNA (dsRNA) is a common by-product of viral infections and acts as a potent trigger of antiviral immunity. In the nematode C. elegans, sid-1 encodes a dsRNA transporter that is highly conserved throughout animal evolution, but the physiological role of SID-1 and its orthologs remains unclear. Here, we show that the mammalian SID-1 ortholog, SIDT2, is required to transport internalized extracellular dsRNA from endocytic compartments into the cytoplasm for immune activation. Sidt2-deficient mice exposed to extracellular dsRNA, encephalomyocarditis virus (EMCV), and herpes simplex virus 1 (HSV-1) show impaired production of antiviral cytokines and-in the case of EMCV and HSV-1-reduced survival. Thus, SIDT2 has retained the dsRNA transport activity of its C. elegans ortholog, and this transport is important for antiviral immunity.

Therapeutic Targeting of the IL-6 Trans-Signaling/Mechanistic Target of Rapamycin Complex 1 Axis in Pulmonary Emphysema

RATIONALE

The potent immunomodulatory cytokine IL-6 is consistently up-regulated in human lungs with emphysema and in mouse emphysema models; however, the mechanisms by which IL-6 promotes emphysema remain obscure. IL-6 signals using two distinct modes: classical signaling via its membrane-bound IL-6 receptor (IL-6R), and trans-signaling via a naturally occurring soluble IL-6R.

OBJECTIVES

To identify whether IL-6 trans-signaling and/or classical signaling contribute to the pathogenesis of emphysema.

METHODS

We used the gp130^F/F genetic mouse model for spontaneous emphysema and cigarette smoke-induced emphysema models. Emphysema in mice was quantified by various methods including in vivo lung function and stereology, and terminal deoxynucleotidyl transferase dUTP nick end labeling assay was used to assess alveolar cell apoptosis. In mouse and human lung tissues, the expression level and location of IL-6 signaling-related genes and proteins were measured, and the levels of IL-6 and related proteins in sera from emphysematous mice and patients were also assessed.

MEASUREMENTS AND MAIN RESULTS

Lung tissues from patients with emphysema, and from spontaneous and cigarette smoke-induced emphysema mouse models, were characterized by excessive production of soluble IL-6R. Genetic blockade of IL-6 trans-signaling in emphysema mouse models and therapy with the IL-6 trans-signaling antagonist sgp130Fc ameliorated emphysema by suppressing augmented alveolar type II cell apoptosis. Furthermore, IL-6 trans-signaling-driven emphysematous changes in the lung correlated with mechanistic target of rapamycin complex 1 hyperactivation, and treatment of emphysema mouse models with the mechanistic target of rapamycin complex 1 inhibitor rapamycin attenuated emphysematous changes.

CONCLUSIONS

Collectively, our data reveal that specific targeting of IL-6 trans-signaling may represent a novel treatment strategy for emphysema.

Suppressor of cytokine signaling (SOCS)5 ameliorates influenza infection via inhibition of EGFR signaling

Influenza virus infections have a significant impact on global human health. Individuals with suppressed immunity, or suffering from chronic inflammatory conditions such as COPD, are particularly susceptible to influenza. Here we show that suppressor of cytokine signaling (SOCS) five has a pivotal role in restricting influenza A virus in the airway epithelium, through the regulation of epidermal growth factor receptor (EGFR). Socs5-deficient mice exhibit heightened disease severity, with increased viral titres and weight loss. Socs5 levels were differentially regulated in response to distinct influenza viruses (H1N1, H3N2, H5N1 and H11N9) and were reduced in primary epithelial cells from COPD patients, again correlating with increased susceptibility to influenza. Importantly, restoration of SOCS5 levels restricted influenza virus infection, suggesting that manipulating SOCS5 expression and/or SOCS5 targets might be a novel therapeutic approach to influenza.

PB1-F2 Peptide Derived from Avian Influenza A Virus H7N9 Induces Inflammation via Activation of the NLRP3 Inflammasome

The emergence of avian H7N9 influenza A virus in humans with associated high mortality has highlighted the threat of a potential pandemic. Fatal H7N9 infections are characterized by hyperinflammation and increased cellular infiltrates in the lung. Currently there are limited therapies to address the pathologies associated with H7N9 infection and the virulence factors that contribute to these pathologies. We have found that PB1-F2 derived from H7N9 activates the NLRP3 inflammasome and induces lung inflammation and cellular recruitment that is NLRP3-dependent. We have also shown that H7N9 and A/Puerto Rico/H1N1 (PR8)PB1-F2 peptide treatment induces significant mitochondrial reactive oxygen production, which contributes to NLRP3 activation. Importantly, treatment of cells or mice with the specific NLRP3 inhibitor MCC950 significantly reduces IL-1β maturation, lung cellular recruitment, and cytokine production. Together, these results suggest that PB1-F2 from H7N9 avian influenza A virus may be a major contributory factor to disease pathophysiology and excessive inflammation characteristic of clinical infections and that targeting the NLRP3 inflammasome may be an effective means to reduce the inflammatory burden associated with H7N9 infections.

Hero turned villain: NLRP3 inflammasome-induced inflammation during influenza A virus infection

The severity of influenza A virus (IAV) infection can range from asymptotic to mild to severe. Infections, such as those seen following outbreaks of avian IAV, are associated with hyperinflammatory responses and the development of fatal disease. There is a continual threat that a novel or pandemic IAV will circulate in humans with high rates of mortality. The neuronal apoptosis inhibitor protein, class 2 transcription activator of the MHC, heterokaryon incompatibility, telomerase-associated protein 1, leucine-rich repeat, and pyrin domain-containing protein 3 (NLRP3) inflammasome is an innate immune sensor that has been shown to be critical for the secretion of the potent proinflammatory cytokines, IL-1β and IL-18, as well as chemokine production and cellular inflammation in vivo following IAV infection. Initial studies illustrated a protective role of NLRP3 during severe IAV infection in mice. However, the NLRP3 inflammasome may be a hero that turns villain in the later stages of severe IAV infection via the promotion of a hyperinflammatory state. Current treatments for patients who present to hospitals with a severe IAV infection are limited. The understanding of the mechanisms involved in the induction of NLRP3-dependent inflammation during severe IAV infections may provide new therapeutic targets that reduce human mortality.

Endogenous Murine BST-2/Tetherin Is Not a Major Restriction Factor of Influenza A Virus Infection

BST-2 (tetherin, CD317, HM1.24) restricts virus growth by tethering enveloped viruses to the cell surface. The role of BST-2 during influenza A virus infection (IAV) is controversial. Here, we assessed the capacity of endogenous BST-2 to restrict IAV in primary murine cells. IAV infection increased BST-2 surface expression by primary macrophages, but not alveolar epithelial cells (AEC). BST-2-deficient AEC and macrophages displayed no difference in susceptibility to IAV infection relative to wild type cells. Furthermore, BST-2 played little role in infectious IAV release from either AEC or macrophages. To examine BST-2 during IAV infection in vivo, we infected BST-2-deficient mice. No difference in weight loss or in viral loads in the lungs and/or nasal tissues were detected between BST-2-deficient and wild type animals. This study rules out a major role for endogenous BST-2 in modulating IAV in the mouse model of infection.

MicroRNA as Type I Interferon-Regulated Transcripts and Modulators of the Innate Immune Response

Type I interferons (IFNs) are an important family of cytokines that regulate innate and adaptive immune responses to pathogens, in cancer and inflammatory diseases. While the regulation and role of protein-coding genes involved in these responses are well characterized, the role of non-coding microRNAs in the IFN responses is less developed. We review the emerging picture of microRNA regulation of the IFN response at the transcriptional and post-transcriptional level. This response forms an important regulatory loop; several microRNAs target transcripts encoding components at many steps of the type I IFN response, both production and action, at the receptor, signaling, transcription factor, and regulated gene level. Not only do IFNs regulate positive signaling molecules but also negative regulators such as SOCS1. In total, 36 microRNA are reported as IFN regulated. Given this apparent multipronged targeting of the IFN response by microRNAs and their well-characterized capacity to “buffer” responses in other situations, the prospects of improved sequencing and microRNA targeting technologies will facilitate the elucidation of the broader regulatory networks of microRNA in this important biological context, and their therapeutic and diagnostic potential.

Sequence-dependent off-target inhibition of TLR7/8 sensing by synthetic microRNA inhibitors

Anti-microRNA (miRNA) oligonucleotides (AMOs) with 2'-O-Methyl (2'OMe) residues are commonly used to study miRNA function and can achieve high potency, with low cytotoxicity. Not withstanding this, we demonstrate the sequence-dependent capacity of 2'OMe AMOs to inhibit Toll-like receptor (TLR) 7 and 8 sensing of immunostimulatory RNA, independent of their miRNA-targeting function. Through a screen of 29 AMOs targeting common miRNAs, we found a subset of sequences highly inhibitory to TLR7 sensing in mouse macrophages. Interspecies conservation of this inhibitory activity was confirmed on TLR7/8 activity in human peripheral blood mononuclear cells. Significantly, we identified a core motif governing the inhibitory activity of these AMOs, which is present in more than 50 AMOs targeted to human miRNAs in miRBaseV20. DNA/locked nucleic acids (LNA) AMOs synthesized with a phosphorothioate backbone also inhibited TLR7 sensing in a sequence-dependent manner, demonstrating that the off-target effects of AMOs are not restricted to 2'OMe modification. Taken together, our work establishes the potential for off-target effects of AMOs on TLR7/8 function, which should be taken into account in their therapeutic development and in vivo application.

Playing hide and seek: how glycosylation of the influenza virus hemagglutinin can modulate the immune response to infection

Seasonal influenza A viruses (IAV) originate from pandemic IAV and have undergone changes in antigenic structure, including addition of glycans to the hemagglutinin (HA) glycoprotein. The viral HA is the major target recognized by neutralizing antibodies and glycans have been proposed to shield antigenic sites on HA, thereby promoting virus survival in the face of widespread vaccination and/or infection. However, addition of glycans can also interfere with the receptor binding properties of HA and this must be compensated for by additional mutations, creating a fitness barrier to accumulation of glycosylation sites. In addition, glycans on HA are also recognized by phylogenetically ancient lectins of the innate immune system and the benefit provided by evasion of humoral immunity is balanced by attenuation of infection. Therefore, a fine balance must exist regarding the optimal pattern of HA glycosylation to offset competing pressures associated with recognition by innate defenses, evasion of humoral immunity and maintenance of virus fitness. In this review, we examine HA glycosylation patterns of IAV associated with pandemic and seasonal influenza and discuss recent advancements in our understanding of interactions between IAV glycans and components of innate and adaptive immunity.

Activation of the NLRP3 inflammasome by IAV virulence protein PB1-F2 contributes to severe pathophysiology and disease

The ability for a host to recognize infection is critical for virus clearance and often begins with induction of inflammation. The PB1-F2 of pathogenic influenza A viruses (IAV) contributes to the pathophysiology of infection, although the mechanism for this is unclear. The NLRP3-inflammasome has been implicated in IAV pathogenesis, but whether IAV virulence proteins can be activators of the complex is unknown. We investigated whether PB1-F2-mediated activation of the NLRP3-inflammasome is a mechanism contributing to overt inflammatory responses to IAV infection. We show PB1-F2 induces secretion of pyrogenic cytokine IL-1β by activating the NLRP3-inflammasome, contributing to inflammation triggered by pathogenic IAV. Compared to infection with wild-type virus, mice infected with reverse engineered PB1-F2-deficient IAV resulted in decreased IL-1β secretion and cellular recruitment to the airways. Moreover, mice exposed to PB1-F2 peptide derived from pathogenic IAV had enhanced IL-1β secretion compared to mice exposed to peptide derived from seasonal IAV. Implicating the NLRP3-inflammasome complex specifically, we show PB1-F2 derived from pathogenic IAV induced IL-1β secretion was Caspase-1-dependent in human PBMCs and NLRP3-dependent in mice. Importantly, we demonstrate PB1-F2 is incorporated into the phagolysosomal compartment, and upon acidification, induces ASC speck formation. We also show that high molecular weight aggregated PB1-F2, rather than soluble PB1-F2, induces IL-1β secretion. Furthermore, NLRP3-deficient mice exposed to PB1-F2 peptide or infected with PB1-F2 expressing IAV were unable to efficiently induce the robust inflammatory response as observed in wild-type mice. In addition to viral pore forming toxins, ion channel proteins and RNA, we demonstrate inducers of NLRP3-inflammasome activation may include disordered viral proteins, as exemplified by PB1-F2, acting as host pathogen ‘danger’ signals. Elucidating immunostimulatory PB1-F2 mediation of NLRP3-inflammasome activation is a major step forward in our understanding of the aetiology of disease attributable to exuberant inflammatory responses to IAV infection.

Influenza virus is a highly contagious respiratory pathogen that can cause pandemics, resulting in the deaths of millions worldwide. Previously we demonstrated that PB1-F2 protein produced by pathogenic influenza induces overwhelming inflammatory responses to infection, which enhances disease. The way in which PB1-F2 causes this overt inflammation is unclear. Recently, influenza virus was shown to be involved in activating the inflammasome, which plays a pivotal role during inflammatory responses to infection. However, whether virulence factors such as PB1-F2 produced by the virus can play a role in activation of the inflammasome is unknown. Here, we investigated whether PB1-F2 could have a role in activation of the inflammasome. Using detection of the inflammatory cytokine IL-1β as a marker for inflammasome complex activation, we definitively show PB1-F2 from a pathogenic strain rapidly induces activation of the inflammasome in humans and mice. Using macrophages from mice lacking components of the inflammasome complex, induction of inflammation was shown to be Caspase-1 and NLRP3-dependent. Inflammation induced by PB1-F2 was abrogated in NLRP3-deficient mice. To our knowledge, this is the first description of the mechanism of PB1-F2-mediated inflammasome complex activation. Our work provides further understanding of the contribution of PB1-F2 to enhancing inflammation during influenza infections.

Increased antigen specific T cell numbers in the absence of altered migration or division rates as a result of mucosal cholera toxin administration

Cholera toxin (CT) is a mucosal adjuvant capable of inducing strong immune responses to co-administered antigens following oral or intranasal immunization of mice. To date, the direct effect of CT on antigen-specific CD4(+) T cell migration and proliferation profiles in vivo is not well characterized. In this study, the effect of CT on the migration pattern and proliferative responses of adoptively transferred, CD4(+) TCR transgenic T cells in orally or intranasally vaccinated mice, was analyzed by flow cytometry. GFP-expressing or CFSE-labeled OT-II lymphocytes were adoptively transferred to naïve C57BL/6 mice, and mice were subsequently vaccinated with OVA with or without CT via the oral or intranasal route. CT did not alter the migration pattern of antigen-specific T cells, regardless of the route of immunization, but increased the number of transgenic CD4(+) T cells in draining lymphoid tissue. This increase in the number of transgenic CD4(+) T cells was not due to cells undergoing more rounds of cellular division in vivo, suggesting that CT may exert an indirect adjuvant effect on CD4(+) T cells. The findings reported here suggest that CT functions as a mucosal adjuvant by increasing the number of antigen specific CD4(+) T cells independent of their migration pattern or kinetics of cellular division.

Addition of glycosylation to influenza A virus hemagglutinin modulates antibody-mediated recognition of H1N1 2009 pandemic viruses

Seasonal influenza A viruses (IAV) originate from pandemic IAV and have undergone changes in antigenic structure, including addition of glycans to the viral hemagglutinin (HA). Glycans on the head of HA promote virus survival by shielding antigenic sites, but highly glycosylated seasonal IAV are inactivated by soluble lectins of the innate immune system. In 2009, human strains of pandemic H1N1 [A(H1N1)pdm] expressed a single glycosylation site (Asn(104)) on the head of HA. Since then, variants with additional glycosylation sites have been detected, and the location of these sites has been distinct to those of recent seasonal H1N1 strains. We have compared wild-type and reverse-engineered A(H1N1)pdm IAV with differing potential glycosylation sites on HA for sensitivity to collectins and to neutralizing Abs. Addition of a glycan (Asn(136)) to A(H1N1)pdm HA was associated with resistance to neutralizing Abs but did not increase sensitivity to collectins. Moreover, variants expressing Asn(136) showed enhanced growth in A(H1N1)pdm-vaccinated mice, consistent with evasion of Ab-mediated immunity in vivo. Thus, a fine balance exists regarding the optimal pattern of HA glycosylation to facilitate evasion of Ab-mediated immunity while maintaining resistance to lectin-mediated defenses of the innate immune system.

Cell-surface receptors on macrophages and dendritic cells for attachment and entry of influenza virus

Review of interactions between influenza A virus and C‐type lectin receptors on macrophages and dendritic cells that may result in virus entry and infection.

Airway MΦ and DCs are important components of innate host defense and can play a critical role in limiting the severity of influenza virus infection. Although it has been well established that cell‐surface SA acts as a primary attachment receptor for IAV, the particular receptor(s) or coreceptor(s) that mediate IAV entry into any cell, including MΦ and DC, have not been clearly defined. Identifying which receptors are involved in attachment and entry of IAV into immune cells may have important implications in regard to understanding IAV tropism and pathogenesis. Recent evidence suggests that specialized receptors on MΦ and DCs, namely CLRs, can act as capture and/or entry receptors for many viral pathogens, including IAV. Herein, we review the early stages of infection of MΦ and DC by IAV. Specifically, we examine the potential role of CLRs expressed on MΦ and DC to act as attachment and/or entry receptors for IAV.

Responses of mouse airway epithelial cells and alveolar macrophages to virulent and avirulent strains of influenza A virus

Influenza A virus (IAV) infection is associated with outcomes ranging from subclinical infection to severe pneumonia. In this study, we compared IAV strains BJx109 (H3N2), HKx31 (H3N2), and PR8 (H1N1), for their ability to elicit innate immune responses from mouse airway cells in vitro and their virulence in mice. The viruses differed markedly in their ability to induce disease in mice (PR8 > HKx31 > BJx109). In particular, PR8 infection was associated with high levels of virus replication and pulmonary inflammation. We next compared the ability of each virus strain to infect and induce inflammatory mediators from mouse airway cells. First, major differences were observed in the ability of viruses to infect and induce chemokines and cytokines from mouse alveolar macrophages (BJx109 > HKx31 > PR8), but not from airway epithelial cells (AEC) in vitro. Second, C-type lectins of the innate immune system in mouse lung fluids blocked the ability of BJx109, but not PR8, to infect mouse macrophages and AEC. The failure of the virulent PR8 virus to elicit responses from airway macrophages, combined with resistance to antiviral proteins in mouse airway fluids, likely contribute to virulence in mice. These findings provide insight into the mechanisms underlying disease severity in the mouse model of influenza infection.

Specific sites of N-linked glycosylation on the hemagglutinin of H1N1 subtype influenza A virus determine sensitivity to inhibitors of the innate immune system and virulence in mice

Oligosaccharides on the hemagglutinin (HA) and neuraminidase of influenza A virus (IAV) are a target for recognition by lectins of the innate immune system, including soluble surfactant protein-D and the macrophage mannose receptor on airway macrophages. Glycans attached to the head of H1 subtype of IAV differ markedly in number and location. A reverse genetic approach was used to define the importance of particular N-glycosylation sites on H1 in determining sensitivity to innate immune defenses and virulence in mice. The HA of A/PR/8/34 (PR8, H1N1) and A/Brazil/11/78 (Brazil, H1N1) express zero and four glycosylation sites on the head of HA, respectively. Site-directed mutagenesis was used to add (PR8) or delete (Brazil) glycosylation sites, and IAV expressing wild-type or mutant HA were generated on a PR8 backbone. Addition or removal of particular glycans modulated sensitivity to mouse lung fluids but was not a major factor determining susceptibility of airway macrophages to infection. PR8 is a mouse-adapted virus, and mutations in multiple IAV genes have been shown to contribute to virulence, yet addition of glycosylation to PR8 HA was sufficient to attenuate disease. In contrast, removal of glycans from Brazil HA resulted in severe disease and death. These studies provide insight regarding the mechanisms by which IAV can induce disease in mice. Moreover, reduced glycosylation of HA is likely to be an important factor associated with adaptation of human IAV to growth in mouse lung.

Receptor specificity of the influenza virus hemagglutinin modulates sensitivity to soluble collectins of the innate immune system and virulence in mice

The hemagglutinin (HA) glycoprotein of influenza virus binds to cell surface sialic acid (SA) to initiate infection. In this study, a mutant of influenza A virus strain BJx109 (H3N2) was plaque-purified from the lungs of virus-infected mice that had been depleted of airway macrophages. Sequence analysis identified a single amino acid substitution (S186I) in the vicinity of the receptor-binding site of HA. This substitution was associated with enhanced binding to α(2,3)-Gal-linked SA and an increased ability to infect murine airway epithelial cells. Mutant viruses were less sensitive to neutralization by mouse airway fluids and less efficient in their ability to infect murine macrophages. Moreover, infection of mice with viruses bearing the S186I substitution led to severe disease, characterized by enhanced virus replication, lung pathology and pulmonary edema. Together, these studies confirm that residue 186 of H3 subtype viruses is a critical determinant of virulence in a mouse model of influenza infection.

The role of neutrophils during mild and severe influenza virus infections of mice

Neutrophils have been implicated in both protective and pathological responses following influenza virus infections. We have used mAb 1A8 (anti-Ly6G) to specifically deplete LyG6(high) neutrophils and induce neutropenia in mice infected with virus strains known to differ in virulence. Mice were also treated with mAb RB6-8C5 (anti-Ly6C/G or anti-Gr-1), a mAb widely used to investigate the role of neutrophils in mice that has been shown to bind and deplete additional leukocyte subsets. Using mAb 1A8, we confirm the beneficial role of neutrophils in mice infected with virus strains of intermediate (HKx31; H3N2) or high (PR8; H1N1) virulence whereas treatment of mice infected with an avirulent strain (BJx109; H3N2) did not affect disease or virus replication. Treatment of BJx109-infected mice with mAb RB6-8C5 was, however, associated with significant weight loss and enhanced virus replication indicating that other Gr-1(+) cells, not neutrophils, limit disease severity during mild influenza infections.

Glycosylation of the hemagglutinin modulates the sensitivity of H3N2 influenza viruses to innate proteins in airway secretions and virulence in mice

Collectins in airway fluids and membrane-associated lectins such as the macrophage mannose receptor (MMR) recognize mannose-rich glycans on the envelope glycoproteins of influenza A viruses. In this study, we used a reverse genetic approach to examine the role of particular N-linked glycosylation sites on the hemagglutinin (HA) of A/Beijing/353/89 (Beij/89, H3N2) in determining sensitivity to lectin-mediated immune defenses and virulence in mice. We generated 7:1 reassortant viruses on an A/PR/8/34 'backbone' with Beij/89 HA or HA lacking one or more glycosylation sites. Asn(165) was an important determinant of sensitivity to mouse collectins and virulence but did not alter susceptibility of airway macrophages to infection. Removal of both Asn(165) and Asn(246) led to a further increase in virulence, characterized by enhanced virus replication, pulmonary inflammation and vascular leak. These studies define the importance of particular glycans on H3 HA in determining sensitivity to airway collectins and virulence in mice.