Both influenza-induced neutrophil dysfunction and neutrophil-independent mechanisms contribute to increased susceptibility to a secondary Streptococcus pneumoniae infection

Intracellular Streptococcus pneumoniae develops enhanced fluoroquinolone persistence during influenza A coinfection

Streptococcus pneumoniae is a major pathogen responsible for severe complications in patients with prior influenza A virus (IAV) infection. We have previously demonstrated that S. pneumoniae exhibits increased intracellular survival within IAV-infected cells. Fluoroquinolones (FQs) are widely used to treat pneumococcal infections. However, our prior work has shown that S. pneumoniae can develop intracellular FQ persistence, a phenomenon triggered by oxidative stress within host cells. This persistence allows the bacteria to withstand high FQ concentrations. In this study, we show that IAV infection enhances pneumococcal FQ persistence during intracellular survival within pneumocytes, macrophages, and neutrophils. This enhancement is partly due to increased oxidative stress induced by the viral infection. We find that this phenotype is particularly pronounced in autophagy-proficient host cells, potentially resulting from IAV-induced blockage of autophagosome-lysosome fusion. Moreover, we identified several S. pneumoniae genes involved in oxidative stress response that contribute to FQ persistence, including sodA (superoxide dismutase), clpL (chaperone), nrdH (glutaredoxin), and psaB (Mn+2 transporter component). Our findings reveal a novel mechanism of antibiotic persistence promoted by viral infection within host cells. This underscores the importance of considering this phenomenon when using FQs to treat pneumococcal infections, especially in patients with concurrent influenza A infection.

Human monoclonal antibodies protect against viral-mediated pneumococcal superinfection

Introduction

Community-acquired pneumonia (CAP) is a global health concern, with 25% of cases attributed to Streptococcus pneumoniae (Spn). Viral infections like influenza A virus (IAV), respiratory syncytial virus (RSV), and human metapneumovirus (hMPV) increase the risk of Spn, leading to severe complications due to compromised host immunity.

Methods

We evaluated the efficacy of an anti-PhtD monoclonal antibody (mAb) cocktail therapy (PhtD3 + 7) in improving survival rates in three viral/bacterial coinfection models: IAV/Spn, hMPV/Spn, and RSV/Spn.

Results

The PhtD3 + 7 mAb cocktail outperformed antiviral mAbs, resulting in prolonged survival. In the IAV/Spn model, it reduced bacterial titers in blood and lungs by 2-4 logs. In the hMPV/Spn model, PhtD3 + 7 provided greater protection than the hMPV-neutralizing mAb MPV467, significantly reducing bacterial titers. In the RSV/Spn model, PhtD3 + 7 offered slightly better protection than the antiviral mAb D25, uniquely decreasing bacterial titers in blood and lungs.

Discussion

Given the threat of antibiotic resistance, our findings highlight the potential of anti-PhtD mAb therapy as an effective option for treating viral and secondary pneumococcal coinfections.

The host transcriptional response to superinfection by influenza A virus and Streptococcus pneumoniae

Secondary bacterial challenges during influenza virus infection "superinfection") cause excessive mortality and hospitalization. Here, we present a longitudinal study of bulk gene expression changes in murine lungs during superinfection, with an initial influenza A virus infection and a subsequent Streptococcus pneumoniae infection. In addition to the well-characterized impairment of the host response, we identified superinfection-specific alterations in the global transcriptional program that are linked to the host's ability to resist the pathogens. Particularly, whereas superinfected mice manifested an excessive rapid induction of the resistance-to-infection program, there was a substantial tissue-level rewiring of this program: upon superinfection, interferon-regulated genes were switched from positive to negative correlations with the host's resistance state, whereas genes of fatty acid metabolism switched from negative to positive correlations with resistance states. Thus, the transcriptional resistance state in superinfection is reprogrammed toward repressed interferon signaling and induced fatty acid metabolism. Our findings suggest new insights into a tissue-level remodeling of the host defense upon superinfection, providing promising targets for future therapeutic interventions.

IMPORTANCE

Secondary bacterial infections are the most frequent complications during influenza A virus (IAV) pandemic outbreaks, contributing to excessive morbidity and mortality in the human population. Most IAV-related deaths are attributed to Streptococcus pneumoniae (SP) infections, which usually begin within the first week of IAV infection in the respiratory tracts. Here, we focused on longitudinal transcriptional responses during a superinfection model consisting of an SP infection that follows an initial IAV infection, comparing superinfection to an IAV-only infection, an SP-only infection, and control treatments. Our longitudinal data allowed a fine analysis of gene expression changes during superinfection. For instance, we found that superinfected mice exhibited rapid gene expression induction or reduction within the first 12 h after encountering the second pathogen. Cell proliferation and immune response activation processes were upregulated, while endothelial processes, vasculogenesis, and angiogenesis were downregulated, providing promising targets for future therapeutic interventions. We further analyzed the longitudinal transcriptional responses in the context of a previously defined spectrum of the host's resistance state, revealing superinfection-specific reprogramming of resistance states, such as reprogramming of fatty acid metabolism and interferon signaling. The reprogrammed functions are compelling new targets for switching the pathogenic superinfection state into a single-infection state.

Diverging patterns in innate immunity against respiratory viruses during a lifetime: lessons from the young and the old

Respiratory viral infections frequently lead to severe respiratory disease, particularly in vulnerable populations such as young children, individuals with chronic lung conditions and older adults, resulting in hospitalisation and, in some cases, fatalities. The innate immune system plays a crucial role in monitoring for, and initiating responses to, viruses, maintaining a state of preparedness through the constant expression of antimicrobial defence molecules. Throughout the course of infection, innate immunity remains actively involved, contributing to viral clearance and damage control, with pivotal contributions from airway epithelial cells and resident and newly recruited immune cells. In instances where viral infections persist or are not effectively eliminated, innate immune components prominently contribute to the resulting pathophysiological consequences. Even though both young children and older adults are susceptible to severe respiratory disease caused by various respiratory viruses, the underlying mechanisms may differ significantly. Children face the challenge of developing and maturing their immunity, while older adults contend with issues such as immune senescence and inflammaging. This review aims to compare the innate immune responses in respiratory viral infections across both age groups, identifying common central hubs that could serve as promising targets for innovative therapeutic and preventive strategies, despite the apparent differences in underlying mechanisms.

Positioning the preventive potential of microbiome treatments for cystic fibrosis in the context of current therapies

Antibiotics and cystic fibrosis transmembrane conductance regulator (CFTR) modulators play a pivotal role in cystic fibrosis (CF) treatment, but both have limitations. Antibiotics are linked to antibiotic resistance and disruption of the airway microbiome, while CFTR modulators are not widely accessible, and structural lung damage and pathogen overgrowth still occur. Complementary strategies that can beneficially modulate the airway microbiome in a preventive way are highly needed. This could be mediated via oral probiotics, which have shown some improvement of lung function and reduction of airway infections and exacerbations, as a cost-effective approach. However, recent data suggest that specific and locally administered probiotics in the respiratory tract might be a more targeted approach to prevent pathogen outgrowth in the lower airways. This review aims to summarize the current knowledge on the CF airway microbiome and possibilities of microbiome treatments to prevent bacterial and/or viral infections and position them in the context of current CF therapies.

Mathematical Modeling of the Lethal Synergism of Coinfecting Pathogens in Respiratory Viral Infections: A Review

Influenza A virus (IAV) infections represent a substantial global health challenge and are often accompanied by coinfections involving secondary viruses or bacteria, resulting in increased morbidity and mortality. The clinical impact of coinfections remains poorly understood, with conflicting findings regarding fatality. Isolating the impact of each pathogen and mechanisms of pathogen synergy during coinfections is challenging and further complicated by host and pathogen variability and experimental conditions. Factors such as cytokine dysregulation, immune cell function alterations, mucociliary dysfunction, and changes to the respiratory tract epithelium have been identified as contributors to increased lethality. The relative significance of these factors depends on variables such as pathogen types, infection timing, sequence, and inoculum size. Mathematical biological modeling can play a pivotal role in shedding light on the mechanisms of coinfections. Mathematical modeling enables the quantification of aspects of the intra-host immune response that are difficult to assess experimentally. In this narrative review, we highlight important mechanisms of IAV coinfection with bacterial and viral pathogens and survey mathematical models of coinfection and the insights gained from them. We discuss current challenges and limitations facing coinfection modeling, as well as current trends and future directions toward a complete understanding of coinfection using mathematical modeling and computer simulation.

A cooperativity between virus and bacteria during respiratory infections

Respiratory tract infections remain the leading cause of morbidity and mortality worldwide. The burden is further increased by polymicrobial infection or viral and bacterial co-infection, often exacerbating the existing condition. Way back in 1918, high morbidity due to secondary pneumonia caused by bacterial infection was known, and a similar phenomenon was observed during the recent COVID-19 pandemic in which secondary bacterial infection worsens the Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2) condition. It has been observed that viruses paved the way for subsequent bacterial infection; similarly, bacteria have also been found to aid in viral infection. Viruses elevate bacterial infection by impairing the host's immune response, disrupting epithelial barrier integrity, expression of surface receptors and adhesion proteins, direct binding of virus to bacteria, altering nutritional immunity, and effecting the bacterial biofilm. Similarly, the bacteria enhance viral infection by altering the host's immune response, up-regulation of adhesion proteins, and activation of viral proteins. During co-infection, respiratory bacterial and viral pathogens were found to adapt and co-exist in the airways of their survival and to benefit from each other, i.e., there is a cooperative existence between the two. This review comprehensively reviews the mechanisms involved in the synergistic/cooperativity relationship between viruses and bacteria and their interaction in clinically relevant respiratory infections.

Antiviral epithelial-macrophage crosstalk permits secondary bacterial infections

IMPORTANCE

Miscommunication of antiviral and antibacterial immune signals drives worsened morbidity and mortality during respiratory viral-bacterial coinfections. Extracellular vesicles (EVs) are a form of intercellular communication with broad implications during infection, and here we show that epithelium-derived EVs released during the antiviral response impair the antibacterial activity of macrophages, an innate immune cell crucial for bacterial control in the airway. Macrophages exposed to antiviral EVs display reduced clearance of Staphylococcus aureus as well as altered inflammatory signaling and anti-inflammatory metabolic reprogramming, thus revealing EVs as a source of dysregulated epithelium-macrophage crosstalk during coinfection. As effective epithelium-macrophage communication is critical in mounting an appropriate immune response, this novel observation of epithelium-macrophage crosstalk shaping macrophage metabolism and antimicrobial function provides exciting new insight and improves our understanding of immune dysfunction during respiratory coinfections.

Immune predisposition drives susceptibility to pneumococcal pneumonia after mild influenza A virus infection in mice

Introduction

A frequent sequela of influenza A virus (IAV) infection is secondary bacterial pneumonia. Therefore, it is clinically important to understand the genetic predisposition to IAV and bacterial coinfection.

Methods

BALB/c and C57BL/6 (B6) mice were infected with high or low-pathogenic IAV and Streptococcus pneumoniae (SPn). The contribution of cellular and molecular immune factors to the resistance/susceptibility of BALB/c and B6 mice were dissected in nonlethal and lethal IAV/SPn coinfection models.

Results

Low-virulent IAV X31 (H3N2) rendered B6 mice extremely susceptible to SPn superinfection, while BALB/c mice remained unaffected. X31 infection alone barely induces IFN-γresponse in two strains of mice; however, SPn superinfection significantly enhances IFN-γ production in the susceptible B6 mice. As a result, IFN-γ signaling inhibits neutrophil recruitment and bacterial clearance, leading to lethal X31/SPn coinfection in B6 mice. Conversely, the diminished IFN-γ and competent neutrophil responses enable BALB/c mice highly resistant to X31/SPn coinfection.

Discussion

The results establish that type 1 immune predisposition plays a key role in lethal susceptibility of B6 mice to pneumococcal pneumonia after mild IAV infection.

Complicated Acute Pericarditis and Peripheral Venous Catheter-Related Bloodstream Infection Caused by Methicillin-Resistant Staphylococcus aureus after Influenza B Virus Infection: A Case Report

Background

In this study, we report the case of a 14-month-old female patient transferred from another hospital to our hospital with a 9-day history of fever and worsening dyspnea. Case Report. The patient tested positive for influenza type B virus 7 days before being transferred to our hospital but was never treated. The physical examination performed at presentation revealed redness and swelling of the skin at the site of the peripheral venous catheter insertion performed at the previous hospital. Her electrocardiogram revealed ST segment elevations in leads II, III, aVF, and V2-V6. An emergent transthoracic echocardiogram revealed pericardial effusion. As ventricular dysfunction due to pericardial effusion was not present, pericardiocentesis was not performed. Furthermore, blood culture revealed methicillin-resistant Staphylococcus aureus (MRSA). Thus, a diagnosis of acute pericarditis complicated with sepsis and peripheral venous catheter-related bloodstream infection (PVC-BSI) due to MRSA was made. Frequent bedside ultrasound examinations were performed to evaluate the outcomes of the treatment. After administering vancomycin, aspirin, and colchicine, the patient's general condition stabilized.

Conclusions

In children, it is crucial to identify the causative organism and provide appropriate targeted therapy to prevent worsening of the condition and mortality due to acute pericarditis. Moreover, it is important to carefully monitor the clinical course for the progression of acute pericarditis to cardiac tamponade and evaluate the treatment outcomes.

The Activation of the RIG-I/MDA5 Signaling Pathway upon Influenza D Virus Infection Impairs the Pulmonary Proinflammatory Response Triggered by Mycoplasma bovis Superinfection

Concurrent infections with multiple pathogens are often described in cattle with respiratory illness. However, how the host-pathogen interactions influence the clinical outcome has been only partially explored in this species. Influenza D virus (IDV) was discovered in 2011. Since then, IDV has been detected worldwide in different hosts. A significant association between IDV and bacterial pathogens in sick cattle was shown in epidemiological studies, especially with Mycoplasma bovis. In an experimental challenge, IDV aggravated M. bovis-induced pneumonia. However, the mechanisms through which IDV drives an increased susceptibility to bacterial superinfections remain unknown. Here, we used the organotypic lung model precision-cut lung slices to study the interplay between IDV and M. bovis coinfection. Our results show that a primary IDV infection promotes M. bovis superinfection by increasing the bacterial replication and the ultrastructural damages in lung pneumocytes. In our model, IDV impaired the innate immune response triggered by M. bovis by decreasing the expression of several proinflammatory cytokines and chemokines that are important for immune cell recruitment and the bacterial clearance. Stimulations with agonists of cytosolic helicases and Toll-like receptors (TLRs) revealed that a primary activation of RIG-I/MDA5 desensitizes the TLR2 activation, similar to what was observed with IDV infection. The cross talk between these two pattern recognition receptors leads to a nonadditive response, which alters the TLR2-mediated cascade that controls the bacterial infection. These results highlight innate immune mechanisms that were not described for cattle so far and improve our understanding of the bovine host-microbe interactions and IDV pathogenesis. IMPORTANCE Since the spread of the respiratory influenza D virus (IDV) infection to the cattle population, the question about the impact of this virus on bovine respiratory disease (BRD) remains still unanswered. Animals affected by BRD are often coinfected with multiple pathogens, especially viruses and bacteria. In particular, viruses are suspected to enhance secondary bacterial superinfections. Here, we use an ex vivo model of lung tissue to study the effects of IDV infection on bacterial superinfections. Our results show that IDV increases the susceptibility to the respiratory pathogen Mycoplasma bovis. In particular, IDV seems to activate immune pathways that inhibit the innate immune response against the bacteria. This may allow M. bovis to increase its proliferation and to delay its clearance from lung tissue. These results suggest that IDV could have a negative impact on the respiratory pathology of cattle.

Synergistic Protection against Secondary Pneumococcal Infection by Human Monoclonal Antibodies Targeting Distinct Epitopes

Streptococcus pneumoniae persists as a leading cause of bacterial pneumonia despite the widespread use of polysaccharide-based vaccines. The limited serotype coverage of current vaccines has led to increased incidence of nonvaccine serotypes, as well as an increase in antibiotic resistance among these serotypes. Pneumococcal infection often follows a primary viral infection such as influenza virus, which hinders host defense and results in bacterial spread to the lungs. We previously isolated human monoclonal Abs (mAbs) against the conserved surface Ag pneumococcal histidine triad protein D (PhtD), and we demonstrated that mAbs to this Ag are protective against lethal pneumococcal challenge prophylactically and therapeutically. In this study, we elucidated the mechanism of protection of a protective anti-pneumococcal human mAb, PhtD3, which is mediated by the presence of complement and macrophages in a mouse model of pneumococcal infection. Treatment with mAb PhtD3 reduced blood and lung bacterial burden in mice, and mAb PhtD3 is able to bind to bacteria in the presence of the capsular polysaccharide, indicating exposure of surface PhtD on encapsulated bacteria. In a mouse model of secondary pneumococcal infection, protection mediated by mAb PhtD3 and another mAb targeting a different epitope, PhtD7, was reduced; however, robust protection was restored by combining mAb PhtD3 with mAb PhtD7, indicating a synergistic effect. Overall, these studies provide new insights into anti-pneumococcal mAb protection and demonstrate, to our knowledge, for the first time, that mAbs to pneumococcal surface proteins can protect against secondary pneumococcal infection in the mouse model.

The Influence of Influenza Virus Infections in Patients with Chronic Obstructive Pulmonary Disease

Introduction

Chronic obstructive pulmonary disease (COPD) is a common disease and is preventable and treatable. A previous study showed that influenza virus infections were also associated with the risk of acute exacerbation in patients with COPD, and other studies showed that the influenza virus might increase the risk of stroke. However, studies on the influence of influenza infection among COPD patients are limited. In this study, we review the role of influenza infection in contributing to mortality, pneumonia, respiratory failure, COPD acute exacerbation, and ischemic stroke among COPD patients.

Materials and Methods

We performed a population-based cohort study of COPD patients using data from Taiwan between January 1, 2011, and December 31, 2019. We excluded patients with lung cancer, lung transplantation and asthma. We also excluded patients who lacked COPD medication prescriptions and those treated with anti-influenza drugs without flu diagnosis records. Patients with missing or incomplete data were also excluded from the study cohort.

Results

After 1:1 matching by age, sex, COPD duration, diagnosed years and comorbidities, we enrolled 10,855 cases and controls for further analysis. The risks of pneumonia, respiratory failure, COPD acute exacerbation, and ischemic stroke were 1.770 (95% CI=1.638–1.860; P<0.0001), 1.097 (95% CI=1.008–1.194; P=0.0319), 1.338 (95% CI=1.248–1.435; P<0.0001), and 1.134 (95% CI=1.039–1.239, P=0.0051), respectively, in the influenza infection group compared with COPD patients without influenza infection.

Conclusion

Influenza infections are linked to an increased risk of ischemic stroke, pneumonia, respiratory failure, and COPD acute exacerbation among COPD patients. In conclusion, patients with COPD need to be closely monitored after having an influenza infection.

Frontline workers: Mediators of mucosal immunity in community acquired pneumonia and COVID-19

The current COVID-19 pandemic has highlighted a need to further understand lung mucosal immunity to reduce the burden of community acquired pneumonia, including that caused by the SARS-CoV-2 virus. Local mucosal immunity provides the first line of defence against respiratory pathogens, however very little is known about the mechanisms involved, with a majority of literature on respiratory infections based on the examination of peripheral blood. The mortality for severe community acquired pneumonia has been rising annually, even prior to the current pandemic, highlighting a significant need to increase knowledge, understanding and research in this field. In this review we profile key mediators of lung mucosal immunity, the dysfunction that occurs in the diseased lung microenvironment including the imbalance of inflammatory mediators and dysbiosis of the local microbiome. A greater understanding of lung tissue-based immunity may lead to improved diagnostic and prognostic procedures and novel treatment strategies aimed at reducing the disease burden of community acquired pneumonia, avoiding the systemic manifestations of infection and excess morbidity and mortality.

Rectal microbiota are coupled with altered cytokine production capacity following community-acquired pneumonia hospitalization

Human studies describing the immunomodulatory role of the intestinal microbiota in systemic infections are lacking. Here, we sought to relate microbiota profiles from 115 patients with community-acquired pneumonia (CAP), both on hospital admission and following discharge, to concurrent circulating monocyte and neutrophil function. Rectal microbiota composition did not explain variation in cytokine responses in acute CAP (median 0%, IQR 0.0%–1.9%), but did one month following hospitalization (median 4.1%, IQR 0.0%–6.6%, p = 0.0035). Gene expression analysis of monocytes showed that undisrupted microbiota profiles following hospitalization were associated with upregulated interferon, interleukin-10, and G-protein-coupled-receptor-ligand-binding pathways. While CAP is characterized by profoundly distorted gut microbiota, the effects of these disruptions on cytokine responses and transcriptional profiles during acute infection were absent or modest. However, rectal microbiota were related to altered cytokine responses one month following CAP hospitalization, which may provide insights into potential mechanisms contributing to the high risk of recurrent infections following hospitalization.

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Rectal microbiota are disrupted at hospitalization for CAP and one month thereafter

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No variation in cytokines is explained by gut microbiota in the acute phase of CAP

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Following recovery, gut microbiota are linked with variation in cytokine responses

Increased virus dissemination leads to enhanced lung injury but not inflammation during influenza-associated secondary bacterial infection

Secondary bacterial infections increase influenza-related morbidity and mortality, particularly if acquired after 5-7 d from the viral onset. Synergistic host responses and direct pathogen-pathogen interactions are thought to lead to a state of hyperinflammation, but the kinetics of the lung pathology have not yet been detailed, and identifying the contribution of different mechanisms to disease is difficult because these may change over time. To address this gap, we examined host-pathogen and lung pathology dynamics following a secondary bacterial infection initiated at different time points after influenza within a murine model. We then used a mathematical approach to quantify the increased virus dissemination in the lung, coinfection time-dependent bacterial kinetics, and virus-mediated and postbacterial depletion of alveolar macrophages. The data showed that viral loads increase regardless of coinfection timing, which our mathematical model predicted and histomorphometry data confirmed was due to a robust increase in the number of infected cells. Bacterial loads were dependent on the time of coinfection and corresponded to the level of IAV-induced alveolar macrophage depletion. Our mathematical model suggested that the additional depletion of these cells following the bacterial invasion was mediated primarily by the virus. Contrary to current belief, inflammation was not enhanced and did not correlate with neutrophilia. The enhanced disease severity was correlated to inflammation, but this was due to a nonlinearity in this correlation. This study highlights the importance of dissecting nonlinearities during complex infections and demonstrated the increased dissemination of virus within the lung during bacterial coinfection and simultaneous modulation of immune responses during influenza-associated bacterial pneumonia.

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

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

Neutrophil Recruitment in Pneumococcal Pneumonia

Streptococcus pneumoniae (Spn) is the primary agent of community-acquired pneumonia. Neutrophils are innate immune cells that are essential for bacterial clearance during pneumococcal pneumonia but can also do harm to host tissue. Neutrophil migration in pneumococcal pneumonia is therefore a major determinant of host disease outcomes. During Spn infection, detection of the bacterium leads to an increase in proinflammatory signals and subsequent expression of integrins and ligands on both the neutrophil as well as endothelial and epithelial cells. These integrins and ligands mediate the tethering and migration of the neutrophil from the bloodstream to the site of infection. A gradient of host-derived and bacterial-derived chemoattractants contribute to targeted movement of neutrophils. During pneumococcal pneumonia, neutrophils are rapidly recruited to the pulmonary space, but studies show that some of the canonical neutrophil migratory machinery is dispensable. Investigation of neutrophil migration is necessary for us to understand the dynamics of pneumococcal infection. Here, we summarize what is known about the pathways that lead to migration of the neutrophil from the capillaries to the lung during pneumococcal infection.

Time-Dependent Increase in Susceptibility and Severity of Secondary Bacterial Infections During SARS-CoV-2

Secondary bacterial infections can exacerbate SARS-CoV-2 infection, but their prevalence and impact remain poorly understood. Here, we established that a mild to moderate infection with the SARS-CoV-2 USA-WA1/2020 strain increased the risk of pneumococcal (type 2 strain D39) coinfection in a time-dependent, but sex-independent, manner in the transgenic K18-hACE2 mouse model of COVID-19. Bacterial coinfection increased lethality when the bacteria was initiated at 5 or 7 d post-virus infection (pvi) but not at 3 d pvi. Bacterial outgrowth was accompanied by neutrophilia in the groups coinfected at 7 d pvi and reductions in B cells, T cells, IL-6, IL-15, IL-18, and LIF were present in groups coinfected at 5 d pvi. However, viral burden, lung pathology, cytokines, chemokines, and immune cell activation were largely unchanged after bacterial coinfection. Examining surviving animals more than a week after infection resolution suggested that immune cell activation remained high and was exacerbated in the lungs of coinfected animals compared with SARS-CoV-2 infection alone. These data suggest that SARS-CoV-2 increases susceptibility and pathogenicity to bacterial coinfection, and further studies are needed to understand and combat disease associated with bacterial pneumonia in COVID-19 patients.

Neutrophil extracellular traps mediate severe lung injury induced by influenza A virus H1N1 in mice coinfected with Staphylococcus aureus

Influenza virus and bacterial infection contributed to massive morbidity and mortality. However, the underlying mechanisms were poorly understood. A coinfected model was generating by using sublethal doses of influenza A virus H1N1 A/FM/1/47(H1N1) and methicillin-resistant Staphylococcus aureus (MRSA). Further, the model was optimized to achieve the highest peak of mortality initiated by intranasal infection with 0.2LD₅₀ H1N1 and 0.16LD₅₀ MRSA at 3 days interval. Excessive neutrophil recruitment, accompanied by high levels of inflammatory cytokines and chemokines, and increased bacterial and viral load were observed in coinfected mice. Under the inflammatory environments triggered by H1N1 and MRSA, the excessive neutrophil recruitment led to the formation of neutrophil extracellular traps (NETs), associated with severe inflammation and vascular endothelial injury. Importantly, the severity of lung injury could be alleviated by treatment with DNase I or a selective neutrophil elastase inhibitor (NEi). Therefore, our data suggested that excessive neutrophil recruitment and NETs formation contributed to severe inflammation and acute lung injury in coinfected animals.

Time-Dependent Increase in Susceptibility and Severity of Secondary Bacterial Infection during SARS-CoV-2 Infection

Secondary bacterial infections can exacerbate SARS-CoV-2 infection, but their prevalence and impact remain poorly understood. Here, we established that a mild to moderate SARS-CoV-2 infection increased the risk of pneumococcal coinfection in a time-dependent, but sexindependent, manner in the transgenic K18-hACE mouse model of COVID-19. Bacterial coinfection was not established at 3 d post-virus, but increased lethality was observed when the bacteria was initiated at 5 or 7 d post-virus infection (pvi). Bacterial outgrowth was accompanied by neutrophilia in the groups coinfected at 7 d pvi and reductions in B cells, T cells, IL-6, IL-15, IL-18, and LIF were present in groups coinfected at 5 d pvi. However, viral burden, lung pathology, cytokines, chemokines, and immune cell activation were largely unchanged after bacterial coinfection. Examining surviving animals more than a week after infection resolution suggested that immune cell activation remained high and was exacerbated in the lungs of coinfected animals compared with SARS-CoV-2 infection alone. These data suggest that SARS-CoV-2 increases susceptibility and pathogenicity to bacterial coinfection, and further studies are needed to understand and combat disease associated with bacterial pneumonia in COVID-19 patients.

Pathogenesis of Respiratory Viral and Fungal Coinfections

Individuals suffering from severe viral respiratory tract infections have recently emerged as "at risk" groups for developing invasive fungal infections. Influenza virus is one of the most common causes of acute lower respiratory tract infections worldwide. Fungal infections complicating influenza pneumonia are associated with increased disease severity and mortality, with invasive pulmonary aspergillosis being the most common manifestation. Strikingly, similar observations have been made during the current coronavirus disease 2019 (COVID-19) pandemic. The copathogenesis of respiratory viral and fungal coinfections is complex and involves a dynamic interplay between the host immune defenses and the virulence of the microbes involved that often results in failure to return to homeostasis. In this review, we discuss the main mechanisms underlying susceptibility to invasive fungal disease following respiratory viral infections. A comprehensive understanding of these interactions will aid the development of therapeutic modalities against newly identified targets to prevent and treat these emerging coinfections.

Antibiotics as immunomodulators: a potential pharmacologic approach for ARDS treatment

First described in the mid-1960s, acute respiratory distress syndrome (ARDS) is a life-threatening form of respiratory failure with an overall mortality rate of approximately 40%. Despite significant advances in the understanding and treatment of ARDS, no substantive pharmacologic therapy has proven to be beneficial, and current management continues to be primarily supportive. Beyond their antibacterial activity, several antibiotics such as macrolides and tetracyclines exert pleiotropic immunomodulatory effects that might be able to rectify the dysregulated inflammatory response present in patients with ARDS. This review aims to provide an overview of preclinical and clinical studies that describe the immunomodulatory effects of antibiotics in ARDS. Moreover, the underlying mechanisms of their immunomodulatory properties will be discussed. Further studies are necessary to investigate their full therapeutic potential and to identify ARDS phenotypes which are most likely to benefit from their immunomodulatory effects.

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.

Viral and Bacterial Co-Infections in the Lungs: Dangerous Liaisons

Respiratory tract infections constitute a significant public health problem, with a therapeutic arsenal that remains relatively limited and that is threatened by the emergence of antiviral and/or antibiotic resistance. Viral–bacterial co-infections are very often associated with the severity of these respiratory infections and have been explored mainly in the context of bacterial superinfections following primary influenza infection. This review summarizes our current knowledge of the mechanisms underlying these co-infections between respiratory viruses (influenza viruses, RSV, and SARS-CoV-2) and bacteria, at both the physiological and immunological levels. This review also explores the importance of the microbiome and the pathological context in the evolution of these respiratory tract co-infections and presents the different in vitro and in vivo experimental models available. A better understanding of the complex functional interactions between viruses/bacteria and host cells will allow the development of new, specific, and more effective diagnostic and therapeutic approaches.

Interleukin-13: A pivotal target against influenza-induced exacerbation of chronic lung diseases

Non-communicable, chronic respiratory diseases (CRDs) affect millions of individuals worldwide. The course of these CRDs (asthma, chronic obstructive pulmonary disease, and cystic fibrosis) are often punctuated by microbial infections that may result in hospitalization and are associated with increased risk of morbidity and mortality, as well as reduced quality of life. Interleukin-13 (IL-13) is a key protein that regulates airway inflammation and mucus hypersecretion. There has been much interest in IL-13 from the last two decades. This cytokine is believed to play a decisive role in the exacerbation of inflammation during the course of viral infections, especially, in those with pre-existing CRDs. Here, we discuss the common viral infections in CRDs, as well as the potential role that IL-13 plays in the virus-induced disease pathogenesis of CRDs. We also discuss, in detail, the immune-modulation potential of IL-13 that could be translated to in-depth studies to develop IL-13-based therapeutic entities.

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.

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.

Dynamic Pneumococcal Genetic Adaptations Support Bacterial Growth and Inflammation during Coinfection with Influenza

Streptococcus pneumoniae (pneumococcus) is one of the primary bacterial pathogens that complicates influenza virus infections. These bacterial coinfections increase influenza-associated morbidity and mortality through a number of immunological and viral-mediated mechanisms, but the specific bacterial genes that contribute to postinfluenza pathogenicity are not known. Here, we used genome-wide transposon mutagenesis (Tn-Seq) to reveal bacterial genes that confer improved fitness in influenza virus-infected hosts. The majority of the 32 genes identified are involved in bacterial metabolism, including nucleotide biosynthesis, amino acid biosynthesis, protein translation, and membrane transport. We generated mutants with single-gene deletions (SGD) of five of the genes identified, SPD1414, SPD2047 (cbiO1), SPD0058 (purD), SPD1098, and SPD0822 (proB), to investigate their effects on in vivo fitness, disease severity, and host immune responses. The growth of the SGD mutants was slightly attenuated in vitro and in vivo, but each still grew to high titers in the lungs of mock- and influenza virus-infected hosts. Despite high bacterial loads, mortality was significantly reduced or delayed with all SGD mutants. Time-dependent reductions in pulmonary neutrophils, inflammatory macrophages, and select proinflammatory cytokines and chemokines were also observed. Immunohistochemical staining further revealed altered neutrophil distribution with reduced degeneration in the lungs of influenza virus-SGD mutant-coinfected animals. These studies demonstrate a critical role for specific bacterial genes and for bacterial metabolism in driving virulence and modulating immune function during influenza-associated bacterial pneumonia.

Altered Signal Transduction in the Immune Response to Influenza Virus and S. pneumoniae or S. aureus Co-Infections

Influenza virus is a well-known respiratory pathogen, which still leads to many severe pulmonary infections in the human population every year. Morbidity and mortality rates are further increased if virus infection coincides with co-infections or superinfections caused by bacteria such as Streptococcus pneumoniae (S. pneumoniae) and Staphylococcus aureus (S. aureus). This enhanced pathogenicity is due to complex interactions between the different pathogens and the host and its immune system and is mainly governed by altered intracellular signaling processes. In this review, we summarize the recent findings regarding the innate and adaptive immune responses during co-infection with influenza virus and S. pneumoniae or S. aureus, describing the signaling pathways involved and how these interactions influence disease outcomes.

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

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

Virus-Induced Changes of the Respiratory Tract Environment Promote Secondary Infections With Streptococcus pneumoniae

Secondary bacterial infections enhance the disease burden of influenza infections substantially. Streptococcus pneumoniae (the pneumococcus) plays a major role in the synergism between bacterial and viral pathogens, which is based on complex interactions between the pathogen and the host immune response. Here, we discuss mechanisms that drive the pathogenesis of a secondary pneumococcal infection after an influenza infection with a focus on how pneumococci senses and adapts to the influenza-modified environment. We briefly summarize what is known regarding secondary bacterial infection in relation to COVID-19 and highlight the need to improve our current strategies to prevent and treat viral bacterial coinfections.

The Effects of Social Distancing Policies on Non-SARS-CoV-2 Respiratory Pathogens

Background

The initial focus of the US public health response to coronavirus disease 2019 (COVID-19) was the implementation of numerous social distancing policies. While COVID-19 was the impetus for imposing these policies, it is not the only respiratory disease affected by their implementation. This study aimed to assess the impact of social distancing policies on non-severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) respiratory pathogens typically circulating across multiple US states.

Methods

Linear mixed-effect models were implemented to explore the effects of 5 social distancing policies on non-SARS-CoV-2 respiratory pathogens across 9 states from January 1 through May 1, 2020. The observed 2020 pathogen detection rates were compared week by week with historical rates to determine when the detection rates were different.

Results

Model results indicate that several social distancing policies were associated with a reduction in total detection rate, by nearly 15%. Policies were associated with decreases in pathogen circulation of human rhinovirus/enterovirus and human metapneumovirus, as well as influenza A, which typically decrease after winter. Parainfluenza viruses failed to circulate at historical levels during the spring. The total detection rate in April 2020 was 35% less than the historical average. Many of the pathogens driving this difference fell below the historical detection rate ranges within 2 weeks of initial policy implementation.

Conclusions

This analysis investigated the effect of multiple social distancing policies implemented to reduce transmission of SARS-CoV-2 on non-SARS-CoV-2 respiratory pathogens. These findings suggest that social distancing policies may be used as an impactful public health tool to reduce communicable respiratory illness.

Streptococcus pneumoniae serotype 22F infection in respiratory syncytial virus infected neonatal lambs enhances morbidity

Respiratory syncytial virus (RSV) is the primary cause of viral bronchiolitis resulting in hospitalization and a frequent cause of secondary respiratory bacterial infection, especially by Streptococcus pneumoniae (Spn) in infants. While murine studies have demonstrated enhanced morbidity during a viral/bacterial co-infection, human meta-studies have conflicting results. Moreover, little knowledge about the pathogenesis of emerging Spn serotype 22F, especially the co-pathologies between RSV and Spn, is known. Here, colostrum-deprived neonate lambs were divided into four groups. Two of the groups were nebulized with RSV M37, and the other two groups were mock nebulized. At day three post-RSV infection, one RSV group (RSV/Spn) and one mock-nebulized group (Spn only) were inoculated with Spn intratracheally. At day six post-RSV infection, bacterial/viral loads were assessed along with histopathology and correlated with clinical symptoms. Lambs dually infected with RSV/Spn trended with higher RSV titers, but lower Spn. Additionally, lung lesions were observed to be more frequent in the RSV/Spn group characterized by increased interalveolar wall thickness accompanied by neutrophil and lymphocyte infiltration and higher myeloperoxidase. Despite lower Spn in lungs, co-infected lambs had more significant morbidity and histopathology, which correlated with a different cytokine response. Thus, enhanced disease severity during dual infection may be due to lesion development and altered immune responses rather than bacterial counts.

Sequential targeting of interferon pathways for increased host resistance to bacterial superinfection during influenza

Bacterial co-infections represent a major clinical complication of influenza. Host-derived interferon (IFN) increases susceptibility to bacterial infections following influenza, but the relative roles of type-I versus type-II IFN remain poorly understood. We have used novel mouse models of co-infection in which colonizing pneumococci were inoculated into the upper respiratory tract; subsequent sublethal influenza virus infection caused the bacteria to enter the lungs and mediate lethal disease. Compared to wild-type mice or mice deficient in only one pathway, mice lacking both IFN pathways demonstrated the least amount of lung tissue damage and mortality following pneumococcal-influenza virus superinfection. Therapeutic neutralization of both type-I and type-II IFN pathways similarly provided optimal protection to co-infected wild-type mice. The most effective treatment regimen was staggered neutralization of the type-I IFN pathway early during co-infection combined with later neutralization of type-II IFN, which was consistent with the expression and reported activities of these IFNs during superinfection. These results are the first to directly compare the activities of type-I and type-II IFN during superinfection and provide new insights into potential host-directed targets for treatment of secondary bacterial infections during influenza.

Bacterial co-infections represent a common and challenging clinical complication of influenza. Type-I and type-II interferon (IFN) pathways enhance susceptibility to influenza-pneumococcal co-infection, leading to increased lung pathology and mortality. However, the comparative importance of type-I versus type-II IFN remains unclear. We have used two novel mouse models of co-infection in which pneumococci were inoculated into the upper respiratory tract followed two days later by influenza virus infection. Virus co-infection caused IFN-dependent inflammation that facilitated spreading of the colonizing bacteria into the lungs, followed by tissue damage and death. In this pneumococcal-influenza virus superinfection model, mice lacking both type-I and type-II IFN pathways demonstrated minimal lung pathology and increased survival compared to wild-type mice and mice deficient in only one pathway. Therapeutic neutralization of both type-I and type-II IFN pathways similarly provided optimal protection to superinfected wild-type mice. The most effective treatment regimen involved neutralization of the type-I IFN pathway early during co-infection combined with later neutralization of the type-II IFN pathway. These results provide new insights into potential host-directed therapy for management of bacterial-viral superinfections.

T cell responses in respiratory viral infections and chronic obstructive pulmonary disease

ABSTRACT

Respiratory viruses are major human pathogens that cause approximately 200 million pneumonia cases annually and induce various comorbidities with chronic obstructive pulmonary disease (COPD), resulting in significant health concerns and economic burdens. Clinical manifestations in respiratory viral infections and inflammations vary from asymptomatic, mild, to severe, depending on host immune cell responses to pathogens and interactions with airway epithelia. We critically review the activation, effector, and regulation of T cells in respiratory virus infections and chronic inflammations associated with COPD. Crosstalk among T cells, innate immune cells, and airway epithelial cells is discussed as essential parts of pathogenesis and protection in viral infections and COPD. We emphasize the specificity of peptide antigens and the functional heterogeneity of conventional CD4+ and CD8+ T cells to shed some light on potential cellular and molecular candidates for the future development of therapeutics and intervention against respiratory viral infections and inflammations.

Respiratory RNA Viruses: How to Be Prepared for an Encounter with New Pandemic Virus Strains

The characteristics of the biology of influenza viruses and coronavirus that determine the implementation of the infectious process are presented. With provision for pathogenesis of infection possible effects of serine proteinase inhibitors, heparin, and inhibitors of heparan sulfate receptors in the prevention of cell contamination by viruses are examined. It has been determined that chelators of metals of variable valency and antioxidants should be used for the reduction of replicative activity of viruses and anti-inflammatory therapy. The possibility of a pH-dependent impairment of glycosylation of cellular and viral proteins was traced for chloroquine and its derivatives. The use of low-toxicity drugs as part of adjunct therapy increases the effectiveness of synthetic antiviral drugs and interferons and ensures the safety of baseline therapy.

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

Background

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

Methods

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

Results

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

Conclusions

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

Flu Virus Attenuates Memory Clearance of Pneumococcus via IFN-γ-Dependent Th17 and Independent Antibody Mechanisms

Bacterial coinfection is a major cause of influenza-associated mortality. Most people have experienced infections with bacterial pathogens commonly associated with influenza A virus (IAV) coinfection before IAV exposure; however, bacterial clearance through the immunological memory response (IMR) in coinfected patients is inefficient, suggesting that the IMR to bacteria is impaired during IAV infection. Adoptive transfer of CD4⁺ T cells from mice that had experienced bacterial infection into IAV-infected mice revealed that memory protection against bacteria was weakened in the latter. Additionally, memory Th17 cell responses were impaired due to an IFN-γ-dependent reduction in Th17 cell proliferation and delayed migration of CD4⁺ T cells into the lungs. A bacterium-specific antibody-mediated memory response was also substantially reduced in coinfected mice, independently of IFN-γ. These findings provide additional perspectives on the pathogenesis of coinfection and suggest additional strategies for the treatment of defective antibacterial immunity and the design of bacterial vaccines against coinfection.

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Memory protection against bacteria was impaired in coinfection

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Memory Th17 response to bacteria was reduced by IAV-induced IFN-γ

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The Th17 reduction was caused by impeded Th17 proliferation and migration

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Bacteria-specific antibody was reduced in coinfection independent of IFN-γ

The past, present and future of RNA respiratory viruses: influenza and coronaviruses

Influenza virus and coronaviruses continue to cause pandemics across the globe. We now have a greater understanding of their functions. Unfortunately, the number of drugs in our armory to defend us against them is inadequate. This may require us to think about what mechanisms to address. Here, we review the biological properties of these viruses, their genetic evolution and antiviral therapies that can be used or have been attempted. We will describe several classes of drugs such as serine protease inhibitors, heparin, heparan sulfate receptor inhibitors, chelating agents, immunomodulators and many others. We also briefly describe some of the drug repurposing efforts that have taken place in an effort to rapidly identify molecules to treat patients with COVID-19. While we put a heavy emphasis on the past and present efforts, we also provide some thoughts about what we need to do to prepare for respiratory viral threats in the future.

Can phytotherapy with polyphenols serve as a powerful approach for the prevention and therapy tool of novel coronavirus disease 2019 (COVID-19)?

Much more serious than the previous severe acute respiratory syndrome (SARS) coronavirus (CoV) outbreaks, the novel SARS-CoV-2 infection has spread speedily, affecting 213 countries and causing ∼17,300,000 cases and ∼672,000 (∼+1,500/day) deaths globally (as of July 31, 2020). The potentially fatal coronavirus disease (COVID-19), caused by air droplets and airborne as the main transmission modes, clearly induces a spectrum of respiratory clinical manifestations, but it also affects the immune, gastrointestinal, hematological, nervous, and renal systems. The dramatic scale of disorders and complications arises from the inadequacy of current treatments and absence of a vaccine and specific anti-COVID-19 drugs to suppress viral replication, inflammation, and additional pathogenic conditions. This highlights the importance of understanding the SARS-CoV-2 mechanisms of actions and the urgent need of prospecting for new or alternative treatment options. The main objective of the present review is to discuss the challenging issue relative to the clinical utility of plants-derived polyphenols in fighting viral infections. Not only is the strong capacity of polyphenols highlighted in magnifying health benefits, but the underlying mechanisms are also stressed. Finally, emphasis is placed on the potential ability of polyphenols to combat SARS-CoV-2 infection via the regulation of its molecular targets of human cellular binding and replication, as well as through the resulting host inflammation, oxidative stress, and signaling pathways.

From virus to inflammation, how influenza promotes lung damage

Despite seasonal vaccines, influenza-related hospitalization and death rates have remained unchanged over the past 5 years. Influenza pathogenesis has 2 crucial clinical components; first, influenza causes acute lung injury that may require hospitalization. Second, acute injury promotes secondary bacterial pneumonia, a leading cause of hospitalization and disease burden in the United States and globally. Therefore, developing an effective therapeutic regimen against influenza requires a comprehensive understanding of the damage-associated immune-mechanisms to identify therapeutic targets for interventions to mitigate inflammation/tissue-damage, improve antiviral immunity, and prevent influenza-associated secondary bacterial diseases. In this review, the pathogenic immune mechanisms implicated in acute lung injury and the possibility of using lung inflammation and barrier crosstalk for developing therapeutics against influenza are highlighted.

Age and Location in Severity of COVID-19 Pathology: Do Lactoferrin and Pneumococcal Vaccination Explain Low Infant Mortality and Regional Differences?

Two conundrums puzzle COVID-19 investigators: 1) morbidity and mortality is rare among infants and young children and 2) rates of morbidity and mortality exhibit large variances across nations, locales, and even within cities. It is found that the higher the rate of pneumococcal vaccination in a nation (or city) the lower the COVID-19 morbidity and mortality. Vaccination rates with Bacillus Calmette-Guerin, poliovirus, and other vaccines do not correlate with COVID-19 risks, nor do COVID-19 case or death rates correlate with number of people in the population with diabetes, obesity, or adults over 65. Infant protection may be due to maternal antibodies and antiviral proteins in milk such as lactoferrin that are known to protect against coronavirus infections. Subsequent protection might then be conferred (and correlate with) rates of Haemophilus influenzae type B (Hib) (universal in infants) and pneumococcal vaccination, the latter varying widely by geography among infants, at-risk adults, and the elderly. Also see the video abstract here https://youtu.be/GODBYRbPL00.

The pneumococcal two-component system SirRH is linked to enhanced intracellular survival of Streptococcus pneumoniae in influenza-infected pulmonary cells

The virus-bacterial synergism implicated in secondary bacterial infections caused by Streptococcus pneumoniae following infection with epidemic or pandemic influenza A virus (IAV) is well documented. However, the molecular mechanisms behind such synergism remain largely ill-defined. In pneumocytes infected with influenza A virus, subsequent infection with S. pneumoniae leads to enhanced pneumococcal intracellular survival. The pneumococcal two-component system SirRH appears essential for such enhanced survival. Through comparative transcriptomic analysis between the ΔsirR and wt strains, a list of 179 differentially expressed genes was defined. Among those, the clpL protein chaperone gene and the psaB Mn+2 transporter gene, which are involved in the stress response, are important in enhancing S. pneumoniae survival in influenza-infected cells. The ΔsirR, ΔclpL and ΔpsaB deletion mutants display increased susceptibility to acidic and oxidative stress and no enhancement of intracellular survival in IAV-infected pneumocyte cells. These results suggest that the SirRH two-component system senses IAV-induced stress conditions and controls adaptive responses that allow survival of S. pneumoniae in IAV-infected pneumocytes.

What came first, the virus or the egg: Innate immunity during viral coinfections

Infections with any pathogen can be severe and present with numerous complications caused by the pathogen or the host immune response to the invading microbe. However, coinfections, also called polymicrobial infections or secondary infections, can further exacerbate disease. Coinfections are more common than is often appreciated. In this review, we focus specifically on coinfections between viruses and other viruses, bacteria, parasites, or fungi. Importantly, innate immune signaling and innate immune cells that facilitate clearance of the initial viral infection can affect host susceptibility to coinfections. Understanding these immune imbalances may facilitate better diagnosis, prevention, and treatment of such coinfections.

Impact of Influenza on Pneumococcal Vaccine Effectiveness during Streptococcus pneumoniae Infection in Aged Murine Lung

Changes in innate and adaptive immune responses caused by viral imprinting can have a significant direct or indirect influence on secondary infections and vaccine responses. The purpose of our current study was to investigate the role of immune imprinting by influenza on pneumococcal vaccine effectiveness during Streptococcus pneumoniae infection in the aged murine lung. Aged adult (18 months) mice were vaccinated with the pneumococcal polyvalent vaccine Pneumovax (5 mg/mouse). Fourteen days post vaccination, mice were instilled with PBS or influenza A/PR8/34 virus (3.5 × 10² PFU). Control and influenza-infected mice were instilled with PBS or S. pneumoniae (1 × 10³ CFU, ATCC 6303) on day 7 of infection and antibacterial immune responses were assessed in the lung. Our results illustrate that, in response to a primary influenza infection, there was diminished bacterial clearance and heightened production of pro-inflammatory cytokines, such as IL6 and IL1β. Vaccination with Pneumovax decreased pro-inflammatory cytokine production by modulating NFҡB expression; however, these responses were significantly diminished after influenza infection. Taken together, the data in our current study illustrate that immune imprinting by influenza diminishes pneumococcal vaccine efficacy and, thereby, may contribute to increased susceptibility of older persons to a secondary infection with S. pneumoniae.

Influenza Suppresses Neutrophil Recruitment to the Lung and Exacerbates Secondary Invasive Pulmonary Aspergillosis

Aspergillus fumigatus is an environmental fungus that can cause invasive pulmonary aspergillosis when spores are inhaled into the respiratory tract and invade airway or lung tissue. Influenza is a common respiratory virus that can cause severe respiratory disease, and postinfluenza invasive pulmonary aspergillosis, which is becoming a well-recognized clinical problem, typically occurs in critically ill patients. Mice challenged with influenza A PR/8/34 H1N1 and subsequently challenged with A. fumigatus had increased fungal burden, viral burden, inflammation, and mortality compared with single infected mice. Neutrophil recruitment in the lung of superinfected mice was decreased; however, mice were not neutropenic, and there was no difference in absolute blood neutrophils between groups. Additionally, CXCL1 and CXCL2 were decreased in lungs of superinfected mice compared with controls. IFN levels were increased in mice that received influenza, and deletion of STAT1 resulted in decreased fungal burden, increased airway and lung neutrophils, and increased CXCL1 compared with wild-type mice, whereas deletion of STAT2 did not change fungal burden or airway neutrophilia compared with wild-type mice. These data demonstrate a mechanism by which influenza A-induced STAT1 signaling inhibits neutrophil recruitment and increases susceptibility to postinfluenza invasive pulmonary aspergillosis.