Postinfluenza bacterial pneumonia: host defenses gone awry

Altered peripheral blood leukocyte subpopulations, function, and gene expression in children with Down syndrome: implications for respiratory tract infection

OBJECTIVES

We tested the hypothesis that aberrant expression of Hsa21-encoded interferon genes in peripheral blood immune cells would correlate to immune cell dysfunction in children with Down syndrome (DS).

STUDY DESIGN

We performed flow cytometry to quantify peripheral blood leukocyte subtypes and measured their ability to migrate and phagocytose. In matched samples, we measured gene expression levels for constituents of interferon signaling pathways. We screened 49 children, of which 29 were individuals with DS.

RESULTS

We show that the percentages of two peripheral blood myeloid cell subtypes (alternatively-activated macrophages and low-density granulocytes) in children with DS differed significantly from typical children, children with DS circulate a very different pattern of cytokines vs. typical individuals, and higher expression levels of type III interferon receptor Interleukin-10Rb in individuals with DS correlated with reduced migratory and phagocytic capacity of macrophages.

CONCLUSIONS

Increased susceptibility to severe and chronic infection in children with DS may result from inappropriate numbers and subtypes of immune cells that are phenotypically and functionally altered due to trisomy 21 associated interferonopathy.

Methyl 2-[3-(4-hydroxyphenyl)prop-2-enoylamino]-3-phenylpropanoate Is a Potent Cell-Permeable Anti-Cytokine Compound To Inhibit Inflammatory Cytokines in Monocyte/Macrophage-Like Cells

Cytokines are signaling molecules involved in inflammation process. Interleukin (IL)-6 is one of pivotal inflammatory cytokines associated with many human diseases. Therefore, there are on-going efforts to find a therapeutic to inhibit IL-6 and other cytokines. Methyl 2-[3-(4-hydroxyphenyl)prop-2-enoylamino]-3-phenylpropanoate (MHPAP) is a phenolic amide ester, transported better than its non-ester form (NEF) in monocyte/macrophage-like cells. However, there is no information about the effects of their cell permeability on cytokines. Therefore, the effects of MHPAP and NEF on cytokines were investigated in lipopolysaccharide (LPS)-stimulated THP-1 and human peripheral blood mononuclear cells (PBMCs). In the THP-1 cells, MHPAP significantly inhibited IL-6, IL-1beta, IL-8, and tumor necrosis factor (TNF)-alpha (P < 0.05), but NEF showed no effects. MHPAP also inhibited nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB) p65 phosphorylation in the THP-1 cells (P < 0.05), without significant effects on c-FOS, ATF-2, and JUN phosphorylations. Because NF-κB p65 is phosphorylated by IκB kinase (IKK), in silico analysis was performed on IKK. MHPAP was found to bind to IKK better than an IKK inhibitor ((E)-2-fluoro-4'-methoxystilbene). Furthermore, MHPAP inhibited the luminescence increased in the LPS-stimulated NF-κB-Luc2 THP-1 cells. As anticipated, MHPAP was also found to inhibit IL-6, IL-1beta, IL-8, and TNF-alpha significantly in LPS-stimulated PBMCs (P < 0.05). Especially, MHPAP inhibited IL-6 and IL-1beta with an IC50 of 0.85 and 0.87 µM, better than IL-8 (1.58 µM) and TNF-alpha (1.22 µM) in the cells. Altogether, the data suggest that cell permeability may have a significant impact on MHPAP's ability to inhibit cytokines and MHPAP may be used as a potent cell-permeable compound to inhibit inflammatory cytokines in monocyte/macrophage-like cells. SIGNIFICANCE STATEMENT: Potential effects of MHPAP and NEF on inflammatory cytokines (IL-6, IL-8, IL-1beta, and TNF-alpha) were investigated in LPS-stimulated THP-1 and PBMCs. Cell transport had a great impact on cytokine inhibition in the cells. MHPAP was also found to inhibit NF-κB pathway, which was supported by in silico and NF-κB reporter (Luc)-THP-1 data. Also, in LPS-stimulated PBMCs, MHPAP significantly inhibited IL-6, IL-1beta, IL-8, and TNF-alpha, suggesting that MHPAP may be a potent cell-permeable compound to inhibit inflammatory cytokines in monocyte/macrophage-like cells.

What Have We Learned by Resurrecting the 1918 Influenza Virus?

The 1918 Spanish influenza pandemic was one of the deadliest infectious disease events in recorded history, resulting in approximately 50-100 million deaths worldwide. The origins of the 1918 virus and the molecular basis for its exceptional virulence remained a mystery for much of the 20th century because the pandemic predated virologic techniques to isolate, passage, and store influenza viruses. In the late 1990s, overlapping fragments of influenza viral RNA preserved in the tissues of several 1918 victims were amplified and sequenced. The use of influenza reverse genetics then permitted scientists to reconstruct the 1918 virus entirely from cloned complementary DNA, leading to new insights into the origin of the virus and its pathogenicity. Here, we discuss some of the advances made by resurrection of the 1918 virus, including the rise of innovative molecular research, which is a topic in the dual use debate.

Lung and Heart Biology of the Dp16 Mouse Model of down Syndrome: Implications for Studying Cardiopulmonary Disease

(1) Background: We sought to investigate the baseline lung and heart biology of the Dp16 mouse model of Down syndrome (DS) as a prelude to the investigation of recurrent respiratory tract infection. (2) Methods: In controls vs. Dp16 mice, we compared peripheral blood cell and plasma analytes. We examined baseline gene expression in lungs and hearts for key parameters related to susceptibility of lung infection. We investigated lung and heart protein expression and performed lung morphometry. Finally, and for the first time each in a model of DS, we performed pulmonary function testing and a hemodynamic assessment of cardiac function. (3) Results: Dp16 mice circulate unique blood plasma cytokines and chemokines. Dp16 mouse lungs over-express the mRNA of triplicated genes, but not necessarily corresponding proteins. We found a sex-specific decrease in the protein expression of interferon α receptors, yet an increased signal transducer and activator of transcription (STAT)-3 and phospho-STAT3. Platelet-activating factor receptor protein was not elevated in Dp16 mice. The lungs of Dp16 mice showed increased stiffness and mean linear intercept and contained bronchus-associated lymphoid tissue. The heart ventricles of Dp16 mice displayed hypotonicity. Finally, Dp16 mice required more ketamine to achieve an anesthetized state. (4) Conclusions: The Dp16 mouse model of DS displays key aspects of lung heart biology akin to people with DS. As such, it has the potential to be an extremely valuable model of recurrent severe respiratory tract infection in DS.

Opportunistic infections in elderly TB patients

Since ancient times, tuberculosis has been a lethal infectious illness. The elderly are particularly susceptible to various illnesses, including tuberculosis. Tuberculosis (TB) and people ageing weaken the immune system, thus increasing the risk of getting other co-infections. Most elderly TB cases are associated with the reactivation of dormant lesions, and these lesions have reactivated due to immunosenescence. Elderly patients have a greater mortality rate from tuberculosis and other co-infections. Active infection signs and symptoms are generally less severe in the elderly. The interaction of structural lung damage, prolonged inflammation, bacterial and fungal colonisation of the respiratory system, and mucociliary insufficiency causes recurrent infections. It is imperative to use all available tools to make a microbiological diagnosis in diagnostic challenges in atypical cases. The therapeutic management of older people presents a significant difficulty in identifying frailty to prevent loss of independence.

A Mouse Model for the Transition of Streptococcus pneumoniae from Colonizer to Pathogen upon Viral Co-Infection Recapitulates Age-Exacerbated Illness

Streptococcus pneumoniae (pneumococcus) is an asymptomatic colonizer of the nasopharynx in most individuals but can progress to a pulmonary and systemic pathogen upon influenza A virus (IAV) infection. Advanced age enhances host susceptibility to secondary pneumococcal pneumonia and is associated with worsened disease outcomes. The host factors driving those processes are not well defined, in part due to a lack of animal models that reproduce the transition from asymptomatic colonization to severe clinical disease. This paper describes a novel mouse model that recreates the transition of pneumococci from asymptomatic carriage to disease upon viral infection. In this model, mice are first intranasally inoculated with biofilm-grown pneumococci to establish asymptomatic carriage, followed by IAV infection of both the nasopharynx and lungs. This results in bacterial dissemination to the lungs, pulmonary inflammation, and obvious signs of illness that can progress to lethality. The degree of disease is dependent on the bacterial strain and host factors. Importantly, this model reproduces the susceptibility of aging, because compared to young mice, old mice display more severe clinical illness and succumb to disease more frequently. By separating carriage and disease into distinct steps and providing the opportunity to analyze the genetic variants of both the pathogen and the host, this S. pneumoniae/IAV co-infection model permits the detailed examination of the interactions of an important pathobiont with the host at different phases of disease progression. This model can also serve as an important tool for identifying potential therapeutic targets against secondary pneumococcal pneumonia in susceptible hosts.

Adults with Trisomy 21 Have Differential Antibody Responses to Influenza A

Down syndrome is caused by an extra copy of chromosome 21. In the past two decades, the life expectancy of individuals with Down syndrome has significantly increased from early 20s to early 60s, creating a population of individuals of which little is known about how well they are protected against infectious disease. The goal of this work is to better understand if adults with Down syndrome are well protected against influenza following vaccination. We obtained plasma samples from 18 adults (average age = 31yo) with Down syndrome and 17 age/gender-matched disomic individuals, all vaccinated against influenza. Antibody concentration to influenza A was measured using ELISA and antibody titers were measured using a hemagglutinin inhibition assay. Statistical analysis was performed using Stata Statistical Software. Adults with Down syndrome had a significantly increased concentration of antibodies to a mixture of influenza A viral proteins; however, they had a significantly decreased titer to the Influenza A/Hong Kong compared to disomic controls. These findings suggest that more vigorous studies of B- and T-cell function in adults with Down syndrome with respect to influenza vaccination are warranted, and that this population may benefit from a high-dose influenza vaccine.

The Contribution of Viral Proteins to the Synergy of Influenza and Bacterial Co-Infection

A severe course of acute respiratory disease caused by influenza A virus (IAV) infection is often linked with subsequent bacterial superinfection, which is difficult to cure. Thus, synergistic influenza-bacterial co-infection represents a serious medical problem. The pathogenic changes in the infected host are accelerated as a consequence of IAV infection, reflecting its impact on the host immune response. IAV infection triggers a complex process linked with the blocking of innate and adaptive immune mechanisms required for effective antiviral defense. Such disbalance of the immune system allows for easier initiation of bacterial superinfection. Therefore, many new studies have emerged that aim to explain why viral-bacterial co-infection can lead to severe respiratory disease with possible fatal outcomes. In this review, we discuss the key role of several IAV proteins-namely, PB1-F2, hemagglutinin (HA), neuraminidase (NA), and NS1-known to play a role in modulating the immune defense of the host, which consequently escalates the development of secondary bacterial infection, most often caused by Streptococcus pneumoniae. Understanding the mechanisms leading to pathological disorders caused by bacterial superinfection after the previous viral infection is important for the development of more effective means of prevention; for example, by vaccination or through therapy using antiviral drugs targeted at critical viral proteins.

Bacterial and viral infections and related inflammatory responses in chronic obstructive pulmonary disease

In chronic obstructive pulmonary disease (COPD) patients, bacterial and viral infections play a relevant role in worsening lung function and, therefore, favour disease progression. The inflammatory response to lung infections may become a specific indication of the bacterial and viral infections. We here review data on the bacterial-viral infections and related airways and lung parenchyma inflammation in stable and exacerbated COPD, focussing our attention on the prevalent molecular pathways in these different clinical conditions. The roles of macrophages, autophagy and NETosis are also briefly discussed in the context of lung infections in COPD. Controlling their combined response may restore a balanced lung homeostasis, reducing the risk of lung function decline. KEY MESSAGE Bacteria and viruses can influence the responses of the innate and adaptive immune system in the lung of chronic obstructive pulmonary disease (COPD) patients. The relationship between viruses and bacterial colonization, and the consequences of the imbalance of these components can modulate the inflammatory state of the COPD lung. The complex actions involving immune trigger cells, which activate innate and cell-mediated inflammatory responses, could be responsible for the clinical consequences of irreversible airflow limitation, lung remodelling and emphysema in COPD patients.

A Murine Model for Enhancement of Streptococcus pneumoniae Pathogenicity upon Viral Infection and Advanced Age

Streptococcus pneumoniae (pneumococcus) resides asymptomatically in the nasopharynx (NP) but can progress from benign colonizer to lethal pulmonary or systemic pathogen. Both viral infection and aging are risk factors for serious pneumococcal infections. Previous work established a murine model that featured the movement of pneumococcus from the nasopharynx to the lung upon nasopharyngeal inoculation with influenza A virus (IAV) but did not fully recapitulate the severe disease associated with human coinfection. We built upon this model by first establishing pneumococcal nasopharyngeal colonization, then inoculating both the nasopharynx and lungs with IAV. In young (2-month-old) mice, coinfection triggered bacterial dispersal from the nasopharynx into the lungs, pulmonary inflammation, disease, and mortality in a fraction of mice. In aged mice (18 to 24 months), coinfection resulted in earlier and more severe disease. Aging was not associated with greater bacterial burdens but rather with more rapid pulmonary inflammation and damage. Both aging and IAV infection led to inefficient bacterial killing by neutrophils ex vivo. Conversely, aging and pneumococcal colonization also blunted alpha interferon (IFN-α) production and increased pulmonary IAV burden. Thus, in this multistep model, IAV promotes pneumococcal pathogenicity by modifying bacterial behavior in the nasopharynx, diminishing neutrophil function, and enhancing bacterial growth in the lung, while pneumococci increase IAV burden, likely by compromising a key antiviral response. Thus, this model provides a means to elucidate factors, such as age and coinfection, that promote the evolution of S. pneumoniae from asymptomatic colonizer to invasive pathogen, as well as to investigate consequences of this transition on antiviral defense.

Clinical Experience With Severe Acute Respiratory Syndrome Coronavirus 2-Related Illness in Children: Hospital Experience in Cape Town, South Africa

BACKGROUND

Children seem relatively protected from serious severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2)-related disease, but little is known about children living in settings with high tuberculosis and human immunodeficiency virus (HIV) burden. This study reflects clinical data on South African children with SARS-CoV-2.

METHODS

We collected clinical data of children aged <13 years with laboratory-confirmed SARS-CoV-2 presenting to Tygerberg Hospital, Cape Town, between 17 April and 24 July 2020.

RESULTS

One hundred fifty-nine children (median age, 48.0 months [interquartile range {IQR}, 12.0-106.0 months]) were included. Hospitalized children (n = 62), with a median age of 13.5 months (IQR, 1.8-43.5 months) were younger than children not admitted (n = 97; median age, 81.0 months [IQR, 34.5-120.5 months]; P < .01.). Thirty-three of 159 (20.8%) children had preexisting medical conditions. Fifty-one of 62 (82.3%) hospitalized children were symptomatic; lower respiratory tract infection was diagnosed in 21 of 51 (41.2%) children, and in 11 of 16 (68.8%) children <3 months of age. Respiratory support was required in 25 of 51 (49.0%) children; 13 of these (52.0%) were <3 months of age. One child was HIV infected and 11 of 51 (21.2%) were HIV exposed but uninfected, and 7 of 51 (13.7%) children had a recent or new diagnosis of tuberculosis.

CONCLUSIONS

Children <1 year of age hospitalized with SARS-CoV-2 in Cape Town frequently required respiratory support. Access to oxygen may be limited in some low- and middle-income countries, which could potentially drive morbidity and mortality. HIV infection was uncommon but a relationship between HIV exposure, tuberculosis, and SARS-CoV-2 should be explored.

Intranasal Vaccine Delivery Technology for Respiratory Tract Disease Application with a Special Emphasis on Pneumococcal Disease

This mini-review will cover recent trends in intranasal (IN) vaccine delivery as it relates to applications for respiratory tract diseases. The logic and rationale for IN vaccine delivery will be compared to methods and applications accompanying this particular administration route. In addition, we will focus extended discussion on the potential role of IN vaccination in the context of respiratory tract diseases, with a special emphasis on pneumococcal disease. Here, elements of this disease, including its prevalence and impact upon the elderly population, will be viewed from the standpoint of improving health outcomes through vaccine design and delivery technology and how IN administration can play a role in such efforts.

Gut Dysbiosis during Influenza Contributes to Pulmonary Pneumococcal Superinfection through Altered Short-Chain Fatty Acid Production

Secondary bacterial infections often complicate viral respiratory infections. We hypothesize that perturbation of the gut microbiota during influenza A virus (IAV) infection might favor respiratory bacterial superinfection. Sublethal infection with influenza transiently alters the composition and fermentative activity of the gut microbiota in mice. These changes are attributed in part to reduced food consumption. Fecal transfer experiments demonstrate that the IAV-conditioned microbiota compromises lung defenses against pneumococcal infection. In mechanistic terms, reduced production of the predominant short-chain fatty acid (SCFA) acetate affects the bactericidal activity of alveolar macrophages. Following treatment with acetate, mice colonized with the IAV-conditioned microbiota display reduced bacterial loads. In the context of influenza infection, acetate supplementation reduces, in a free fatty acid receptor 2 (FFAR2)-dependent manner, local and systemic bacterial loads. This translates into reduced lung pathology and improved survival rates of double-infected mice. Lastly, pharmacological activation of the SCFA receptor FFAR2 during influenza reduces bacterial superinfection.

High burden of viral respiratory co-infections in a cohort of children with suspected pulmonary tuberculosis

Background

The presentation of pulmonary tuberculosis (PTB) in young children is often clinically indistinguishable from other common respiratory illnesses, which are frequently infections of viral aetiology. As little is known about the role of viruses in children with PTB, we investigated the prevalence of respiratory viruses in children with suspected PTB at presentation and follow-up.

Methods

In an observational cohort study, children < 13 years were routinely investigated for suspected PTB in Cape Town, South Africa between December 2015 and September 2017 and followed up for 24 weeks. Nasopharyngeal aspirates (NPAs) were tested for respiratory viruses using multiplex PCR at enrolment, week 4 and 8.

Results

Seventy-three children were enrolled [median age 22.0 months; (interquartile range 10.0–48.0); 56.2% male and 17.8% HIV-infected. Anti-tuberculosis treatment was initiated in 54.8%; of these 50.0% had bacteriologically confirmed TB. At enrolment, ≥1 virus were detected in 95.9% (70/73) children; most commonly human rhinovirus (HRV) (74.0%). HRV was more frequently detected in TB cases (85%) compared to ill controls (60.6%) (p = 0.02). Multiple viruses were detected in 71.2% of all children; 80% of TB cases and 60.6% of ill controls (p = 0.07). At follow-up, ≥1 respiratory virus was detected in 92.2% (47/51) at week 4, and 94.2% (49/52) at week 8.

Conclusions

We found a high prevalence of viral respiratory co-infections in children investigated for PTB, irrespective of final PTB diagnosis, which remained high during follow up. Future work should include investigating the whole respiratory ecosystem in combination with pathogen- specific immune responses.

Epidemic and pandemic viral infections: impact on tuberculosis and the lung: A consensus by the World Association for Infectious Diseases and Immunological Disorders (WAidid), Global Tuberculosis Network (GTN), and members of the European Society of Clinical Microbiology and Infectious Diseases Study Group for Mycobacterial Infections (ESGMYC)

Major epidemics, including some that qualify as pandemics, such as severe acute respiratory syndrome (SARS), Middle East respiratory syndrome (MERS), HIV, influenza A (H1N1)pdm/09 and most recently COVID-19, affect the lung. Tuberculosis (TB) remains the top infectious disease killer, but apart from syndemic TB/HIV little is known regarding the interaction of viral epidemics and pandemics with TB. The aim of this consensus-based document is to describe the effects of viral infections resulting in epidemics and pandemics that affect the lung (MERS, SARS, HIV, influenza A (H1N1)pdm/09 and COVID-19) and their interactions with TB. A search of the scientific literature was performed. A writing committee of international experts including the European Centre for Disease Prevention and Control Public Health Emergency (ECDC PHE) team, the World Association for Infectious Diseases and Immunological Disorders (WAidid), the Global Tuberculosis Network (GTN), and members of the European Society of Clinical Microbiology and Infectious Diseases (ESCMID) Study Group for Mycobacterial Infections (ESGMYC) was established. Consensus was achieved after multiple rounds of revisions between the writing committee and a larger expert group. A Delphi process involving the core group of authors (excluding the ECDC PHE team) identified the areas requiring review/consensus, followed by a second round to refine the definitive consensus elements. The epidemiology and immunology of these viral infections and their interactions with TB are discussed with implications for diagnosis, treatment and prevention of airborne infections (infection control, viral containment and workplace safety). This consensus document represents a rapid and comprehensive summary on what is known on the topic.

Bacterial co-infections with SARS-CoV-2

The pandemic coronavirus disease 2019 (COVID‐19), caused by Severe Acute Respiratory Syndrome Coronavirus 2 (SARS‐CoV‐2), has affected millions of people worldwide. To date, there are no proven effective therapies for this virus. Efforts made to develop antiviral strategies for the treatment of COVID‐19 are underway. Respiratory viral infections, such as influenza, predispose patients to co‐infections and these lead to increased disease severity and mortality. Numerous types of antibiotics such as azithromycin have been employed for the prevention and treatment of bacterial co‐infection and secondary bacterial infections in patients with a viral respiratory infection (e.g., SARS‐CoV‐2). Although antibiotics do not directly affect SARS‐CoV‐2, viral respiratory infections often result in bacterial pneumonia. It is possible that some patients die from bacterial co‐infection rather than virus itself. To date, a considerable number of bacterial strains have been resistant to various antibiotics such as azithromycin, and the overuse could render those or other antibiotics even less effective. Therefore, bacterial co‐infection and secondary bacterial infection are considered critical risk factors for the severity and mortality rates of COVID‐19. Also, the antibiotic‐resistant as a result of overusing must be considered. In this review, we will summarize the bacterial co‐infection and secondary bacterial infection in some featured respiratory viral infections, especially COVID‐19.

The immune response to influenza in older humans: beyond immune senescence

Despite widespread influenza vaccination programs, influenza remains a major cause of morbidity and mortality in older adults. Age-related changes in multiple aspects of the adaptive immune response to influenza have been well-documented including a decline in antibody responses to influenza vaccination and changes in the cell-mediated response associated with immune senescence. This review will focus on T cell responses to influenza and influenza vaccination in older adults, and how increasing frailty or coexistence of multiple (≥2) chronic conditions contributes to the loss of vaccine effectiveness for the prevention of hospitalization. Further, dysregulation of the production of pro- and anti-inflammatory mediators contributes to a decline in the generation of an effective CD8 T cell response needed to clear influenza virus from the lungs. Current influenza vaccines provide only a weak stimulus to this arm of the adaptive immune response and rely on re-stimulation of CD8 T cell memory related to prior exposure to influenza virus. Efforts to improve vaccine effectiveness in older adults will be fruitless until CD8 responses take center stage.

Down Syndrome and COVID-19: A Perfect Storm?

People with Down syndrome show signs of chronic immune dysregulation, including a higher prevalence of autoimmune disorders, increased rates of hospitalization during respiratory viral infections, and higher mortality rates from pneumonia and sepsis. At the molecular and cellular levels, they show markers of chronic autoinflammation, including interferon hyperactivity, elevated levels of many inflammatory cytokines and chemokines, and changes in diverse immune cell types reminiscent of inflammatory conditions observed in the general population. However, the impact of this immune dysregulation in severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection and CoV disease of 2019 (COVID-19) remains unknown. This Perspective outlines why individuals with Down syndrome should be considered an at-risk population for severe COVID-19. Specifically, the immune dysregulation caused by trisomy 21 may result in an exacerbated cytokine release syndrome relative to that observed in the euploid population, thus justifying additional monitoring and specialized care for this vulnerable population.

Viral strategies predisposing to respiratory bacterial superinfections

Acute respiratory infections are amongst the leading causes of childhood morbidity and mortality globally. Viruses are the predominant cause of such infections, but mixed etiologies with bacteria has for decades raised the question of the interplay between them in causality and determination of the outcome of such infections. In this review, we examine recent microbiological, biochemical, and immunological advances that contribute to elucidating the mechanisms by which infections by specific viruses enable bacterial infections in the airway, and exacerbate them. We analyze specific domains in which viruses play such facilitating role including enhancement of bacterial adhesion by unmasking cryptic receptors and upregulation of adhesion proteins, disruption of tight junction integrity favoring paracellular transmigration of bacteria and loss of epithelial barrier integrity, increased availability of nutrient, such as mucins and iron, alteration of innate and adaptive immune responses, and disabling defense against bacteria, and lastly, changes in airway microbiome that render the lung more vulnerable to pathogens. Separate exhaustive analysis of each domain focuses on individuals with cystic fibrosis (CF), in whom viruses may play a key role in paving the way for the primary injury that leads to permanence of bacterial pathogens, viruses may then serve as triggers for "CF exacerbations"; these constituting the signature and ultimately the outcome determinants of these patients.

16HBE Cell Lipid Mediator Responses to Mono and Co-Infections with Respiratory Pathogens

Respiratory tract infections are a global health problem. The main causative agents of these infections are influenza A virus (IAV), Staphylococcus aureus (S. aureus), and Streptococcus pneumoniae (S. pneumoniae). Major research focuses on genetics and immune responses in these infections. Eicosanoids and other oxylipins are host-derived lipid mediators that play an important role in the activation and resolution of inflammation. In this study, we assess, for the first time, the different intracellular profiles of these bioactive lipid mediators during S. aureus LUG2012, S. pneumoniae TIGR4, IAV, and corresponding viral and bacterial co-infections of 16HBE cells. We observed a multitude of altered lipid mediators. Changes in the amount of 5-hydroxyeicosatetraenoic acid (5-HETE) were prominent for all bacterial infections. The infection with S. pneumoniae showed the strongest impact on bioactive lipid production and led to alterations in the amount of PPARγ ligands and precursors of pro-resolving lipid mediators.

Post tuberculosis treatment infectious complications

Following greater attention and follow-up of patients with treated pulmonary tuberculosis (TB), it has emerged that infections are more likely to occur in this cohort of patients. This comes as no surprise, as pulmonary TB is a destructive process that leads to cicatrization, alteration of parenchyma, bronchiectasis, and scarring of the lung, with reduction of lung volumes and an impact on pulmonary function. In addition to relapse and re-infection with TB, other pathogens are increasingly recognized in post-TB patients. This paper serves as a summary and guide on how to approach the post-TB patient with new signs and symptoms of pulmonary infection in order to ensure optimal management and rehabilitation.

A Porcine Ex Vivo Lung Perfusion Model To Investigate Bacterial Pathogenesis

The implementation of infection models that approximate human disease is essential to understand infections and for testing new therapies before they enter into clinical stages. Rodents are used in most preclinical studies, although the differences between mice and humans have fueled the conclusion that murine studies are unreliable predictors of human outcomes. In this study, we have developed a whole-lung porcine model of infection using the ex vivo lung perfusion (EVLP) system established to recondition human lungs for transplant. As a proof of principle, we provide evidence demonstrating that infection of the porcine EVLP with the human pathogen Klebsiella pneumoniae recapitulates the known features of Klebsiella-triggered pneumonia. Moreover, our data revealed that the porcine EVLP model is useful to reveal features of the virulence of K. pneumoniae, including the manipulation of immune cells. Together, the findings of this study support the utility of the EVLP model using pig lungs as a surrogate host for assessing respiratory infections.

The use of animal infection models is essential to understand microbial pathogenesis and to develop and test treatments. Insects and two-dimensional (2D) and 3D tissue models are increasingly being used as surrogates for mammalian models. However, there are concerns about whether these models recapitulate the complexity of host-pathogen interactions. In this study, we developed the ex vivo lung perfusion (EVLP) model of infection using porcine lungs to investigate Klebsiella pneumoniae-triggered pneumonia as a model of respiratory infections. The porcine EVLP model recapitulates features of K. pneumoniae-induced pneumonia lung injury. This model is also useful to assess the pathogenic potential of K. pneumoniae, as we observed that the attenuated Klebsiella capsule mutant strain caused less pathological tissue damage with a concomitant decrease in the bacterial burden compared to that in lungs infected with the wild type. The porcine EVLP model allows assessment of inflammatory responses following infection; similar to the case with the mouse pneumonia model, we observed an increase of il-10 in the lungs infected with the wild type and an increase of ifn-γ in lungs infected with the capsule mutant. This model also allows monitoring of phenotypes at the single-cell level. Wild-type K. pneumoniae skews macrophages toward an M2-like state. In vitro experiments probing pig bone marrow-derived macrophages uncovered the role for the M2 transcriptional factor STAT6 and that Klebsiella-induced il-10 expression is controlled by p38 and extracellular signal-regulated kinase (ERK). Klebsiella-induced macrophage polarization is dependent on the capsule. Together, the findings of this study support the utility of the EVLP model using pig lungs as a platform to investigate the infection biology of respiratory pathogens.

Influenza and tuberculosis co-infection: A systematic review

Introduction

There are limited data on risk of severe disease or outcomes in patients with influenza and pulmonary tuberculosis (PTB) co‐infection compared to those with single infection.

Methods

We conducted a systematic review of published literature on the interaction of influenza viruses and PTB. Studies were eligible for inclusion if they presented data on prevalence, disease association, presentation or severity of laboratory‐confirmed influenza among clinically diagnosed or laboratory‐confirmed PTB cases. We searched eight databases from inception until December 2018. Summary characteristics of each study were extracted, and a narrative summary was presented. Cohort or case‐control studies were assessed for potential bias using the Newcastle‐Ottawa scale.

Results

We assessed 5154 abstracts, reviewed 146 manuscripts and included 19 studies fulfilling selection criteria (13 human and six animal). Of seven studies reporting on the possible effect of the underlying PTB disease in patients with influenza, three of four analytical studies reported no association with disease severity of influenza infection in those with PTB, whilst one study reported PTB as a risk factor for influenza‐associated hospitalization.

An association between influenza infection and PTB disease was found in three of five analytical studies; whereas the two other studies reported a high frequency of PTB disease progression and complications among patients with seasonal influenza co‐infection.

Conclusion

Human analytical studies of an association between co‐infection and severe influenza‐ or PTB‐associated disease or increased prevalence of influenza co‐infection in individuals' hospitalized for PTB were not conclusive. Data are limited from large, high‐quality, analytical epidemiological studies with laboratory‐confirmed endpoints.

Immunogenicity and safety of 13-valent pneumococcal conjugate vaccine in HIV-infected adults in the era of highly active antiretroviral therapy: analysis stratified by CD4 T-cell count

HIV-infected patients are 30- to 100-fold more susceptible to invasive pneumococcal diseases than are healthy adults. Pneumococcal vaccination may be the best way to decrease the large pneumococcal disease burden, but the optimal timing of vaccination is still unclear. In this study, HIV-infected subjects aged ≥18 years were recruited and divided into 2 age-matched groups: group 1 (subjects with CD4 T-cell count ≥350 cells/µL) and group 2 (CD4 T-cell count <350 cells/µL). Multiplex opsonophagocytic killing assay was used to compare immunogenicity after immunization with 13-valent pneumococcal conjugate vaccine (PCV13). Among 70 subjects, 67 (group 1, N = 34; group 2, N = 33) were available for the assessment of immunogenicity and safety. With respect to the post-vaccination geometric mean titer (GMT) ratios, the non-inferiority criteria were not met. Post-vaccination GMTs were significantly lower in group 2 compared to group 1 for all 4 pneumococcal serotypes (5, 6B, 18C, and 19A) tested. PCV13 was safe and well tolerated in HIV-infected patients irrespective of immune status. In conclusion, PCV13 showed significantly inferior immunogenicity among HIV-infected patients with CD4 T-cell count <350 cells/µL compared to those with a higher CD4 T-cell count.

Pharmacological effects of ginseng on infectious diseases

Ginseng has been traditionally used as an herbal nutritional supplement in Asian countries, including Korea, China, Japan, and Vietnam for several millennia. Most studies have focused on the role of ginseng on anti-oxidative stress, anti-inflammatory, and anti-cancer activities. Recently, modulator activities of ginseng on the immune responses during pathogenic bacterial and viral infections and beneficial effects of ginseng in infectious diseases have been elucidated. In vivo and in vitro studies revealed the potential of ginseng extracts and ginsenosides Rg1, Rg3, Rb1, Rb2, Rb3, compound K, Re, Rd, Rh2 for treatment of several infectious diseases. The molecular mechanisms of these effects mainly involve inflammatory cytokines (TNF-α, IL-6, IL-1β, IFN-γ, IL-10), apoptotic pathway (bcl-2, bcl-xL), PI3K/Akt pathway, MAPKs pathway, JAK2/STAT5, NF-κB pathway, and the inflammasome. In this review, we will summarize the current knowledge on the effects of ginseng in the immune responses during the infections and its bioactivities on the prevention of infectious diseases as well as its underlying mechanisms. Moreover, the therapeutic potential of ginseng as an anti-bacterial and anti-viral medication and vaccine adjuvant will be discussed as well.

The Development and Use of Reporter Influenza B Viruses

Influenza B viruses (IBVs) are major contributors to total human influenza disease, responsible for ~1/3 of all infections. These viruses, however, are relatively less studied than the related influenza A viruses (IAVs). While it has historically been assumed that the viral biology and mechanisms of pathogenesis for all influenza viruses were highly similar, studies have shown that IBVs possess unique characteristics. Relative to IAV, IBV encodes distinct viral proteins, displays a different mutational rate, has unique patterns of tropism, and elicits different immune responses. More work is therefore required to define the mechanisms of IBV pathogenesis. One valuable approach to characterize mechanisms of microbial disease is the use of genetically modified pathogens that harbor exogenous reporter genes. Over the last few years, IBV reporter viruses have been developed and used to provide new insights into the host response to infection, viral spread, and the testing of antiviral therapeutics. In this review, we will highlight the history and study of IBVs with particular emphasis on the use of genetically modified viruses and discuss some remaining gaps in knowledge that can be addressed using reporter expressing IBVs.

Antibody Treatment against Angiopoietin-Like 4 Reduces Pulmonary Edema and Injury in Secondary Pneumococcal Pneumonia

Secondary bacterial lung infection by Streptococcus pneumoniae (S. pneumoniae) poses a serious health concern, especially in developing countries. We posit that the emergence of multiantibiotic-resistant strains will jeopardize current treatments in these regions. Deaths arising from secondary infections are more often associated with acute lung injury, a common consequence of hypercytokinemia, than with the infection per se Given that secondary bacterial pneumonia often has a poor prognosis, newer approaches to improve treatment outcomes are urgently needed to reduce the high levels of morbidity and mortality. Using a sequential dual-infection mouse model of secondary bacterial lung infection, we show that host-directed therapy via immunoneutralization of the angiopoietin-like 4 c-isoform (cANGPTL4) reduced pulmonary edema and damage in infected mice. RNA sequencing analysis revealed that anti-cANGPTL4 treatment improved immune and coagulation functions and reduced internal bleeding and edema. Importantly, anti-cANGPTL4 antibody, when used concurrently with either conventional antibiotics or antipneumolysin antibody, prolonged the median survival of mice compared to monotherapy. Anti-cANGPTL4 treatment enhanced immune cell phagocytosis of bacteria while restricting excessive inflammation. This modification of immune responses improved the disease outcomes of secondary pneumococcal pneumonia. Taken together, our study emphasizes that host-directed therapeutic strategies are viable adjuncts to standard antimicrobial treatments.IMPORTANCE Despite extensive global efforts, secondary bacterial pneumonia still represents a major cause of death in developing countries and is an important cause of long-term functional disability arising from lung tissue damage. Newer approaches to improving treatment outcomes are needed to reduce the significant morbidity and mortality caused by infectious diseases. Our study, using an experimental mouse model of secondary S. pneumoniae infection, shows that a multimodal treatment that concurrently targets host and pathogen factors improved lung tissue integrity and extended the median survival time of infected mice. The immunoneutralization of host protein cANGPTL4 reduced the severity of pulmonary edema and damage. We show that host-directed therapeutic strategies as well as neutralizing antibodies against pathogen virulence factors are viable adjuncts to standard antimicrobial treatments such as antibiotics. In view of their different modes of action compared to antibiotics, concurrent immunotherapies using antibodies are potentially efficacious against secondary pneumococcal pneumonia caused by antibiotic-resistant pathogens.

Klebsiella pneumoniae infection biology: living to counteract host defences

Klebsiella species cause a wide range of diseases including pneumonia, urinary tract infections (UTIs), bloodstream infections and sepsis. These infections are particularly a problem among neonates, elderly and immunocompromised individuals. Klebsiella is also responsible for a significant number of community-acquired infections. A defining feature of these infections is their morbidity and mortality, and the Klebsiella strains associated with them are considered hypervirulent. The increasing isolation of multidrug-resistant strains has significantly narrowed, or in some settings completely removed, the therapeutic options for the treatment of Klebsiella infections. Not surprisingly, this pathogen has then been singled out as an 'urgent threat to human health' by several organisations. This review summarises the tremendous progress that has been made to uncover the sophisticated immune evasion strategies of K. pneumoniae. The co-evolution of Klebsiella in response to the challenge of an activated immune has made Klebsiella a formidable pathogen exploiting stealth strategies and actively suppressing innate immune defences to overcome host responses to survive in the tissues. A better understanding of Klebsiella immune evasion strategies in the context of the host-pathogen interactions is pivotal to develop new therapeutics, which can be based on antagonising the anti-immune strategies of this pathogen.

Immune Function and Micronutrient Requirements Change over the Life Course

As humans age, the risk and severity of infections vary in line with immune competence according to how the immune system develops, matures, and declines. Several factors influence the immune system and its competence, including nutrition. A bidirectional relationship among nutrition, infection and immunity exists: changes in one component affect the others. For example, distinct immune features present during each life stage may affect the type, prevalence, and severity of infections, while poor nutrition can compromise immune function and increase infection risk. Various micronutrients are essential for immunocompetence, particularly vitamins A, C, D, E, B2, B6, and B12, folic acid, iron, selenium, and zinc. Micronutrient deficiencies are a recognized global public health issue, and poor nutritional status predisposes to certain infections. Immune function may be improved by restoring deficient micronutrients to recommended levels, thereby increasing resistance to infection and supporting faster recovery when infected. Diet alone may be insufficient and tailored micronutrient supplementation based on specific age-related needs necessary. This review looks at immune considerations specific to each life stage, the consequent risk of infection, micronutrient requirements and deficiencies exhibited over the life course, and the available evidence regarding the effects of micronutrient supplementation on immune function and infection.

Back to the Future: Lessons Learned From the 1918 Influenza Pandemic

2018 marks the 100-year anniversary of the 1918 influenza pandemic, which killed ~50 million people worldwide. The severity of this pandemic resulted from a complex interplay between viral, host, and societal factors. Here, we review the viral, genetic and immune factors that contributed to the severity of the 1918 pandemic and discuss the implications for modern pandemic preparedness. We address unresolved questions of why the 1918 influenza H1N1 virus was more virulent than other influenza pandemics and why some people survived the 1918 pandemic and others succumbed to the infection. While current studies suggest that viral factors such as haemagglutinin and polymerase gene segments most likely contributed to a potent, dysregulated pro-inflammatory cytokine storm in victims of the pandemic, a shift in case-fatality for the 1918 pandemic toward young adults was most likely associated with the host's immune status. Lack of pre-existing virus-specific and/or cross-reactive antibodies and cellular immunity in children and young adults likely contributed to the high attack rate and rapid spread of the 1918 H1N1 virus. In contrast, lower mortality rate in in the older (>30 years) adult population points toward the beneficial effects of pre-existing cross-reactive immunity. In addition to the role of humoral and cellular immunity, there is a growing body of evidence to suggest that individual genetic differences, especially involving single-nucleotide polymorphisms (SNPs), contribute to differences in the severity of influenza virus infections. Co-infections with bacterial pathogens, and possibly measles and malaria, co-morbidities, malnutrition or obesity are also known to affect the severity of influenza disease, and likely influenced 1918 H1N1 disease severity and outcomes. Additionally, we also discuss the new challenges, such as changing population demographics, antibiotic resistance and climate change, which we will face in the context of any future influenza virus pandemic. In the last decade there has been a dramatic increase in the number of severe influenza virus strains entering the human population from animal reservoirs (including highly pathogenic H7N9 and H5N1 viruses). An understanding of past influenza virus pandemics and the lessons that we have learnt from them has therefore never been more pertinent.

Natural Killer T Cells and Mucosal-Associated Invariant T Cells in Lung Infections

The immune system has been traditionally divided into two arms called innate and adaptive immunity. Typically, innate immunity refers to rapid defense mechanisms that set in motion within minutes to hours following an insult. Conversely, the adaptive immune response emerges after several days and relies on the innate immune response for its initiation and subsequent outcome. However, the recent discovery of immune cells displaying merged properties indicates that this distinction is not mutually exclusive. These populations that span the innate-adaptive border of immunity comprise, among others, CD1d-restricted natural killer T cells and MR1-restricted mucosal-associated invariant T cells. These cells have the unique ability to swiftly activate in response to non-peptidic antigens through their T cell receptor and/or to activating cytokines in order to modulate many aspects of the immune response. Despite they recirculate all through the body via the bloodstream, these cells mainly establish residency at barrier sites including lungs. Here, we discuss the current knowledge into the biology of these cells during lung (viral and bacterial) infections including activation mechanisms and functions. We also discuss future strategies targeting these cell types to optimize immune responses against respiratory pathogens.

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.

Treatment with broadly neutralizing influenza antibodies reduces severity of secondary pneumococcal pneumonia in mice

Secondary bacterial pneumonia is a frequent complication of influenza, associated with high morbidity and mortality. We hypothesized that treatment with neutralizing influenza A antibody AT10_002 protects against severe secondary pneumococcal infection in a mouse model of influenza A infection. Influenza A (H3N2) virus–infected male C57Bl6 mice were treated intravenously with either AT10_002 or a control 2 days postinfection. Seven days later, both groups were infected with Streptococcus pneumoniae and killed 18 hours later. Mice receiving AT10_002 showed less loss of bodyweight compared with controls (+1% vs −12%, P < .001), lower viral loads in bronchoalveolar lavage fluids (BALFs) (7 vs 194 RNA copies per µL; P < .001), and reduced bacterial outgrowth in lung homogenates (3.3 × 10¹ vs 2.5 × 10⁵ colony‐forming units per mg; P < .001). The treatment group showed lower pulmonary wet weights, lower cell counts, and lower protein levels in BALF compared with controls. Treatment with AT10_002 was associated with lower levels of tumor necrosis factor‐α, interleukin (IL)‐6, cytokine‐induced neutrophil chemoattractant (KC), and interferon‐γ in BALF and lower IL‐6 and KC in lung homogenates. Treatment with anti‐influenza antibody AT10_002 is associated with reduced weight loss, viral load, bacterial outgrowth, and lung injury in a murine model of secondary pneumococcal pneumonia following influenza infection.

Effects of influenza immunization on pneumonia in the elderly

Influenza virus is a common pathogen implicated in respiratory tract infections, annually affecting up to 20% of the general population, and pneumonia is a leading cause of death after influenza infection. Post-influenza pneumonia is especially common in the elderly and chronically ill patients. The risk of post-influenza pneumonia is significantly increased according to the number of concurrent comorbidities. Vaccination is the primary measure used to abate influenza epidemics and associated complications. In meta-analyses, influenza vaccine significantly reduces pneumonia- and influenza-related hospitalizations, with a vaccine effectiveness of 25-53%. However, considering the poor effectiveness of conventional influenza vaccines in the elderly, several highly immunogenic influenza vaccines have been developed. Further evaluations of the comparative effectiveness of diverse vaccine formulations are warranted to assess their utility for preventing influenza infection, post-influenza pneumonia, and related hospitalization/mortality. Based on cost-effectiveness and budget impact analysis, influenza vaccination strategies should be tailored in the elderly.

Bacterial pneumonia as an influenza complication

PURPOSE OF REVIEW

The pathogenesis and impact of coinfection, in particular bacterial coinfection, in influenza are incompletely understood. This review summarizes results from studies on bacterial coinfection in the recent pandemic influenza outbreak.

RECENT FINDINGS

Systemic immune mechanisms play a key role in the development of coinfection based on the complexity of the interaction of the host and the viral and bacterial pathogens. Several studies were performed to determine the point prevalence of bacterial coinfection in influenza. Coinfection in influenza is frequent in critically ill patients with Streptococcus pneumoniae being the most frequent bacterial pathogen and higher rates of potentially resistant pathogens over the years.

SUMMARY

Bacterial pneumonia is certainly an influenza complication. The recent epidemiology findings have helped to partially resolve the contribution of different pathogens. Immunosuppression is a risk factor for bacterial coinfection in influenza, and the epidemiology of coinfection has changed over the years during the last influenza pandemic, and these recent findings should be taken into account during present outbreaks.

Pneumococcal and influenza vaccination status of hospitalized adults with community acquired pneumonia and the effects of vaccination on clinical presentation

BACKGROUND

Previous reports have shown that vaccination rates of adult at-risk populations are low in Turkey. There are differing reports with regards to the effectiveness of the influenza and the pneumococcal polysaccharide vaccine (PPSV23) on the clinical outcomes of community acquired pneumonia (CAP). The purpose of this study was to analyze the influenza (FV) and pneumococcal vaccination (PV) status, the factors that influence the receipt of influenza/pneumococcal vaccine and the effects of prior vaccination on the clinical outcomes in adults hospitalized with CAP.

PATIENTS AND METHODS

Patients hospitalized with CAP between March 2009 and October 2013 and registered at the web-based Turkish Thoracic Society Pneumonia Database (TURCAP) were included in this multicentric, observational study. Of a total of 787 cases, data were analyzed for 466 patients for whom self-reported information on PV and FV was available.

RESULTS

In this adult population with CAP, the vaccination rate with both the pneumococcal and influenza vaccines was found to be 6%. Prior FV was found to be the sole variable that was associated with the receipt of PV [OR 17.8, 95% CI (25-75:8.56-37.01), p < 0.001]. Conversely, being vaccinated with PPSV23 was the only predictor of receipt of FV [OR 18.1, 95% CI (25 - 75:8.75 - 37.83), p < 0.001]. Compared to the unvaccinated cases, the chest radiograms of the vaccinated patients revealed less consolidation. The latter also reported fatigue, muscle pain and gastrointestinal symptoms less frequently. Although there was a trend for lower 30-day mortality and for lower rates of intensive care unit (ICU) admission, these did not reach statistical significance. A pneumonia severity index (PSI) score ≥ 90, CURB-65 score ≥3 and multilobar involvement, but not the vaccination status, were identified as independent determinants of ICU admission.

CONCLUSIONS

This study showed that, among patients hospitalized with CAP, the FV and/or PV rates are low. Prior vaccination does not appear to significantly affect the clinical outcomes.

Legionnaire's Disease and Influenza

Legionella pneumophila and influenza types A and B viruses can cause either community-acquired pneumonia with respiratory failure, or Legionella infection could attribute to influenza infection with potentially fatal prognosis. Copathogenesis between pandemic influenza and bacteria is characterized by complex interactions between coinfecting pathogens and the host. Understanding the underlying reason of the emersion of the secondary bacterial infection during an influenza infection is challenging. The dual infection has an impact on viral control and may delay viral clearance. Effective vaccines and antiviral therapy are crucial to increase resistance toward influenza, decrease the prevalence of influenza, and possibly interrupt the potential secondary bacterial infections.

RIG-I Activation Protects and Rescues from Lethal Influenza Virus Infection and Bacterial Superinfection

Influenza A virus infection causes substantial morbidity and mortality in seasonal epidemic outbreaks, and more efficient treatments are urgently needed. Innate immune sensing of viral nucleic acids stimulates antiviral immunity, including cell-autonomous antiviral defense mechanisms that restrict viral replication. RNA oligonucleotide ligands that potently activate the cytoplasmic helicase retinoic-acid-inducible gene I (RIG-I) are promising candidates for the development of new antiviral therapies. Here, we demonstrate in an Mx1-expressing mouse model of influenza A virus infection that a single intravenous injection of low-dose RIG-I ligand 5'-triphosphate RNA (3pRNA) completely protected mice from a lethal challenge with influenza A virus for at least 7 days. Furthermore, systemic administration of 3pRNA rescued mice with pre-established fulminant influenza infection and prevented the fatal effects of a streptococcal superinfection. Type I interferon, but not interferon-λ, was required for the therapeutic effect. Our results suggest that the use of RIG-I activating oligonucleotide ligands has the clinical potential to confine influenza epidemics when a strain-specific vaccine is not yet available and to reduce lethality of influenza in severely infected patients.

Free actin impairs macrophage bacterial defenses via scavenger receptor MARCO interaction with reversal by plasma gelsolin

Lung injury can release intracellular actin into the alveolar milieu and is also associated with increased susceptibility to secondary infections. We investigated the effect of free (extracellular) actin on lung macrophage host defense functions. Western blot analysis demonstrated free actin release into the lung lavage fluids of mouse models of ozone injury, influenza infection, and secondary pneumococcal pneumonia and in samples from patients following burn and inhalation injury. Using levels comparable with those observed in lung injury, we found that free actin markedly inhibited murine lung macrophage binding and uptake in vitro of S. pneumoniae, S. aureus, and E. coli, (e.g., S. pneumoniae, mean %inhibition, actin vs. vehicle: 85 ± 0.3 (SD); n = 22, P < .001). Similar effects were observed on the ability of primary human macrophages to bind and ingest fluorescent Saureus (~75% inhibition). Plasma gelsolin (pGSN), a protein that functions to bind and cleave actin, restored bacterial binding and uptake by both murine and human macrophages. Scavenger receptor inhibitors reduced binding of fluorescent actin by murine macrophages [fluorescence index (×10^-3) after incubation with vehicle, actin, or actin + polyinosinic acid, respectively: 0.8 ± 0.7, 101.7 ± 50.7, or 52.7 ± 16.9; n = 5-6, P < 0.05]. In addition, actin binding was reduced in a MARCO/SR-AI/II-deficient cell line and by normal AMs obtained from MARCO^-/- mice. After release from injured cells during lung injury, free actin likely contributes to impaired host defense by blocking scavenger receptor binding of bacteria. This mechanism for increased risk of secondary infections after lung injury or inflammation may represent another target for therapeutic intervention with pGSN.

Postviral Complications: Bacterial Pneumonia

Secondary bacterial pneumonia after viral respiratory infection remains a significant source of morbidity and mortality. Susceptibility is mediated by a variety of viral and bacterial factors, and complex interactions with the host immune system. Prevention and treatment strategies are limited to influenza vaccination and antibiotics/antivirals respectively. Novel approaches to identifying the individuals with influenza who are at increased risk for secondary bacterial pneumonias are urgently needed. Given the threat of further pandemics and the heightened prevalence of these viruses, more research into the immunologic mechanisms of this disease is warranted with the hope of discovering new potential therapies.

Viral-bacterial co-infections in the respiratory tract

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Viruses predispose to secondary bacterial infection throughout the respiratory tract.

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Viral damage to airway epithelium and aberrant inflammatory responses play key roles.

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Dysregulation of both innate and acquired immune effectors contribute to co-infection.

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Viral co-infection promotes bacterial invasion of sterile sites within the airway.

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Optimal treatment likely requires control of both bacterial growth and host responses.

Preceding or concurrent viral respiratory tract infection can predispose to secondary bacterial co-infection throughout the airway. The mechanisms by which viruses promote these superinfections are diverse and replete. Whereas we understand much as to how viruses damage the airway and dysregulate both innate and acquired immune responses which, in turn, supports bacterial growth, adherence and invasion into normally sterile sites within the respiratory tract, new information regarding these co-infections is being gained from recent advances in microbiome research and our enhanced appreciation of the contribution of bacterial biofilms, among others. The advanced understanding obtained by continued research efforts in all aspects of viral–bacterial co-infections of the respiratory tract will allow us to devise novel approaches for disease prevention as well as to develop more effective therapeutics.

Hierarchical effects of pro-inflammatory cytokines on the post-influenza susceptibility to pneumococcal coinfection

In the course of influenza A virus (IAV) infections, a secondary bacterial infection frequently leads to serious respiratory conditions provoking high hospitalization and death tolls. Although abundant pro-inflammatory responses have been reported as key contributing factors for these severe dual infections, the relative contributions of cytokines remain largely unclear. In the current study, mathematical modelling based on murine experimental data dissects IFN-γ as a cytokine candidate responsible for impaired bacterial clearance, thereby promoting bacterial growth and systemic dissemination during acute IAV infection. We also found a time-dependent detrimental role of IL-6 in curtailing bacterial outgrowth which was not as distinct as for IFN-γ. Our numerical simulations suggested a detrimental effect of IFN-γ alone and in synergism with IL-6 but no conclusive pathogenic effect of IL-6 and TNF-α alone. This work provides a rationale to understand the potential impact of how to manipulate temporal immune components, facilitating the formulation of hypotheses about potential therapeutic strategies to treat coinfections.

Influenza A Virus Infection Predisposes Hosts to Secondary Infection with Different Streptococcus pneumoniae Serotypes with Similar Outcome but Serotype-Specific Manifestation

Influenza A virus (IAV) and Streptococcus pneumoniae are major causes of respiratory tract infections, particularly during coinfection. The synergism between these two pathogens is characterized by a complex network of dysregulated immune responses, some of which last until recovery following IAV infection. Despite the high serotype diversity of S. pneumoniae and the serotype replacement observed since the introduction of conjugate vaccines, little is known about pneumococcal strain dependency in the enhanced susceptibility to severe secondary S. pneumoniae infection following IAV infection. Thus, we studied how preinfection with IAV alters host susceptibility to different S. pneumoniae strains with various degrees of invasiveness using a highly invasive serotype 4 strain, an invasive serotype 7F strain, and a carrier serotype 19F strain. A murine model of pneumococcal coinfection during the acute phase of IAV infection showed a significantly increased degree of pneumonia and mortality for all tested pneumococcal strains at otherwise sublethal doses. The incidence and kinetics of systemic dissemination, however, remained bacterial strain dependent. Furthermore, we observed strain-specific alterations in the pulmonary levels of alveolar macrophages, neutrophils, and inflammatory mediators ultimately affecting immunopathology. During the recovery phase following IAV infection, bacterial growth in the lungs and systemic dissemination were enhanced in a strain-dependent manner. Altogether, this study shows that acute IAV infection predisposes the host to lethal S. pneumoniae infection irrespective of the pneumococcal serotype, while the long-lasting synergism between IAV and S. pneumoniae is bacterial strain dependent. These results hold implications for developing tailored therapeutic treatment regimens for dual infections during future IAV outbreaks.

Bacterial-viral load and the immune response in stable and exacerbated COPD: significance and therapeutic prospects

Chronic obstructive pulmonary disease (COPD) is characterized by persistent airflow limitation and an abnormal inflammatory response of the lung. Bacteria and viruses are a major cause of COPD exacerbations and may contribute to COPD progression by perpetuating the inflammatory response in the airways. Bacterial variety diminishes with increasing COPD severity. Respiratory viruses can colonize the lower respiratory tract in stable COPD, altering the respiratory microbiome and facilitating secondary bacterial infections. In this review, we present the most updated information about the role of bacteria and viruses in stable and exacerbated COPD. In our opinion, to optimize therapeutic strategies, the dynamic events involving bacterial-viral infections and related immune response in COPD phenotypes need to be better clarified. Our paper would address these points that we consider of great importance for the clinical management of COPD.

Influenza-induced type I interferon enhances susceptibility to gram-negative and gram-positive bacterial pneumonia in mice

Suppression of type 17 immunity by type I interferon (IFN) during influenza A infection has been shown to enhance susceptibility to secondary bacterial pneumonia. Although this mechanism has been described in coinfection with gram-positive bacteria, it is unclear whether similar mechanisms may impair lung defense against gram-negative infections. Furthermore, precise delineation of the duration of type I IFN-associated susceptibility to bacterial infection remains underexplored. Therefore, we investigated the effects of preceding influenza A virus infection on subsequent challenge with the gram-negative bacteria Escherichia coli or Pseudomonas aeruginosa and the temporal association between IFN expression with susceptibility to Staphylococcus aureus challenge in a mouse model of influenza and bacterial coinfection. Here we demonstrate that preceding influenza A virus led to increased lung E. coli and P. aeruginosa bacterial burden, which was associated with suppression of type 17 immunity and attenuation of antimicrobial peptide expression. Enhanced susceptibility to S. aureus coinfection ceased at day 14 of influenza infection, when influenza-associated type I IFN levels had returned to baseline levels, further suggesting a key role for type I IFN in coinfection pathogenesis. These findings further implicate type I IFN-associated suppression of type 17 immunity and antimicrobial peptide production as a conserved mechanism for enhanced susceptibility to both gram-positive and gram-negative bacterial coinfection during influenza infection.

Angiopoietin-like 4 Increases Pulmonary Tissue Leakiness and Damage during Influenza Pneumonia

Excessive host inflammatory responses negatively impact disease outcomes in respiratory infection. Host-pathogen interactions during the infective phase of influenza are well studied, but little is known about the host's response during the repair stage. Here, we show that influenza infection stimulated the expression of angiopoietin-like 4 (ANGPTL4) via a direct IL6-STAT3-mediated mechanism. ANGPTL4 enhanced pulmonary tissue leakiness and exacerbated inflammation-induced lung damage. Treatment of infected mice with neutralizing anti-ANGPTL4 antibodies significantly accelerated lung recovery and improved lung tissue integrity. ANGPTL4-deficient mice also showed reduced lung damage and recovered faster from influenza infection when compared to their wild-type counterparts. Retrospective examination of human lung biopsy specimens from infection-induced pneumonia with tissue damage showed elevated expression of ANGPTL4 when compared to normal lung samples. These observations underscore the important role that ANGPTL4 plays in lung infection and damage and may facilitate future therapeutic strategies for the treatment of influenza pneumonia.

Why is coinfection with influenza virus and bacteria so difficult to control?

Influenza viruses are genetically labile pathogens which avoid immune detection by constantly changing their coat proteins. Most human infections are caused by mildly pathogenic viruses which rarely cause life-threatening disease in healthy people, but some individuals with a weakened immune system can experience severe complications. Widespread infections with highly pathogenic strains of influenza virus are less common, but have the potential to cause enormous death tolls among healthy adults if infection rates reach pandemic proportions. Increased virulence has been attributed to a variety of factors, including enhanced susceptibility to coinfection with common strains of bacteria. The mechanisms that facilitate dual infection are a major focus of current research, as preventative measures are needed to avert future pandemics.

Prospective cohort study on the effectiveness of influenza and pneumococcal vaccines in preventing pneumonia development and hospitalization

Pneumonia and acute exacerbation of chronic illness are leading causes of influenza-related hospitalization. Therefore, influenza and pneumococcal vaccinations are strongly recommended for adults with comorbidities. Using a hospital-based influenza surveillance system, we performed a multicenter, prospective cohort study of patients visiting emergency rooms with influenza-like illness (ILI) during the influenza epidemic period in 2013 to 2014. Patients aged ≥ 19 years were enrolled, and clinical data were collected. Multivariate analyses were performed to estimate the effectiveness of influenza and pneumococcal vaccination in preventing pneumonia development and hospitalization. During study periods, 2,262 patients with ILI were registered. Among 2,217 patients with available vaccination records, 31.9% (707 patients) and 9.7% (216 patients) had received influenza and pneumococcal vaccines, respectively. Among patients who had been administered a pneumococcal vaccine, 94.4% had received the 23-valent polysaccharide vaccine (PPV23). The adjusted rates of effectiveness of the influenza vaccine for preventing pneumonia development and hospitalization were 64.0% (95% confidence interval [CI] = 29% to 81%) and 35.0% (95% CI = 12% to 52%), respectively. Pneumococcal vaccination did not reduce pneumonia development or hospitalization. In conclusion, influenza rather than PPV23 vaccination may reduce pneumonia development and hospitalization in patients with preceding ILI.