Critical role of natural killer cells in lung immunopathology during influenza infection in mice

Pulmonary delivery of silver nanoparticles prevents influenza infection by recruiting and activating lymphoid cells

Silver nanoparticles (AgNPs) are a potential antiviral agent due to their ability to disrupt the viral particle or alter the virus metabolism inside the host cell. In vitro, AgNPs exhibit antiviral activity against the most common human respiratory viruses. However, their capacity to modulate immune responses during respiratory viral infections has yet to be explored. This study demonstrates that administering AgNPs directly into the lungs prior to infection can reduce viral loads and therefore virus-induced cytokines in mice infected with influenza virus or murine pneumonia virus. The prophylactic effect was diminished in mice with depleted lymphoid cells. We showed that AgNPs-treatment resulted in the recruitment and activation of lymphocytes in the lungs, particularly natural killer (NK) cells. Mechanistically, AgNPs enhanced the ability of alveolar macrophages to promote both NK cell migration and IFN-γ production. By contrast, following infection, in mice treated with AgNPs, NK cells exhibited decreased activation, indicating that these nanoparticles can regulate the potentially deleterious activation of these cells. Overall, the data suggest that AgNPs may possess prophylactic antiviral properties by recruiting and controlling the activation of lymphoid cells through interaction with alveolar macrophages.

Natural killer cells in the lung: potential role in asthma and virus-induced exacerbation?

Asthma is a chronic inflammatory airway disorder whose pathophysiological and immunological mechanisms are not completely understood. Asthma exacerbations are mostly driven by respiratory viral infections and characterised by worsening of symptoms. Despite current therapies, asthma exacerbations can still be life-threatening. Natural killer (NK) cells are innate lymphoid cells well known for their antiviral activity and are present in the lung as circulating and resident cells. However, their functions in asthma and its exacerbations are still unclear. In this review, we will address NK cell activation and functions, which are particularly relevant for asthma and virus-induced asthma exacerbations. Then, the role of NK cells in the lungs at homeostasis in healthy individuals will be described, as well as their functions during pulmonary viral infections, with an emphasis on those associated with asthma exacerbations. Finally, we will discuss the involvement of NK cells in asthma and virus-induced exacerbations and examine the effect of asthma treatments on NK cells.

Understanding the treatment benefit of hyperimmune anti-influenza intravenous immunoglobulin (Flu-IVIG) for severe human influenza

BACKGROUNDAntibody-based therapies for respiratory viruses are of increasing importance. The INSIGHT 006 trial administered anti-influenza hyperimmune intravenous immunoglobulin (Flu-IVIG) to patients hospitalized with influenza. Flu-IVIG treatment improved outcomes in patients with influenza B but showed no benefit for influenza A.METHODSTo probe potential mechanisms of Flu-IVIG utility, sera collected from patients hospitalized with influenza A or B viruses (IAV or IBV) were analyzed for antibody isotype/subclass and Fcγ receptor (FcγR) binding by ELISA, bead-based multiplex, and NK cell activation assays.RESULTSInfluenza-specific FcγR-binding antibodies were elevated in Flu-IVIG-infused IBV- and IAV-infected patients. In IBV-infected participants (n = 62), increased IgG3 and FcγR binding were associated with more favorable outcomes. Flu-IVIG therapy also improved the odds of a more favorable outcome in patients with low levels of anti-IBV Fc-functional antibody. Higher FcγR-binding antibody was associated with less favorable outcomes in IAV-infected patients (n = 50), and Flu-IVIG worsened the odds of a favorable outcome in participants with low levels of anti-IAV Fc-functional antibody.CONCLUSIONThese detailed serological analyses provide insights into antibody features and mechanisms required for a successful humoral response against influenza, suggesting that IBV-specific, but not IAV-specific, antibodies with Fc-mediated functions may assist in improving influenza outcome. This work will inform development of improved influenza immunotherapies.TRIAL REGISTRATIONClinicalTrials.gov NCT02287467.FUNDINGFunding for this research was provided by subcontract 13XS134 under Leidos Biomedical Research Prime Contract HHSN261200800001E and HHSN261201500003I, NCI/NIAID.

Identification of natural killer markers associated with fatal outcome in COVID-19 patients

Introduction

Increasing evidence has shown that coronavirus disease 19 (COVID-19) severity is driven by a dysregulated immunological response. Previous studies have demonstrated that natural killer (NK) cell dysfunction underpins severe illness in COVID-19 patients, but have lacked an in-depth analysis of NK cell markers as a driver of death in the most critically ill patients.

Methods

We enrolled 50 non-vaccinated hospitalized patients infected with the initial virus or the alpha variant of SARS-CoV-2 with moderate or severe illness, to evaluate phenotypic and functional features of NK cells.

Results

Here, we show that, consistent with previous studies, evolution NK cells from COVID-19 patients are more activated, with the decreased activation of natural cytotoxicity receptors and impaired cytotoxicity and IFN-γ production, in association with disease regardless of the SARS-CoV-2 strain. Fatality was observed in 6 of 17 patients with severe disease; NK cells from all of these patients displayed a peculiar phenotype of an activated memory-like phenotype associated with massive TNF-α production.

Discussion

These data suggest that fatal COVID-19 infection is driven by an uncoordinated inflammatory response in part mediated by a specific subset of activated NK cells.

Bioactive Constituents of Verbena officinalis Alleviate Inflammation and Enhance Killing Efficiency of Natural Killer Cells

Natural killer (NK) cells play key roles in eliminating pathogen-infected cells. Verbena officinalis (V. officinalis) has been used as a medical plant in traditional and modern medicine for its anti-tumor and anti-inflammatory activities, but its effects on immune responses remain largely elusive. This study aimed to investigate the potential of V. officinalis extract (VO extract) to regulate inflammation and NK cell functions. We examined the effects of VO extract on lung injury in a mouse model of influenza virus infection. We also investigated the impact of five bioactive components of VO extract on NK killing functions using primary human NK cells. Our results showed that oral administration of VO extract reduced lung injury, promoted the maturation and activation of NK cells in the lung, and decreased the levels of inflammatory cytokines (IL-6, TNF-α and IL-1β) in the serum. Among five bioactive components of VO extract, Verbenalin significantly enhanced NK killing efficiency in vitro, as determined by real-time killing assays based on plate-reader or high-content live-cell imaging in 3D using primary human NK cells. Further investigation showed that treatment of Verbenalin accelerated the killing process by reducing the contact time of NK cells with their target cells without affecting NK cell proliferation, expression of cytotoxic proteins, or lytic granule degranulation. Together, our findings suggest that VO extract has a satisfactory anti-inflammatory effect against viral infection in vivo, and regulates the activation, maturation, and killing functions of NK cells. Verbenalin from V. officinalis enhances NK killing efficiency, suggesting its potential as a promising therapeutic to fight viral infection.

Antiviral Responses of Tissue-resident CD49a+ Lung Natural Killer Cells Are Dysregulated in Chronic Obstructive Pulmonary Disease

Rationale: Tissue-resident natural killer (trNK) cells have been identified in numerous organs, but little is known about their functional contribution to respiratory immunity, in particular during chronic lung diseases such as chronic obstructive pulmonary disease (COPD). Objectives: To investigate the phenotype and antiviral responses of trNK cells in murine cigarette smoke-induced experimental COPD and in human lung parenchyma from COPD donors. Methods: Mice were exposed to cigarette smoke for 12 weeks to induce COPD-like lung disease. Lung trNK cell phenotypes and function were analyzed by flow cytometry in both murine and human disease with and without challenge with influenza A virus. Measurements and Main Results: In the mouse lung, CD49a+CD49b+EOMES+ and CD49a+CD49b-EOMESlo NK cell populations had a distinct phenotype compared with CD49a- circulating NK cells. CD49a+ NK cells were more extensively altered earlier in disease onset than circulating NK cells, and increased proportions of CD49a+ NK cells correlated with worsening disease in both murine and human COPD. Furthermore, the presence of lung disease delayed both circulating and trNK cell functional responses to influenza infection. CD49a+ NK cells markedly increased their NKG2D, CD103, and CD69 expression in experimental COPD after influenza infection, and human CD49a+ NK cells were hyperactive to ex vivo influenza infection in COPD donors. Conclusions: Collectively, these results demonstrate that trNK cell function is altered in cigarette smoke-induced disease and suggests that smoke exposure may aberrantly prime trNK cell responsiveness to viral infection. This may contribute to excess inflammation during viral exacerbations of COPD.

COVID-19 lung disease shares driver AT2 cytopathic features with Idiopathic pulmonary fibrosis

BACKGROUND

In the aftermath of Covid-19, some patients develop a fibrotic lung disease, i.e., post-COVID-19 lung disease (PCLD), for which we currently lack insights into pathogenesis, disease models, or treatment options.

METHODS

Using an AI-guided approach, we analyzed > 1000 human lung transcriptomic datasets associated with various lung conditions using two viral pandemic signatures (ViP and sViP) and one covid lung-derived signature. Upon identifying similarities between COVID-19 and idiopathic pulmonary fibrosis (IPF), we subsequently dissected the basis for such similarity from molecular, cytopathic, and immunologic perspectives using a panel of IPF-specific gene signatures, alongside signatures of alveolar type II (AT2) cytopathies and of prognostic monocyte-driven processes that are known drivers of IPF. Transcriptome-derived findings were used to construct protein-protein interaction (PPI) network to identify the major triggers of AT2 dysfunction. Key findings were validated in hamster and human adult lung organoid (ALO) pre-clinical models of COVID-19 using immunohistochemistry and qPCR.

FINDINGS

COVID-19 resembles IPF at a fundamental level; it recapitulates the gene expression patterns (ViP and IPF signatures), cytokine storm (IL15-centric), and the AT2 cytopathic changes, e.g., injury, DNA damage, arrest in a transient, damage-induced progenitor state, and senescence-associated secretory phenotype (SASP). These immunocytopathic features were induced in pre-clinical COVID models (ALO and hamster) and reversed with effective anti-CoV-2 therapeutics in hamsters. PPI-network analyses pinpointed ER stress as one of the shared early triggers of both diseases, and IHC studies validated the same in the lungs of deceased subjects with COVID-19 and SARS-CoV-2-challenged hamster lungs. Lungs from tg-mice, in which ER stress is induced specifically in the AT2 cells, faithfully recapitulate the host immune response and alveolar cytopathic changes that are induced by SARS-CoV-2.

INTERPRETATION

Like IPF, COVID-19 may be driven by injury-induced ER stress that culminates into progenitor state arrest and SASP in AT2 cells. The ViP signatures in monocytes may be key determinants of prognosis. The insights, signatures, disease models identified here are likely to spur the development of therapies for patients with IPF and other fibrotic interstitial lung diseases.

FUNDING

This work was supported by the National Institutes for Health grants R01- GM138385 and AI155696 and funding from the Tobacco-Related disease Research Program (R01RG3780).

COVID-19 lung disease shares driver AT2 cytopathic features with Idiopathic pulmonary fibrosis

Background

In the aftermath of Covid-19, some patients develop a fibrotic lung disease, i.e., p ost- C OVID-19 l ung d isease (PCLD), for which we currently lack insights into pathogenesis, disease models, or treatment options.

Method

Using an AI-guided approach, we analyzed > 1000 human lung transcriptomic datasets associated with various lung conditions using two viral pandemic signatures (ViP and sViP) and one covid lung-derived signature. Upon identifying similarities between COVID-19 and idiopathic pulmonary fibrosis (IPF), we subsequently dissected the basis for such similarity from molecular, cytopathic, and immunologic perspectives using a panel of IPF-specific gene signatures, alongside signatures of alveolar type II (AT2) cytopathies and of prognostic monocyte-driven processes that are known drivers of IPF. Transcriptome-derived findings were used to construct protein-protein interaction (PPI) network to identify the major triggers of AT2 dysfunction. Key findings were validated in hamster and human adult lung organoid (ALO) pre-clinical models of COVID-19 using immunohistochemistry and qPCR.

Findings

COVID-19 resembles IPF at a fundamental level; it recapitulates the gene expression patterns (ViP and IPF signatures), cytokine storm (IL15-centric), and the AT2 cytopathic changes, e.g., injury, DNA damage, arrest in a transient, damage-induced progenitor state, and senescence-associated secretory phenotype (SASP). These immunocytopathic features were induced in pre-clinical COVID models (ALO and hamster) and reversed with effective anti-CoV-2 therapeutics in hamsters. PPI-network analyses pinpointed ER stress as one of the shared early triggers of both diseases, and IHC studies validated the same in the lungs of deceased subjects with COVID-19 and SARS-CoV-2-challenged hamster lungs. Lungs from tg - mice, in which ER stress is induced specifically in the AT2 cells, faithfully recapitulate the host immune response and alveolar cytopathic changes that are induced by SARS-CoV-2.

Interpretation

Like IPF, COVID-19 may be driven by injury-induced ER stress that culminates into progenitor state arrest and SASP in AT2 cells. The ViP signatures in monocytes may be key determinants of prognosis. The insights, signatures, disease models identified here are likely to spur the development of therapies for patients with IPF and other fibrotic interstitial lung diseases.

Funding

This work was supported by the National Institutes for Health grants R01-GM138385 and AI155696 and funding from the Tobacco-Related disease Research Program (R01RG3780).

One Sentence Summary

Severe COVID-19 triggers cellular processes seen in fibrosing Interstitial Lung Disease.

RESEARCH IN CONTEXT

Evidence before this study: In its aftermath, the COVID-19 pandemic has left many survivors, almost a third of those who recovered, with a mysterious long-haul form of the disease which culminates in a fibrotic form of interstitial lung disease (post-COVID-19 ILD). Post-COVID-19 ILD remains a largely unknown entity. Currently, we lack insights into the core cytopathic features that drive this condition.Added value of this study: Using an AI-guided approach, which involves the use of sets of gene signatures, protein-protein network analysis, and a hamster model of COVID-19, we have revealed here that COVID-19 -lung fibrosis resembles IPF, the most common form of ILD, at a fundamental levelâ€"showing similar gene expression patterns in the lungs and blood, and dysfunctional AT2 processes (ER stress, telomere instability, progenitor cell arrest, and senescence). These findings are insightful because AT2 cells are known to contain an elegant quality control network to respond to intrinsic or extrinsic stress; a failure of such quality control results in diverse cellular phenotypes, of which ER stress appears to be a point of convergence, which appears to be sufficient to drive downstream fibrotic remodeling in the lung.Implications of all the available evidence: Because unbiased computational methods identified the shared fundamental aspects of gene expression and cellular processes between COVID-19 and IPF, the impact of our findings is likely to go beyond COVID-19 or any viral pandemic. The insights, tools (disease models, gene signatures, and biomarkers), and mechanisms identified here are likely to spur the development of therapies for patients with IPF and, other fibrotic interstitial lung diseases, all of whom have limited or no treatment options. To dissect the validated prognostic biomarkers to assess and track the risk of pulmonary fibrosis and develop therapeutics to halt fibrogenic progression.

Lymphocytic Airway Inflammation in Lung Allografts

Lung transplant remains a key therapeutic option for patients with end stage lung disease but short- and long-term survival lag other solid organ transplants. Early ischemia-reperfusion injury in the form of primary graft dysfunction (PGD) and acute cellular rejection are risk factors for chronic lung allograft dysfunction (CLAD), a syndrome of airway and parenchymal fibrosis that is the major barrier to long term survival. An increasing body of research suggests lymphocytic airway inflammation plays a significant role in these important clinical syndromes. Cytotoxic T cells are observed in airway rejection, and transcriptional analysis of airways reveal common cytotoxic gene patterns across solid organ transplant rejection. Natural killer (NK) cells have also been implicated in the early allograft damage response to PGD, acute rejection, cytomegalovirus, and CLAD. This review will examine the roles of lymphocytic airway inflammation across the lifespan of the allograft, including: 1) The contribution of innate lymphocytes to PGD and the impact of PGD on the adaptive immune response. 2) Acute cellular rejection pathologies and the limitations in identifying airway inflammation by transbronchial biopsy. 3) Potentiators of airway inflammation and heterologous immunity, such as respiratory infections, aspiration, and the airway microbiome. 4) Airway contributions to CLAD pathogenesis, including epithelial to mesenchymal transition (EMT), club cell loss, and the evolution from constrictive bronchiolitis to parenchymal fibrosis. 5) Protective mechanisms of fibrosis involving regulatory T cells. In summary, this review will examine our current understanding of the complex interplay between the transplanted airway epithelium, lymphocytic airway infiltration, and rejection pathologies.

Recruited and Tissue-Resident Natural Killer Cells in the Lung During Infection and Cancer

Natural killer (NK) cells are an important component of the innate immune system, and have a key role in host defense against infection and in tumor surveillance. Tumors and viruses employ remarkably similar strategies to avoid recognition and killing by NK cells and so much can be learnt by comparing NK cells in these disparate diseases. The lung is a unique tissue environment and immune cells in this organ, including NK cells, exist in a hypofunctional state to prevent activation against innocuous stimuli. Upon infection, rapid NK cell infiltration into the lung occurs, the amplitude of which is determined by the extent of inflammation and damage. Activated NK cells kill infected cells and produce pro-inflammatory cytokines and chemokines to recruit cells of the adaptive immune system. More recent evidence has shown that NK cells also play an additional role in resolution of inflammation. In lung cancer however, NK cell recruitment is impaired and those that are present have reduced functionality. The majority of lung NK cells are circulatory, however recently a small population of tissue-resident lung NK cells has been described. The specific role of this subset is yet to be determined, but they show similarity to resident memory T cell subsets. Whether resident or recruited, NK cells are important in the control of pulmonary infections, but equally, can drive excessive inflammation if not regulated. In this review we discuss how NK cells are recruited, controlled and retained in the specific environment of the lung in health and disease. Understanding these mechanisms in the context of infection may provide opportunities to promote NK cell recruitment and function in the lung tumor setting.

Human placental hematopoietic stem cell derived natural killer cells (CYNK-001) mediate protection against influenza a viral infection

Influenza A virus (IAV) infections are associated with a high healthcare burden around the world and there is an urgent need to develop more effective therapies. Natural killer (NK) cells have been shown to play a pivotal role in reducing IAV-induced pulmonary infections in preclinical models; however, little is known about the therapeutic potential of adoptively transferred NK cells for IAV infections. Here, we investigated the effects of CYNK-001, human placental hematopoietic stem cell derived NK cells that exhibited strong cytolytic activity against a range of malignant cells and expressed high levels of activating receptors, against IAV infections. In a severe IAV-induced acute lung injury model, mice treated with CYNK-001 showed a milder body weight loss and clinical symptoms, which led to a delayed onset of mortality, thus demonstrating their antiviral protection in vivo. Analysis of bronchoalveolar lavage fluid (BALF) revealed that CYNK-001 reduced proinflammatory cytokines and chemokines highlighting CYNK-001’s anti-inflammatory actions in viral induced-lung injury. Furthermore, CYNK-001-treated mice had altered immune responses to IAV with reduced number of neutrophils in BALF yet increased number of CD8+ T cells in the BALF and lung compared to vehicle-treated mice. Our results demonstrate that CYNK-001 displays protective functions against IAV via its anti-inflammatory and immunomodulating activities, which leads to alleviation of disease burden and progression in a severe IAV-infected mice model. The potential of adoptive NK therapy for IAV infections warrants clinical investigation.

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

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

Immune Responses to IAV Infection and the Roles of L-Selectin and ADAM17 in Lymphocyte Homing

Influenza A virus (IAV) infection is a global public health burden causing up to 650,000 deaths per year. Yearly vaccination programmes and anti-viral drugs currently have limited benefits; therefore, research into IAV is fundamental. Leukocyte trafficking is a crucial process which orchestrates the immune response to infection to protect the host. It involves several homing molecules and receptors on both blood vessels and leukocytes. A key mediator of this process is the transmembrane glycoprotein L-selectin, which binds to vascular addressins on blood vessel endothelial cells. L-selectin classically mediates homing of naïve and central memory lymphocytes to lymph nodes via high endothelial venules (HEVs). Recent studies have found that L-selectin is essential for homing of activated CD8⁺ T cells to influenza-infected lungs and reduction in virus load. A disintegrin and metalloproteinase 17 (ADAM17) is the primary regulator of cell surface levels of L-selectin. Understanding the mechanisms that regulate these two proteins are central to comprehending recruitment of T cells to sites of IAV infection. This review summarises the immune response to IAV infection in humans and mice and discusses the roles of L-selectin and ADAM17 in T lymphocyte homing during IAV infection.

The Dual Role of CCR5 in the Course of Influenza Infection: Exploring Treatment Opportunities

Influenza is one of the most relevant respiratory viruses to human health causing annual epidemics, and recurrent pandemics. Influenza disease is principally associated with inappropriate activation of the immune response. Chemokine receptor 5 (CCR5) and its cognate chemokines CCL3, CCL4 and CCL5 are rapidly induced upon influenza infection, contributing to leukocyte recruitment into the airways and a consequent effective antiviral response. Here we discuss the existing evidence for CCR5 role in the host immune responses to influenza virus. Complete absence of CCR5 in mice revealed the receptor’s role in coping with influenza via the recruitment of early memory CD8+ T cells, B cell activation and later recruitment of activated CD4+ T cells. Moreover, CCR5 contributes to inflammatory resolution by enhancing alveolar macrophages survival and reprogramming macrophages to pro-resolving phenotypes. In contrast, CCR5 activation is associated with excessive recruitment of neutrophils, inflammatory monocytes, and NK cells in models of severe influenza pneumonia. The available data suggests that, while CCL5 can play a protective role in influenza infection, CCL3 may contribute to an overwhelming inflammatory process that can harm the lung tissue. In humans, the gene encoding CCR5 might contain a 32-base pair deletion, resulting in a truncated protein. While discordant data in literature regarding this CCR5 mutation and influenza severity, the association of CCR5delta32 and HIV resistance fostered the development of different CCR5 inhibitors, now being tested in lung inflammation therapy. The potential use of CCR5 inhibitors to modulate the inflammatory response in severe human influenza infections is to be addressed.

The Immune Response to Respiratory Viruses: From Start to Memory

Biomedical research has long strived to improve our understanding of the immune response to respiratory viral infections, an effort that has become all the more important as we live through the consequences of a pandemic. The disease course of these infections is shaped in large part by the actions of various cells of the innate and adaptive immune systems. While these cells are crucial in clearing viral pathogens and establishing long-term immunity, their effector mechanisms may also escalate into excessive, tissue-destructive inflammation detrimental to the host. In this review, we describe the breadth of the immune response to infection with respiratory viruses such as influenza and respiratory syncytial virus. Throughout, we focus on the host rather than the pathogen and try to describe shared patterns in the host response to different viruses. We start with the local cells of the airways, onto the recruitment and activation of innate and adaptive immune cells, followed by the establishment of local and systemic memory cells key in protection against reinfection. We end by exploring how respiratory viral infections can predispose to bacterial superinfection.

Could Interleukin-33 (IL-33) Govern the Outcome of an Equine Influenza Virus Infection? Learning from Other Species

Influenza A viruses (IAVs) are important respiratory pathogens of horses and humans. Infected individuals develop typical respiratory disorders associated with the death of airway epithelial cells (AECs) in infected areas. Virulence and risk of secondary bacterial infections vary among IAV strains. The IAV non-structural proteins, NS1, PB1-F2, and PA-X are important virulence factors controlling AEC death and host immune responses to viral and bacterial infection. Polymorphism in these proteins impacts their function. Evidence from human and mouse studies indicates that upon IAV infection, the manner of AEC death impacts disease severity. Indeed, while apoptosis is considered anti-inflammatory, necrosis is thought to cause pulmonary damage with the release of damage-associated molecular patterns (DAMPs), such as interleukin-33 (IL-33). IL-33 is a potent inflammatory mediator released by necrotic cells, playing a crucial role in anti-viral and anti-bacterial immunity. Here, we discuss studies in human and murine models which investigate how viral determinants and host immune responses control AEC death and subsequent lung IL-33 release, impacting IAV disease severity. Confirming such data in horses and improving our understanding of early immunologic responses initiated by AEC death during IAV infection will better inform the development of novel therapeutic or vaccine strategies designed to protect life-long lung health in horses and humans, following a One Health approach.

Disease Tolerance during Viral-Bacterial Co-Infections

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

Deficiency of eIF4B Increases Mouse Mortality and Impairs Antiviral Immunity

Eukaryotic translation initiation factor 4B (eIF4B) plays an important role in mRNA translation initiation, cell survival and proliferation in vitro. However, its function in vivo is poorly understood. Here, we identified that eIF4B knockout (KO) in mice led to embryonic lethality, and the embryos displayed severe liver damage. Conditional KO (CKO) of eIF4B in adulthood profoundly increased the mortality of mice, characterized by severe pathological changes in several organs and reduced number of peripheral blood lymphocytes. Strikingly, eIF4B CKO mice were highly susceptible to viral infection with severe pulmonary inflammation. Selective deletion of eIF4B in lung epithelium also markedly promoted replication of influenza A virus (IAV) in the lung of infected animals. Furthermore, we observed that eIF4B deficiency significantly enhanced the expression of several important inflammation-associated factors and chemokines, including serum amyloid A1 (Saa1), Marco, Cxcr1, Ccl6, Ccl8, Ccl20, Cxcl2, Cxcl17 that are implicated in recruitment and activation of neutrophiles and macrophages. Moreover, the eIF4B-deficient mice exhibited impaired natural killer (NK) cell-mediated cytotoxicity during the IAV infection. Collectively, the results reveal that eIF4B is essential for mouse survival and host antiviral responses, and establish previously uncharacterized roles for eIF4B in regulating normal animal development and antiviral immunity in vivo.

Maternal natural killer cells at the intersection between reproduction and mucosal immunity

Many maternal immune cells populate the decidua, which is the mucosal lining of the uterus transformed during pregnancy. Here, abundant natural killer (NK) cells and macrophages help the uterine vasculature adapt to fetal demands for gas and nutrients, thereby supporting fetal growth. Fetal trophoblast cells budding off the forming placenta and invading deep into maternal tissues come into contact with these and other immune cells. Besides their homeostatic functions, decidual NK cells can respond to pathogens during infection, but in doing so, they may become conflicted between destroying the invader and sustaining fetoplacental growth. We review how maternal NK cells balance their double duty both in the local microenvironment of the uterus and systemically, during toxoplasmosis, influenza, cytomegalovirus, malaria and other infections that threat pregnancy. We also discuss recent developments in the understanding of NK-cell responses to SARS-Cov-2 infection and the possible dangers of COVID-19 during pregnancy.

Single-cell RNA-seq Analysis Reveals That Prenatal Arsenic Exposure Results in Long-term, Adverse Effects on Immune Gene Expression in Response to Influenza A Infection

Arsenic exposure via drinking water is a serious environmental health concern. Epidemiological studies suggest a strong association between prenatal arsenic exposure and subsequent childhood respiratory infections, as well as morbidity from respiratory diseases in adulthood, long after systemic clearance of arsenic. We investigated the impact of exclusive prenatal arsenic exposure on the inflammatory immune response and respiratory health after an adult influenza A virus (IAV) lung infection. C57BL/6J mice were exposed to 100 ppb sodium arsenite in utero, and subsequently infected with IAV (H1N1) after maturation to adulthood. Assessment of lung tissue and bronchoalveolar lavage fluid at various time points post-IAV infection reveals greater lung damage and inflammation in arsenic-exposed mice versus control mice. Single-cell RNA sequencing analysis of immune cells harvested from IAV-infected lungs suggests that the enhanced inflammatory response is mediated by dysregulation of innate immune function of monocyte-derived macrophages, neutrophils, natural killer cells, and alveolar macrophages. Our results suggest that prenatal arsenic exposure results in lasting effects on the adult host innate immune response to IAV infection, long after exposure to arsenic, leading to greater immunopathology. This study provides the first direct evidence that exclusive prenatal exposure to arsenic in drinking water causes predisposition to a hyperinflammatory response to IAV infection in adult mice, which is associated with significant lung damage.

Natural killer-cell immunoglobulin-like receptors trigger differences in immune response to SARS-CoV-2 infection

Background

The diversity in the clinical course of COVID-19 has been related to differences in innate and adaptative immune response mechanisms. Natural killer (NK) lymphocytes are critical protagonists of human host defense against viral infections. It would seem that reduced circulating levels of these cells have an impact on COVID-19 progression and severity. Their activity is strongly regulated by killer-cell immuno-globulin-like receptors (KIRs) expressed on the NK cell surface. The present study’s focus was to investigate the impact of KIRs and their HLA Class I ligands on SARS-CoV-2 infection.

Methods

KIR gene frequencies, KIR haplotypes, KIR ligands and combinations of KIRs and their HLA Class I ligands were investigated in 396 Sardinian patients with SARS-CoV-2 infection. Comparisons were made between 2 groups of patients divided according to disease severity: 240 patients were symptomatic or paucisymptomatic (Group A), 156 hospitalized patients had severe disease (Group S). The immunogenetic characteristics of patients were also compared to a population group of 400 individuals from the same geographical areas.

Results

Substantial differences were obtained for KIR genes, KIR haplotypes and KIR-HLA ligand combinations when comparing patients of Group S to those of Group A. Patients in Group S had a statistically significant higher frequency of the KIR A/A haplotype compared to patients in Group A [34.6% vs 23.8%, OR = 1.7 (95% CI 1.1–2.6); P = 0.02, Pc = 0.04]. Moreover, the KIR2DS2/HLA C1 combination was poorly represented in the group of patients with severe symptoms compared to those of the asymptomatic-paucisymptomatic group [33.3% vs 50.0%, OR = 0.5 (95% CI 0.3–0.8), P = 0.001, Pc = 0.002]. Multivariate analysis confirmed that, regardless of the sex and age of the patients, the latter genetic variable correlated with a less severe disease course [OR_M = 0.4 (95% CI 0.3–0.7), P_M = 0.0005, P_MC = 0.005].

Conclusions

The KIR2DS2/HLA C1 functional unit resulted to have a strong protective effect against the adverse outcomes of COVID-19. Combined to other well known factors such as advanced age, male sex and concomitant autoimmune diseases, this marker could prove to be highly informative of the disease course and thus enable the timely intervention needed to reduce the mortality associated with the severe forms of SARS-CoV-2 infection. However, larger studies in other populations as well as experimental functional studies will be needed to confirm our findings and further pursue the effect of KIR receptors on NK cell immune-mediated response to SARS-Cov-2 infection.

Hypoxia-inducible factor-dependent induction of myeloid-derived netrin-1 attenuates natural killer cell infiltration during endotoxin-induced lung injury

Sepsis and sepsis‐associated lung inflammation significantly contribute to the morbidity and mortality of critical illness. Here, we examined the hypothesis that neuronal guidance proteins could orchestrate inflammatory events during endotoxin‐induced lung injury. Through a targeted array, we identified netrin‐1 as the top upregulated neuronal guidance protein in macrophages treated with lipopolysaccharide (LPS). Furthermore, we found that netrin‐1 is highly enriched in infiltrating myeloid cells, particularly in macrophages during LPS‐induced lung injury. Transcriptional studies implicate hypoxia‐inducible factor HIF‐1α in the transcriptional induction of netrin‐1 during LPS treatment. Subsequently, the deletion of netrin‐1 in the myeloid compartment (Ntn1^loxp/loxp LysM Cre) resulted in exaggerated mortality and lung inflammation. Surprisingly, further studies revealed enhanced natural killer cells (NK cells) infiltration in Ntn1^loxp/loxp LysM Cre mice, and neutralization of NK cell chemoattractant chemokine (C‐C motif) ligand 2 (CCL2) reversed the exaggerated lung inflammation. Together, these studies provide functional insight into myeloid cell‐derived netrin‐1 in controlling lung inflammation through the modulation of CCL2‐dependent infiltration of NK cells.

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.

An overview of some potential immunotherapeutic options against COVID-19

After the advent of the novel severe acute respiratory syndrome coronavirus 2 (SARS-CoV2) in the late 2019, the resulting severe and pernicious syndrome (COVID-19) immediately was deployed all around the world. To date, despite relentless efforts to control the disease by drug repurposing, there is no approved specific therapy for COVID-19. Given the role of innate and acquired immune components in the control and elimination of viral infections and inflammatory mutilations during SARS-CoV2 pathogenesis, immunotherapeutic strategies appear to be beneficent. Passive immunotherapies such as convalescent plasma, which has received much attention especially in severe cases, as well as suppressing inflammatory cytokines, interferon administration, inhibition of kinases and complement cascade, virus neutralization with key engineered products, cell-based therapies, immunomodulators and anti-inflammatory drugs are among the key immunotherapeutic approaches to deal with COVID-19, which is discussed in this review. Also, details of leading COVID-19 vaccine candidates as the most potent immunotherapy have been provided. However, despite salient improvements, there is still a lack of completely assured vaccines for universal application. Therefore, adopting proper immunotherapies according to the cytokine pattern and involved immune responses, alongside engineered biologics specially ACE2-Fc to curb SARS-CoV2 infection until achieving a tailored vaccine is probably the best strategy to better manage this pandemic. Therefore, gaining knowledge about the mechanism of action, potential targets, as well as the effectiveness of immune-based approaches to confront COVID-19 in the form of a well-ordered review study is highly momentous.

AI-guided discovery of the invariant host response to viral pandemics

BACKGROUND

Coronavirus Disease 2019 (Covid-19) continues to challenge the limits of our knowledge and our healthcare system. Here we sought to define the host immune response, a.k.a, the "cytokine storm" that has been implicated in fatal COVID-19 using an AI-based approach.

METHOD

Over 45,000 transcriptomic datasets of viral pandemics were analyzed to extract a 166-gene signature using ACE2 as a 'seed' gene; ACE2 was rationalized because it encodes the receptor that facilitates the entry of SARS-CoV-2 (the virus that causes COVID-19) into host cells. An AI-based approach was used to explore the utility of the signature in navigating the uncharted territory of Covid-19, setting therapeutic goals, and finding therapeutic solutions.

FINDINGS

The 166-gene signature was surprisingly conserved across all viral pandemics, including COVID-19, and a subset of 20-genes classified disease severity, inspiring the nomenclatures ViP and severe-ViP signatures, respectively. The ViP signatures pinpointed a paradoxical phenomenon wherein lung epithelial and myeloid cells mount an IL15 cytokine storm, and epithelial and NK cell senescence and apoptosis determine severity/fatality. Precise therapeutic goals could be formulated; these goals were met in high-dose SARS-CoV-2-challenged hamsters using either neutralizing antibodies that abrogate SARS-CoV-2•ACE2 engagement or a directly acting antiviral agent, EIDD-2801. IL15/IL15RA were elevated in the lungs of patients with fatal disease, and plasma levels of the cytokine prognosticated disease severity.

INTERPRETATION

The ViP signatures provide a quantitative and qualitative framework for titrating the immune response in viral pandemics and may serve as a powerful unbiased tool to rapidly assess disease severity and vet candidate drugs.

FUNDING

This work was supported by the National Institutes for Health (NIH) [grants CA151673 and GM138385 (to DS) and AI141630 (to P.G), DK107585-05S1 (SD) and AI155696 (to P.G, D.S and S.D), U19-AI142742 (to S.

C, CCHI

Cooperative Centers for Human Immunology)]; Research Grants Program Office (RGPO) from the University of California Office of the President (UCOP) (R00RG2628 & R00RG2642 to P.G, D.S and S.D); the UC San Diego Sanford Stem Cell Clinical Center (to P.G, D.S and S.D); LJI Institutional Funds (to S.C); the VA San Diego Healthcare System Institutional funds (to L.C.A). GDK was supported through The American Association of Immunologists Intersect Fellowship Program for Computational Scientists and Immunologists.

ONE SENTENCE SUMMARY

The host immune response in COVID-19.

AI-guided discovery of the invariant host response to viral pandemics

We sought to define the host immune response, a.k.a, the "cytokine storm" that has been implicated in fatal COVID-19 using an AI-based approach. Over 45,000 transcriptomic datasets of viral pandemics were analyzed to extract a 166-gene signature using ACE2 as a 'seed' gene; ACE2 was rationalized because it encodes the receptor that facilitates the entry of SARS-CoV-2 (the virus that causes COVID-19) into host cells. Surprisingly, this 166-gene signature was conserved in all vi ral p andemics, including COVID-19, and a subset of 20-genes classified disease severity, inspiring the nomenclatures ViP and severe-ViP signatures, respectively. The ViP signatures pinpointed a paradoxical phenomenon wherein lung epithelial and myeloid cells mount an IL15 cytokine storm, and epithelial and NK cell senescence and apoptosis determines severity/fatality. Precise therapeutic goals were formulated and subsequently validated in high-dose SARS-CoV-2-challenged hamsters using neutralizing antibodies that abrogate SARS-CoV-2•ACE2 engagement or a directly acting antiviral agent, EIDD-2801. IL15/IL15RA were elevated in the lungs of patients with fatal disease, and plasma levels of the cytokine tracked with disease severity. Thus, the ViP signatures provide a quantitative and qualitative framework for titrating the immune response in viral pandemics and may serve as a powerful unbiased tool to rapidly assess disease severity and vet candidate drugs.

One Sentence Summary

The host immune response in COVID-19.

PANEL RESEARCH IN CONTEXT

Evidence before this study: The SARS-CoV-2 pandemic has inspired many groups to find innovative methodologies that can help us understand the host immune response to the virus; unchecked proportions of such immune response have been implicated in fatality. We searched GEO and ArrayExpress that provided many publicly available gene expression data that objectively measure the host immune response in diverse conditions. However, challenges remain in identifying a set of host response events that are common to every condition. There are no studies that provide a reproducible assessment of prognosticators of disease severity, the host response, and therapeutic goals. Consequently, therapeutic trials for COVID-19 have seen many more 'misses' than 'hits'. This work used multiple (> 45,000) gene expression datasets from GEO and ArrayExpress and analyzed them using an unbiased computational approach that relies upon fundamentals of gene expression patterns and mathematical precision when assessing them.Added value of this study: This work identifies a signature that is surprisingly conserved in all viral pandemics, including Covid-19, inspiring the nomenclature ViP-signature. A subset of 20-genes classified disease severity in respiratory pandemics. The ViP signatures pinpointed the nature and source of the 'cytokine storm' mounted by the host. They also helped formulate precise therapeutic goals and rationalized the repurposing of FDA-approved drugs.Implications of all the available evidence: The ViP signatures provide a quantitative and qualitative framework for assessing the immune response in viral pandemics when creating pre-clinical models; they serve as a powerful unbiased tool to rapidly assess disease severity and vet candidate drugs.

Deregulated cellular circuits driving immunoglobulins and complement consumption associate with the severity of COVID-19 patients

SARS‐CoV‐2 infection causes an abrupt response by the host immune system, which is largely responsible for the outcome of COVID‐19. We investigated whether the specific immune responses in the peripheral blood of 276 patients were associated with the severity and progression of COVID‐19. At admission, dramatic lymphopenia of T, B, and NK cells is associated with severity. Conversely, the proportion of B cells, plasmablasts, circulating follicular helper T cells (cTfh) and CD56^–CD16⁺ NK‐cells increased. Regarding humoral immunity, levels of IgM, IgA, and IgG were unaffected, but when degrees of severity were considered, IgG was lower in severe patients. Compared to healthy donors, complement C3 and C4 protein levels were higher in mild and moderate, but not in severe patients, while the activation peptide of C5 (C5a) increased from the admission in every patient, regardless of their severity. Moreover, total IgG, the IgG1 and IgG3 isotypes, and C4 decreased from day 0 to day 10 in patients who were hospitalized for more than two weeks, but not in patients who were discharged earlier. Our study provides important clues to understand the immune response observed in COVID‐19 patients, associating severity with an imbalanced humoral response, and identifying new targets for therapeutic intervention.

Influenza A Virus Hemagglutinin and Other Pathogen Glycoprotein Interactions with NK Cell Natural Cytotoxicity Receptors NKp46, NKp44, and NKp30

Natural killer (NK) cells are part of the innate immunity repertoire, and function in the recognition and destruction of tumorigenic and pathogen-infected cells. Engagement of NK cell activating receptors can lead to functional activation of NK cells, resulting in lysis of target cells. NK cell activating receptors specific for non-major histocompatibility complex ligands are NKp46, NKp44, NKp30, NKG2D, and CD16 (also known as FcγRIII). The natural cytotoxicity receptors (NCRs), NKp46, NKp44, and NKp30, have been implicated in functional activation of NK cells following influenza virus infection via binding with influenza virus hemagglutinin (HA). In this review we describe NK cell and influenza A virus biology, and the interactions of influenza A virus HA and other pathogen lectins with NK cell natural cytotoxicity receptors (NCRs). We review concepts which intersect viral immunology, traditional virology and glycobiology to provide insights into the interactions between influenza virus HA and the NCRs. Furthermore, we provide expert opinion on future directions that would provide insights into currently unanswered questions.

Single-cell profiling reveals the trajectories of natural killer cell differentiation in bone marrow and a stress signature induced by acute myeloid leukemia

Natural killer (NK) cells are innate cytotoxic lymphoid cells (ILCs) involved in the killing of infected and tumor cells. Among human and mouse NK cells from the spleen and blood, we previously identified by single-cell RNA sequencing (scRNAseq) two similar major subsets, NK1 and NK2. Using the same technology, we report here the identification, by single-cell RNA sequencing (scRNAseq), of three NK cell subpopulations in human bone marrow. Pseudotime analysis identified a subset of resident CD56^bright NK cells, NK0 cells, as the precursor of both circulating CD56^dim NK1-like NK cells and CD56^bright NK2-like NK cells in human bone marrow and spleen under physiological conditions. Transcriptomic profiles of bone marrow NK cells from patients with acute myeloid leukemia (AML) exhibited stress-induced repression of NK cell effector functions, highlighting the profound impact of this disease on NK cell heterogeneity. Bone marrow NK cells from AML patients exhibited reduced levels of CD160, but the CD160^high group had a significantly higher survival rate.

Natural killer cell responses to emerging viruses of zoonotic origin

Emerging viral diseases pose a major threat to public health worldwide. Nearly all emerging viruses, including Ebola, Dengue, Nipah, West Nile, Zika, and coronaviruses (including SARS-Cov2, the causative agent of the current COVID-19 pandemic), have zoonotic origins, indicating that animal-to-human transmission constitutes a primary mode of acquisition of novel infectious diseases. Why these viruses can cause profound pathologies in humans, while natural reservoir hosts often show little evidence of disease is not completely understood. Differences in the host immune response, especially within the innate compartment, have been suggested to be involved in this divergence. Natural killer (NK) cells are innate lymphocytes that play a critical role in the early antiviral response, secreting effector cytokines and clearing infected cells. In this review, we will discuss the mechanisms through which NK cells interact with viruses, their contribution towards maintaining equilibrium between the virus and its natural host, and their role in disease progression in humans and other non-natural hosts.

Innate Immune Responses to Acute Viral Infection During Pregnancy

Immunological adaptations in pregnancy allow maternal tolerance of the semi-allogeneic fetus but also increase maternal susceptibility to infection. At implantation, the endometrial stroma, glands, arteries and immune cells undergo anatomical and functional transformation to create the decidua, the specialized secretory endometrium of pregnancy. The maternal decidua and the invading fetal trophoblast constitute a dynamic junction that facilitates a complex immunological dialogue between the two. The decidual and peripheral immune systems together assume a pivotal role in regulating the critical balance between tolerance and defense against infection. Throughout pregnancy, this equilibrium is repeatedly subjected to microbial challenge. Acute viral infection in pregnancy is associated with a wide spectrum of adverse consequences for both mother and fetus. Vertical transmission from mother to fetus can cause developmental anomalies, growth restriction, preterm birth and stillbirth, while the mother is predisposed to heightened morbidity and maternal death. A rapid, effective response to invasive pathogens is therefore essential in order to avoid overwhelming maternal infection and consequent fetal compromise. This sentinel response is mediated by the innate immune system: a heritable, highly evolutionarily conserved system comprising physical barriers, antimicrobial peptides (AMP) and a variety of immune cells—principally neutrophils, macrophages, dendritic cells, and natural killer cells—which express pattern-receptors that detect invariant molecular signatures unique to pathogenic micro-organisms. Recognition of these signatures during acute infection triggers signaling cascades that enhance antimicrobial properties such as phagocytosis, secretion of pro-inflammatory cytokines and activation of the complement system. As well as coordinating the initial immune response, macrophages and dendritic cells present microbial antigens to lymphocytes, initiating and influencing the development of specific, long-lasting adaptive immunity. Despite extensive progress in unraveling the immunological adaptations of pregnancy, pregnant women remain particularly susceptible to certain acute viral infections and continue to experience mortality rates equivalent to those observed in pandemics several decades ago. Here, we focus specifically on the pregnancy-induced vulnerabilities in innate immunity that contribute to the disproportionately high maternal mortality observed in the following acute viral infections: Lassa fever, Ebola virus disease (EVD), dengue fever, hepatitis E, influenza, and novel coronavirus infections.

Pathogenesis of COVID-19-induced ARDS: implications for an ageing population

The coronavirus disease 2019 (COVID-19) pandemic has elicited a swift response by the scientific community to elucidate the pathogenesis of severe acute respiratory syndrome-coronavirus-2 (SARS-CoV-2)-induced lung injury and develop effective therapeutics. Clinical data indicate that severe COVID-19 most commonly manifests as viral pneumonia-induced acute respiratory distress syndrome (ARDS), a clinical entity mechanistically understood best in the context of influenza A virus-induced pneumonia. Similar to influenza, advanced age has emerged as the leading host risk factor for developing severe COVID-19. In this review we connect the current understanding of the SARS-CoV-2 replication cycle and host response to the clinical presentation of COVID-19, borrowing concepts from influenza A virus-induced ARDS pathogenesis and discussing how these ideas inform our evolving understanding of COVID-19-induced ARDS. We also consider important differences between COVID-19 and influenza, mainly the protean clinical presentation and associated lymphopenia of COVID-19, the contrasting role of interferon-γ in mediating the host immune response to these viruses, and the tropism for vascular endothelial cells of SARS-CoV-2, commenting on the potential limitations of influenza as a model for COVID-19. Finally, we explore hallmarks of ageing that could explain the association between advanced age and susceptibility to severe COVID-19.

Review of viral ARDS pathogenesis, how it informs evolving models of COVID-19, and how hallmarks of ageing explain the age-related morbidity and mortality of severe COVID-19https://bit.ly/39Ca0c0

Harnessing Memory NK Cell to Protect Against COVID-19

The worldwide struggle against the coronavirus disease 2019 (COVID-19) as a public health crisis continues to sweep across the globe. Up to now, effective antiviral treatment against COVID-19 is not available. Therefore, throughout virus infections, a thorough clarification of the virus-host immune system interactions will be most probably helpful to encounter these challenges. Emerging evidence suggests that just like SARS and MERS, COVID-19 primarily suppresses the innate immune system, enabling its stable propagation during the early stage of infection. Consequently, proinflammatory cytokines and chemokines have been increasing during infection progression associated with severe lung pathology. It is imperative to consider hyper inflammation in vaccine designing, as vaccine-induced immune responses must have a protective role against infection without leading to immunopathology. Among the front-line responders to viral infections, Natural Killer (NK) cells have immense therapeutic potential, forming a bridge between innate and adaptive responses. A subset of NK cells exhibits putatively increased effector functions against viruses following pathogen-specific and immunization. Memory NK cells have higher cytotoxicity and effector activity, compared with the conventional NK cells. As a pioneering strategy, prompt accumulation and long-term maintenance of these memory NK cells could be an efficacious viral treatment. According to the high prevalence of human cytomegalovirus (HCMV) infection in the world, it remains to be determined whether HCMV adaptive NK cells could play a protective role against this new emerging virus. In addition, the new adaptive-like KIR+NKG2C+ NK cell subset (the adaptive-like lung tissue residue [tr]NK cell) in the context of the respiratory infection at this site could specifically exhibit the expansion upon COVID-19. Another aspect of NK cells we should note, utilizing modified NK cells such as allogeneic off-the-shelf CAR-NK cells as a state-of-the-art strategy for the treatment of COVID-19. In this line, we speculate introducing NKG2C into chimeric antigen receptors in NK cells might be a potential approach in future viral immunotherapy for emerging viruses. In this contribution, we will briefly discuss the current status and future perspective of NK cells, which provide to successfully exploit NK cell-mediated antiviral activity that may offer important new tools in COVID-19 treatment.

Dynamic Natural Killer Cell and T Cell Responses to Influenza Infection

Influenza viruses have perplexed scientists for over a hundred years. Yearly vaccines limit their spread, but they do not prevent all infections. Therapeutic treatments for those experiencing severe infection are limited; further advances are held back by insufficient understanding of the fundamental immune mechanisms responsible for immunopathology. NK cells and T cells are essential in host responses to influenza infection. They produce immunomodulatory cytokines and mediate the cytotoxic response to infection. An imbalance in NK and T cell responses can lead to two outcomes: excessive inflammation and tissue damage or insufficient anti-viral functions and uncontrolled infection. The main cause of death in influenza patients is the former, mediated by hyperinflammatory responses termed “cytokine storm.” NK cells and T cells contribute to cytokine storm, but they are also required for viral clearance. Many studies have attempted to distinguish protective and pathogenic components of the NK cell and T cell influenza response, but it has become clear that they are dynamic and integrated processes. This review will analyze how NK cell and T cell effector functions during influenza infection affect the host response and correlate with morbidity and mortality outcomes.

Flattening the COVID-19 Curve With Natural Killer Cell Based Immunotherapies

Natural Killer (NK) cells are innate immune responders critical for viral clearance and immunomodulation. Despite their vital role in viral infection, the contribution of NK cells in fighting SARS-CoV-2 has not yet been directly investigated. Insights into pathophysiology and therapeutic opportunities can therefore be inferred from studies assessing NK cell phenotype and function during SARS, MERS, and COVID-19. These studies suggest a reduction in circulating NK cell numbers and/or an exhausted phenotype following infection and hint toward the dampening of NK cell responses by coronaviruses. Reduced circulating NK cell levels and exhaustion may be directly responsible for the progression and severity of COVID-19. Conversely, in light of data linking inflammation with coronavirus disease severity, it is necessary to examine NK cell potential in mediating immunopathology. A common feature of coronavirus infections is that significant morbidity and mortality is associated with lung injury and acute respiratory distress syndrome resulting from an exaggerated immune response, of which NK cells are an important component. In this review, we summarize the current understanding of how NK cells respond in both early and late coronavirus infections, and the implication for ongoing COVID-19 clinical trials. Using this immunological lens, we outline recommendations for therapeutic strategies against COVID-19 in clearing the virus while preventing the harm of immunopathological responses.

NK cells: A double edge sword against SARS-CoV-2

Natural killer (NK) cells are pivotal effectors of the innate immunity protecting an individual from microbes. They are the first line of defense against invading viruses, given their substantial ability to directly target infected cells without the need for specific antigen presentation. By establishing cellular networks with a variety of cell types such as dendritic cells, NK cells can also amplify and modulate antiviral adaptive immune responses. In this review, we will examine the role of NK cells in SARS-COV2 infections causing the ongoing COVID19 pandemic, keeping in mind the controversial role of NK cells specifically in viral respiratory infections and in inflammatory-driven lung damage. We discuss lessons learnt from previous coronavirus outbreaks in humans (caused by SARS-CoV-1 and MERS-COV).

Host Protective Immune Responses against Influenza A Virus Infection

Influenza viruses cause infectious respiratory disease characterized by fever, myalgia, and congestion, ranging in severity from mild to life-threating. Although enormous efforts have aimed to prevent and treat influenza infections, seasonal and pandemic influenza outbreaks remain a major public health concern. This is largely because influenza viruses rapidly undergo genetic mutations that restrict the long-lasting efficacy of vaccine-induced immune responses and therapeutic regimens. In this review, we discuss the virological features of influenza A viruses and provide an overview of current knowledge of the innate sensing of invading influenza viruses and the protective immune responses in the host.

Host-Virus Interaction: How Host Cells Defend against Influenza A Virus Infection

Influenza A viruses (IAVs) are highly contagious pathogens infecting human and numerous animals. The viruses cause millions of infection cases and thousands of deaths every year, thus making IAVs a continual threat to global health. Upon IAV infection, host innate immune system is triggered and activated to restrict virus replication and clear pathogens. Subsequently, host adaptive immunity is involved in specific virus clearance. On the other hand, to achieve a successful infection, IAVs also apply multiple strategies to avoid be detected and eliminated by the host immunity. In the current review, we present a general description on recent work regarding different host cells and molecules facilitating antiviral defenses against IAV infection and how IAVs antagonize host immune responses.

Natural Killer Cells Dampen the Pathogenic Features of Recall Responses to Influenza Infection

Despite evidence of augmented Natural Killer (NK) cell responses after influenza vaccination, the role of these cells in vaccine-induced immunity remains unclear. Here, we hypothesized that NK cells might increase viral clearance but possibly at the expense of increased severity of pathology. On the contrary, we found that NK cells serve a homeostatic role during influenza virus infection of vaccinated mice, allowing viral clearance with minimal pathology. Using a diphtheria toxin receptor transgenic mouse model, we were able to specifically deplete NKp46+ NK cells through the administration of diphtheria toxin. Using this model, we assessed the effect of NK cell depletion prior to influenza challenge in vaccinated and unvaccinated mice. NK-depleted, vaccinated animals lost significantly more weight after viral challenge than vaccinated NK intact animals, indicating that NK cells ameliorate disease in vaccinated animals. However, there was also a significant reduction in viral load in NK-depleted, unvaccinated animals indicating that NK cells also constrain viral clearance. Depletion of NK cells after vaccination, but 21 days before infection, did not affect viral clearance or weight loss—indicating that it is the presence of NK cells during the infection itself that promotes homeostasis. Further work is needed to identify the mechanism(s) by which NK cells regulate adaptive immunity in influenza-vaccinated animals to allow efficient and effective virus control whilst simultaneously minimizing inflammation and pathology.

Pregnancy-Induced Alterations in NK Cell Phenotype and Function

Pregnant women are particularly susceptible to complications of influenza A virus infection, which may result from pregnancy-induced changes in the function of immune cells, including natural killer (NK) cells. To better understand NK cell function during pregnancy, we assessed the ability of the two main subsets of NK cells, CD56^dim, and CD56^bright NK cells, to respond to influenza-virus infected cells and tumor cells. During pregnancy, CD56^dim and CD56^bright NK cells displayed enhanced functional responses to both infected and tumor cells, with increased expression of degranulation markers and elevated frequency of NK cells producing IFN-γ. To better understand the mechanisms driving this enhanced function, we profiled CD56^dim and CD56^bright NK cells from pregnant and non-pregnant women using mass cytometry. NK cells from pregnant women displayed significantly increased expression of several functional and activation markers such as CD38 on both subsets and NKp46 on CD56^dim NK cells. NK cells also displayed diminished expression of the chemokine receptor CXCR3 during pregnancy. Overall, these data demonstrate that functional and phenotypic shifts occur in NK cells during pregnancy that can influence the magnitude of the immune response to both infections and tumors.

Natural Killer Cells in the Lungs

The lungs, a special site that is frequently challenged by tumors, pathogens and other environmental insults, are populated by large numbers of innate immune cells. Among these, natural killer (NK) cells are gaining increasing attention. Recent studies have revealed that NK cells are heterogeneous populations consisting of distinct subpopulations with diverse characteristics, some of which are determined by their local tissue microenvironment. Most current information about NK cells comes from studies of NK cells from the peripheral blood of humans and NK cells from the spleen and bone marrow of mice. However, the functions and phenotypes of lung NK cells differ from those of NK cells in other tissues. Here, we provide an overview of human and mouse lung NK cells in the context of homeostasis, pathogenic infections, asthma, chronic obstructive pulmonary disease (COPD) and lung cancer, mainly focusing on their phenotype, function, frequency, and their potential role in pathogenesis or immune defense. A comprehensive understanding of the biology of NK cells in the lungs will aid the development of NK cell-based immunotherapies for the treatment of lung diseases.

Immunomodulatory and prophylactic effects of Bifidobacterium bifidum probiotic strain on influenza infection in mice

The limited efficacy of available influenza vaccines against rapidly emerging new viral strains stresses the need for the development of new antigen-independent prophylactic treatment for enhancing immunity against influenza infection. Recent studies suggest that probiotics possess immunomodulatory properties and can reduce the severity of respiratory infections. Here, we investigated the potential of prophylactic Bifidobacterium bifidum in improving anti-influenza immune responses in an experimental lethal mouse-adapted influenza A (H1N1) infection in a BALB/c mouse model. One week after viral challenge, splenocyte proliferation assay (MTT), IFN-gamma, IL-12, and IL-4 in spleen and IL-6 in the lung homogenates were conducted using ELISA assays. Sera samples were collected to measure IgG1 and IgG2a levels. Furthermore, the mice challenged with lethal influenza virus were assessed for survival rate. The findings demonstrated a strong induction of both humoral and cellular immunities, as well as decreased level of IL-6 production in the lung and an increase in survival rate in the mice receiving Bifidobacterium than those of the control group were observed. Taken together, the results indicate a robust potential for Bifidobacterium to modulate humoral and cellular immune responses and induce balanced Th1/Th2 immune responses against influenza infection.

Mechanisms of Bacterial Superinfection Post-influenza: A Role for Unconventional T Cells

Despite the widespread application of vaccination programs and antiviral drug treatments, influenza viruses are still among the most harmful human pathogens. Indeed, influenza results in significant seasonal and pandemic morbidity and mortality. Furthermore, severe bacterial infections can occur in the aftermath of influenza virus infection, and contribute substantially to the excess morbidity and mortality associated with influenza. Here, we review the main features of influenza viruses and current knowledge about the mechanical and immune mechanisms that underlie post-influenza secondary bacterial infections. We present the emerging literature describing the role of "innate-like" unconventional T cells in post-influenza bacterial superinfection. Unconventional T cell populations span the border between the innate and adaptive arms of the immune system, and are prevalent in mucosal tissues (including the airways). They mainly comprise Natural Killer T cells, mucosal-associated invariant T cells and γδ T cells. We provide an overview of the principal functions that these cells play in pulmonary barrier functions and immunity, highlighting their unique ability to sense environmental factors and promote protection against respiratory bacterial infections. We focus on two major opportunistic pathogens involved in superinfections, namely Streptococcus pneumoniae and Staphylococcus aureus. We discuss mechanisms through which influenza viruses alter the antibacterial activity of unconventional T cells. Lastly, we discuss recent fundamental advances and possible therapeutic approaches in which unconventional T cells would be targeted to prevent post-influenza bacterial superinfections.

Influenza Virus Infection Enhances Antibody-Mediated NK Cell Functions via Type I Interferon-Dependent Pathways

Natural killer (NK) cells are an important component in the control of influenza virus infection, acting to both clear virus-infected cells and release antiviral cytokines. Engagement of CD16 on NK cells by antibody-coated influenza virus-infected cells results in antibody-dependent cellular cytotoxicity (ADCC). Increasing the potency of antibody-mediated NK cell activity could ultimately lead to improved control of influenza virus infection. To understand if NK cells can be functionally enhanced following exposure to influenza virus-infected cells, we cocultured human peripheral blood mononuclear cells (PBMCs) with influenza virus-infected human alveolar epithelial (A549) cells and evaluated the capacity of NK cells to mediate antibody-dependent functions. Preincubation of PBMCs with influenza virus-infected cells markedly enhanced the ability of NK cells to respond to immune complexes containing hemagglutinin (HA) and anti-HA antibodies or transformed allogeneic cells in the presence or absence of a therapeutic monoclonal antibody. Cytokine multiplex, RNA sequencing, supernatant transfer, Transwell, and cytokine-blocking/cytokine supplementation experiments showed that type I interferons released from PBMCs were primarily responsible for the influenza virus-induced enhancement of antibody-mediated NK cell functions. Importantly, the influenza virus-mediated increase in antibody-dependent NK cell functionality was mimicked by the type I interferon agonist poly(I·C). We conclude that the type I interferon secretion induced by influenza virus infection enhances the capacity of NK cells to mediate ADCC and that this pathway could be manipulated to alter the potency of anti-influenza virus therapies and vaccines.IMPORTANCE Protection from severe influenza may be assisted by antibodies that engage NK cells to kill infected cells through ADCC. Studies have primarily focused on antibodies that have ADCC activity, rather than the capacity of NK cells to become activated and mediate ADCC during an influenza virus infection. We found that type I interferon released in response to influenza virus infection primes NK cells to become highly reactive to anti-influenza virus ADCC antibodies. Enhancing the capacity of NK cells to mediate ADCC could assist in controlling influenza virus infections.

Virus genes and host correlates of pathology are markedly reduced during respiratory syncytial and influenza virus co-infection in BALB/c mice

Globally, influenza A virus (IAV) and respiratory syncytial virus (RSV) infection remain very high. There is also a high burden of IAV and RSV co-infection in developing countries. To develop universally protective vaccines against these infections, it is imperative that viral genes and immune correlates of pathology are elucidated. As such, we profiled virus genes expressions, histopathology and immunological responses of BALB/c mice infected with RSV and/or IAV in this study. RSV A2 and/or influenza A/H3N2/Perth/16/09 (Pr/H3N2) were induced over a seven-day period in BALB/c mice. Anaesthetized BALB/c mice (12-14 g) were divided into six groups (15-20 mice per group), inoculated with 32 μl each of 3LD50 Pr/H3N2 and/or 100 TCID50 RSV. Two groups (R or I) received RSV or Pr/H3N2 intranasally. Prior infection with either RSV or Pr/H3N2 was followed with a second challenge of the other virus 24 hours post inoculation in RI and IR groups. Another set was exposed to the two viruses simultaneously (I + R group) while the last group served as healthy controls. Five to seven mice per group were euthanized at days 2, 4 and 7. Lung and spleen organs were harvested for virus genes quantitation and immune cells phenotyping respectively. I + R group showed progressive downregulation of RSV F, G, NS1 and NS2 genes. IAV PB2 and M genes had high fold increase on day 2 and 4 post infections. However, by day 7 post infection, M and PB2 fold increase was lower. Also, increased proportions of NKT and T cell subsets were observed throughout the period in I + R group. Conversely, I group was characterized by reduced NKT cell counts and enhanced CD8 T cells levels while R group only showed an increased proportion of CD8 T cells towards the peak of infection. This study shows that RSV and IAV co-infection lead to reduced virulence and pathology compared to single infections. This information is very useful in combinatorial RSV/IAV vaccine design and development.

Enhancing the natural killer cell activity and anti-influenza effect of heat-treated Lactobacillus plantarum nF1-fortified yogurt in mice

Influenza A virus (IAV) infection is a global public health concern. It causes respiratory diseases ranging from mild illness to fatal disease. Natural killer (NK) cells are an innate immune component that kill infected cells and secrete cytokines to modulate the adaptive immune system; they constitute the first-line defense and play important roles in controlling IAV infection. This study evaluated the effect of daily administration of heat-treated Lactobacillus plantarum nF1-fortified yogurt on immunity and protection against IAV infection. Mice administered with heat-treated L. plantarum nF1-fortified yogurt showed elevated NK cell-related cytokine expression levels. Daily administration of the L. plantarum nF1-fortified yogurt before IAV infection also enhanced splenic NK activity, lung inflammatory cytokine responses, and survival rate. Thus, daily administration of nF1-fortified yogurt enhances host immunity and helps prevent IAV infection.

Differential Induction of IFN-α and Modulation of CD112 and CD54 Expression Govern the Magnitude of NK Cell IFN-γ Response to Influenza A Viruses

In human and murine studies, IFN-γ is a critical mediator immunity to influenza. IFN-γ production is critical for viral clearance and the development of adaptive immune responses, yet excessive production of IFN-γ and other cytokines as part of a cytokine storm is associated with poor outcomes of influenza infection in humans. As NK cells are the main population of lung innate immune cells capable of producing IFN-γ early in infection, we set out to identify the drivers of the human NK cell IFN-γ response to influenza A viruses. We found that influenza triggers NK cells to secrete IFN-γ in the absence of T cells and in a manner dependent upon signaling from both cytokines and receptor-ligand interactions. Further, we discovered that the pandemic A/California/07/2009 (H1N1) strain elicits a seven-fold greater IFN-γ response than other strains tested, including a seasonal A/Victoria/361/2011 (H3N2) strain. These differential responses were independent of memory NK cells. Instead, we discovered that the A/Victoria/361/2011 influenza strain suppresses the NK cell IFN-γ response by downregulating NK-activating ligands CD112 and CD54 and by repressing the type I IFN response in a viral replication-dependent manner. In contrast, the A/California/07/2009 strain fails to repress the type I IFN response or to downregulate CD54 and CD112 to the same extent, which leads to the enhanced NK cell IFN-γ response. Our results indicate that influenza implements a strain-specific mechanism governing NK cell production of IFN-γ and identifies a previously unrecognized influenza innate immune evasion strategy.

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.

Human CD49a+ Lung Natural Killer Cell Cytotoxicity in Response to Influenza A Virus

Influenza A virus (IAV) is a major global public health burden due to its routine evasion of immunization strategies. Natural killer (NK) cells are innate cytotoxic cells with important antiviral activity in the human body, yet the function of these cells in the control of IAV infection is unclear. The aim of this study was to determine the role of lung NK cell cytotoxic responses to IAV. Human lung explants were infected ex vivo with IAV, and lung NK cell activation was analyzed by flow cytometry. Cytotoxic responses of NK cell subsets against IAV-infected macrophages were measured by flow cytometry and ELISA. Despite reports of hypofunctionality in the pulmonary environment, human lung-associated NK cells responded rapidly to ex vivo IAV infection, with upregulation of surface CD107a 24 h post-infection. The lung NK cell phenotype is similar in maturity and differentiation to NK cells of the peripheral blood but a unique CD56^brightCD49a⁺CD103⁺CD69⁺ NK cell population was identified in the lung, indicating NK cell residency within this organ. In response to ex vivo IAV infection a greater proportion of resident CD56^brightCD49a⁺ NK cells expressed surface CD107a compared with CD56^brightCD49a⁻ NK cells, suggesting a hyperfunctional NK cell population may be present within human lung tissue and could be the result of innate immunological training. Furthermore, NK cells provided significant antiviral, cytotoxic activity following contact with influenza-infected cells, including the production and release of IFN-γ and granzyme-B resulting in macrophage cell death. These results suggest that a resident, trained NK cell population are present in the human lung and may provide early and important control of viral infection. A greater understanding of this resident mucosal population may provide further insight into the role of these cells in controlling viral infection and generating appropriate adaptive immunity to IAV.