TLR7 contributes to the rapid progression but not to the overall fatal outcome of secondary pneumococcal disease following influenza A virus infection

Propofol directly binds to and inhibits TLR7

Sedatives/anesthetics are important medical tools to facilitate medical care and increase patients' comfort. Increasingly, there is recognition that sedatives/anesthetics can modulate immune functions. Toll-like receptors (TLRs) are major pattern recognition receptors involved in the recognition of microbial components. TLR7 recognizes single-strand RNA virus such as influenza and SARS-CoV2 viruses and initiates interferon (IFN) responses. IFN production triggered by TLR7 stimulation is a critical anti-viral response. For example, patients with TLR7 variants including loss-of- function variants were associated with severe COVID-19. Taken together, it is important to determine if sedatives/anesthetics mitigate TLR7 function. We have previously showed that TLR7-mediated activation was not affected by volatile anesthetics. However, we found that propofol attenuated TLR7 activation among intravenous sedatives in the reporter assay. TLR7 agonist R837 stimulation increased TNF-α, IL-1β, IL-6, IL-10, and IFN-β mRNA levels in bone marrow-derived dendritic cells, while these levels were attenuated by propofol. Our murine lung slice experiments showed that propofol attenuated IFN production. R837 increased IFN-β expression in the lungs, and propofol attenuated IFN-β expression in an in vivo model of R837 intranasal instillation. We also found that propofol directly bound to and hindered its association of TLR7 with MyD88. Our analysis using fropofol, propofol derivative showed that the hydroxyl group in propofol was important for propofol-TLR7 interaction.

Exogenous and Endogenous Triggers Differentially Stimulate Pigr Expression and Antibacterial Secretory Immunity in the Murine Respiratory Tract

PURPOSE

Transport of secretory immunoglobulin A (SIgA) through the airway epithelial cell barrier into the mucosal lumen by the polymeric immunoglobulin receptor (pIgR) is an important mechanism of respiratory mucosal host defense. Identification of immunomodulating substances that regulate secretory immunity might have therapeutic implications with regard to an improved immune exclusion. Thus, we sought to analyze secretory immunity under homeostatic and immunomodulating conditions in different compartments of the murine upper and lower respiratory tract (URT&LRT).

METHODS

Pigr gene expression in lung, trachea, and nasal-associated lymphoid tissue (NALT) of germ-free mice, specific pathogen-free mice, mice with an undefined microbiome, as well as LPS- and IFN-γ-treated mice was determined by quantitative real-time PCR. IgA levels in bronchoalveolar lavage (BAL), nasal lavage (NAL), and serum were determined by ELISA. LPS- and IFN-γ-treated mice were colonized with Streptococcus pneumoniae and bacterial CFUs were determined in URT and LRT.

RESULTS

Respiratory Pigr expression and IgA levels were dependent on the degree of exposure to environmental microbial stimuli. While immunostimulation with LPS and IFN-γ differentially impacts respiratory Pigr expression and IgA in URT vs. LRT, only prophylactic IFN-γ treatment reduces nasal colonization with S. pneumoniae.

CONCLUSION

Airway-associated secretory immunity can be partly modulated by exposure to microbial ligands and proinflammatory stimuli. Prophylactic IFN-γ-treatment modestly improves antibacterial immunity in the URT, but this does not appear to be mediated by SIgA or pIgR.

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.

TLR7 Expression Aggravates Invasive Pulmonary Aspergillosis by Suppressing Anti-Aspergillus Immunity of Macrophages

Toll-like receptors (TLRs) play a critical role in early immune recognition of Aspergillus, which can regulate host defense during invasive pulmonary Aspergillosis (IPA). However, the role of TLR7 in the pathogenesis of IPA remains unknown. In this study, an in vivo model of IPA was established to investigate the contribution of TLR7 to host anti-Aspergillus immunity upon invasive pulmonary Aspergillus fumigatus infection. The effects of TLR7 on phagocytosis and killing capacities of A. fumigatus by macrophages and neutrophils were investigated in vitro We found that TLR7 knockout mice exhibited lower lung inflammatory response and tissue injury, higher fungal clearance, and greater survival in an in vivo model of IPA compared with wild-type mice. TLR7 activation by R837 ligand led to wild-type mice being more susceptible to invasive pulmonary Aspergillus fumigatus infection. Macrophages, but not neutrophils, were required for the protection against IPA observed in TLR7 knockout mice. Mechanistically, TLR7 impaired phagocytosis and killing of A. fumigatus by macrophages but not neutrophils. Together, these data identify TLR7 as an important negative regulator of anti-Aspergillus innate immunity in IPA, and we propose that targeting TLR7 will be beneficial in the treatment of IPA.

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.

Resolved Influenza A Virus Infection Has Extended Effects on Lung Homeostasis and Attenuates Allergic Airway Inflammation in a Mouse Model

Allergic airway inflammation (AAI) involves T helper cell type 2 (Th2) and pro-inflammatory responses to aeroallergens and many predisposing factors remain elusive. Influenza A virus (IAV) is a major human pathogen that causes acute respiratory infections and induces specific immune responses essential for viral clearance and resolution of the infection. Beyond acute infection, IAV has been shown to persistently affect lung homeostasis and respiratory immunity. Here we asked how resolved IAV infection affects subsequently induced AAI. Mice infected with a sublethal dose of IAV were sensitized and challenged in an ovalbumin mediated mouse model for AAI after resolution of the acute viral infection. Histological changes, respiratory leukocytes, cytokines and airway hyperreactivity were analyzed in resolved IAV infection alone and in AAI with and without previous IAV infection. More than five weeks after infection, we detected persistent pneumonia with increased activated CD4⁺ and CD8⁺ lymphocytes as well as dendritic cells and MHCII expressing macrophages in the lung. Resolved IAV infection significantly affected subsequently induced AAI on different levels including morphological changes, respiratory leukocytes and lymphocytes as well as the pro-inflammatory cytokine responses, which was clearly diminished. We conclude that IAV has exceptional persisting effects on respiratory immunity with substantial consequences for subsequently induced AAI.

Combined immunization with attenuated live influenza vaccine and chimeric pneumococcal recombinant protein improves the outcome of virus-bacterial infection in mice

Influenza and its bacterial complications are a leading cause of morbidity and mortality worldwide. The effect of combined immunization with live influenza vaccine and recombinant chimeric pneumococcal protein in dual infection caused by influenza H1N1 and S. pneumoniae (serotype 3) has been studied. The combined vaccine consisted of the strain A/California/2009/38 (H1N1) pdm and chimeric recombinant protein PSPF composed of immunodominant fragments of the surface virulence factors of S. pneumoniae—PsaA, PspA, and Shr1875—associated with modified salmonella flagellin. Vaccinated mice were infected with the influenza virus 24 hours before or 24 hours after the onset of pneumococcal infection. The protective effect of combined vaccination was shown on both models of viral-bacterial infection.

In vivo Neutralization of Pro-inflammatory Cytokines During Secondary Streptococcus pneumoniae Infection Post Influenza A Virus Infection

An overt pro-inflammatory immune response is a key factor contributing to lethal pneumococcal infection in an influenza pre-infected host and represents a potential target for therapeutic intervention. However, there is a paucity of knowledge about the level of contribution of individual cytokines. Based on the predictions of our previous mathematical modeling approach, the potential benefit of IFN-γ- and/or IL-6-specific antibody-mediated cytokine neutralization was explored in C57BL/6 mice infected with the influenza A/PR/8/34 strain, which were subsequently infected with the Streptococcus pneumoniae strain TIGR4 on day 7 post influenza. While single IL-6 neutralization had no effect on respiratory bacterial clearance, single IFN-γ neutralization enhanced local bacterial clearance in the lungs. Concomitant neutralization of IFN-γ and IL-6 significantly reduced the degree of pneumonia as well as bacteremia compared to the control group, indicating a positive effect for the host during secondary bacterial infection. The results of our model-driven experimental study reveal that the predicted therapeutic value of IFN-γ and IL-6 neutralization in secondary pneumococcal infection following influenza infection is tightly dependent on the experimental protocol while at the same time paving the way toward the development of effective immune therapies.

RIG-I Signaling via MAVS Is Dispensable for Survival in Lethal Influenza Infection In Vivo

Retinoic acid-inducible gene I (RIG-I) is an important regulator of virus-induced antiviral interferons (IFNs) and proinflammatory cytokines. It requires interaction with an adaptor molecule, mitochondrial antiviral-signaling protein (MAVS), to activate downstream signaling pathways. To elucidate the mechanism(s) by which RIG-I-dependent recognition of IAV infection in vivo triggers innate immune responses, we infected mutant mice lacking RIG-I or MAVS with influenza A virus (IAV) and measured their innate immune responses. As has previously been demonstrated with isolated deletion of the virus recognition receptors TLR3, TLR7, and NOD2, RIG-I or MAVS knockout (KO) did not result in higher mortality and did not reduce IAV-induced cytokine responses in mice. Infected RIG-I KO animals displayed similar lung inflammation profiles as did WT mice, in terms of the protein concentration, total cell count, and inflammatory cell composition in the bronchoalveolar lavage fluid. RNA-Seq results demonstrated that all types of mice exhibited equivalent antiviral and inflammatory gene responses following IAV infection. Together, the results indicated that although RIG-I is important in innate cytokine responses in vitro, individual deletion of the genes encoding RIG-I or MAVS did not change survival or innate responses in vivo after IAV infection in mice.

Respiratory Influenza A Virus Infection Triggers Local and Systemic Natural Killer Cell Activation via Toll-Like Receptor 7

The innate immune system senses influenza A virus (IAV) through different pathogen-recognition receptors including Toll-like receptor 7 (TLR7). Downstream of viral recognition natural killer (NK) cells are activated as part of the anti-IAV immune response. Despite the known decisive role of TLR7 for NK cell activation by therapeutic immunostimulatory RNAs, the contribution of TLR7 to the NK cell response following IAV infection has not been addressed. We have analyzed lung cytokine responses as well as the activation, interferon (IFN)-γ production, and cytotoxicity of lung and splenic NK cells following sublethal respiratory IAV infection in wild-type and TLR7ko mice. Early airway IFN-γ levels as well as the induction of lung NK cell CD69 expression and IFN-γ production in response to IAV infection were significantly attenuated in TLR7-deficient hosts. Strikingly, respiratory IAV infection also primed splenic NK cells for IFN-γ production, degranulation, and target cell lysis, all of which were fully dependent on TLR7. At the same time, lung type I IFN levels were significantly reduced in TLR7ko mice early following IAV infection, displaying a potential upstream mechanism of the attenuated NK cell activation observed. Taken together, our data clearly demonstrate a specific role for TLR7 signaling in local and systemic NK cell activation following respiratory IAV infection despite the presence of redundant innate IAV-recognition pathways.

Chronic lung inflammation primes humoral immunity and augments antipneumococcal resistance

Airway epithelial cells (AECs) display remarkable plasticity in response to infectious stimuli and their functional adaptations are critical for antimicrobial immunity. However, the roles of AECs and humoral mediators to host defense in non-communicable lung inflammation remain elusive. We dissected pulmonary defense against Streptococcus pneumoniae in hosts with pre-existing inflammatory conditions (SPC-HAxTCR-HA mice). Lung tissue transcriptomics and bronchoalveolar lavage fluid (BALF) proteomics revealed an induction of humoral defense mechanisms in inflamed lungs. Accordingly, besides antibacterial proteins and complement components being overrepresented in inflamed lungs, elevated polymeric immunoglobulin receptor (pIgR)-expression in AECs correlated with increased secretory immunoglobulin (SIg) transport. Consequently, opsonization assays revealed augmented pneumococcal coverage by SIgs present in the BALF of SPC-HAxTCR-HA mice, which was associated with enhanced antipneumococcal resistance. These findings emphasize the immunologic potential of AECs as well as their central role in providing antibacterial protection and put forward pIgR as potential target for therapeutic manipulation in infection-prone individuals.

Alveolar Type II Epithelial Cells Contribute to the Anti-Influenza A Virus Response in the Lung by Integrating Pathogen- and Microenvironment-Derived Signals

Influenza A virus (IAV) periodically causes substantial morbidity and mortality in the human population. In the lower lung, the primary targets for IAV replication are type II alveolar epithelial cells (AECII), which are increasingly recognized for their immunological potential. So far, little is known about their reaction to IAV and their contribution to respiratory antiviral immunity in vivo. Therefore, we characterized the AECII response during early IAV infection by analyzing transcriptional regulation in cells sorted from the lungs of infected mice. We detected rapid and extensive regulation of gene expression in AECII following in vivo IAV infection. The comparison to transcriptional regulation in lung tissue revealed a strong contribution of AECII to the respiratory response. IAV infection triggered the expression of a plethora of antiviral factors and immune mediators in AECII with a high prevalence for interferon-stimulated genes. Functional pathway analyses revealed high activity in pathogen recognition, immune cell recruitment, and antigen presentation. Ultimately, our analyses of transcriptional regulation in AECII and lung tissue as well as interferon I/III levels and cell recruitment indicated AECII to integrate signals provided by direct pathogen recognition and surrounding cells. Ex vivo analysis of AECII proved a powerful tool to increase our understanding of their role in respiratory immune responses, and our results clearly show that AECII need to be considered a part of the surveillance and effector system of the lower respiratory tract.

In order to confront the health hazard posed by IAV, we need to complete our understanding of its pathogenesis. AECII are primary targets for IAV replication in the lung, and while we are beginning to understand their importance for respiratory immunity, the in vivo AECII response during IAV infection has not been analyzed. In contrast to studies addressing the response of AECII infected with IAV ex vivo, we have performed detailed gene transcriptional profiling of AECII isolated from the lungs of infected mice. Thereby, we have identified an exceptionally rapid and versatile response to IAV infection that is shaped by pathogen-derived as well as microenvironment-derived signals and aims at the induction of antiviral measures and the recruitment and activation of immune cells. In conclusion, our study presents AECII as active players in antiviral defense in vivo that need to be considered part of the sentinel and effector immune system of the lung.

Modeling Influenza Virus Infection: A Roadmap for Influenza Research

Influenza A virus (IAV) infection represents a global threat causing seasonal outbreaks and pandemics. Additionally, secondary bacterial infections, caused mainly by Streptococcus pneumoniae, are one of the main complications and responsible for the enhanced morbidity and mortality associated with IAV infections. In spite of the significant advances in our knowledge of IAV infections, holistic comprehension of the interplay between IAV and the host immune response (IR) remains largely fragmented. During the last decade, mathematical modeling has been instrumental to explain and quantify IAV dynamics. In this paper, we review not only the state of the art of mathematical models of IAV infection but also the methodologies exploited for parameter estimation. We focus on the adaptive IR control of IAV infection and the possible mechanisms that could promote a secondary bacterial coinfection. To exemplify IAV dynamics and identifiability issues, a mathematical model to explain the interactions between adaptive IR and IAV infection is considered. Furthermore, in this paper we propose a roadmap for future influenza research. The development of a mathematical modeling framework with a secondary bacterial coinfection, immunosenescence, host genetic factors and responsiveness to vaccination will be pivotal to advance IAV infection understanding and treatment optimization.

Intravenous peramivir inhibits viral replication, and leads to bacterial clearance and prevention of mortality during murine bacterial co-infection caused by influenza A(H1N1)pdm09 virus and Streptococcus pneumoniae

INTRODUCTION

Influenza virus infection increases susceptibility to bacterial infection and mortality in humans. Although the efficacy of approved intravenous peramivir, a neuraminidase (NA) inhibitor, against influenza virus infection has been reported, its efficacy against bacterial co-infection, which occurs during the period of viral shedding, was not fully investigated. To further understand the significance of treatment with peramivir, we assessed the efficacy of peramivir against a bacterial co-infection model in mice caused by clinically isolated influenza A(H1N1)pdm09 virus and Streptococcus pneumoniae.

METHODS

Mice were infected with influenza A(H1N1)pdm09. Peramivir was intravenously administered after the viral infection. At 2days post viral infection, the mice were infected with S. pneumoniae. Peramivir efficacy was measured by the survival rates and viral titers, bacterial titers, or proinflammatory cytokine concentrations in lung homogenates.

RESULTS

Peramivir treatment reduced the mortality of mice infected with influenza virus and S. pneumoniae. The survival rate in the peramivir-treated group was significantly higher than that in the oseltamivir-treated group. Viral titers and proinflammatory cytokine responses in the peramivir-treated group were significantly lower than those in the oseltamivir-treated group until at 2days post viral infection. Bacterial titer was significantly lower in the peramivir-treated group than in the oseltamivir-treated group at 4days post viral infection.

CONCLUSION

These results demonstrated that peramivir inhibits viral replication, consequently leading to bacterial clearance and prevention of mortality during severe murine bacterial co-infection, which occurs during the period of viral shedding, with the efficacy of peramivir being superior to that of oseltamivir.

Desialylation of airway epithelial cells during influenza virus infection enhances pneumococcal adhesion via galectin binding

The continued threat of worldwide influenza pandemics, together with the yearly emergence of antigenically drifted influenza A virus (IAV) strains, underscore the urgent need to elucidate not only the mechanisms of influenza virulence, but also those mechanisms that predispose influenza patients to increased susceptibility to subsequent infection with Streptococcus pneumoniae. Glycans displayed on the surface of epithelia that are exposed to the external environment play important roles in microbial recognition, adhesion, and invasion. It is well established that the IAV hemagglutinin and pneumococcal adhesins enable their attachment to the host epithelia. Reciprocally, the recognition of microbial glycans by host carbohydrate-binding proteins (lectins) can initiate innate immune responses, but their relevance in influenza or pneumococcal infections is poorly understood. Galectins are evolutionarily conserved lectins characterized by affinity for β-galactosides and a unique sequence motif, with critical regulatory roles in development and immune homeostasis. In this study, we examined the possibility that galectins expressed in the airway epithelial cells might play a significant role in viral or pneumococcal adhesion to airway epithelial cells. Our results in a mouse model for influenza and pneumococcal infection revealed that the murine lung expresses a diverse galectin repertoire, from which selected galectins, including galectin 1 (Gal1) and galectin 3 (Gal3), are released to the bronchoalveolar space. Further, the results showed that influenza and subsequent S. pneumoniae infections significantly alter the glycosylation patterns of the airway epithelial surface and modulate galectin expression. In vitro studies on the human airway epithelial cell line A549 were consistent with the observations made in the mouse model, and further revealed that both Gal1 and Gal3 bind strongly to IAV and S. pneumoniae, and that exposure of the cells to viral neuraminidase or influenza infection increased galectin-mediated S. pneumoniae adhesion to the cell surface. Our results suggest that upon influenza infection, pneumococcal adhesion to the airway epithelial surface is enhanced by an interplay among the host galectins and viral and pneumococcal neuraminidases. The observed enhancement of pneumococcal adhesion may be a contributing factor to the observed hypersusceptibility to pneumonia of influenza patients.