Therapeutic effect of anti-macrophage inflammatory protein 2 antibody on influenza virus-induced pneumonia in mice

Blocking CCL3-mediated neutrophil recruitment into the brain alleviates immunopathology following severe enterovirus 71 infection

Inflammatory cells infiltration in the cerebrospinal fluid is a hallmark of severe enterovirus 71 (EV71) infection, but which type of immune cells are critical for severe EV71 infection remains unclear. Here, we observe that both neutrophils and macrophages are increased in the brains of patients and mice with severe EV71 infection, and the depletion of neutrophils but not macrophages results in a marked enhancement of survival of EV71-infected mice. Furthermore, CCR1/3 may play an important role in CCL3 facilitating the accumulation of neutrophils in the brains of patients. Inhibition of CCL3 by anti-CCL3 antibodies or selected miRNAs significantly reduces the neutrophils infiltration in brains and the mortality of EV71-infected mice. Collectively, CCL3-mediated neutrophils recruitment into the brain contributes to the severe immunopathology of EV71 infection, which provides a potential diagnostic and therapeutic target for EV71 infection.

Preclinical animal models to evaluate therapeutic antiviral antibodies

Despite the availability of effective preventative vaccines and potent small-molecule antiviral drugs, effective non-toxic prophylactic and therapeutic measures are still lacking for many viruses. The use of monoclonal and polyclonal antibodies in an antiviral context could fill this gap and provide effective virus-specific medical interventions. In order to develop these therapeutic antibodies, preclinical animal models are of utmost importance. Due to the variability in viral pathogenesis, immunity and overall characteristics, the most representative animal model for human viral infection differs between virus species. Therefore, throughout the years researchers sought to find the ideal preclinical animal model for each virus. The most used animal models in preclinical research include rodents (mice, ferrets, …) and non-human primates (macaques, chimpanzee, ….). Currently, antibodies are tested for antiviral efficacy against a variety of viruses including different hepatitis viruses, human immunodeficiency virus (HIV), influenza viruses, respiratory syncytial virus (RSV), severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) and rabies virus. This review provides an overview of the current knowledge about the preclinical animal models that are used for the evaluation of therapeutic antibodies for the abovementioned viruses.

Paradoxical immunodeficiencies-When failures of innate immunity cause immunopathology

Innate immunity facilitates immediate defense against invading pathogens throughout all organs and tissues but also mediates tissue homeostasis and repair, thereby playing a key role in health and development. Recognition of pathogens is mediated by germline-encoded PRRs. Depending on the specific PRRs triggered, ligand binding leads to phagocytosis and pathogen killing and the controlled release of immune-modulatory factors such as IFNs, cytokines, or chemokines. PRR-mediated and other innate immune responses do not only prevent uncontrolled replication of intruding pathogens but also contribute to the tailoring of an effective adaptive immune response. Therefore, hereditary or acquired immunodeficiencies impairing innate responses may paradoxically cause severe immunopathology in patients. This can occur in the context of, but also independently of an increased microbial burden. It can include pathogen-dependent organ damage, autoinflammatory syndromes, and neurodevelopmental or neurodegenerative diseases. Here, we discuss the current state of research of several different such immune paradoxes. Understanding the underlying mechanisms causing immunopathology as a consequence of failures of innate immunity may help to prevent life-threatening disease.

Identification of Nifurtimox and Chrysin as Anti-Influenza Virus Agents by Clinical Transcriptome Signature Reversion

The rapid development in the field of transcriptomics provides remarkable biomedical insights for drug discovery. In this study, a transcriptome signature reversal approach was conducted to identify the agents against influenza A virus (IAV) infection through dissecting gene expression changes in response to disease or compounds’ perturbations. Two compounds, nifurtimox and chrysin, were identified by a modified Kolmogorov–Smirnov test statistic based on the transcriptional signatures from 81 IAV-infected patients and the gene expression profiles of 1309 compounds. Their activities were verified in vitro with half maximal effective concentrations (EC₅₀s) from 9.1 to 19.1 μM against H1N1 or H3N2. It also suggested that the two compounds interfered with multiple sessions in IAV infection by reversing the expression of 28 IAV informative genes. Through network-based analysis of the 28 reversed IAV informative genes, a strong synergistic effect of the two compounds was revealed, which was confirmed in vitro. By using the transcriptome signature reversion (TSR) on clinical datasets, this study provides an efficient scheme for the discovery of drugs targeting multiple host factors regarding clinical signs and symptoms, which may also confer an opportunity for decelerating drug-resistant variant emergence.

Disease Tolerance during Viral-Bacterial Co-Infections

Disease tolerance has emerged as an alternative way, in addition to host resistance, to survive viral-bacterial co-infections. Disease tolerance plays an important role not in reducing pathogen burden, but in maintaining tissue integrity and controlling organ damage. A common co-infection is the synergy observed between influenza virus and Streptococcus pneumoniae that results in superinfection and lethality. Several host cytokines and cells have shown promise in promoting tissue protection and damage control while others induce severe immunopathology leading to high levels of morbidity and mortality. The focus of this review is to describe the host cytokines and innate immune cells that mediate disease tolerance and lead to a return to host homeostasis and ultimately, survival during viral-bacterial co-infection.

Targeting Macrophage Dysregulation for Viral Infections: Novel Targets for Immunomodulators

A major barrier to human immunodeficiency virus (HIV-1) cure is the latent viral reservoir, which persists despite antiretroviral therapy (ART), including across the non-dividing myeloid reservoir which is found systemically in sanctuary sites across tissues and the central nervous system (CNS). Unlike activated CD4+ T cells that undergo rapid cell death during initial infection (due to rapid viral replication kinetics), viral replication kinetics are delayed in non-dividing myeloid cells, resulting in long-lived survival of infected macrophages and macrophage-like cells. Simultaneously, persistent inflammation in macrophages confers immune dysregulation that is a key driver of co-morbidities including cardiovascular disease (CVD) and neurological deficits in people living with HIV-1 (PLWH). Macrophage activation and dysregulation is also a key driver of disease progression across other viral infections including SARS-CoV-2, influenza, and chikungunya viruses, underscoring the interplay between macrophages and disease progression, pathogenesis, and comorbidity in the viral infection setting. This review discusses the role of macrophages in persistence and pathogenesis of HIV-1 and related comorbidities, SARS-CoV-2 and other viruses. A special focus is given to novel immunomodulatory targets for key events driving myeloid cell dysregulation and reservoir maintenance across a diverse array of viral infections.

Rho signaling inhibition mitigates lung injury via targeting neutrophil recruitment and selectin-AKT signaling

Neutrophils, the early responders of the immune system, eliminate intruders, but their over-activation can also instigate tissue damage leading to various autoimmune and inflammatory disease conditions. As approaches causing neutropenia are associated with immunodeficiency, targeting aberrant neutrophil infiltration offers an attractive strategy in neutrophil-centered diseases including acute lung injury. Rho GTPase family proteins Rho, Rac and Cdc42 play important role as regulators of chemotaxis in diverse systems. Rho inhibitors protected against lung injuries, while genetic Rho-deficiency exhibited neutrophil hyperactivity and exacerbated lung injury. These differential outcomes might be due to distinct effects on different cell types or activation/ inhibition of specific signaling pathways responsible for neutrophil polarity, migration and functions. In this study, we explored neutrophil centric effects of Rho signaling mitigation. Consistent with previous reports, Rho signaling inhibitor Y-27632 provided protection against acute lung injury, but without regulating LPS mediated systemic increase of neutrophils in the circulation. Interestingly, the adoptive transfer approach identified a specific defect in neutrophil migration capacity after Rho signaling mitigation. These defects were associated with loss of polarity and altered actin dynamics identified using time-lapse in vitro studies. Further analysis revealed a rescue of stimulation-dependent L-selectin shedding on neutrophils with Rho signaling inhibitor. Surprisingly, functional blocking of L-selectin (CD62L) led to defective recruitment of neutrophils into inflamed lungs. Further, single-cell level analyses identified MAPK signaling as downstream mechanism of Rho signaling and L-selectin mediated effects. p-AKT levels were diminished in detergent resistance membrane-associated signalosome upon Rho signaling inhibition and blockade of selectin. Moreover, inhibition of AKT signaling as well as selectin blocking led to defects in neutrophil polarity. Together, this study identified Rho-dependent distinct L-selectin and AKT signaling mediated regulation of neutrophil recruitment to inflamed lung tissue.

Complement Decay-Accelerating Factor is a modulator of influenza A virus lung immunopathology

Clearance of viral infections, such as SARS-CoV-2 and influenza A virus (IAV), must be fine-tuned to eliminate the pathogen without causing immunopathology. As such, an aggressive initial innate immune response favors the host in contrast to a detrimental prolonged inflammation. The complement pathway bridges innate and adaptive immune system and contributes to the response by directly clearing pathogens or infected cells, as well as recruiting proinflammatory immune cells and regulating inflammation. However, the impact of modulating complement activation in viral infections is still unclear. In this work, we targeted the complement decay-accelerating factor (DAF/CD55), a surface protein that protects cells from non-specific complement attack, and analyzed its role in IAV infections. We found that DAF modulates IAV infection in vivo, via an interplay with the antigenic viral proteins hemagglutinin (HA) and neuraminidase (NA), in a strain specific manner. Our results reveal that, contrary to what could be expected, DAF potentiates complement activation, increasing the recruitment of neutrophils, monocytes and T cells. We also show that viral NA acts on the heavily sialylated DAF and propose that the NA-dependent DAF removal of sialic acids exacerbates complement activation, leading to lung immunopathology. Remarkably, this mechanism has no impact on viral loads, but rather on the host resilience to infection, and may have direct implications in zoonotic influenza transmissions.

Exacerbated complement activation and immune deregulation are at the basis of several pathologies induced by respiratory viruses. Here, we report that complement decay-accelerating factor (DAF), which inhibits complement activation in healthy cells, increases disease severity upon influenza A virus (IAV) infection. Remarkably, DAF interaction with IAV proteins, hemagglutinin (HA) and neuraminidase (NA), resulted in excessive complement activation and recruitment of innate and adaptive immune cells, without affecting viral loads. Furthermore, we observed that viral NA directly cleaves DAF and promotes complement activation, providing a possible link between IAV-DAF interaction and pathology. Therefore, our results unveil a novel pathway that could modulate disease severity, which may help to understand the increased pathogenicity of zoonotic and pandemic IAV infections.

CCR2 mediates the adverse effects of LPS in the pregnant mouse

In our earlier work, we found that intrauterine (i.u.) and intraperitoneal (i.p.) injection of LPS (10-μg serotype 0111:B4) induced preterm labor (PTL) with high pup mortality, marked systemic inflammatory response and hypotension. Here, we used both i.u. and i.p. LPS models in pregnant wild-type (wt) and CCR2 knockout (CCR2-/-) mice on E16 to investigate the role played by the CCL2/CCR2 system in the response to LPS. Basally, lower numbers of monocytes and macrophages and higher numbers of neutrophils were found in the myometrium, placenta, and blood of CCR2-/- vs. wt mice. After i.u. LPS, parturition occurred at 14 h in both groups of mice. At 7 h post-injection, 70% of wt pups were dead vs. 10% of CCR2-/- pups, but at delivery 100% of wt and 90% of CCR2-/- pups were dead. Myometrial and placental monocytes and macrophages were generally lower in CCR2-/- mice, but this was less consistent in the circulation, lung, and liver. At 7 h post-LPS, myometrial ERK activation was greater and JNK and p65 lower and the mRNA levels of chemokines were higher and of inflammatory cytokines lower in CCR2-/- vs. wt mice. Pup brain and placental inflammation were similar. Using the IP LPS model, we found that all measures of arterial pressure increased in CCR2-/- but declined in wt mice. These data suggest that the CCL2/CCR2 system plays a critical role in the cardiovascular response to LPS and contributes to pup death but does not influence the onset of inflammation-induced PTL.

Immunopathological similarities between COVID-19 and influenza: Investigating the consequences of Co-infection

Coronavirus disease 2019 (COVID-19) caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has been a global public health emergency since December 2019, and so far, more than 980,000 people (until September 24, 2020) around the world have died. SARS-CoV-2 mimics the influenza virus regarding methods and modes of transmission, clinical features, related immune responses, and seasonal coincidence. Accordingly, co-infection by these viruses is imaginable because some studies have reported several cases with SARS-CoV-2 and influenza virus co-infection. Given the importance of the mentioned co-infection and the coming influenza season, it is essential to recognize the similarities and differences between the symptoms, immunopathogenesis and treatment of SARS-CoV-2 and influenza virus. Therefore, we reviewed the virology, clinical features, and immunopathogenesis of both influenza virus and SARS-CoV-2 and evaluated outcomes in cases with SARS-CoV-2 and influenza virus co-infection.

Comparison of cytokine profiles induced by nonlethal and lethal doses of influenza A virus in mice

Influenza viruses are among the most common human pathogens and are responsible for causing extensive seasonal morbidity and mortality. To investigate the immunological factors associated with severe influenza infection, the immune responses in mice infected with nonlethal (LD0) doses of A/PR/8/34 (H1N1) influenza virus were compared with those of mice infected with a lethal dose (LD100) of the virus. The virus titer and activation of retinoic acid-inducible gene (RIG)-I-like receptor signaling pathways were similar in the mice infected with LD0 and LD100 at 2 days post-infection; however, mice infected with LD100 exhibited a greater abundance of cytokines and a more diverse cytokine profile. Infection with LD100 induced the expression of the following factors: Interleukins (ILs), IL-4, IL-7, IL-10, IL-11, IL-12p40, IL-13 and IL-15; inflammatory chemokines, C-C motif chemokine ligand (CCL)2, CCL3/4, CCL12, CCL17, CCL19; and lung injury-associated cytokines, leptin, leukaemia inhibitory factor, macrophage colony stimulating factor, pentraxin (PTX)2 and PTX3, WNT1-inducible-signaling pathway protein 1, matrix metallopeptidase (MMP)-2, MMP-3, proprotein convertase subtilisin/kexin type 9, and T cell immunoglobulin and mucin domain. Switching in macrophage polarization from M1 to M2 was evidenced by the increase in M2 markers, including arginase-1 (Arg1) and early growth response protein 2 (Egr2), in the lungs of mice infected with LD100. Since IL-12 and interferon-γ are the major T helper (Th)1 cytokines, increased expression of interferon regulatory factor 4, IL-4, IL-10 and IL-13 promoted the differentiation of naïve CD4⁺ T cells into Th2 cells. In conclusion, the present study identified key cytokines involved in the pathogenicity of influenza infection, and demonstrated that lethal influenza virus infection induces a mixed Th1/Th2 response.

Bacterial Outer Membrane Vesicles Provide Broad-Spectrum Protection against Influenza Virus Infection via Recruitment and Activation of Macrophages

Influenza A virus (IAV) poses a constant worldwide threat to human health. Although conventional vaccines are available, their protective efficacy is type or strain specific, and their production is time-consuming. For the control of an influenza pandemic in particular, agents that are immediately effective against a wide range of virus variants should be developed. Although pretreatment of various Toll-like receptor (TLR) ligands have already been reported to be effective in the defense against subsequent IAV infection, the efficacy was limited to specific subtypes, and safety concerns were also raised. In this study, we investigated the protective effect of an attenuated bacterial outer membrane vesicle -harboring modified lipid A moiety of lipopolysaccharide (fmOMV) against IAV infection and the underlying mechanisms. Administration of fmOMV conferred significant protection against a lethal dose of pandemic H1N1, PR8, H5N2, and highly pathogenic H5N1 viruses; this broad antiviral activity was dependent on macrophages but independent of neutrophils. fmOMV induced recruitment and activation of macrophages and elicited type I IFNs. Intriguingly, fmOMV showed a more significant protective effect than other TLR ligands tested in previous reports, without exhibiting any adverse effect. These results show the potential of fmOMV as a prophylactic agent for the defense against influenza virus infection.

Surviving Deadly Lung Infections: Innate Host Tolerance Mechanisms in the Pulmonary System

Much research on infectious diseases focuses on clearing the pathogen through the use of antimicrobial drugs, the immune response, or a combination of both. Rapid clearance of pathogens allows for a quick return to a healthy state and increased survival. Pathogen-targeted approaches to combating infection have inherent limitations, including their pathogen-specific nature, the potential for antimicrobial resistance, and poor vaccine efficacy, among others. Another way to survive an infection is to tolerate the alterations to homeostasis that occur during a disease state through a process called host tolerance or resilience, which is independent from pathogen burden. Alterations in homeostasis during infection are numerous and include tissue damage, increased inflammation, metabolic changes, temperature changes, and changes in respiration. Given its importance and sensitivity, the lung is a good system for understanding host tolerance to infectious disease. Pneumonia is the leading cause of death for children under five worldwide. One reason for this is because when the pulmonary system is altered dramatically it greatly impacts the overall health and survival of a patient. Targeting host pathways involved in maintenance of pulmonary host tolerance during infection could provide an alternative therapeutic avenue that may be broadly applicable across a variety of pathologies. In this review, we will summarize recent findings on tolerance to host lung infection. We will focus on the involvement of innate immune responses in tolerance and how an initial viral lung infection may alter tolerance mechanisms in leukocytic, epithelial, and endothelial compartments to a subsequent bacterial infection. By understanding tolerance mechanisms in the lung we can better address treatment options for deadly pulmonary infections.

Neutrophils drive alveolar macrophage IL-1β release during respiratory viral infection

BACKGROUND

Alveolar macrophages are sentinels of the airways that must exhibit immune restraint to innocuous antigens but elicit a robust inflammatory response to pathogenic threats. How distinction between these dichotomous functions is controlled is poorly defined.Neutrophils are the first responders to infection, and we hypothesised that they may free alveolar macrophages from their hyporesponsive state, promoting their activation. Activation of the inflammasome and interleukin (IL)-1β release is a key early inflammatory event that must be tightly regulated. Thus, the role of neutrophils in defining inflammasome activation in the alveolar macrophage was assessed.

METHODS

Mice were infected with the X31 strain of influenza virus and the role of neutrophils in alveolar macrophage activation established through administration of a neutrophil-depleting (1A8) antibody.

RESULTS

Influenza elicited a robust IL-1β release that correlated (r=0.6849; p<0.001) with neutrophil infiltrate and was ablated by neutrophil depletion. Alveolar macrophages were shown to be the prominent source of IL-1β during influenza infection, and virus triggered the expression of Nod-like receptor protein 3 (NLRP3) inflammasome and pro-IL-1β in these cells. However, subsequent activation of the inflammasome complex and release of mature IL-1β from alveolar macrophages were critically dependent on the provision of a secondary signal, in the form of antimicrobial peptide mCRAMP, from infiltrating neutrophils.

CONCLUSIONS

Neutrophils are critical for the activation of the NLRP3 inflammasome in alveolar macrophages during respiratory viral infection. Accordingly, we rationalise that neutrophils are recruited to the lung to confront a viable pathogenic threat and subsequently commit alveolar macrophages to a pro-inflammatory phenotype to combat infection.

Contribution of Regulatory T Cells in Nucleotide-Binding Oligomerization Domain 2 Response to Influenza Virus Infection

Influenza A virus (IAV) is recognized to cause severe pulmonary illnesses in humans, particularly in elderly and children. One of the features associated with IAV infection is an excessive lung inflammation due to an uncontrolled immune response. The nucleotide-binding oligomerization domain 2 (NOD2) receptor is known to recognize ssRNA viruses such as IAV, but its role in the inflammatory process during viral infections remains to be clarified. In a previous report, we have shown that activation of NOD2 with muramyl dipeptide (MDP) significantly reduces both viral loads and lung inflammation and also improves pulmonary function during IAV infection. These findings prompted us to further investigate whether NOD2 receptor may contribute to regulate inflammation during viral infection. In the present study, we show that administration of MDP to mice infected with IAV stimulates the migration of regulatory T (Treg) cells to the lungs. Such a presence of Treg cells was also accompanied with a reduction of neutrophils in the lungs during IAV infection, which correlated, with a significant decrease of Th17 cells. In our model, Treg cell recruitment is dependent of CXCL12 and CCL5 chemokines. Moreover, we show that the presence of Ly6C^low patrolling monocytes is required for Treg cells mobilization to the lung of mice treated with MDP. In fact, following monocyte depletion by administration of clodronate liposome, mobilization of Treg cells to the lungs of treated mice was found to occur when circulating Ly6C^low monocytes begin to reemerge. In addition, we also detected an increased production of TGF-β, a cytokine contributing to Treg activity when blood Ly6C^low monocytes are restored. Together, our results demonstrate that MDP treatment can promote an anti-inflammatory environment through the mobilization of Treg cells to the lung, a mechanism that requires the presence of Ly6C^low monocytes during IAV infection. Overall, our results suggest that activation of NOD2 receptor could be an appealing approach to control pulmonary inflammation in patients infected with IAV.

Differential Ability of Pandemic and Seasonal H1N1 Influenza A Viruses To Alter the Function of Human Neutrophils

A long-standing notion is that IAV inhibits normal neutrophil function and thereby predisposes individuals to secondary bacterial infections. Here we report that seasonal H1N1 IAV primes human neutrophils for enhanced killing of Staphylococcus aureus. Moreover, we provide a comprehensive view of the changes in neutrophil gene expression during interaction with seasonal or pandemic IAV and report how these changes relate to functions such as bactericidal activity. This study expands our knowledge of IAV interactions with human neutrophils.

Neutrophils are essential cells of host innate immunity. Although the role of neutrophils in defense against bacterial and fungal infections is well characterized, there is a relative paucity of information about their role against viral infections. Influenza A virus (IAV) infection can be associated with secondary bacterial coinfection, and it has long been posited that the ability of IAV to alter normal neutrophil function predisposes individuals to secondary bacterial infections. To better understand this phenomenon, we evaluated the interaction of pandemic or seasonal H1N1 IAV with human neutrophils isolated from healthy persons. These viruses were ingested by human neutrophils and elicited changes in neutrophil gene expression that are consistent with an interferon-mediated immune response. The viability of neutrophils following coculture with either pandemic or seasonal H1N1 IAV was similar for up to 18 h of culture. Notably, neutrophil exposure to seasonal (but not pandemic) IAV primed these leukocytes for enhanced functions, including production of reactive oxygen species and bactericidal activity. Taken together, our results are at variance with the universal idea that IAV impairs neutrophil function directly to predispose individuals to secondary bacterial infections. Rather, we suggest that some strains of IAV prime neutrophils for enhanced bacterial clearance.

IMPORTANCE A long-standing notion is that IAV inhibits normal neutrophil function and thereby predisposes individuals to secondary bacterial infections. Here we report that seasonal H1N1 IAV primes human neutrophils for enhanced killing of Staphylococcus aureus. Moreover, we provide a comprehensive view of the changes in neutrophil gene expression during interaction with seasonal or pandemic IAV and report how these changes relate to functions such as bactericidal activity. This study expands our knowledge of IAV interactions with human neutrophils.

Nlrp12 Mediates Adverse Neutrophil Recruitment during Influenza Virus Infection

Exaggerated inflammatory responses during influenza A virus (IAV) infection are typically associated with severe disease. Neutrophils are among the immune cells that can drive this excessive and detrimental inflammation. In moderation, however, neutrophils are necessary for optimal viral control. In this study, we explore the role of the nucleotide-binding domain leucine-rich repeat containing receptor family member Nlrp12 in modulating neutrophilic responses during lethal IAV infection. Nlrp12^-/- mice are protected from lethality during IAV infection and show decreased vascular permeability, fewer pulmonary neutrophils, and a reduction in levels of neutrophil chemoattractant CXCL1 in their lungs compared with wild-type mice. Nlrp12^-/- neutrophils and dendritic cells within the IAV-infected lungs produce less CXCL1 than their wild-type counterparts. Decreased CXCL1 production by Nlrp12^-/- dendritic cells was not due to a difference in CXCL1 protein stability, but instead to a decrease in Cxcl1 mRNA stability. Together, these data demonstrate a previously unappreciated role for Nlrp12 in exacerbating the pathogenesis of IAV infection through the regulation of CXCL1-mediated neutrophilic responses.

Host-directed therapies for bacterial and viral infections

Host-directed therapy (HDT) is a novel approach in the field of anti-infectives for overcoming antimicrobial resistance.

HDT aims to interfere with host cell factors that are required by a pathogen for replication or persistence, to enhance protective immune responses against a pathogen, to reduce exacerbated inflammation and to balance immune reactivity at sites of pathology.

HDTs encompassing the 'shock and kill' strategy or the delivery of recombinant interferons are possible approaches to treat HIV infections. HDTs that suppress the cytokine storm that is induced by some acute viral infections represent a promising concept.

In tuberculosis, HDT aims to enhance the antimicrobial activities of phagocytes through phagosomal maturation, autophagy and antimicrobial peptides. HDTs also curtail inflammation through interference with soluble (such as eicosanoids or cytokines) or cellular (co-stimulatory molecules) factors and modulate granulomas to allow the access of antimicrobials or to restrict tissue damage.

Numerous parallels between the immunological abnormalities that occur in sepsis and cancer indicate that the HDTs that are effective in oncology may also hold promise in sepsis.

Advances in immune phenotyping, genetic screening and biosignatures will help to guide drug therapy to optimize the host response. Combinations of canonical pathogen-directed drugs and novel HDTs will become indispensable in treating emerging infections and diseases caused by drug-resistant pathogens.

Host-directed therapy (HDT) aims to interfere with host cell factors that are required by a pathogen for replication or persistence. In this Review, Kaufmannet al. describe recent progress in the development of HDTs for the treatment of viral and bacterial infections and the challenges in bringing these approaches to the clinic.

Despite the recent increase in the development of antivirals and antibiotics, antimicrobial resistance and the lack of broad-spectrum virus-targeting drugs are still important issues and additional alternative approaches to treat infectious diseases are urgently needed. Host-directed therapy (HDT) is an emerging approach in the field of anti-infectives. The strategy behind HDT is to interfere with host cell factors that are required by a pathogen for replication or persistence, to enhance protective immune responses against a pathogen, to reduce exacerbated inflammation and to balance immune reactivity at sites of pathology. Although HDTs encompassing interferons are well established for the treatment of chronic viral hepatitis, novel strategies aimed at the functional cure of persistent viral infections and the development of broad-spectrum antivirals against emerging viruses seem to be crucial. In chronic bacterial infections, such as tuberculosis, HDT strategies aim to enhance the antimicrobial activities of phagocytes and to curtail inflammation through interference with soluble factors (such as eicosanoids and cytokines) or cellular factors (such as co-stimulatory molecules). This Review describes current progress in the development of HDTs for viral and bacterial infections, including sepsis, and the challenges in bringing these new approaches to the clinic.

The inflammatory response triggered by Influenza virus: a two edged sword

Influenza A virus (IAV) is a relevant respiratory tract pathogen leading to a great number of deaths and hospitalizations worldwide. Secondary bacterial infections are a very common cause of IAV associated morbidity and mortality. The robust inflammatory response that follows infection is important for the control of virus proliferation but is also associated with lung damage, morbidity and death. The role of the different components of immune response underlying protection or disease during IAV infection is not completely elucidated. Overall, in the context of IAV infection, inflammation is a 'double edge sword' necessary to control infection but causing disease. Therefore, a growing number of studies suggest that immunomodulatory strategies may improve disease outcome without affecting the ability of the host to deal with infection. This review summarizes recent aspects of the inflammatory responses triggered by IAV that are preferentially involved in causing severe pulmonary disease and the anti-inflammatory strategies that have been suggested to treat influenza induced immunopathology.

The Influenza Virus Protein PB1-F2 Increases Viral Pathogenesis through Neutrophil Recruitment and NK Cells Inhibition

The influenza A virus (IAV) PB1-F2 protein is a virulence factor contributing to the pathogenesis observed during IAV infections in mammals. In this study, using a mouse model, we compared the host response associated with PB1-F2 with an early transcriptomic signature that was previously associated with neutrophils and consecutively fatal IAV infections. This allowed us to show that PB1-F2 is partly involved in neutrophil-related mechanisms leading to death. Using neutropenic mice, we confirmed that the harmful effect of PB1-F2 is due to an excessive inflammation mediated by an increased neutrophil mobilization. We identified the downstream effects of this PB1-F2-exacerbated neutrophil recruitment. PB1-F2 had no impact on the lymphocyte recruitment in the airways at day 8 pi. However, functional genomics analysis and flow cytometry in broncho-alveolar lavages at 4 days pi revealed that PB1-F2 induced a NK cells deficiency. Thus, our results identify PB1-F2 as an important immune disruptive factor during the IAV infection.

Antimicrobial peptides alter early immune response to influenza A virus infection in C57BL/6 mice

Influenza is a disease of the respiratory system caused by single stranded RNA viruses with varying genotypes. Immunopathogenesis to influenza viruses differs based on virus strain, dose, and mouse strain used in laboratory models. Although effective mucosal immune defenses are important in early host defense against influenza, information on the kinetics of these immune defense mechanisms during the course of influenza infection is limited. We investigated changes to antimicrobial peptides and primary innate immune cells at early time points after infection and compared these variables between two prominent H1N1 influenza A virus (IAV) strains, A/CA/04/2009 and A/PR/08/1934 in C57BL/6 mice. Alveolar and parenchymal macrophage ratios were altered after IAV infection and pro-inflammatory cytokine production in macrophages was induced after IAV infection. Genes encoding antimicrobial peptides, β-defensin (Defb4), bactericidal-permeability increasing protein (Bpifa1), and cathelicidin antimicrobial peptide (Camp), were differentially regulated after IAV infection and the kinetics of Defb4 expression differed in response to each virus strain. Beta-defensin reduced infectivity of A/CA/04/2009 virus but not A/PR/08/1934. Beta defensins also changed the innate immune cell profile wherein mice pre-treated with β-defensin had increased alveolar macrophages and CD103(+) dendritic cells, and reduced CD11b(+) dendritic cells and neutrophils. In addition to highlighting that immune responses may vary based on influenza virus strain used, our data suggest an important role for antimicrobial peptides in host defense against influenza virus.

Consecutive oral administration of Bifidobacterium longum MM-2 improves the defense system against influenza virus infection by enhancing natural killer cell activity in a murine model

Bifidobacterium, one of the major components of intestinal microflora, shows anti-influenza virus (IFV) potential as a probiotic, partly through enhancement of innate immunity by modulation of the intestinal immune system. Bifidobacterium longum MM-2 (MM-2), a very safe bacterium in humans, was isolated from healthy humans and its protective effect against IFV infection in a murine model shown. In mice that were intranasally inoculated with IFV, oral administration of MM-2 for 17 consecutive days improved clinical symptoms, reduced mortality, suppressed inflammation in the lower respiratory tract, and decreased virus titers, cell death, and pro-inflammatory cytokines such as IL-6 and TNF-α in bronchoalveolar lavage fluid. The anti-IFV mechanism of MM-2 involves innate immunity through significant increases in NK cell activities in the lungs and spleen and a significant increase in pulmonary gene expression of NK cell activators such as IFN-γ, IL-2, IL-12 and IL-18. Even in non-infected mice, MM-2 administration also induced significant enhancement of both IFN-γ production by Peyer's patch cells (PPs) and splenetic NK cell activity. Oral administration of MM-2 for 17 days activates systemic immunoreactivity in PPs, which contributes to innate immunity, including NK cell activation, resulting in an anti-IFV effect. MM-2 as a probiotic may function as a prophylactic agent in the management of an IFV epidemic.

Transcriptomic and Epigenetic Profiling of the Lung of Influenza-Infected Pigs: A Comparison of Different Birth Weight and Susceptibility Groups

Influenza viruses are a common cause of respiratory disease in swine. Infections range in severity from asymptomatic to causing significant morbidity. The main objective of this study was to compare lung transcriptomic and epigenetic responses to influenza infection in pigs from high or low birth weight litters. The latter is a potential indicator of intrauterine growth restriction, a significant risk factor for prenatal programming effects. Individual pigs from high (HBW) or low birth weight (LBW) litters (n = 17) were inoculated with influenza A virus and euthanized 48 hours later. Lesion severity and viral loads were assessed as previously described. The transcriptional response to infection in LBW and HBW groups (n = 16) was assessed by microarray. A separate analysis of pigs classified as 'Resilient' (RES) or 'Susceptible' (SUS) (n = 6) on the basis of severity of lung pathology was also conducted. Eight genes were confirmed as differentially expressed for the birth weight comparison, including three antiviral genes with lower expression in LBW: ISG15, OAS1, and OAS2 (P<0.05). The promoter region methylation status of these three genes was assessed for each birth weight group, and no differences were found. These expression data are consistent with our previous finding that LBW pigs had less severe lesion scores and a trend towards lower viral titres in lung than the HBW cohort. The SUS v RES comparison identified 91 differentially expressed genes (FDR<0.05) that were enriched with functional annotation terms and pathways associated with inflammation. The cytokine genes IL6, IL8, and CCL2 were all upregulated in SUS pigs, and may have driven disease severity in these animals. In conclusion, this study found no evidence that the transcriptional immune response to influenza was adversely affected by low litter birth weight, but did identify several candidate genes for driving disease pathology.

Influenza virus-induced lung injury: pathogenesis and implications for treatment

The influenza viruses are some of the most important human pathogens, causing substantial seasonal and pandemic morbidity and mortality. In humans, infection of the lower respiratory tract of can result in flooding of the alveolar compartment, development of acute respiratory distress syndrome and death from respiratory failure. Influenza-mediated damage of the airway, alveolar epithelium and alveolar endothelium results from a combination of: 1) intrinsic viral pathogenicity, attributable to its tropism for host airway and alveolar epithelial cells; and 2) a robust host innate immune response, which, while contributing to viral clearance, can worsen the severity of lung injury. In this review, we summarise the molecular events at the virus-host interface during influenza virus infection, highlighting some of the important cellular responses. We discuss immune-mediated viral clearance, the mechanisms promoting or perpetuating lung injury, lung regeneration after influenza-induced injury, and recent advances in influenza prevention and therapy.

Protective role of LGP2 in influenza virus pathogenesis

Influenza A virus triggers a contagious respiratory disease that can cause considerable morbidity and mortality. Using an in vitro approach, we previously demonstrated that the pattern recognition receptor retinoic acid-inducible gene I (RIG-I) plays a key role in influenza A virus-mediated immune response. However, the importance of RIG-I signaling in vivo has not been thoroughly examined, because of the lack of an appropriate mouse models. To circumvent this issue, we generated a new transgenic mouse overexpressing LGP2 (hereafter, "LGP2 TG mice"), a major regulator of the RIG-I signaling pathway. The time course of several parameters was compared in infected wild-type and LGP2 TG mice. We found that LGP2 TG mice displayed significantly reduced inflammatory mediators and a lower leukocyte infiltration into the bronchoalveolar airspace. More importantly, LGP2 TG mice had a significant survival advantage. Hence, our in vivo study reveals that LGP2 is a major downregulator of the influenza A virus-triggered detrimental inflammatory response.

Innate immunity to influenza virus: implications for future therapy

Innate immunity is critical in the early containment of influenza virus infection. The innate response is surprisingly complex. A variety of soluble innate inhibitors in respiratory secretions provide an initial barrier to infection. Dendritic cells, phagocytes and natural killer cells mediate viral clearance and promote further innate and adaptive responses. Toll-like receptors 3 and 7 and cytoplasmic RNA sensors are critical for activating these responses. In general, the innate response restricts viral replication without injuring the lung; however, the 1918 pandemic and H5N1 strains cause more profound, possibly harmful, innate responses. In this review, we discuss the implications of burgeoning knowledge of innate immunity for therapy of influenza.

Neutrophils are needed for an effective immune response against pulmonary rat coronavirus infection, but also contribute to pathology

Polymorphonuclear neutrophils (PMN) infiltrate the respiratory tract early after viral infection and can contribute to both host defence and pathology. Coronaviruses are important causes of respiratory tract infections, ranging from mild to severe depending on the viral strain. This study evaluated the role of PMN during a non-fatal pulmonary coronavirus infection in the natural host. Rat coronavirus (RCoV) causes respiratory disease in adult rats, characterized by an early PMN response, viral replication and inflammatory lesions in the lungs, mild weight loss and effective resolution of infection. To determine their role during RCoV infection, PMN were depleted and the effects on disease progression, viral replication, inflammatory response and lung pathology were analysed. Compared with RCoV infection in control animals, PMN-depleted rats had worsened disease with weight loss, clinical signs, mortality and prolonged pulmonary viral replication. PMN-depleted animals had fewer macrophages and lymphocytes in the respiratory tract, corresponding to lower chemokine levels. Combined with in vitro experiments showing that PMN express cytokines and chemokines in response to RCoV-infected alveolar epithelial cells, these findings support a role for PMN in eliciting an inflammatory response to RCoV infection. Despite their critical role in the protection from severe disease, the presence of PMN was correlated with haemorrhagic lesions, epithelial barrier permeability and cellular inflammation in the lungs. This study demonstrated that while PMN are required for an effective antiviral response, they also contribute to lung pathology during RCoV infection.

Pathogenesis of influenza-induced acute respiratory distress syndrome

Acute respiratory distress syndrome (ARDS) is a fatal complication of influenza infection. In this Review we provide an integrated model for its pathogenesis. ARDS involves damage to the epithelial-endothelial barrier, fluid leakage into the alveolar lumen, and respiratory insufficiency. The most important part of the epithelial-endothelial barrier is the alveolar epithelium, strengthened by tight junctions. Influenza virus targets these epithelial cells, reducing sodium pump activity, damaging tight junctions, and killing infected cells. Infected epithelial cells produce cytokines that attract leucocytes--neutrophils and macrophages--and activate adjacent endothelial cells. Activated endothelial cells and infiltrated leucocytes stimulate further infiltration, and leucocytes induce production of reactive oxygen species and nitric oxide that damage the barrier. Activated macrophages also cause direct apoptosis of epithelial cells. This model for influenza-induced ARDS differs from the classic model, which is centred on endothelial damage, and provides a rationale for therapeutic intervention to moderate host response in influenza-induced ARDS.

The respiratory syncytial virus fusion protein and neutrophils mediate the airway mucin response to pathogenic respiratory syncytial virus infection

Respiratory syncytial virus (RSV) is the leading cause of death due to a viral etiology in infants. RSV disease is characterized by epithelial desquamation, neutrophilic bronchiolitis and pneumonia, and obstructive pulmonary mucus. It has been shown that infection of BALB/cJ mice with RSV clinical isolate A2001/2-20 (2-20) results in a higher early viral load, greater airway necrosis, and higher levels of interleukin-13 (IL-13) and airway mucin expression than infection with RSV laboratory strain A2. We hypothesized that the fusion (F) protein of RSV 2-20 is a mucus-inducing viral factor. In vitro, the fusion activity of 2-20 F but not that of A2 F was enhanced by expression of RSV G. We generated a recombinant F-chimeric RSV by replacing the F gene of A2 with the F gene of 2-20, generating A2-2-20F. Similar to the results obtained with the parent 2-20 strain, infection of BALB/cJ mice with A2-2-20F resulted in a higher early viral load and higher levels of subsequent pulmonary mucin expression than infection with the A2 strain. A2-2-20F infection induced greater necrotic airway damage and neutrophil infiltration than A2 infection. We hypothesized that the neutrophil response to A2-2-20F infection is involved in mucin expression. Antibody-mediated depletion of neutrophils in RSV-infected mice resulted in lower tumor necrosis factor alpha levels, fewer IL-13-expressing CD4 T cells, and less airway mucin production in the lung. Our data are consistent with a model in which the F and attachment (G) glycoprotein functional interaction leads to enhanced fusion and F is a key factor in airway epithelium infection, pathogenesis, and subsequent airway mucin expression.

Influenza-infected neutrophils within the infected lungs act as antigen presenting cells for anti-viral CD8(+) T cells

Influenza A virus (IAV) is a leading cause of respiratory tract disease worldwide. Anti-viral CD8⁺ T lymphocytes responding to IAV infection are believed to eliminate virally infected cells by direct cytolysis but may also contribute to pulmonary inflammation and tissue damage via the release of pro-inflammatory mediators following recognition of viral antigen displaying cells. We have previously demonstrated that IAV antigen expressing inflammatory cells of hematopoietic origin within the infected lung interstitium serve as antigen presenting cells (APC) for infiltrating effector CD8⁺ T lymphocytes; however, the spectrum of inflammatory cell types capable of serving as APC was not determined. Here, we demonstrate that viral antigen displaying neutrophils infiltrating the IAV infected lungs are an important cell type capable of acting as APC for effector CD8⁺ T lymphocytes in the infected lungs and that neutrophils expressing viral antigen as a result of direct infection by IAV exhibit the most potent APC activity. Our findings suggest that in addition to their suggested role in induction of the innate immune responses to IAV, virus clearance, and the development of pulmonary injury, neutrophils can serve as APCs to anti-viral effector CD8⁺ T cells within the infected lung interstitium.

Implication of inflammatory macrophages, nuclear receptors, and interferon regulatory factors in increased virulence of pandemic 2009 H1N1 influenza A virus after host adaptation

While pandemic 2009 H1N1 influenza A viruses were responsible for numerous severe infections in humans, these viruses do not typically cause corresponding severe disease in mammalian models. However, the generation of a virulent 2009 H1N1 virus following serial lung passage in mice has allowed for the modeling of human lung pathology in this species. Genetic determinants of mouse-adapted 2009 H1N1 viral pathogenicity have been identified, but the molecular and signaling characteristics of the host response following infection with this adapted virus have not been described. Here we compared the gene expression response following infection of mice with A/CA/04/2009 (CA/04) or the virulent mouse-adapted strain (MA-CA/04). Microarray analysis revealed that increased pathogenicity of MA-CA/04 was associated with the following: (i) an early and sustained inflammatory and interferon response that could be driven in part by interferon regulatory factors (IRFs) and increased NF-κB activation, as well as inhibition of the negative regulator TRIM24, (ii) early and persistent infiltration of immune cells, including inflammatory macrophages, and (iii) the absence of activation of lipid metabolism later in infection, which may be mediated by inhibition of nuclear receptors, including PPARG and HNF1A and -4A, with proinflammatory consequences. Further investigation of these signatures in the host response to other H1N1 viruses of various pathogenicities confirmed their general relevance for virulence of influenza virus and suggested that lung response to MA-CA/04 virus was similar to that following infection with lethal H1N1 r1918 influenza virus. This study links differential activation of IRFs, nuclear receptors, and macrophage infiltration with influenza virulence in vivo.

Matrix metalloprotease 9 mediates neutrophil migration into the airways in response to influenza virus-induced toll-like receptor signaling

The early inflammatory response to influenza virus infection contributes to severe lung disease and continues to pose a serious threat to human health. The mechanisms by which neutrophils gain entry to the respiratory tract and their role during pathogenesis remain unclear. Here, we report that neutrophils significantly contributed to morbidity in a pathological mouse model of influenza virus infection. Using extensive immunohistochemistry, bone marrow transfers, and depletion studies, we identified neutrophils as the predominant pulmonary cellular source of the gelatinase matrix metalloprotease (MMP) 9, which is capable of digesting the extracellular matrix. Furthermore, infection of MMP9-deficient mice showed that MMP9 was functionally required for neutrophil migration and control of viral replication in the respiratory tract. Although MMP9 release was toll-like receptor (TLR) signaling-dependent, MyD88-mediated signals in non-hematopoietic cells, rather than neutrophil TLRs themselves, were important for neutrophil migration. These results were extended using multiplex analyses of inflammatory mediators to show that neutrophil chemotactic factor, CCL3, and TNFα were reduced in the Myd88^−/− airways. Furthermore, TNFα induced MMP9 secretion by neutrophils and blocking TNFα in vivo reduced neutrophil recruitment after infection. Innate recognition of influenza virus therefore provides the mechanisms to induce recruitment of neutrophils through chemokines and to enable their motility within the tissue via MMP9-mediated cleavage of the basement membrane. Our results demonstrate a previously unknown contribution of MMP9 to influenza virus pathogenesis by mediating excessive neutrophil migration into the respiratory tract in response to viral replication that could be exploited for therapeutic purposes.

Influenza-associated morbidity and mortality due to yearly epidemics and sporadic, devastating pandemics are a significant health and economic burden. Severe complications arising from highly virulent viruses are associated with rapid, massive inflammatory cell infiltration. Although neutrophils are the predominant cell population recruited to the lung in response to pandemic influenza viruses, the mechanisms by which they gain entry to the respiratory tract remain unclear. In this study, we show a previously unknown contribution of MMP9 to influenza pathogenesis by mediating excessive neutrophil migration into the lung, which not only controls viral replication, but also contributes to morbidity. The in vivo relevance of MMP9-derived enzymatic activity in neutrophils is controversial and understudied, but our data provide new evidence that innate recognition of influenza virus attracts neutrophils that secrete MMP9, which enables them to traverse the basement membrane of the lung by digesting the extracellular matrix. The dichotomy of MMP9 function in immunity versus pathology provides real challenges for targeting MMP9 for therapeutic purposes. Nevertheless, finding the balance to modulate neutrophil numbers following influenza virus infection will allow for innate immunity to be boosted whilst preventing pathology associated with pandemic strains.

Quelling the storm: utilization of sphingosine-1-phosphate receptor signaling to ameliorate influenza virus-induced cytokine storm

Initial and early tissue injury associated with severe influenza virus infection is the result of both virus-mediated lysis of infected pulmonary cells coupled with an exuberant immune response generated against the virus. The excessive host immune response associated with influenza virus infection has been termed "cytokine storm." Therapies that target virus replication are available; however, the selective pressure by such antiviral drugs on the virus often results in mutation and the escape of virus progeny now resistant to the antiviral regimen, thereby rendering such treatments ineffective. This event highlights the necessity for developing novel methods to combat morbidity and mortality caused by influenza virus infection. One potential method is restricting the host's immune response. However, prior treatment regimens employing drugs like corticosteroids that globally suppress the host's immune response were found unsatisfactory in large part because they disrupted the host's ability to control virus replication. Here, we discuss a novel therapy that utilizes sphingosine-1-phosphate (S1P) receptor signaling that has the ability to significantly limit immunopathologic injury caused by the host's innate and adaptive immune response, thereby significantly aborting morbidity and mortality associated with influenza virus infection. Moreover, S1P analog therapy allows for sufficient anti-influenza T cell and antibody formation to control infection. We review the anti-inflammatory effects of S1P signaling pathways and how modulation of these pathways during influenza virus infection restricts immunopathology. Finally, we discuss that combinatorial administration of S1P simultaneously with a current antiviral enhances the treatment efficacy for virulent influenza virus infections above that of either drug treatment alone. Interestingly, the scope of S1P receptor therapy reported here is likely to extend beyond influenza virus infection and could prove useful for the treatment of multiple maladies like other viral infections and autoimmune diseases where the host's inflammatory response is a major component in the disease process.

Suppression of cytokine storm with a sphingosine analog provides protection against pathogenic influenza virus

Human pandemic H1N1 2009 influenza virus rapidly infected millions worldwide and was associated with significant mortality. Antiviral drugs that inhibit influenza virus replication are the primary therapy used to diminish disease; however, there are two significant limitations to their effective use: (i) antiviral drugs exert selective pressure on the virus, resulting in the generation of more fit viral progeny that are resistant to treatment; and (ii) antiviral drugs do not directly inhibit immune-mediated pulmonary injury that is a significant component of disease. Here we show that dampening the host's immune response against influenza virus using an immunomodulatory drug, AAL-R, provides significant protection from mortality (82%) over that of the neuraminidase inhibitor oseltamivir alone (50%). AAL-R combined with oseltamivir provided maximum protection against a lethal challenge of influenza virus (96%). Mechanistically, AAL-R inhibits cellular and cytokine/chemokine responses to limit immunopathologic damage, while maintaining host control of virus replication. With cytokine storm playing a role in the pathogenesis of a wide assortment of viral, bacterial, and immunologic diseases, a therapeutic approach using sphingosine analogs is of particular interest.

MARCO regulates early inflammatory responses against influenza: a useful macrophage function with adverse outcome

Lung macrophages use the scavenger receptor MARCO to bind and ingest bacteria, particulate matter, and post cellular debris. We investigated the role of MARCO in influenza A virus (IAV) pneumonia. In contrast to higher susceptibility to bacterial infection, MARCO(-/-) mice had lower morbidity and mortality from influenza pneumonia than wild-type (WT) mice. The early course of influenza in MARCO(-/-) lungs was marked by an enhanced but transient neutrophilic inflammatory response and significantly lower viral replication compared with the WT mice. At later time points, no significant differences in lung histopathology or absolute numbers of T lymphocyte influx were evident. Uptake of IAV by WT and MARCO(-/-) bronchoalveolar lavage macrophages in vitro was similar. By LPS coadministration, we demonstrated that rapid neutrophil and monocyte influx during the onset of influenza suppressed viral replication, indicating a protective role of early inflammation. We hypothesized that the presence of increased basal proinflammatory post cellular debris in the absence of scavenging function lowered the inflammatory response threshold to IAV in MARCO(-/-) mice. Indeed, MARCO(-/-) mice showed increased accumulation of proinflammatory oxidized lipoproteins in the bronchoalveolar lavage early in the infection process, which are the potential mediators of the observed enhanced inflammation. These results indicate that MARCO suppresses a protective early inflammatory response to influenza, which modulates viral clearance and delays recovery.

Excessive neutrophils and neutrophil extracellular traps contribute to acute lung injury of influenza pneumonitis

Complications of acute respiratory distress syndrome (ARDS) are common among critically ill patients infected with highly pathogenic influenza viruses. Macrophages and neutrophils constitute the majority of cells recruited into infected lungs, and are associated with immunopathology in influenza pneumonia. We examined pathological manifestations in models of macrophage- or neutrophil-depleted mice challenged with sublethal doses of influenza A virus H1N1 strain PR8. Infected mice depleted of macrophages displayed excessive neutrophilic infiltration, alveolar damage, and increased viral load, later progressing into ARDS-like pathological signs with diffuse alveolar damage, pulmonary edema, hemorrhage, and hypoxemia. In contrast, neutrophil-depleted animals showed mild pathology in lungs. The brochoalveolar lavage fluid of infected macrophage-depleted mice exhibited elevated protein content, T1-α, thrombomodulin, matrix metalloproteinase-9, and myeloperoxidase activities indicating augmented alveolar-capillary damage, compared to neutrophil-depleted animals. We provide evidence for the formation of neutrophil extracellular traps (NETs), entangled with alveoli in areas of tissue injury, suggesting their potential link with lung damage. When co-incubated with infected alveolar epithelial cells in vitro, neutrophils from infected lungs strongly induced NETs generation, and augmented endothelial damage. NETs induction was abrogated by anti-myeloperoxidase antibody and an inhibitor of superoxide dismutase, thus implying that NETs generation is induced by redox enzymes in influenza pneumonia. These findings support the pathogenic effects of excessive neutrophils in acute lung injury of influenza pneumonia by instigating alveolar-capillary damage.

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.

Type I interferon signaling regulates Ly6C(hi) monocytes and neutrophils during acute viral pneumonia in mice

Type I interferon (IFN-I) plays a critical role in the homeostasis of hematopoietic stem cells and influences neutrophil influx to the site of inflammation. IFN-I receptor knockout (Ifnar1^−/−) mice develop significant defects in the infiltration of Ly6C^hi monocytes in the lung after influenza infection (A/PR/8/34, H1N1). Ly6C^hi monocytes of wild-type (WT) mice are the main producers of MCP-1 while the alternatively generated Ly6C^int monocytes of Ifnar1^−/− mice mainly produce KC for neutrophil influx. As a consequence, Ifnar1^−/− mice recruit more neutrophils after influenza infection than do WT mice. Treatment of IFNAR1 blocking antibody on the WT bone marrow (BM) cells in vitro failed to differentiate into Ly6C^hi monocytes. By using BM chimeric mice (WT BM into Ifnar1^−/− and vice versa), we confirmed that IFN-I signaling in hematopoietic cells is required for the generation of Ly6C^hi monocytes. Of note, WT BM reconstituted Ifnar1^−/− chimeric mice with increased numbers of Ly6C^hi monocytes survived longer than influenza-infected Ifnar1^−/− mice. In contrast, WT mice that received Ifnar1^−/− BM cells with alternative Ly6C^int monocytes and increased numbers of neutrophils exhibited higher mortality rates than WT mice given WT BM cells. Collectively, these data suggest that IFN-I contributes to resistance of influenza infection by control of monocytes and neutrophils in the lung.

Type I interferon (IFN-I) was originally reported as a molecule that interferes with influenza virus replication. Various IFN-I inducible antiviral proteins contribute to dampening virus replication and dissemination. Thus, loss of IFN-I signaling attenuates antiviral response and aggravates disease. Recent studies suggest the possible role of IFN-I in hematopoiesis, which subsequently might have an effect on the immune cell response at the site of infection. Indeed, IFN-I signaling-defective mice have been shown to develop aberrant cell populations. The aim of this current study was to clarify the mechanisms of IFN-I signaling in the regulation of monocytes and neutrophils. We show that IFN-I is directly involved in monocyte differentiation and that loss of IFN-I signaling allows mice to generate monocytes whose gene profile is significantly different. We found that monocytes are an important source of chemokines for further monocyte recruitment, but IFN-I-defective monocytes produce chemokines for neutrophil recruitment. As a result, mice lacking IFN-I signaling recruit more neutrophils and a reduced number of alternatively generated monocytes. Thus, our findings indicate that authentic monocyte differentiation, which requires IFN-I signaling, is critical in controlling neutrophils and protecting mice against influenza virus infection.

Inhibition of lung serine proteases in mice: a potentially new approach to control influenza infection

BACKGROUND

Host serine proteases are essential for the influenza virus life cycle because the viral haemagglutinin is synthesized as a precursor which requires proteolytic maturation. Therefore, we studied the activity and expression of serine proteases in lungs from mice infected with influenza and evaluated the effect of serine protease inhibitors on virus replication both in cell culture and in infected mice.

RESULTS

Two different inbred mouse strains were investigated: DBA/2J as a highly susceptible and C57Bl/6J as a more resistant strain to influenza virus infection. The serine proteases from lung homogenates of mice exhibited pH optima of 10.00. Using the substrate Bz-Val-Gly-Arg-p-nitroanilide or in zymograms, the intensities of proteolysis increased in homogenates from both mouse strains with time post infection (p.i.) with the mouse-adapted influenza virus A/Puerto Rico/8/34 (H1N1; PR8). In zymograms at day 7 p.i., proteolytic bands were stronger and numerous in lung homogenates from DBA/2J than C57Bl/6J mice. Real-time PCR results confirmed differential expression of several lung proteases before and after infecting mice with the H1N1 virus. The most strongly up-regulated proteases were Gzma, Tmprss4, Elane, Ctrl, Gzmc and Gzmb. Pretreatment of mouse and human lung cell lines with the serine protease inhibitors AEBSF or pAB or a cocktail of both prior to infection with the H1N1 or the A/Seal/Massachusetts/1/80 (H7N7; SC35M) virus resulted in a decrease in virus replication. Pretreatment of C57Bl/6J mice with either AEBSF or a cocktail of AEBSF and pAB prior to infection with the H1N1 virus significantly reduced weight loss and led to a faster recovery of treated versus untreated mice while pAB alone exerted a very poor effect. After infection with the H7N7 virus, the most significant reduction of weight loss was obtained upon pretreatment with either the protease inhibitor cocktail or pAB. Furthermore, pretreatment of C57BL/6J mice with AEBSF prior to infection resulted in a significant reduction in the levels of both the H1N1 and H7N7 nucleoproteins in mice lungs and also a significant reduction in the levels of the HA transcript in the lungs of the H1N1--but not the H7N7-infected mice.

CONCLUSION

Multiple serine protease activities might be implicated in mediating influenza infection. Blocking influenza A virus infection in cultured lung epithelia and in mice by the used serine protease inhibitors may provide an alternative approach for treatment of influenza infection.

Platelet-activating factor receptor plays a role in lung injury and death caused by Influenza A in mice

Influenza A virus causes annual epidemics which affect millions of people worldwide. A recent Influenza pandemic brought new awareness over the health impact of the disease. It is thought that a severe inflammatory response against the virus contributes to disease severity and death. Therefore, modulating the effects of inflammatory mediators may represent a new therapy against Influenza infection. Platelet activating factor (PAF) receptor (PAFR) deficient mice were used to evaluate the role of the gene in a model of experimental infection with Influenza A/WSN/33 H1N1 or a reassortant Influenza A H3N1 subtype. The following parameters were evaluated: lethality, cell recruitment to the airways, lung pathology, viral titers and cytokine levels in lungs. The PAFR antagonist PCA4248 was also used after the onset of flu symptoms. Absence or antagonism of PAFR caused significant protection against flu-associated lethality and lung injury. Protection was correlated with decreased neutrophil recruitment, lung edema, vascular permeability and injury. There was no increase of viral load and greater recruitment of NK1.1⁺ cells. Antibody responses were similar in WT and PAFR-deficient mice and animals were protected from re-infection. Influenza infection induces the enzyme that synthesizes PAF, lyso-PAF acetyltransferase, an effect linked to activation of TLR7/8. Therefore, it is suggested that PAFR is a disease-associated gene and plays an important role in driving neutrophil influx and lung damage after infection of mice with two subtypes of Influenza A. Further studies should investigate whether targeting PAFR may be useful to reduce lung pathology associated with Influenza A virus infection in humans.

Influenza virus causes disease that affects people from different age, gender or social conditions. The illness spreads easily and affects millions of people every year. Vaccines are effective preventive approaches, but the high degree of viral antigenic drift requires annual formulation. Anti-viral drugs are used as therapy, but are only effective at the very early stages of disease. The main symptoms that lead to hospitalizations and deaths are associated with the severe inflammatory host immune response triggered by the virus infection. Our approach was to decrease the inflammatory events associated with the viral infection by targeting a molecule, Platelet Activating Factor receptor (PAFR), known to induce several inflammatory events, including leukocyte recruitment and leakage. We found that PAFR deficient mice or wild type mice treated with a PAFR antagonist had less pulmonary inflammation, pulmonary injury and lethality rates when infected by two subtypes of Influenza A virus. In contrast, the immune response against the virus, as assessed by viral loads and specific antibodies, were not decreased. Our findings concur with the idea that severe inflammation plays an important role in flu morbidity and mortality and show that PAFR is a major driver of the exacerbated inflammation in mice infected with Influenza A virus.

Excess neutrophil infiltration during cytomegalovirus brain infection of interleukin-10-deficient mice

Wild-type mice control murine cytomegalovirus (MCMV) brain infection, but identical infection is lethal to animals deficient in interleukin (IL)-10. Here, we report that MCMV-infected IL-10 knockout (KO) mice displayed a marked increase in neutrophil infiltration into the infected, IL-10-deficient brain when compared to wild-type animals. Enhanced microglial cell activation, determined by MHC class II up-regulation, overexpression of CXCL2, and elevated P-selectin mRNA levels were observed. In vivo blocking of CXCL2 attenuated neutrophil infiltration and significantly improved the outcome of infection. Collectively, these data indicate that the absence of IL-10 results in pathologic neutrophil infiltration into MCMV-infected brains.

Salmonella flagellins are potent adjuvants for intranasally administered whole inactivated influenza vaccine

Bacterial flagellins are potent inducers of innate immune responses in the mouse lung because they bind to TLR5 expressed on the apical surfaces of airway epithelial cells. TLR engagement leads to the initiation of a signaling cascade that results in a pro-inflammatory response with subsequent up-regulation of several cytokines and leads to adaptive immune responses. We examined the ability of two soluble flagellins, a monomeric flagellin expressed in Escherichia coli and a highly purified polymeric flagellin directly isolated from Salmonella, to enhance the efficacy of influenza vaccines in mice. Here we demonstrate that both flagellins co-administered intranasally with inactivated A/PR/8/34 (PR8) virus induced robust increases of systemic influenza-specific IgA and IgG titers and resulted in a more comprehensive humoral response as indicated by the increase of IgG2a and IgG2b subclass responses. Groups immunized with the adjuvanted vaccines were fully protected against high dose lethal challenge by homologous virus whereas inactivated PR8 alone conferred only partial protection. Finally we show that shortly after immunization the adjuvanted vaccines induced a dramatic increase in pro-inflammatory cytokines in the lung, resulting in extensive lung infiltration by granulocytes and monocytes/macrophages. Our results reveal a promising perspective for the use of both soluble monomeric and polymeric flagellin as mucosal vaccine adjuvants to improve protection against influenza epidemics as well as a range of other infectious diseases.

Receptor for advanced glycation end products is detrimental during influenza A virus pneumonia

Pneumonia caused by influenza A virus (IAV) can have devastating effects, resulting in respiratory failure and death. The idea that a new influenza pandemic might occur in the near future has triggered renewed interests in IAV infection. The receptor for advanced glycation end products (RAGE) is expressed on different cell types and plays a key role in diverse inflammatory processes. We here investigated the role of RAGE in the host response to IAV pneumonia using wild-type (wt) and RAGE deficient ((-/-)) mice. Whereas strong RAGE was constitutively expressed in the lungs of uninfected wt mice, in particular on endothelium, IAV pneumonia was associated with enhanced expression on endothelium and de novo expression on bronchial epithelium. Additionally, the high-affinity RAGE ligand high mobility group box 1 was upregulated during IAV pneumonia. RAGE(-/-) mice were relatively protected from IAV induced mortality and showed an improved viral clearance and enhanced cellular T cell response and activation of neutrophils. These data suggest that RAGE is detrimental during IAV pneumonia.

CXCR3-deficiency protects influenza-infected CCR5-deficient mice from mortality

Mice lacking the chemokine receptor CCR5 are susceptible to mortality from a normally non-lethal influenza infection. Here we found that CXCR3-deficiency rescued CCR5-deficient (CCR5(-/-)) mice from influenza-induced mortality. The number of mononuclear phagocytes in the airways was transiently increased in CCR5(-/-) mice but not in CXCR3-CCR5 double-deficient mice. Antigen-specific CXCR3-CCR5 double-deficient CD8 effector cells were less efficient at entering the airways compared with WT or CCR5(-/-) CD8 effector cells. The decrease in inflammatory cell infiltrates in CXCR3-CCR5 double-deficient-infected mice correlated with a decrease in CCL2 and IFN-gamma production in the airways. Finally, CXCR3-CCR5 double-deficient mice that survived the primary viral challenge were protected from a lethal secondary challenge, indicating that T-cell-mediated protective memory was not compromised in mice lacking these chemokine receptors. In conclusion, CXCR3-deficiency attenuated the lethal cellular immune response in CCR5(-/-) influenza-infected mice without hindering viral clearance or long-term immunity.

Cigarette smoke worsens lung inflammation and impairs resolution of influenza infection in mice

BACKGROUND

Cigarette smoke has both pro-inflammatory and immunosuppressive effects. Both active and passive cigarette smoke exposure are linked to an increased incidence and severity of respiratory virus infections, but underlying mechanisms are not well defined. We hypothesized, based on prior gene expression profiling studies, that upregulation of pro-inflammatory mediators by short term smoke exposure would be protective against a subsequent influenza infection.

METHODS

BALB/c mice were subjected to whole body smoke exposure with 9 cigarettes/day for 4 days. Mice were then infected with influenza A (H3N1, Mem71 strain), and analyzed 3 and 10 days later (d3, d10). These time points are the peak and resolution (respectively) of influenza infection.

RESULTS

Inflammatory cell influx into the bronchoalveolar lavage (BALF), inflammatory mediators, proteases, histopathology, viral titres and T lymphocyte profiles were analyzed. Compared to smoke or influenza alone, mice exposed to smoke and then influenza had more macrophages, neutrophils and total lymphocytes in BALF at d3, more macrophages in BALF at d10, lower net gelatinase activity and increased activity of tissue inhibitor of metalloprotease-1 in BALF at d3, altered profiles of key cytokines and CD4+ and CD8+ T lymphocytes, worse lung pathology and more virus-specific, activated CD8+ T lymphocytes in BALF. Mice smoke exposed before influenza infection had close to 10-fold higher lung virus titres at d3 than influenza alone mice, although all mice had cleared virus by d10, regardless of smoke exposure. Smoke exposure caused temporary weight loss and when smoking ceased after viral infection, smoke and influenza mice regained significantly less weight than smoke alone mice.

CONCLUSION

Smoke induced inflammation does not protect against influenza infection.In most respects, smoke exposure worsened the host response to influenza. This animal model may be useful in studying how smoke worsens respiratory viral infections.

Manipulation of acute inflammatory lung disease

Inflammatory lung disease to innocuous antigens or infectious pathogens is a common occurrence and in some cases, life threatening. Often, the inflammatory infiltrate that accompanies these events contributes to pathology by deleterious effects on otherwise healthy tissue and by compromising lung function by consolidating (blocking) the airspaces. A fine balance, therefore, exists between a lung immune response and immune-mediated damage, and in some the "threshold of ignorance" may be set too low. In most cases, the contributing, potentially offending, cell population or immune pathway is known, as are factors that regulate them. Why then are targeted therapeutic strategies to manipulate them not more commonplace in clinical medicine? This review highlights immune homeostasis in the lung, how and why this is lost during acute lung infection, and strategies showing promise as future immune therapeutics.

Monocyte chemoattractant protein 1 contributes to an adequate immune response in influenza pneumonia

Monocyte chemoattractant protein 1 (MCP-1) and its receptor CCR2 have been shown to play an import role in leukocyte recruitment to sites of infection and inflammation. To investigate the role of MCP-1 during infection with influenza we inoculated wild-type (WT) and MCP-1 knockout (KO) mice with a non-lethal dose of a mouse adapted strain of influenza A. Influenza infection of WT mice resulted in a profound increase in pulmonary MCP-1 levels. MCP-1 KO mice had enhanced weight loss and did not fully regain their body weight during the 14-day observation period. In addition, MCP-1 KO mice demonstrated elevated viral loads 8 days after infection, which was accompanied by reduced leukocyte recruitment into the infected lungs, primarily caused by a diminished influx of macrophages and granulocytes. Moreover, pulmonary levels of IgA were reduced in MCP-1 KO mice. The pulmonary concentrations of tumor necrosis factor-alpha, interleukin-6, macrophage inflammatory protein 2 and interferon-gamma were higher in MCP-1 KO mice. This study shows that MCP-1 contributes to an adequate protective immune response against influenza infection in mice.

Impact of neutrophils on antiviral activity of human bronchoalveolar lavage fluid

Surfactant protein D (SP-D) and neutrophils participate in the early innate immune response to influenza A virus (IAV) infection. SP-D increases neutrophil uptake of IAV and modulates neutrophil respiratory burst responses to IAV; however, neutrophil proteases have been shown to degrade SP-D, and human neutrophil peptide defensins bind to SP-D and can cause precipitation of SP-D from bronchoalveolar lavage fluid (BALF). BALF has significant antiviral activity against IAV. We first added neutrophils to BALF during incubation with IAV. Addition of neutrophils to BALF caused significantly greater clearance of IAV from culture supernatants than from BALF alone, and this effect was significantly more pronounced when neutrophils were activated during incubation with the virus. In contrast, if activated neutrophils were incubated with BALF before addition of virus, they reduced antiviral activity of BALF. This effect correlated with depletion of SP-D from BALF. Activation of neutrophils with agonists that induce primary granule release (including release of human neutrophil peptide defensins) caused SP-D depletion, but activation with PMA, which causes only secondary granule release, did not. The ability of activated neutrophils to deplete SP-D from BALF was partially, but not fully, corrected with protease inhibitors but was unaffected by inhibition of neutrophil respiratory burst responses. These results suggest that chronic neutrophilic inflammation (e.g., as in chronic smoking or cystic fibrosis) may reduce SP-D levels and predispose to IAV infection. In contrast, acute inflammation, as occurs in the early phase of IAV infection, may promote neutrophil-mediated viral clearance.