Excessive neutrophil levels in the lung underlie the age-associated increase in influenza mortality

Pathogenesis of pneumonia and acute lung injury

Pneumonia and its sequelae, acute lung injury, present unique challenges for pulmonary and critical care healthcare professionals, and these challenges have recently garnered global attention due to the ongoing Sars-CoV-2 pandemic. One limitation to translational investigation of acute lung injury, including its most severe manifestation (acute respiratory distress syndrome, ARDS) has been heterogeneity resulting from the clinical and physiologic diagnosis that represents a wide variety of etiologies. Recent efforts have improved our understanding and approach to heterogeneity by defining sub-phenotypes of ARDS although significant gaps in knowledge remain. Improving our mechanistic understanding of acute lung injury and its most common cause, infectious pneumonia, can advance our approach to precision targeted clinical interventions. Here, we review the pathogenesis of pneumonia and acute lung injury, including how respiratory infections and lung injury disrupt lung homoeostasis, and provide an overview of respiratory microbial pathogenesis, the lung microbiome, and interventions that have been demonstrated to improve outcomes-or not-in human clinical trials.

Tanshinone IIA prevents acute lung injury by regulating macrophage polarization

OBJECTIVE

Acute lung injury (ALI) is a serious respiratory dysfunction caused by pathogen or physical invasion. The strong induced inflammation often causes death. Tanshinone IIA (Tan-IIA) is the major constituent of Salvia miltiorrhiza Bunge and has been shown to display anti-inflammatory effects. The aim of the current study was to investigate the effects of Tan-IIA on ALI.

METHODS

A murine model of lipopolysaccharide (LPS)-induced ALI was used. The lungs and serum samples of mice were extracted at 3 days after treatment. ALI-induced inflammatory damages were confirmed from cytokine detections and histomorphology observations. Effects of Tan-IIA were investigated using in vivo and in vitro ALI models. Tan-IIA mechanisms were investigated by performing Western blot and flow cytometry experiments. A wound-healing assay was performed to confirm the Tan-IIA function.

RESULTS

The cytokine storm induced by LPS treatment was detected at 3 days after LPS treatment, and alveolar epithelial damage and lymphocyte aggregation were observed. Tan-IIA treatment attenuated the LPS-induced inflammation and reduced the levels of inflammatory cytokines released not only by inhibiting neutrophils, but also by macrophage. Moreover, we found that macrophage activation and polarization after LPS treatment were abrogated after applying the Tan-IIA treatment. An in vitro assay also confirmed that including the Tan-IIA supplement increased the relative amount of the M2 subtype and decreased that of M1. Rebalanced macrophages and Tan-IIA inhibited activations of the nuclear factor-κB and hypoxia-inducible factor pathways. Including Tan-IIA and macrophages also improved alveolar epithelial repair by regulating macrophage polarization.

CONCLUSION

This study found that while an LPS-induced cytokine storm exacerbated ALI, including Tan-IIA could prevent ALI-induced inflammation and improve the alveolar epithelial repair, and do so by regulating macrophage polarization.

Polysalicylic Acid Polymer Microparticle Decoys Therapeutically Treat Acute Respiratory Distress Syndrome

Acute lung injury (ALI) and acute respiratory distress syndrome (ARDS) remain problematic due to high mortality rates and lack of effective treatments. Neutrophilic injury contributes to mortality in ALI/ARDS. Here, technology for rapid ARDS intervention is developed and evaluated, where intravenous salicylic acid-based polymer microparticles, i.e., Poly-Aspirin (Poly-A), interfere with neutrophils in blood, reducing lung neutrophil infiltration and injury in vivo in mouse models of ALI/ARDS. Importantly, Poly-A particles reduce multiple inflammatory cytokines in the airway and bacterial load in the bloodstream in a live bacteria lung infection model of ARDS, drastically improving survival. It is observed that phagocytosis of the Poly-A microparticles, with salicylic acid in the polymer backbone, alters the neutrophil surface expression of adhesion molecules, potentially contributing to their added therapeutic benefits. Given the proven safety profile of the microparticle degradation products-salicylic acid and adipic acid-it is anticipated that the Poly-A particles represent a therapeutic strategy in ARDS with a rare opportunity for rapid clinical translation.

Impact of Coinfection With SARS-CoV-2 and Influenza on Disease Severity: A Systematic Review and Meta-Analysis

Background: Although coinfection with influenza in COVID-19 patients has drawn considerable attention, it is still not completely understood whether simultaneously infected with these two viruses influences disease severity. We therefore aimed to estimate the impact of coinfected with SARS-CoV-2 and influenza on the disease outcomes compared with the single infection of SARS-CoV-2. Materials and Methods: We searched the PubMed, Web of Science, Embase, Cochrane Library, China National Knowledge Infrastructure Database (CNKI) to identify relevant articles up to July 9, 2021. Studies that assessed the effect of SARS-CoV-2 and influenza coinfection on disease outcomes or those with sufficient data to calculate risk factors were included. Risk effects were pooled using fixed or random effects model. Results: We ultimately identified 12 studies with 9,498 patients to evaluate the risk effects of SARS-CoV-2 and influenza coinfection on disease severity. Results indicated that coinfection was not significantly associated with mortality (OR = 0.85, 95%CI: 0.51, 1.43; p = 0.55, I ² = 76.00%). However, mortality was found significantly decreased in the studies from China (OR = 0.51, 95%CI: 0.39, 0.68; I ² = 26.50%), while significantly increased outside China (OR = 1.56, 95%CI: 1.12, 2.19; I ² = 1.00%). Moreover, a lower risk for critical outcomes was detected among coinfection patients (OR = 0.64, 95%CI: 0.43, 0.97; p = 0.04, I ² = 0.00%). Additionally, coinfection patients presented different laboratory indexes compared with the single SARS-CoV-2 infection, including lymphocyte counts and APTT. Conclusion: Our study revealed that coinfection with SARS-CoV-2 and influenza had no effect on overall mortality. However, risk for critical outcomes was lower in coinfection patients and different associations were detected in the studies from different regions and specific laboratory indexes. Further studies on influenza strains and the order of infection were warranted. Systematic testing for influenza coinfection in COVID-19 patients and influenza vaccination should be recommended.

Atherosclerosis is a major human killer and non-resolving inflammation is a prime suspect

The resolution of inflammation (or inflammation-resolution) is an active and highly coordinated process. Inflammation-resolution is governed by several endogenous factors, and specialized pro-resolving mediators (SPMs) are one such class of molecules that have robust biological function. Non-resolving inflammation is associated with a variety of human diseases, including atherosclerosis. Moreover, non-resolving inflammation is a hallmark of ageing, an inevitable process associated with increased risk for cardiovascular disease. Uncovering mechanisms as to why inflammation-resolution is impaired in ageing and in disease and identifying useful biomarkers for non-resolving inflammation are unmet needs. Recent work has pointed to a critical role for balanced ratios of SPMs and pro-inflammatory lipids (i.e. leucotrienes and/or specific prostaglandins) as a key determinant of timely inflammation resolution. This review will focus on the accumulating findings that support the role of non-resolving inflammation and imbalanced pro-resolving and pro-inflammatory mediators in atherosclerosis. We aim to provide insight as to why these imbalances occur, the importance of ageing in disease progression, and how haematopoietic function impacts inflammation-resolution and atherosclerosis. We highlight open questions regarding therapeutic strategies and mechanisms of disease to provide a framework for future studies that aim to tackle this important human disease.

Tpl2 Ablation Leads to Hypercytokinemia and Excessive Cellular Infiltration to the Lungs During Late Stages of Influenza Infection

Tumor progression locus 2 (Tpl2) is a serine-threonine kinase known to promote inflammation in response to various pathogen-associated molecular patterns (PAMPs), inflammatory cytokines and G-protein-coupled receptors and consequently aids in host resistance to pathogens. We have recently shown that Tpl2-/- mice succumb to infection with a low-pathogenicity strain of influenza (x31, H3N2) by an unknown mechanism. In this study, we sought to characterize the cytokine and immune cell profile of influenza-infected Tpl2-/- mice to gain insight into its host protective effects. Although Tpl2-/- mice display modestly impaired viral control, no virus was observed in the lungs of Tpl2-/- mice on the day of peak morbidity and mortality suggesting that morbidity is not due to virus cytopathic effects but rather to an overactive antiviral immune response. Indeed, increased levels of interferon-β (IFN-β), the IFN-inducible monocyte chemoattractant protein-1 (MCP-1, CCL2), Macrophage inflammatory protein 1 alpha (MIP-1α; CCL3), MIP-1β (CCL4), RANTES (CCL5), IP-10 (CXCL10) and Interferon-γ (IFN-γ) was observed in the lungs of influenza-infected Tpl2-/- mice at 7 days post infection (dpi). Elevated cytokine and chemokines were accompanied by increased infiltration of the lungs with inflammatory monocytes and neutrophils. Additionally, we noted that increased IFN-β correlated with increased CCL2, CXCL1 and nitric oxide synthase (NOS2) expression in the lungs, which has been associated with severe influenza infections. Bone marrow chimeras with Tpl2 ablation localized to radioresistant cells confirmed that Tpl2 functions, at least in part, within radioresistant cells to limit pro-inflammatory response to viral infection. Collectively, this study suggests that Tpl2 tempers inflammation during influenza infection by constraining the production of interferons and chemokines which are known to promote the recruitment of detrimental inflammatory monocytes and neutrophils.

Immune response to COVID-19 in older adults

The severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) is the third highly pathogenic coronavirus to emerge in the human population in last two decades. SARS-CoV-2 spread from Wuhan, China, across the globe, causing an unprecedented public healthcare crisis. The virus showed remarkable age dependent pathology, with symptoms resembling common cold in most adults and children while causing more severe respiratory distress and significant mortality in older and frail humans. Even before the SARS-CoV-2 outbreak infectious diseases represented one of the major causes of death of older adults. Loss of immune function and reduced protection from infectious agents with age - immunosenescence - is a result of complex mechanisms affecting production and maintenance of immune cells as well as the initiation, maintenance and termination of properly directed immune responses. Here we briefly discuss the current knowledge on how this process affects age-dependent outcomes of SARS-CoV-2 infection.

Viral coinfections in COVID-19

The most consequential challenge raised by coinfection is perhaps the inappropriate generation of recombinant viruses through the exchange of genetic material among different strains. These genetically similar viruses can interfere with the replication process of each other and even compete for the metabolites required for the maintenance of the replication cycle. Due to the similarity in clinical symptoms of most viral respiratory tract infections, and their coincidence with COVID-19, caused by SARS-CoV-2, it is recommended to develop a comprehensive diagnostic panel for detection of respiratory and nonrespiratory viruses through the evaluation of patient samples. Given the resulting changes in blood markers, such as coagulation factors and white blood cell count following virus infection, these markers can be of diagnostic value in the detection of mixed infection in individuals already diagnosed with a certain viral illness. In this review, we seek to investigate the coinfection of SARS-CoV-2 with other respiratory and nonrespiratory viruses to provide novel insights into the development of highly sensitive diagnostics and effective treatment modalities.

Immunoregulatory Functions of Interferons During Genital HSV-2 Infection

Herpes simplex virus type 2 (HSV-2) infection is one of the most prevalent sexually transmitted infections that disproportionately impacts women worldwide. Currently, there are no vaccines or curative treatments, resulting in life-long infection. The mucosal environment of the female reproductive tract (FRT) is home to a complex array of local immune defenses that must be carefully coordinated to protect against genital HSV-2 infection, while preventing excessive inflammation to prevent disease symptoms. Crucial to the defense against HSV-2 infection in the FRT are three classes of highly related and integrated cytokines, type I, II, and III interferons (IFN). These three classes of cytokines control HSV-2 infection and reduce tissue damage through a combination of directly inhibiting viral replication, as well as regulating the function of resident immune cells. In this review, we will examine how interferons are induced and their critical role in how they shape the local immune response to HSV-2 infection in the FRT.

Neutrophils at the crossroads of acute viral infections and severity

Neutrophils are versatile immune effector cells essential for mounting a first-line defense against invading pathogens. However, uncontrolled activation can lead to severe life-threatening complications. Neutrophils exist as a heterogeneous population, and their interaction with pathogens and other immune cells may shape the outcome of the host immune response. Diverse classes of viruses, including the recently identified novel SARS-CoV-2, have shown to alter the various aspects of neutrophil biology, offering possibilities for selective intervention. Here, we review heterogeneity within the neutrophil population, highlighting the functional consequences of circulating phenotypes and their critical involvement in exaggerating protective and pathological immune responses against the viruses. We discuss the recent findings of neutrophil extracellular traps (NETs) in COVID-19 pathology and cover other viruses, where neutrophil biology and NETs are crucial for developing disease severity. In the end, we have also pointed out the areas where neutrophil-mediated responses can be finely tuned to outline opportunities for therapeutic manipulation in controlling inflammation against viral infection.

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.

Age-related changes in the local milieu of inflamed tissues cause aberrant neutrophil trafficking and subsequent remote organ damage

Aging is associated with dysregulated immune functions. Here, we investigated the impact of age on neutrophil diapedesis. Using confocal intravital microscopy, we found that in aged mice, neutrophils adhered to vascular endothelium in inflamed tissues but exhibited a high frequency of reverse transendothelial migration (rTEM). This retrograde breaching of the endothelium by neutrophils was governed by enhanced production of the chemokine CXCL1 from mast cells that localized at endothelial cell (EC) junctions. Increased EC expression of the atypical chemokine receptor 1 (ACKR1) supported this pro-inflammatory milieu in aged venules. Accumulation of CXCL1 caused desensitization of the chemokine receptor CXCR2 on neutrophils and loss of neutrophil directional motility within EC junctions. Fluorescent tracking revealed that in aged mice, neutrophils undergoing rTEM re-entered the circulation and disseminated to the lungs where they caused vascular leakage. Thus, neutrophils stemming from a local inflammatory site contribute to remote organ damage, with implication to the dysregulated systemic inflammation associated with aging.

Neutrophils in respiratory viral infections

Viral respiratory infections are a common cause of severe disease, especially in infants, people who are immunocompromised, and in the elderly. Neutrophils, an important innate immune cell, infiltrate the lungs rapidly after an inflammatory insult. The most well-characterized effector mechanisms by which neutrophils contribute to host defense are largely extracellular and the involvement of neutrophils in protection from numerous bacterial and fungal infections is well established. However, the role of neutrophils in responses to viruses, which replicate intracellularly, has been less studied. It remains unclear whether and, by which underlying immunological mechanisms, neutrophils contribute to viral control or confer protection against an intracellular pathogen. Furthermore, neutrophils need to be tightly regulated to avoid bystander damage to host tissues. This is especially relevant in the lung where damage to delicate alveolar structures can compromise gas exchange with life-threatening consequences. It is inherently less clear how neutrophils can contribute to host immunity to viruses without causing immunopathology and/or exacerbating disease severity. In this review, we summarize and discuss the current understanding of how neutrophils in the lung direct immune responses to viruses, control viral replication and spread, and cause pathology during respiratory viral infections.

Aging and respiratory viral infection: from acute morbidity to chronic sequelae

The altered immune response in aged hosts play a vital role in contributing to their increased morbidity and mortality during respiratory virus infections. The aged hosts display impaired antiviral immune response as well as increased risk for long-term pulmonary sequelae post virus clearance. However, the underlying cellular and molecular mechanisms driving these alterations of the immune compartment have not been fully elucidated. During the era of COVID-19 pandemic, a better understanding of such aspects is urgently needed to provide insight that will benefit the geriatric patient care in prevention as well as treatment. Here, we review the current knowledge about the unique immune characteristics of aged hosts during homeostasis and respiratory virus infections.

Too young to die? How aging affects cellular innate immune responses to influenza virus and disease severity

Influenza is a respiratory viral infection that causes significant morbidity and mortality worldwide. The innate immune cell response elicited during influenza A virus (IAV) infection forms the critical first line of defense, which typically is impaired as we age. As such, elderly individuals more commonly succumb to influenza-associated complications, which is reflected in most aged animal models of IAV infection. Here, we review the important roles of several major innate immune cell populations in influenza pathogenesis, some of which being deleterious to the host, and the current knowledge of how age-associated numerical, phenotypic and functional cell changes impact disease development. Further investigation into age-related modulation of innate immune cell responses, using appropriate animal models, will help reveal how immunity to IAV may be compromised by aging and inform the development of novel therapies, tailored for use in this vulnerable group.

Pro-Resolving Ligands Orchestrate Phagocytosis

The resolution of inflammation is a tissue protective program that is governed by several factors including specialized pro-resolving mediators (SPMs), proteins, gasses and nucleotides. Pro-resolving mediators activate counterregulatory programs to quell inflammation and promote tissue repair in a manner that does not compromise host defense. Phagocytes like neutrophils and macrophages play key roles in the resolution of inflammation because of their ability to remove debris, microbes and dead cells through processes including phagocytosis and efferocytosis. Emerging evidence suggests that failed resolution of inflammation and defective phagocytosis or efferocytosis underpins several prevalent human diseases. Therefore, understanding factors and mechanisms associated with enhancing these processes is a critical need. SPMs enhance phagocytosis and efferocytosis and this review will highlight mechanisms associated with their actions.

Neutrophils and Influenza: A Thin Line between Helpful and Harmful

Influenza viruses are one of the most prevalent respiratory pathogens known to humans and pose a significant threat to global public health each year. Annual influenza epidemics are responsible for 3-5 million infections worldwide and approximately 500,000 deaths. Presently, yearly vaccinations represent the most effective means of combating these viruses. In humans, influenza viruses infect respiratory epithelial cells and typically cause localized infections of mild to moderate severity. Neutrophils are the first innate cells to be recruited to the site of the infection and possess a wide range of effector functions to eliminate viruses. Some well-described effector functions include phagocytosis, degranulation, the production of reactive oxygen species (ROS), and the formation of neutrophil extracellular traps (NETs). However, while these mechanisms can promote infection resolution, they can also contribute to the pathology of severe disease. Thus, the role of neutrophils in influenza viral infection is nuanced, and the threshold at which protective functions give way to immunopathology is not well understood. Moreover, notable differences between human and murine neutrophils underscore the need to exercise caution when applying murine findings to human physiology. This review aims to provide an overview of neutrophil characteristics, their classic effector functions, as well as more recently described antibody-mediated effector functions. Finally, we discuss the controversial role these cells play in the context of influenza virus infections and how our knowledge of this cell type can be leveraged in the design of universal influenza virus vaccines.

A sustained type I IFN-neutrophil-IL-18 axis drives pathology during mucosal viral infection

Neutrophil responses against pathogens must be balanced between protection and immunopathology. Factors that determine these outcomes are not well-understood. In a mouse model of genital herpes simplex virus-2 (HSV-2) infection, which results in severe genital inflammation, antibody-mediated neutrophil depletion reduced disease. Comparative single-cell RNA-sequencing analysis of vaginal cells against a model of genital HSV-1 infection, which results in mild inflammation, demonstrated sustained expression of interferon-stimulated genes (ISGs) only after HSV-2 infection primarily within the neutrophil population. Both therapeutic blockade of IFNα/β receptor 1 (IFNAR1) and genetic deletion of IFNAR1 in neutrophils concomitantly decreased HSV-2 genital disease severity and vaginal IL-18 levels. Therapeutic neutralization of IL-18 also diminished genital inflammation, indicating an important role for this cytokine in promoting neutrophil-dependent immunopathology. Our study reveals that sustained type I interferon (IFN) signaling is a driver of pathogenic neutrophil responses and identifies IL-18 as a novel component of disease during genital HSV-2 infection.

The aging lung: Physiology, disease, and immunity

The population is aging at a rate never seen before in human history. As the number of elderly adults grows, it is imperative we expand our understanding of the underpinnings of aging biology. Human lungs are composed of a unique panoply of cell types that face ongoing chemical, mechanical, biological, immunological, and xenobiotic stress over a lifetime. Yet, we do not fully appreciate the mechanistic drivers of lung aging and why age increases the risk of parenchymal lung disease, fatal respiratory infection, and primary lung cancer. Here, we review the molecular and cellular aspects of lung aging, local stress response pathways, and how the aging process predisposes to the pathogenesis of pulmonary disease. We place these insights into context of the COVID-19 pandemic and discuss how innate and adaptive immunity within the lung is altered with age.

Aging and Interferons: Impacts on Inflammation and Viral Disease Outcomes

As highlighted by the COVID-19 global pandemic, elderly individuals comprise the majority of cases of severe viral infection outcomes and death. A combined inability to control viral replication and exacerbated inflammatory immune activation in elderly patients causes irreparable immune-mediated tissue pathology in response to infection. Key to these responses are type I, II, and III interferons (IFNs), which are involved in inducing an antiviral response, as well as controlling and suppressing inflammation and immunopathology. IFNs support monocyte/macrophage-stimulated immune responses that clear infection and promote their immunosuppressive functions that prevent excess inflammation and immune-mediated pathology. The timing and magnitude of IFN responses to infection are critical towards their immunoregulatory functions and ability to prevent immunopathology. Aging is associated with multiple defects in the ability of macrophages and dendritic cells to produce IFNs in response to viral infection, leading to a dysregulation of inflammatory immune responses. Understanding the implications of aging on IFN-regulated inflammation will give critical insights on how to treat and prevent severe infection in vulnerable individuals. In this review, we describe the causes of impaired IFN production in aging, and the evidence to suggest that these impairments impact the regulation of the innate and adaptive immune response to infection, thereby causing disease pathology.

Older but Not Wiser: the Age-Driven Changes in Neutrophil Responses during Pulmonary Infections

Elderly individuals are at increased risk of life-threatening pulmonary infections. Neutrophils are a key determinant of the disease course of pathogen-induced pneumonia. Optimal host defense balances initial robust pulmonary neutrophil responses to control pathogen numbers, ultimately followed by the resolution of inflammation to prevent pulmonary damage. Recent evidence suggests that phenotypic and functional heterogeneity in neutrophils impacts host resistance to pulmonary pathogens. Apart from their apparent role in innate immunity, neutrophils also orchestrate subsequent adaptive immune responses during infection. Thus, the outcome of pulmonary infections can be shaped by neutrophils. This review summarizes the age-driven impairment of neutrophil responses and the contribution of these cells to the susceptibility of the elderly to pneumonia. We describe how aging is accompanied by changes in neutrophil recruitment, resolution, and function. We discuss how systemic and local changes alter the neutrophil phenotype in aged hosts. We highlight the gap in knowledge of whether these changes in neutrophils also contribute to the decline in adaptive immunity seen with age. We further detail the factors that drive dysregulated neutrophil responses in the elderly and the pathways that may be targeted to rebalance neutrophil activity and boost host resistance to pulmonary infections.

Regulatory mechanisms of neutrophil migration from the circulation to the airspace

The neutrophil, a short-lived effector leukocyte of the innate immune system best known for its proteases and other degradative cargo, has unique, reciprocal physiological interactions with the lung. During health, large numbers of ‘marginated’ neutrophils reside within the pulmonary vasculature, where they patrol the endothelial surface for pathogens and complete their life cycle. Upon respiratory infection, rapid and sustained recruitment of neutrophils through the endothelial barrier, across the extravascular pulmonary interstitium, and again through the respiratory epithelium into the airspace lumen, is required for pathogen killing. Overexuberant neutrophil trafficking to the lung, however, causes bystander tissue injury and underlies several acute and chronic lung diseases. Due in part to the unique architecture of the lung’s capillary network, the neutrophil follows a microanatomic passage into the distal airspace unlike that observed in other end-organs that it infiltrates. Several of the regulatory mechanisms underlying the stepwise recruitment of circulating neutrophils to the infected lung have been defined over the past few decades; however, fundamental questions remain. In this article, we provide an updated review and perspective on emerging roles for the neutrophil in lung biology, on the molecular mechanisms that control the trafficking of neutrophils to the lung, and on past and ongoing efforts to design therapeutics to intervene upon pulmonary neutrophilia in lung disease.

Teaching old mice new tricks: the utility of aged mouse models of C. difficile infection to study pathogenesis and rejuvenate immune response

BACKGROUND

Clostridioides difficile is a serious problem for the aging population. Aged mouse model of C. difficile infection (CDI) has emerged as a valuable tool to evaluate the mechanism of aging in CDI.

METHODS

We reviewed five published studies utilizing aged mice (7-28 months) for CDI model for findings that may advance our understanding of how aging influences outcome from CDI.

RESULTS

Aged mouse models of CDI uniformly demonstrated more severe disease in the old compared to young mice. Diminished neutrophil recruitment to intestinal tissue in aged mice is the most consistent finding. Differences in innate and humoral immune responses were also observed. The effects of aging on the outcome of infection were reversed by pharmacologic or microbiota-targeted interventions.

CONCLUSION

The aged mouse presents an important in vivo model to study CDI and elucidate the mechanisms underlying advanced age as an important risk factor for severe disease.

Tissue-resident CD8+ T cells drive age-associated chronic lung sequelae after viral pneumonia

Lower respiratory viral infections, such as influenza virus and severe acute respiratory syndrome coronavirus 2 infections, often cause severe viral pneumonia in aged individuals. Here, we report that influenza viral pneumonia leads to chronic nonresolving lung pathology and exacerbated accumulation of CD8⁺ tissue-resident memory T cells (T_RM) in the respiratory tract of aged hosts. T_RM cell accumulation relies on elevated TGF-β present in aged tissues. Further, we show that T_RM cells isolated from aged lungs lack a subpopulation characterized by expression of molecules involved in TCR signaling and effector function. Consequently, T_RM cells from aged lungs were insufficient to provide heterologous protective immunity. The depletion of CD8⁺ T_RM cells dampens persistent chronic lung inflammation and ameliorates tissue fibrosis in aged, but not young, animals. Collectively, our data demonstrate that age-associated T_RM cell malfunction supports chronic lung inflammatory and fibrotic sequelae after viral pneumonia.

Cellular senescence: friend or foe to respiratory viral infections?

Cellular senescence permanently arrests the replication of various cell types and contributes to age-associated diseases. In particular, cellular senescence may enhance chronic lung diseases including COPD and idiopathic pulmonary fibrosis. However, the role cellular senescence plays in the pathophysiology of acute inflammatory diseases, especially viral infections, is less well understood. There is evidence that cellular senescence prevents viral replication by increasing antiviral cytokines, but other evidence shows that senescence may enhance viral replication by downregulating antiviral signalling. Furthermore, cellular senescence leads to the secretion of inflammatory mediators, which may either promote host defence or exacerbate immune pathology during viral infections. In this Perspective article, we summarise how senescence contributes to physiology and disease, the role of senescence in chronic lung diseases, and how senescence impacts acute respiratory viral infections. Finally, we develop a potential framework for how senescence may contribute, both positively and negatively, to the pathophysiology of viral respiratory infections, including severe acute respiratory syndrome due to the coronavirus SARS-CoV-2.

Immunity to acute virus infections with advanced age

New infections in general, and new viral infections amongst them, represent a serious challenge to an older organism. This review discusses the age-related alterations in responsiveness to infection from the standpoint of virus:host relationship and the host physiological whole-organism and specific immune response to the virus. Changes with age in the innate and adaptive immune system homeostasis and function are reviewed briefly. This is followed by a review of specific alterations and defects in the response of older organisms (chiefly mice and humans) to acute (particularly emerging and re-emerging) viral infections, with a very brief summary of the response to latent persistent infections. Finally, we provide a brief summary of the perspectives for possible interventions to enhance antiviral immunity.

Innate Immunity and Influenza A Virus Pathogenesis: Lessons for COVID-19

There is abundant evidence that the innate immune response to influenza A virus (IAV) is highly complex and plays a key role in protection against IAV induced infection and illness. Unfortunately it also clear that aspects of innate immunity can lead to severe morbidity or mortality from IAV, including inflammatory lung injury, bacterial superinfection, and exacerbation of reactive airways disease. We review broadly the virus and host factors that result in adverse outcomes from IAV and show evidence that inflammatory responses can become damaging even apart from changes in viral replication per se, with special focus on the positive and adverse effects of neutrophils and monocytes. We then evaluate in detail the role of soluble innate inhibitors including surfactant protein D and antimicrobial peptides that have a potential dual capacity for down-regulating viral replication and also inhibiting excessive inflammatory responses and how these innate host factors could possibly be harnessed to treat IAV infection. Where appropriate we draw comparisons and contrasts the SARS-CoV viruses and IAV in an effort to point out where the extensive knowledge existing regarding severe IAV infection could help guide research into severe COVID 19 illness or vice versa.

The untwining of immunosenescence and aging

From a holistic point of view, aging results from the cumulative erosion of the various systems. Among these, the immune system is interconnected to the rest as immune cells are present in all organs and recirculate through bloodstream. Immunosenescence is the term used to define the remodelling of immune changes during aging. Because immune cells-and particularly lymphocytes-can further differentiate after their maturation in response to pathogen recognition, it is therefore unclear when senescence is induced in these cells. Additionally, it is also unclear which signals triggers senescence in immune cells (i) aging per se, (ii) specific response to pathogens, (iii) underlying conditions, or (iv) inflammaging. In this review, we will cover the current knowledge and concepts linked to immunosenescence and we focus this review on lymphocytes and T cells, which represent the typical model for replicative senescence. With the evidence presented, we propose to disentangle the senescence of immune cells from chronological aging.

Coagulation Abnormalities and Thrombosis in Patients Infected With SARS-CoV-2 and Other Pandemic Viruses

The world is amid a pandemic caused by severe acute respiratory syndrome-coronavirus 2. Severe acute respiratory syndrome-coronavirus causes serious respiratory tract infections that can lead to viral pneumonia, acute respiratory distress syndrome, and death. Some patients with coronavirus disease 2019 (COVID-19) have an activated coagulation system characterized by elevated plasma levels of d-dimer-a biomarker of fibrin degradation. Importantly, high levels of D-dimer on hospital admission are associated with increased risk of mortality. Venous thromboembolism is more common than arterial thromboembolism in hospitalized COVID-19 patients. Pulmonary thrombosis and microvascular thrombosis are observed in autopsy studies, and this may contribute to the severe hypoxia observed in COVID-19 patients. It is likely that multiple systems contribute to thrombosis in COVID-19 patients, such as activation of coagulation, platelet activation, hypofibrinolysis, endothelial cell dysfunction, inflammation, neutrophil extracellular traps, and complement. Targeting these different pathways may reduce thrombosis and improve lung function in COVID-19 patients.

Profile of children with COVID-19 infection: a cross sectional study from North-East Nigeria

INTRODUCTION

available evidence suggests that children infected with COVID-19 tend to have a less severe form of the disease. However, most of the studies that have established this largely emanate from outside sub-Saharan Africa. The pandemic nature of the infection makes it instructive to evaluate its pattern among children across different climes, including ours. This study was set out to describe the clinical characteristics of children with COVID-19 in Bauchi State, North-East Nigeria.

METHODS

this was a cross sectional study that involved 53 children between the ages of 0 and 18 years, who had RT-PCR confirmed COVID-19 infection between March and June 2020 in Bauchi State, Nigeria. Data on epidemiological and clinical characteristics was analysed using IBM SPSS Statistics V 21.® Relationship between categorical variables was established using the chi square test. The level of statistical significance was set at < 0.05, at a confidence interval (CI) of 95%.

RESULTS

the mean age was 12.63 ± 4.31 years with a slight preponderance of males (1.1: 1). Majority were asymptomatic (60.4%), while 32.1% and 7.5% had mild and moderate diseases respectively. The most common symptoms were cough (20.8%), fever (17%), and sneezing (15.1%). Five children (9.4%) complained of loss of taste while anosmia was documented in one child (1.9%). We observed a significant relationship between age category and the presence of symptoms. In fact, children younger than 10 years (pre-adolescents) were five times more likely to be symptomatic when compared to those above this age (p = 0.029, C I 1.08-21.56).

CONCLUSION

our findings have shown a mild pattern of disease and good outcome among infected children. However, we must be mindful of the higher vulnerability among younger children, especially those below 10 years.

Role of Aging and the Immune Response to Respiratory Viral Infections: Potential Implications for COVID-19

Aging impairs immunity to promote diseases, especially respiratory viral infections. The current COVID-19 pandemic, resulting from SARS-CoV-2, induces acute pneumonia, a phenotype that is alarmingly increased with aging. In this article, we review findings of how aging alters immunity to respiratory viral infections to identify age-impacted pathways common to several viral pathogens, permitting us to speculate about potential mechanisms of age-enhanced mortality to COVID-19. Aging generally leads to exaggerated innate immunity, particularly in the form of elevated neutrophil accumulation across murine and large animal studies of influenza infection. COVID-19 patients who succumb exhibit a 2-fold increase in neutrophilia, suggesting that exaggerated innate immunity contributes to age-enhanced mortality to SARS-CoV-2 infection. Further investigation in relevant experimental models will elucidate the mechanisms by which aging impacts respiratory viral infections, including SARS-CoV-2. Such investigation could identify therapies to reduce the suffering of the population at large, but especially among older people, infected with respiratory viruses.

A delicate balancing act: immunity and immunopathology in human H7N9 influenza virus infections

PURPOSE OF REVIEW

A delicate balance exists between a protective and detrimental immune response to an invading viral pathogen. Here, we review the latest advancements in our understanding of immunity and immunopathology during H7N9 influenza A virus (IAV) infections and its relevance to disease management and diagnosis.

RECENT FINDINGS

Recent studies have highlighted the role of specific leukocytes in the pathogenesis of H7N9 IAV infections and potential diagnostic role that host cytokine profiles can play in forecasting disease severity. Furthermore, alterations in diet have emerged as a possible preventive measure for severe IAV infections.

SUMMARY

The recent emergence and continued evolution of H7N9 IAVs have emphasized the threat that these avian viruses pose to human health. Understanding the role of the host immune response in both disease protection and pathogenesis is an essential first step in the creation of novel therapeutic and preventive measures for H7N9 IAV infections.

Clinical characteristics of critically ill patients co-infected with SARS-CoV-2 and the influenza virus in Wuhan, China

OBJECTIVE

To delineate the clinical characteristics of critically ill COVID-19 patients co-infected with influenza.

METHODS

This study included adult patients with laboratory-confirmed COVID-19 form Tongji Hospital (Wuhan, China), with or without influenza, and compared their clinical characteristics.

RESULTS

Among 93 patients, 44 died and 49 were discharged. Forty-four (47.3%) were infected with influenza virus A and two (2.2%) with influenza virus B. Twenty-two (50.0%) of the non-survivors and 24 (49.0%) of the survivors were infected with the influenza virus. Critically ill COVID-19 patients with influenza were more prone to cardiac injury than those without influenza. For the laboratory indicators at admission the following were higher in non-survivors with influenza than in those without influenza: white blood cell counts, neutrophil counts, levels of tumor necrosis factor-α, D-dimer value, and proportion of elevated creatinine.

CONCLUSION

The results showed that a high proportion of COVID-19 patients were co-infected with influenza in Tongji Hospital, with no significant difference in the proportion of co-infection between survivors and non-survivors. The critically ill COVID-19 patients with influenza exhibited more severe inflammation and organ injury, indicating that co-infection with the influenza virus may induce an earlier and more frequently occurring cytokine storm.

Clinical and Immune Features of Hospitalized Pediatric Patients With Coronavirus Disease 2019 (COVID-19) in Wuhan, China

IMPORTANCE

The epidemiologic and clinical characteristics of pediatric patients with coronavirus disease 2019 (COVID-19) have been reported, but information on immune features associated with disease severity is scarce.

OBJECTIVE

To delineate and compare the immunologic features of mild and moderate COVID-19 in pediatric patients.

DESIGN, SETTING, AND PARTICIPANTS

This single-center case series included 157 pediatric patients admitted to Wuhan Children's Hospital with laboratory-confirmed severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). Data were collected from January 25 to April 18, 2020.

EXPOSURES

Documented SARS-CoV-2 infection.

MAIN OUTCOMES AND MEASURES

Clinical and immunologic characteristics were collected and analyzed. Outcomes were observed until April 18, 2020.

RESULTS

Of the 157 pediatric patients with COVID-19, 60 (38.2%) had mild clinical type with pneumonia, 88 (56.1%) had moderate cases, 6 (3.8%) had severe cases, and 3 (1.9%) were critically ill. The 148 children with mild or moderate disease had a median (interquartile range [IQR]) age of 84 (18-123) months, and 88 (59.5%) were girls. The most common laboratory abnormalities were increased levels of alanine aminotransferase (ALT) (median [IQR], 16.0 [12.0-26.0] U/L), aspartate aminotransferase (AST) (median [IQR], 30.0 [23.0-41.8] U/L), creatine kinase MB (CK-MB) activity (median [IQR], 24.0 [18.0-34.0] U/L), and lactate dehydrogenase (LDH) (median [IQR], 243.0 [203.0-297.0] U/L), which are associated with liver and myocardial injury. Compared with mild cases, levels of inflammatory cytokines including interleukin 6, tumor necrosis factor α, and interferon γ were unchanged, whereas the level of immune suppressive interleukin 10 was markedly increased in moderate cases compared with mild cases (median [IQR], 3.96 [3.34-5.29] pg/mL vs 3.58 [3.10-4.36] pg/mL; P = .048). There was no statistically significant difference in absolute number of lymphocytes (including T cells and B cells) between mild and moderate cases, but moderate cases were associated with a decrease in neutrophil levels compared with mild cases (median [IQR], 2310/μL [1680/μL-3510/μL] vs 3120/μL [2040/μL-4170/μL]; P = .01). Immunoglobin G and the neutrophil to lymphocyte ratio were negatively associated with biochemical indices related to liver and myocardial injury (immunoglobulin G, ALT: r, -0.3579; AST: r, -0.5280; CK-MB activity: r, -0.4786; LDH: r, -0.4984; and neutrophil to lymphocyte ratio, ALT: r, -0.1893; AST: r, -0.3912; CK-MB activity: r, -0.3428; LDH: r, -0.3234), while counts of lymphocytes, CD4+ T cells, and interleukin 10 showed positive associations (lymphocytes, ALT: r, 0.2055; AST: r, 0.3615; CK-MB activity: r, 0.338; LDH: r, 0.3309; CD4+ T cells, AST: r, 0.4701; CK-MB activity: r, 0.4151; LDH: r, 0.4418; interleukin 10, ALT: r, 0.2595; AST: r, 0.3386; CK-MB activity: r, 0.3948; LDH: r, 0.3794).

CONCLUSIONS AND RELEVANCE

In this case series, systemic inflammation rarely occurred in pediatric patients with COVID-19, in contrast with the lymphopenia and aggravated inflammatory responses frequently observed in adults with COVID-19. Gaining a deeper understanding of the role of neutrophils, CD4+ T cells, and B cells in the pathogenesis of SARS-CoV-2 infection could be important for the clinical management of COVID-19.

Early Amplified Respiratory Bioactive Lipid Response Is Associated With Worse Outcomes in Pediatric Influenza-Related Respiratory Failure

Background

Biomarkers are needed for early identification of patients at risk of severe complications from influenza infection, including prolonged respiratory failure and death. Eicosanoids are bioactive lipid mediators with pro- and anti-inflammatory properties produced in response to infection. This study assessed the relationships between the host bioactive lipid response, influenza viral load, and clinical outcomes.

Methods

Influenza-positive, intubated children ≤18 years old were enrolled across 26 US pediatric intensive care units (PICUs). Mass spectrometry was used to measure >100 lipid metabolites in endotracheal and nasopharyngeal samples. Influenza viral load was measured by quantitative polymerase chain reaction.

Results

Age and bacterial co-infection were associated with multiple bioactive lipids (P < .05). Influenza viral load was lower in patients with bacterial co-infection compared with those without, and pro-inflammatory bioactive lipids positively correlated with viral load in bacterially co-infected children (P < .05). Lipids associated with disease resolution correlated with viral load in patients without bacterial co-infection (P < .01). After adjusting for age and bacterial co-infection status, elevated pro- and anti-inflammatory lipids measured early in the intensive care unit course were associated with higher mortality, whereas influenza viral load and endotracheal cytokine levels were not associated with clinical outcomes. Prostaglandin E₂, arachidonic acid, docosahexaenoic acid, and 12-hydroxyeicosatetraenoic acid measured within 72 hours of PICU admission predicted death or prolonged (≥28 days) mechanical ventilator support (area under the curve, 0.72-0.79; P < .02) not explained by admission illness severity.

Conclusions

Children with influenza-related complications have early bioactive lipid responses that may reflect lung disease severity. Respiratory bioactive lipids are candidate prognostic biomarkers to identify children with the most severe clinical outcomes.

Two waves of pro-inflammatory factors are released during the influenza A virus (IAV)-driven pulmonary immunopathogenesis

Influenza A virus (IAV) infection is a complicated process. After IAVs spread to the lung, extensive pro-inflammatory cytokines and chemokines are released, which largely determine the outcome of infection. Using a single-cell RNA sequencing (scRNA-seq) assay, we systematically and sequentially analyzed the transcriptome of more than 16,000 immune cells in the pulmonary tissue of infected mice, and demonstrated that two waves of pro-inflammatory factors were released. A group of IAV-infected PD-L1⁺ neutrophils were the major contributor to the first wave at an earlier stage (day 1–3 post infection). Notably, at a later stage (day 7 post infection) when IAV was hardly detected in the immune cells, a group of platelet factor 4-positive (Pf4⁺)-macrophages generated another wave of pro-inflammatory factors, which were probably the precursors of alveolar macrophages (AMs). Furthermore, single-cell signaling map identified inter-lineage crosstalk between different clusters and helped better understand the signature of PD-L1⁺ neutrophils and Pf4⁺-macrophages. Our data characteristically clarified the infiltrated immune cells and their production of pro-inflammatory factors during the immunopathogenesis development, and deciphered the important mechanisms underlying IAV-driven inflammatory reactions in the lung.

Influenza A virus (IAV) infections cause acute respiratory disease in many species, including human, mammals and birds, and are responsible for a number of pandemics among humans, resulting in substantial morbidity and mortality. High morbidity and mortality of IAV-driven pneumonia reflects the deficient immunity of the hosts against IAV infection, and the inefficiency of available prevention and treatment strategies. Thus, in depth exploration of IAV pathogenesis is necessary. In our study, using the transverse (cells to cells) and longitudinal (day to day) analysis of immune cells in the lung, we monitored the whole immunopathogenesis during IAV infection, and identified several cell types as contributors for the release of pro-inflammatory factors. Therefore, our study potentially provides new therapeutic targets for IAV treatment.

Neutrophil Adaptations upon Recruitment to the Lung: New Concepts and Implications for Homeostasis and Disease

Neutrophils have a prominent role in all human immune responses against any type of pathogen or stimulus. The lungs are a major neutrophil reservoir and neutrophilic inflammation is a primary response to both infectious and non-infectious challenges. While neutrophils are well known for their essential role in clearance of bacteria, they are also equipped with specific mechanisms to counter viruses and fungi. When these defense mechanisms become aberrantly activated in the absence of infection, this commonly results in debilitating chronic lung inflammation. Clearance of bacteria by phagocytosis is the hallmark role of neutrophils and has been studied extensively. New studies on neutrophil biology have revealed that this leukocyte subset is highly adaptable and fulfills diverse roles. Of special interest is how these adaptations can impact the outcome of an immune response in the lungs due to their potent capacity for clearing infection and causing damage to host tissue. The adaptability of neutrophils and their propensity to influence the outcome of immune responses implicates them as a much-needed target of future immunomodulatory therapies. This review highlights the recent advances elucidating the mechanisms of neutrophilic inflammation, with a focus on the lung environment due to the immense and growing public health burden of chronic lung diseases such as cystic fibrosis (CF) and chronic obstructive pulmonary disease (COPD), and acute lung inflammatory diseases such as transfusion-related acute lung injury (TRALI).

Neutrophils do not impact viral load or the peak of disease severity during RSV infection

Lung and airway neutrophils are a hallmark of severe disease in infants with respiratory syncytial virus (RSV)-induced lower respiratory tract infections. Despite their abundance in the lungs during RSV infection of both mice and man, the role of neutrophils in viral control and in immune pathology is not clear. Here, antibody mediated neutrophil depletion was used to investigate the degree to which neutrophils impact the lung immune environment, the control of viral replication and the peak severity of disease after RSV infection of mice. Neutrophil depletion did not substantially affect the levels of inflammatory mediators such as type I interferons, IL-6, TNF-α or IL-1β in response to RSV. In addition, the lack of neutrophils did not change the viral load during RSV infection. Neither neutrophil depletion nor the enhancement of lung neutrophils by administration of the chemoattractant CXCL1 during RSV infection affected disease severity as measured by weight loss. Therefore, in this model of RSV infection, lung neutrophils do not offer obvious benefits to the host in terms of increasing anti-viral inflammatory responses or restricting viral replication and neutrophils do not contribute to disease severity.

Isoflavone-mediated radioprotection involves regulation of early endothelial cell death and inflammatory signaling in Radiation-Induced lung injury

Purpose: Vascular damage and inflammation are limiting toxic effects of lung cancer radiotherapy, which lead to pneumonitis and pulmonary fibrosis. We have demonstrated that soy isoflavones (SIF) mitigate these toxic effects at late time points after radiation. However, the process by which SIF impacts the onset of radiation-induced inflammation remains to be elucidated. We have now investigated early events of radiation-induced inflammation and identified cellular and molecular signaling patterns by endothelial cells that could be modified by SIF to control vascular damage and the initiation of lung inflammation.Materials and methods: Histopathological, cellular and molecular studies were performed on mouse lungs from C57Bl/6 mice treated with 10 Gy of thoracic radiation (XRT) in conjunction with daily oral SIF treatment given prior and after radiation. Parallel studies were performed in-vitro using EA.hy926 endothelial cell line with SIF and radiation. Immunohistochemistry, western blots analysis, and flow cytometry were performed on lung tissue or EA.hy926 cells to analyze endothelial cells, their patterns of cell death or survival, and signaling molecules involved in inflammatory events.Results: Histopathological differences in inflammatory infiltrates and vascular injury in lungs, including vascular endothelial cells, were observed with SIF treatment at early time points post-XRT. XRT-induced expression of proinflammatory adhesion molecule ICAM-1 cells was reduced by SIF in-vitro and in-vivo in endothelial cells. Molecular changes in endothelial cells with SIF treatment in conjunction with XRT included increased DNA damage, reduced cell viability and cyclin B1, and inhibition of nuclear translocation of NF-κB. Analysis of cell death showed that SIF treatment promoted apoptotic endothelial cell death and decreased XRT-induced type III cell death. In-vitro molecular studies indicated that SIF + XRT increased apoptotic caspase-9 activation and production of IFNβ while reducing the release of inflammatory HMGB-1 and IL-1α, the cleavage of pyroptotic gasdermin D, and the release of active IL-1β, which are all events associated with type III cell death.Conclusions: SIF + XRT caused changes in patterns of endothelial cell death and survival, proinflammatory molecule release, and adhesion molecule expression at early time points post-XRT associated with early reduction of immune cell recruitment. These findings suggest that SIF could mediate its radioprotective effects in irradiated lungs by limiting excessive immune cell homing via vascular endothelium into damaged lung tissue and curtailing the overall inflammatory response to radiation.