Alveolar macrophage-derived type I interferons orchestrate innate immunity to RSV through recruitment of antiviral monocytes

Suppression of IRG-1 Reduces Inflammatory Cell Infiltration and Lung Injury in Respiratory Syncytial Virus Infection by Reducing Production of Reactive Oxygen Species

Respiratory syncytial virus (RSV) infection is a common cause of lower respiratory tract illness in infants and children. RSV is a negative-sense, single-strand RNA (ssRNA) virus that mainly infects airway epithelial cells. Accumulating evidence indicates that reactive oxygen species (ROS) production is a major factor for pulmonary inflammation and tissue damage of RSV disease. We investigated immune-responsive gene-1 (IRG1) expression during RSV infection, since IRG1 has been shown to mediate innate immune response to intracellular bacterial pathogens by modulating ROS and itaconic acid production. We found that RSV infection induced IRG1 expression in human A549 cells and in the lung tissues of RSV-infected mice. RSV infection or IRG1 overexpression promoted ROS production. Accordingly, knockdown of IRG1 induction blocked RSV-induced ROS production and proinflammatory cytokine gene expression. Finally, we showed that suppression of IRG1 induction reduced immune cell infiltration and prevented lung injury in RSV-infected mice. These results therefore link IRG1 induction to ROS production and immune lung injury after RSV infection.

IMPORTANCE

RSV infection is among the most common causes of childhood diseases. Recent studies identify ROS production as a factor contributing to RSV disease. We investigated the cause of ROS production and identified IRG1 as a critical factor linking ROS production to immune lung injury after RSV infection. We found that IRG1 was induced in A549 alveolar epithelial cells and in mouse lungs after RSV infection. Importantly, suppression of IRG1 induction reduced inflammatory cell infiltration and lung injury in mice. This study links IRG1 induction to oxidative damage and RSV disease. It also uncovers a potential therapeutic target in reducing RSV-caused lung injury.

Alveolar Macrophages Can Control Respiratory Syncytial Virus Infection in the Absence of Type I Interferons

Respiratory syncytial virus (RSV) is a common cause of lower respiratory tract infections. Immunity to RSV is initiated upon detection of the virus by pattern recognition receptors, such as RIG-I-like receptors. RIG-I-like receptors signal via MAVS to induce the synthesis of proinflammatory mediators, including type I interferons (IFNs), which trigger and shape antiviral responses and protect cells from infection. Alveolar macrophages (AMs) are amongst the first cells to encounter invading viruses and the ones producing type I IFNs. However, it is unclear whether IFNs act to prevent AMs from serving as vehicles for viral replication. In this study, primary AMs from MAVS (Mavs-/-)- or type I IFN receptor (Ifnar1-/-)-deficient mice were exposed to RSV ex vivo. Wild-type (wt) AMs but not Mavs-/- and Ifnar1-/- AMs produced inflammatory mediators in response to RSV. Furthermore, Mavs-/- and Ifnar1-/- AMs accumulated more RSV proteins than wt AMs, but the infection was abortive. Thus, RIG-I-like receptor-MAVS and IFNAR signalling are important for the induction of proinflammatory mediators from AMs upon RSV infection, but this signalling is not central for controlling viral replication. The ability to restrict viral replication makes AMs ideal sensors of RSV infection and important initiators of immune responses in the lung.

HSV-1 ICP27 targets the TBK1-activated STING signalsome to inhibit virus-induced type I IFN expression

Herpes simplex virus (HSV) 1 stimulates type I IFN expression through the cGAS-STING-TBK1 signaling axis. Macrophages have recently been proposed to be an essential source of IFN during viral infection. However, it is not known how HSV-1 inhibits IFN expression in this cell type. Here, we show that HSV-1 inhibits type I IFN induction through the cGAS-STING-TBK1 pathway in human macrophages, in a manner dependent on the conserved herpesvirus protein ICP27. This viral protein was expressed de novo in macrophages with early nuclear localization followed by later translocation to the cytoplasm where ICP27 prevented activation of IRF3. ICP27 interacted with TBK1 and STING in a manner that was dependent on TBK1 activity and the RGG motif in ICP27. Thus, HSV-1 inhibits expression of type I IFN in human macrophages through ICP27-dependent targeting of the TBK1-activated STING signalsome.

Differential Type I Interferon Signaling Is a Master Regulator of Susceptibility to Postinfluenza Bacterial Superinfection

Bacterial superinfections are a primary cause of death during influenza pandemics and epidemics. Type I interferon (IFN) signaling contributes to increased susceptibility of mice to bacterial superinfection around day 7 post-influenza A virus (IAV) infection. Here we demonstrate that the reduced susceptibility to methicillin-resistant Staphylococcus aureus (MRSA) at day 3 post-IAV infection, which we previously reported was due to interleukin-13 (IL-13)/IFN-γ responses, is also dependent on type I IFN signaling and its subsequent requirement for protective IL-13 production. We found, through utilization of blocking antibodies, that reduced susceptibility to MRSA at day 3 post-IAV infection was IFN-β dependent, whereas the increased susceptibility at day 7 was IFN-α dependent. IFN-β signaling early in IAV infection was required for MRSA clearance, whereas IFN-α signaling late in infection was not, though it did mediate increased susceptibility to MRSA at that time. Type I IFN receptor (IFNAR) signaling in CD11c⁺ and Ly6G⁺ cells was required for the observed reduced susceptibility at day 3 post-IAV infection. Depletion of Ly6G⁺ cells in mice in which IFNAR signaling was either blocked or deleted indicated that Ly6G⁺ cells were responsible for the IFNAR signaling-dependent susceptibility to MRSA superinfection at day 7 post-IAV infection. Thus, during IAV infection, the temporal differences in type I IFN signaling increased bactericidal activity of both CD11c⁺ and Ly6G⁺ cells at day 3 and reduced effector function of Ly6G⁺ cells at day 7. The temporal differential outcomes induced by IFN-β (day 3) and IFN-α (day 7) signaling through the same IFNAR resulted in differential susceptibility to MRSA at 3 and 7 days post-IAV infection.

Approximately 114,000 hospitalizations and 40,000 annual deaths in the United States are associated with influenza A virus (IAV) infections. Frequently, these deaths are due to community-acquired Gram-positive bacterial species, many of which show increasing resistance to antibiotic therapy. Severe complications, including parapneumonic empyema and necrotizing pneumonia, can arise, depending on virulence factors expressed by either the virus or bacteria. Unfortunately, we are unable to control the expression of these virulence factors, making host responses a logical target for therapeutic interventions. Moreover, interactions between virus, host, and bacteria that exacerbate IAV-related morbidities and mortalities are largely unknown. Here, we show that type I interferon (IFN) expression can modulate susceptibility to methicillin-resistant Staphylococcus aureus (MRSA) infection, with IFN-β reducing host susceptibility to MRSA infection while IFN-α increases susceptibility. Our data indicate that treatments designed to augment IFN-β and/or inhibit IFN-α production around day 7 post-IAV infection could reduce susceptibility to deadly superinfections.

Molecular and epigenetic basis of macrophage polarized activation

Macrophages are unique cells for origin, heterogeneity and plasticity. At steady state most of macrophages are derived from fetal sources and maintained in adulthood through self-renewing. Despite sharing common progenitors, a remarkable heterogeneity characterized tissue-resident macrophages indicating that local signals educate them to express organ-specific functions. Macrophages are extremely plastic: chromatin landscape and transcriptional programs can be dynamically re-shaped in response to microenvironmental changes. Owing to their ductility, macrophages are crucial orchestrators of both initiation and resolution of immune responses and key supporters of tissue development and functions in homeostatic and pathological conditions. Herein, we describe current understanding of heterogeneity and plasticity of macrophages using the M1-M2 dichotomy as operationally useful simplification of polarized activation. We focused on the complex network of signaling cascades, metabolic pathways, transcription factors, and epigenetic changes that control macrophage activation. In particular, this network was addressed in sepsis, as a paradigm of a pathological condition determining dynamic macrophage reprogramming.

Actin- and clathrin-dependent mechanisms regulate interferon gamma release after stimulation of human immune cells with respiratory syncytial virus

Background

Respiratory syncytial virus (RSV) can cause recurrent and severe respiratory tract infections. Cytoskeletal proteins are often involved during viral infections, either for cell entry or the initiation of the immune response. The importance of actin and clathrin dynamics for cell entry and the initiation of the cellular immune response against RSV in human immune cells is not known yet. The aim of this study was to investigate the role of actin and clathrin on cell entry of RSV and the subsequent effect on T cell activation and interferon gamma release in human immune cells.

Methods

Peripheral blood mononuclear cells and purified monocytes were isolated from healthy adults and stimulated in vitro with RSV. Actin and clathrin dynamics were inhibited with respectively cytochalasin D and chlorpromazine. T cell receptor signaling was inhibited with cyclosporin A. Flow cytometry was used to determine the role of actin and clathrin on cell entry and T cell activation by RSV. Enzyme-linked immunosorbent assays were used to investigate the contribution of actin and clathrin on the release of interferon gamma.

Results

Cell entry, virus gene transcription and interferon gamma release are actin-dependent. Post-endocytic processes like the increased expression of major histocompatibility complex II on monocytes , T cell activation and the release of interferon gamma are clathrin-dependent. Finally, T cell receptor signaling affects T cell activation, whereas soluble interleukin 18 is dispensable.

Conclusion

Analysis of cell entry and interferon gamma release after infection with RSV reveals the importance of actin- and clathrin-dependent signaling in human immune cells. Insights into the cellular biology of the human immune response against respiratory syncytial virus will provide a better understanding of disease pathogenesis and may prove useful in the development of preventive strategies.

Cathelicidins Have Direct Antiviral Activity against Respiratory Syncytial Virus In Vitro and Protective Function In Vivo in Mice and Humans

Respiratory syncytial virus (RSV) is a leading cause of respiratory tract infection in infants, causing significant morbidity and mortality. No vaccine or specific, effective treatment is currently available. A more complete understanding of the key components of effective host response to RSV and novel preventative and therapeutic interventions are urgently required. Cathelicidins are host defense peptides, expressed in the inflamed lung, with key microbicidal and modulatory roles in innate host defense against infection. In this article, we demonstrate that the human cathelicidin LL-37 mediates an antiviral effect on RSV by inducing direct damage to the viral envelope, disrupting viral particles and decreasing virus binding to, and infection of, human epithelial cells in vitro. In addition, exogenously applied LL-37 is protective against RSV-mediated disease in vivo, in a murine model of pulmonary RSV infection, demonstrating maximal efficacy when applied concomitantly with virus. Furthermore, endogenous murine cathelicidin, induced by infection, has a fundamental role in protection against disease in vivo postinfection with RSV. Finally, higher nasal levels of LL-37 are associated with protection in a healthy human adult RSV infection model. These data lead us to propose that cathelicidins are a key, nonredundant component of host defense against pulmonary infection with RSV, functioning as a first point of contact antiviral shield and having additional later-phase roles in minimizing the severity of disease outcome. Consequently, cathelicidins represent an inducible target for preventative strategies against RSV infection and may inform the design of novel therapeutic analogs for use in established infection.

The host immune response in respiratory virus infection: balancing virus clearance and immunopathology

The respiratory tract is constantly exposed to the external environment, and therefore, must be equipped to respond to and eliminate pathogens. Viral clearance and resolution of infection requires a complex, multi-faceted response initiated by resident respiratory tract cells and innate immune cells and ultimately resolved by adaptive immune cells. Although an effective immune response to eliminate viral pathogens is essential, a prolonged or exaggerated response can damage the respiratory tract. Immune-mediated pulmonary damage is manifested clinically in a variety of ways depending on location and extent of injury. Thus, the antiviral immune response represents a balancing act between the elimination of virus and immune-mediated pulmonary injury. In this review, we highlight major components of the host response to acute viral infection and their role in contributing to mitigating respiratory damage. We also briefly describe common clinical manifestations of respiratory viral infection and morphological correlates. The continuing threat posed by pandemic influenza as well as the emergence of novel respiratory viruses also capable of producing severe acute lung injury such as SARS-CoV, MERS-CoV, and enterovirus D68, highlights the need for an understanding of the immune mechanisms that contribute to virus elimination and immune-mediated injury.

Mitochondria in lung disease

Mitochondria are a distinguishing feature of eukaryotic cells. Best known for their critical function in energy production via oxidative phosphorylation (OXPHOS), mitochondria are essential for nutrient and oxygen sensing and for the regulation of critical cellular processes, including cell death and inflammation. Such diverse functional roles for organelles that were once thought to be simple may be attributed to their distinct heteroplasmic genome, exclusive maternal lineage of inheritance, and ability to generate signals to communicate with other cellular organelles. Mitochondria are now thought of as one of the cell's most sophisticated and dynamic responsive sensing systems. Specific signatures of mitochondrial dysfunction that are associated with disease pathogenesis and/or progression are becoming increasingly important. In particular, the centrality of mitochondria in the pathological processes and clinical phenotypes associated with a range of lung diseases is emerging. Understanding the molecular mechanisms regulating the mitochondrial processes of lung cells will help to better define phenotypes and clinical manifestations associated with respiratory disease and to identify potential diagnostic and therapeutic targets.

Reduced Expression of HLA-DR on Monocytes During Severe Respiratory Syncytial Virus Infections

BACKGROUND

Respiratory syncytial virus (RSV) is a common cause of bronchiolitis in infants with a wide spectrum of disease severity. Besides environmental and genetic factors, it is thought that the innate immune system plays a pivotal role. The aim of this study was to investigate the expression of immune receptors on monocytes and the in vitro responsiveness from infants with severe RSV infections.

METHODS

Peripheral blood mononuclear cells (PBMCs) from infants with RSV infections were isolated. Classical, intermediate and nonclassical monocytes were immunophenotyped for the expression of CD14, CD16, human leukocyte antigen (HLA)-ABC and HLA-DR. PBMCs were stimulated with lipopolysaccharide to determine the secretion of tumor necrosis factor and interleukin (IL)-10 with enzyme-linked immunosorbent assay.

RESULTS

During RSV infection, intermediate monocytes are increased in the peripheral blood, whereas classical and nonclassical monocytes are reduced. The expression of CD14 and HLA-ABC is increased on monocytes, whereas the expression of HLA-DR is suppressed. Low HLA-DR expression is correlated with increased disease severity. PBMCs from infants with severe RSV infections show an impaired IL-10 response in vitro.

CONCLUSIONS

Phenotyping subpopulations of monocytes combined with in vitro responsiveness reveals significant differences between nonsevere and severe RSV infections. Reduced HLA-DR expression and impaired IL-10 production in vitro during severe RSV infections indicate that an imbalanced innate immune response may play an important role in disease severity.

Flt3 ligand improves the innate response to respiratory syncytial virus and limits lung disease upon RSV reexposure in neonate mice

Respiratory syncytial virus (RSV) causes severe bronchiolitis in infants worldwide. The immunological factors responsible for RSV susceptibility in infants are poorly understood. Here, we used the BALB/c mouse model of neonatal RSV infection to study the mechanisms leading to severe disease upon reexposure to the virus when adults. Two major deficiencies in neonatal lung innate responses were found: a poor DCs mobilization, and a weak engagement of the IFNI pathway. The administration of Flt3 ligand (Flt3-L), a growth factor that stimulates the proliferation of hematopoietic cells, to neonates before RSV-infection, resulted in increased lung DC number, and reconditioned the IFNI pathway upon RSV neonatal infection. Besides, neonates treated with Flt3-L were protected against exacerbated airway disease upon adult reexposure to RSV. This was associated with a reorientation of RSV-specific responses toward Th1-mediated immunity. Thus, the poor lung DCs and IFNI responses to RSV in neonates may be partly responsible for the deleterious long-term consequences revealed upon adult reexposure to RSV, which could be prevented by Flt3-L treatment. These results open new perspectives for developing neonatal immuno-modulating strategies to reduce the burden of bronchiolitis.

ISG15 Is Upregulated in Respiratory Syncytial Virus Infection and Reduces Virus Growth through Protein ISGylation

Human respiratory syncytial virus (RSV), for which neither a vaccine nor an effective therapeutic treatment is currently available, is the leading cause of severe lower respiratory tract infections in children. Interferon-stimulated gene 15 (ISG15) is a ubiquitin-like protein that is highly increased during viral infections and has been reported to have an antiviral or a proviral activity, depending on the virus. Previous studies from our laboratory demonstrated strong ISG15 upregulation during RSV infection in vitro. In this study, an in-depth analysis of the role of ISG15 in RSV infection is presented. ISG15 overexpression and small interfering RNA (siRNA)-silencing experiments, along with ISG15 knockout (ISG15(-/-)) cells, revealed an anti-RSV effect of the molecule. Conjugation inhibition assays demonstrated that ISG15 exerts its antiviral activity via protein ISGylation. This antiviral activity requires high levels of ISG15 to be present in the cells before RSV infection. Finally, ISG15 is also upregulated in human respiratory pseudostratified epithelia and in nasopharyngeal washes from infants infected with RSV, pointing to a possible antiviral role of the molecule in vivo. These results advance our understanding of the innate immune response elicited by RSV and open new possibilities to control infections by the virus.

IMPORTANCE

At present, no vaccine or effective treatment for human respiratory syncytial virus (RSV) is available. This study shows that interferon-stimulated gene 15 (ISG15) lowers RSV growth through protein ISGylation. In addition, ISG15 accumulation highly correlates with the RSV load in nasopharyngeal washes from children, indicating that ISG15 may also have an antiviral role in vivo. These results improve our understanding of the innate immune response to RSV and identify ISG15 as a potential target for virus control.

Functional Interplay between Type I and II Interferons Is Essential to Limit Influenza A Virus-Induced Tissue Inflammation

Host control of influenza A virus (IAV) is associated with exuberant pulmonary inflammation characterized by the influx of myeloid cells and production of proinflammatory cytokines including interferons (IFNs). It is unclear, however, how the immune system clears the virus without causing lethal immunopathology. Here, we demonstrate that in addition to its known anti-viral activity, STAT1 signaling coordinates host inflammation during IAV infection in mice. This regulatory mechanism is dependent on both type I IFN and IFN-γ receptor signaling and, importantly, requires the functional interplay between the two pathways. The protective function of type I IFNs is associated with not only the recruitment of classical inflammatory Ly6C^hi monocytes into IAV-infected lungs, but also the prevention of excessive monocyte activation by IFN-γ. Unexpectedly, type I IFNs preferentially regulate IFN-γ signaling in Ly6C^lo rather than inflammatory Ly6C^hi mononuclear cell populations. In the absence of type I IFN signaling, Ly6C^lo monocytes/macrophages, become phenotypically and functionally more proinflammatory than Ly6C^hi cells, revealing an unanticipated function of the Ly6C^lo mononuclear cell subset in tissue inflammation. In addition, we show that type I IFNs employ distinct mechanisms to regulate monocyte and neutrophil trafficking. Type I IFN signaling is necessary, but not sufficient, for preventing neutrophil recruitment into the lungs of IAV-infected mice. Instead, the cooperation of type I IFNs and lymphocyte-produced IFN-γ is required to regulate the tissue neutrophilic response to IAV. Our study demonstrates that IFN interplay links innate and adaptive anti-viral immunity to orchestrate tissue inflammation and reveals an additional level of complexity for IFN-dependent regulatory mechanisms that function to prevent excessive immunopathology while preserving anti-microbial functions.

Influenza A virus (IAV) is a leading cause of respiratory infection and induces a strong acute inflammation manifested by the recruitment of monocytes and neutrophils as well as the production of proinflammatory cytokines in infected lungs. The interferons (IFNs) are strongly induced by IAV and are known to mediate host resistance to the infection. However, in contrast to their well-studied inhibitory effect on viral replication, the effects of IFNs on host inflammatory responses are less well understood. In this manuscript, we demonstrate that anti-viral IFN signaling is also required for the orchestration of a tissue response associated with the protection against IAV infection in mice. Importantly, we identify that type I IFNs cross-regulate and cooperate with IFN-γ to inhibit monocyte activation and neutrophil infiltration, respectively. This study also demonstrates that Ly6C^lo monocytes/macrophages can potentially mediate influenza virus-induced inflammation, suggesting that IFNs dictate the homeostasis versus inflammatory function of mononuclear phagocytes in viral infection. Our study reveals a novel IFN-dependent regulatory mechanism designed to prevent the excessive immunopathology while preserving its anti-microbial functions. Moreover, these observations have particular relevance for understanding the mechanisms underlying the strong inflammatory response associated with lethal IAV strains and have implications for the development of new immunotherapies to treat influenza.

RSV-specific airway resident memory CD8+ T cells and differential disease severity after experimental human infection

In animal models, resident memory CD8+ T (Trm) cells assist in respiratory virus elimination but their importance in man has not been determined. Here, using experimental human respiratory syncytial virus (RSV) infection, we investigate systemic and local virus-specific CD8+ T-cell responses in adult volunteers. Having defined the immunodominance hierarchy, we analyse phenotype and function longitudinally in blood and by serial bronchoscopy. Despite rapid clinical recovery, we note surprisingly extensive lower airway inflammation with persistent viral antigen and cellular infiltrates. Pulmonary virus-specific CD8+ T cells display a CD69+CD103+ Trm phenotype and accumulate to strikingly high frequencies into convalescence without continued proliferation. While these have a more highly differentiated phenotype, they express fewer cytotoxicity markers than in blood. Nevertheless, their abundance before infection correlates with reduced symptoms and viral load, implying that CD8+ Trm cells in the human lung can confer protection against severe respiratory viral disease when humoral immunity is overcome.

T cell responses are elicited against Respiratory Syncytial Virus in the absence of signalling through TLRs, RLRs and IL-1R/IL-18R

Pattern recognition receptors (PRRs) and cytokine receptors are key players in the initiation of immune responses to infection. PRRs detecting viral RNA, such as toll like receptor (TLR)-3, -7/8, and RIG-I like receptors (RLRs; RIG-I and MDA-5), as well as cytokine receptors such as interleukin 1 receptor (IL-1R), have been implicated in responses to RNA viruses that infect the airways. The latter includes respiratory syncytial virus (RSV), a human pathogen that can cause severe lower respiratory tract infections, especially in infants. To evaluate the collective contribution of PRRs and IL-1R signalling to RSV immunity, we generated Myd88/Trif/Mavs^−/− mice that are deficient in signalling by all TLRs, RLRs and IL-1R, as well as other cytokine receptors such as IL-18 receptor. Early production of pro-inflammatory mediators and lung infiltration by immune cells were completely abrogated in infected Myd88/Trif/Mavs^−/− mice. However, RSV-specific CD8⁺ T cells were elicited and recruited into the lungs and airways. Consistent with these findings, Myd88/Trif/Mavs^−/− mice survived RSV infection but displayed higher viral load and weight loss. These data highlight an unappreciated level of redundancy in pathways that couple innate virus sensing to adaptive immunity, providing the host with remarkable resilience to infection.

Priming of the Respiratory Tract with Immunobiotic Lactobacillus plantarum Limits Infection of Alveolar Macrophages with Recombinant Pneumonia Virus of Mice (rK2-PVM)

Pneumonia virus of mice (PVM) is a natural rodent pathogen that replicates in bronchial epithelial cells and reproduces many clinical and pathological features of the more severe forms of disease associated with human respiratory syncytial virus. In order to track virus-target cell interactions during acute infection in vivo, we developed rK2-PVM, bacterial artificial chromosome-based recombinant PVM strain J3666 that incorporates the fluorescent tag monomeric Katushka 2 (mKATE2). The rK2-PVM pathogen promotes lethal infection in BALB/c mice and elicits characteristic cytokine production and leukocyte recruitment to the lung parenchyma. Using recombinant virus, we demonstrate for the first time PVM infection of both dendritic cells (DCs; CD11c(+) major histocompatibility complex class II(+)) and alveolar macrophages (AMs; CD11c(+) sialic acid-binding immunoglobulin-like lectin F(+)) in vivo and likewise detect mKATE2(+) DCs in mediastinal lymph nodes from infected mice. AMs support both active virus replication and production of infectious virions. Furthermore, we report that priming of the respiratory tract with immunobiotic Lactobacillus plantarum, a regimen that results in protection against the lethal inflammatory sequelae of acute respiratory virus infection, resulted in differential recruitment of neutrophils, DCs, and lymphocytes to the lungs in response to rK2-PVM and a reduction from ∼ 40% to <10% mKATE2(+) AMs in association with a 2-log drop in the release of infectious virions. In contrast, AMs from L. plantarum-primed mice challenged with virus ex vivo exhibited no differential susceptibility to rK2-PVM. Although the mechanisms underlying Lactobacillus-mediated viral suppression remain to be fully elucidated, this study provides insight into the cellular basis of this response.

IMPORTANCE

Pneumonia virus of mice (PVM) is a natural mouse pathogen that serves as a model for severe human respiratory syncytial virus disease. We have developed a fully functional recombinant PVM strain with a fluorescent reporter protein (rK2-PVM) that permits us to track infection of target cells in vivo. With rK2-PVM, we demonstrate infection of leukocytes in the lung, notably, dendritic cells and alveolar macrophages. Alveolar macrophages undergo productive infection and release infectious virions. We have shown previously that administration of immunobiotic Lactobacillus directly to the respiratory mucosa protects mice from the lethal sequelae of PVM infection in association with profound suppression of the virus-induced inflammatory response. We show here that Lactobacillus administration also limits infection of leukocytes in vivo and results in diminished release of infectious virions from alveolar macrophages. This is the first study to provide insight into the cellular basis of the antiviral impact of immunobiotic L. plantarum.

Innate and adaptive cellular phenotypes contributing to pulmonary disease in mice after respiratory syncytial virus immunization and infection

Respiratory syncytial virus (RSV) is the major leading cause of infantile viral bronchiolitis. However, cellular phenotypes contributing to the RSV protection and vaccine-enhanced disease remain largely unknown. Upon RSV challenge, we analyzed phenotypes and cellularity in the lung of mice that were naïve, immunized with formalin inactivated RSV (FI-RSV), or re-infected with RSV. In comparison with naïve and live RSV re-infected mice, the high levels of eosinophils, neutrophils, plasmacytoid and CD11b(+) dendritic cells, and IL-4(+) CD4(+) T cells were found to be contributing to pulmonary inflammation in FI-RSV immune mice despite lung viral clearance. Alveolar macrophages appeared to play differential roles in protection and inflammation upon RSV infection of different RSV immune mice. These results suggest that multiple innate and adaptive immune components differentially contribute to RSV disease and inflammation.

Partial Attenuation of Respiratory Syncytial Virus with a Deletion of a Small Hydrophobic Gene Is Associated with Elevated Interleukin-1β Responses

The small hydrophobic (SH) gene of respiratory syncytial virus (RSV), a major cause of infant hospitalization, encodes a viroporin of unknown function. SH gene knockout virus (RSV ΔSH) is partially attenuated in vivo, but not in vitro, suggesting that the SH protein may have an immunomodulatory role. RSV ΔSH has been tested as a live attenuated vaccine in humans and cattle, and here we demonstrate that it protected against viral rechallenge in mice. We compared the immune response to infection with RSV wild type and RSV ΔSH in vivo using BALB/c mice and in vitro using epithelial cells, neutrophils, and macrophages. Strikingly, the interleukin-1β (IL-1β) response to RSV ΔSH infection was greater than to wild-type RSV, in spite of a decreased viral load, and when IL-1β was blocked in vivo, the viral load returned to wild-type levels. A significantly greater IL-1β response to RSV ΔSH was also detected in vitro, with higher-magnitude responses in neutrophils and macrophages than in epithelial cells. Depleting macrophages (with clodronate liposome) and neutrophils (with anti-Ly6G/1A8) demonstrated the contribution of these cells to the IL-1β response in vivo, the first demonstration of neutrophilic IL-1β production in response to viral lung infection. In this study, we describe an increased IL-1β response to RSV ΔSH, which may explain the attenuation in vivo and supports targeting the SH gene in live attenuated vaccines.

IMPORTANCE

There is a pressing need for a vaccine for respiratory syncytial virus (RSV). A number of live attenuated RSV vaccine strains have been developed in which the small hydrophobic (SH) gene has been deleted, even though the function of the SH protein is unknown. The structure of the SH protein has recently been solved, showing it is a pore-forming protein (viroporin). Here, we demonstrate that the IL-1β response to RSV ΔSH is greater in spite of a lower viral load, which contributes to the attenuation in vivo. This potentially suggests a novel method by which viruses can evade the host response. As all Pneumovirinae and some Paramyxovirinae carry similar SH genes, this new understanding may also enable the development of live attenuated vaccines for both RSV and other members of the Paramyxoviridae.