Allergic lung inflammation alters neither susceptibility to Streptococcus pneumoniae infection nor inducibility of innate resistance in mice

IL-33-mediated Eosinophilia Protects against Acute Lung Injury

Pneumonia-induced lung injury and acute respiratory distress syndrome can develop because of an inappropriate inflammatory response to acute infections, leading to a compromised alveolar barrier. Recent work suggests that hospitalized patients with allergies/asthma are less likely to die of pulmonary infections and that there is a correlation between survival from acute respiratory distress syndrome and higher eosinophil counts; thus, we hypothesized that eosinophils associated with a type 2 immune response may protect against pneumonia-induced acute lung injury. To test this hypothesis, mice were treated with the type 2–initiating cytokine IL-33 intratracheally 3 days before induction of pneumonia with airway administration of a lethal dose of Staphylococcus aureus. Interestingly, IL-33 pretreatment promoted survival by inhibiting acute lung injury: amount of BAL fluid proinflammatory cytokines and pulmonary edema were both reduced, with an associated increase in oxygen saturation. Pulmonary neutrophilia was also reduced, whereas eosinophilia was strongly increased. This eosinophilia was key to protection; eosinophil reduction eliminated both IL-33–mediated protection against mortality and inhibition of neutrophilia and pulmonary edema. Together, these data reveal a novel role for eosinophils in protection against lung injury and suggest that modulation of pulmonary type 2 immunity may represent a novel therapeutic strategy.

Animal Models of Pneumococcal pneumonia

Streptococcus pneumoniae remains the most common bacterial pathogen causing lower respiratory tract infections and is a leading cause of morbidity and mortality worldwide, especially in children and the elderly. Another important aspect related to pneumococcal infections is the persistent rate of penicillin and macrolide resistance. Therefore, animal models have been developed to better understand the pathogenesis of pneumococcal disease and test new therapeutic agents and vaccines. This narrative review will focus on the characteristics of the different animal pneumococcal pneumonia models. The assessment of the different animal models will include considerations regarding pneumococcal strains, microbiology properties, procedures used for bacterial inoculation, pathogenesis, clinical characteristics, diagnosis, treatment, and preventive approaches.

Osteopontin protects against pneumococcal infection in a murine model of allergic airway inflammation

BACKGROUND

In atopic asthma, chronic Th2-biased inflammation is associated with an increased risk of pneumococcal infection. The anionic phosphoglycoprotein osteopontin (OPN) is highly expressed in asthma and has been ascribed several roles during inflammation. This study aimed to investigate whether OPN affects inflammation and vulnerability to pneumococcal infection in atopic asthma.

METHODS

House dust mite (HDM) extract was used to induce allergic airway inflammation in both wild-type (Spp1^+/+ ) and OPN knockout (Spp1^-/- ) C57BL/6J mice, and the airway was then infected with Streptococcus pneumoniae. Parameters reflecting inflammation, tissue injury, and bacterial burden were measured. In addition, samples from humans with allergic asthma were analyzed.

RESULTS

Both allergen challenge in individuals with allergic asthma and the intranasal instillation of HDM in mice resulted in increased OPN levels in bronchoalveolar lavage fluid (BALF). More immune cells (including alveolar macrophages, neutrophils, eosinophils, and lymphocytes) and higher levels of proinflammatory cytokines were found in Spp1^-/- mice than in Spp1^+/+ mice. Moreover, OPN-deficient mice exhibited increased levels of markers reflecting tissue injury. Upon infection with S. pneumoniae, Spp1^+/+ mice with allergic airway inflammation had a significantly lower bacterial burden in both BALF and lung tissue than did Spp1^-/- mice. Furthermore, Spp1^-/- mice had higher levels of cytokines and immune cells in BALF than did Spp1^+/+ mice.

CONCLUSION

OPN reduces inflammation, decreases tissue injury, and reduces bacterial loads during concurrent pneumococcal infection and allergic airway inflammation in a murine model. These findings suggest that OPN significantly affects vulnerability to pneumococcal infection in atopic asthma.

Allergic Asthma Favors Brucella Growth in the Lungs of Infected Mice

Allergic asthma is a chronic Th2 inflammatory disease of the lower airways affecting a growing number of people worldwide. The impact of infections and microbiota composition on allergic asthma has been investigated frequently. Until now, however, there have been few attempts to investigate the impact of asthma on the control of infectious microorganisms and the underlying mechanisms. In this work, we characterize the consequences of allergic asthma on intranasal (i.n.) infection by Brucella bacteria in mice. We observed that i.n. sensitization with extracts of the house dust mite Dermatophagoides farinae or the mold Alternaria alternata (Alt) significantly increased the number of Brucella melitensis, Brucella suis, and Brucella abortus in the lungs of infected mice. Microscopic analysis showed dense aggregates of infected cells composed mainly of alveolar macrophages (CD11c+ F4/80+ MHCII+) surrounded by neutrophils (Ly-6G+). Asthma-induced Brucella susceptibility appears to be dependent on CD4+ T cells, the IL-4/STAT6 signaling pathway and IL-10, and is maintained in IL-12- and IFN-γR-deficient mice. The effects of the Alt sensitization protocol were also tested on Streptococcus pneumoniae and Mycobacterium tuberculosis pulmonary infections. Surprisingly, we observed that Alt sensitization strongly increases the survival of S. pneumoniae infected mice by a T cell and STAT6 independent signaling pathway. In contrast, the course of M. tuberculosis infection is not affected in the lungs of sensitized mice. Our work demonstrates that the impact of the same allergic sensitization protocol can be neutral, negative, or positive with regard to the resistance of mice to bacterial infection, depending on the bacterial species.

Allergic Lung Inflammation Reduces Tissue Invasion and Enhances Survival from Pulmonary Pneumococcal Infection in Mice, Which Correlates with Increased Expression of Transforming Growth Factor β1 and SiglecF(low) Alveolar Macrophages

Asthma is generally thought to confer an increased risk for invasive pneumococcal disease (IPD) in humans. However, recent reports suggest that mortality rates from IPD are unaffected in patients with asthma and that chronic obstructive pulmonary disease (COPD), a condition similar to asthma, protects against the development of complicated pneumonia. To clarify the effects of asthma on the subsequent susceptibility to pneumococcal infection, ovalbumin (OVA)-induced allergic lung inflammation (ALI) was induced in mice followed by intranasal infection with A66.1 serotype 3 Streptococcus pneumoniae. Surprisingly, mice with ALI were significantly more resistant to lethal infection than non-ALI mice. The heightened resistance observed following ALI correlated with enhanced early clearance of pneumococci from the lung, decreased bacterial invasion from the airway into the lung tissue, a blunted inflammatory cytokine and neutrophil response to infection, and enhanced expression of transforming growth factor β1 (TGF-β1). Neutrophil depletion prior to infection had no effect on enhanced early bacterial clearance or resistance to IPD in mice with ALI. Although eosinophils recruited into the lung during ALI appeared to be capable of phagocytizing bacteria, neutralization of interleukin-5 (IL-5) to inhibit eosinophil recruitment likewise had no effect on early clearance or survival following infection. However, enhanced resistance was associated with an increase in levels of clodronate-sensitive, phagocytic SiglecF(low) alveolar macrophages within the airways following ALI. These findings suggest that, while the risk of developing IPD may actually be decreased in patients with acute asthma, additional clinical data are needed to better understand the risk of IPD in patients with different asthma phenotypes.

Eotaxin-3 (CCL26) exerts innate host defense activities that are modulated by mast cell proteases

BACKGROUND

During bacterial infections of the airways, a Th1-profiled inflammation promotes the production of several host defense proteins and peptides with antibacterial activities including β-defensins, ELR-negative CXC chemokines, and the cathelicidin LL-37. These are downregulated by Th2 cytokines of the allergic response. Instead, the eosinophil-recruiting chemokines eotaxin-1/CCL11, eotaxin-2/CCL24, and eotaxin-3/CCL26 are expressed. This study set out to investigate whether these chemokines could serve as innate host defense molecules during allergic inflammation.

METHODS

Antibacterial activities of the eotaxins were investigated using viable count assays, electron microscopy, and methods assessing bacterial permeabilization. Fragments generated by mast cell proteases were characterized, and their potential antibacterial, receptor-activating, and lipopolysaccharide-neutralizing activities were investigated.

RESULTS

CCL11, CCL24, and CCL26 all showed potent bactericidal activity, mediated through membrane disruption, against the airway pathogens Streptococcus pneumoniae, Staphylococcus aureus, Nontypeable Haemophilus influenzae, and Pseudomonas aeruginosa. CCL26 retained bactericidal activity in the presence of salt at physiologic concentrations, and the region holding the highest bactericidal activity was the cationic and amphipathic COOH-terminus. Proteolysis of CCL26 by chymase and tryptase, respectively, released distinct fragments of the COOH- and NH2 -terminal regions. The COOH-terminal fragment retained antibacterial activity while the NH2 -terminal had potent LPS-neutralizing properties in the order of CCL26 full-length protein. An identical fragment to NH2 -terminal fragment generated by tryptase was obtained after incubation with supernatants from activated mast cells. None of the fragments activated the CCR3-receptor.

CONCLUSIONS

Taken together, the findings show that the eotaxins can contribute to host defense against common airway pathogens and that their activities are modulated by mast cell proteases.

Allergic airway inflammation decreases lung bacterial burden following acute Klebsiella pneumoniae infection in a neutrophil- and CCL8-dependent manner

The Th17 cytokines interleukin-17A (IL-17A), IL-17F, and IL-22 are critical for the lung immune response to a variety of bacterial pathogens, including Klebsiella pneumoniae. Th2 cytokine expression in the airways is a characteristic feature of asthma and allergic airway inflammation. The Th2 cytokines IL-4 and IL-13 diminish ex vivo and in vivo IL-17A protein expression by Th17 cells. To determine the effect of IL-4 and IL-13 on IL-17-dependent lung immune responses to acute bacterial infection, we developed a combined model in which allergic airway inflammation and lung IL-4 and IL-13 expression were induced by ovalbumin sensitization and challenge prior to acute lung infection with K. pneumoniae. We hypothesized that preexisting allergic airway inflammation decreases lung IL-17A expression and airway neutrophil recruitment in response to acute K. pneumoniae infection and thereby increases the lung K. pneumoniae burden. As hypothesized, we found that allergic airway inflammation decreased the number of K. pneumoniae-induced airway neutrophils and lung IL-17A, IL-17F, and IL-22 expression. Despite the marked reduction in postinfection airway neutrophilia and lung expression of Th17 cytokines, allergic airway inflammation significantly decreased the lung K. pneumoniae burden and postinfection mortality. We showed that the decreased lung K. pneumoniae burden was independent of IL-4, IL-5, and IL-17A and partially dependent on IL-13 and STAT6. Additionally, we demonstrated that the decreased lung K. pneumoniae burden associated with allergic airway inflammation was both neutrophil and CCL8 dependent. These findings suggest a novel role for CCL8 in lung antibacterial immunity against K. pneumoniae and suggest new mechanisms of orchestrating lung antibacterial immunity.

Lung epithelial cells are essential effectors of inducible resistance to pneumonia

Infectious pneumonias are the leading cause of death worldwide, particularly among immunocompromised patients. Therapeutic stimulation of the lungs' intrinsic defenses with a unique combination of inhaled Toll-like receptor (TLR) agonists broadly protects mice against otherwise lethal pneumonias. As the survival benefit persists despite cytotoxic chemotherapy-related neutropenia, the cells required for protection were investigated. The inducibility of resistance was tested in mice with deficiencies of leukocyte lineages due to genetic deletions and in wild-type mice with leukocyte populations significantly reduced by antibodies or toxins. Surprisingly, these serial reductions in leukocyte lineages did not appreciably impair inducible resistance, but targeted disruption of TLR signaling in the lung epithelium resulted in complete abrogation of the protective effect. Isolated lung epithelial cells were also induced to kill pathogens in the absence of leukocytes. Proteomic and gene expression analyses of isolated epithelial cells and whole lungs revealed highly congruent antimicrobial responses. Taken together, these data indicate that lung epithelial cells are necessary and sufficient effectors of inducible resistance. These findings challenge conventional paradigms about the role of epithelia in antimicrobial defense and offer a novel potential intervention to protect patients with impaired leukocyte-mediated immunity from fatal pneumonias.

Bacterial superinfection following lung inflammatory disorders

The lung environment is designed to prevent innate responses to harmless commensal microorganisms and environmental antigens. Features of an intact respiratory epithelium are critical to this process. A damaged or altered lung epithelial surface will therefore remove or alter the suppressive signals delivered to local innate immune cells, and inflammation ensues. Timely resolution of inflammation is important to prevent bystander tissue damage. However, if resolving pathways themselves are prolonged or repeated, they too can cause undesirable consequences, including bacterial superinfections, which we discuss here.

Antiinflammatory role of MUC1 mucin during infection with nontypeable Haemophilus influenzae

MUC1 (or Muc1 in nonhuman species) is a membrane-tethered mucin expressed on the apical surface of mucosal epithelia (including those of the airways) that suppresses Toll-like receptor (TLR) signaling. We sought to determine whether the anti-inflammatory effect of MUC1 is operative during infection with nontypeable Haemophilus influenzae (NTHi), and if so, which TLR pathway was affected. Our results showed that: (1) a lysate of NTHi increased the early release of IL-8 and later production of MUC1 protein by A549 cells in dose-dependent and time-dependent manners, compared with vehicle control; (2) both effects were attenuated after transfection of the cells with a TLR2-targeting small interfering (si) RNA, compared with a control siRNA; (3) the NTHi-induced release of IL-8 was suppressed by an overexpression of MUC1, and was enhanced by the knockdown of MUC1; (4) the TNF-α released after treatment with NTHi was sufficient to up-regulate MUC1, which was completely inhibited by pretreatment with a soluble TNF-α receptor; and (5) primary murine tracheal surface epithelial (MTSE) cells from Muc1 knockout mice exhibited an increased in vitro production of NTHi-stimulated keratinocyte chemoattractant compared with MTSE cells from Muc1-expressing animals. These results suggest a hypothetical feedback loop model whereby NTHi activates TLRs (mainly TLR2) in airway epithelial cells, leading to the increased production of TNF-α and IL-8, which subsequently up-regulate the expression of MUC1, resulting in suppressed TLR signaling and decreased production of IL-8. This report is the first, to the best of our knowledge, demonstrating that the inflammatory response in airway epithelial cells during infection with NTHi is controlled by MUC1 mucin, mainly through the suppression of TLR2 signaling.

Synergistic TLR2/6 and TLR9 activation protects mice against lethal influenza pneumonia

Lower respiratory tract infections caused by influenza A continue to exact unacceptable worldwide mortality, and recent epidemics have emphasized the importance of preventative and containment strategies. We have previously reported that induction of the lungs' intrinsic defenses by aerosolized treatments can protect mice against otherwise lethal challenges with influenza A virus. More recently, we identified a combination of Toll like receptor (TLR) agonists that can be aerosolized to protect mice against bacterial pneumonia. Here, we tested whether this combination of synthetic TLR agonists could enhance the survival of mice infected with influenza A/HK/8/68 (H3N2) or A/California/04/2009 (H1N1) influenza A viruses. We report that the TLR treatment enhanced survival whether given before or after the infectious challenge, and that protection tended to correlate with reductions in viral titer 4 d after infection. Surprisingly, protection was not associated with induction of interferon gene expression. Together, these studies suggest that synergistic TLR interactions can protect against influenza virus infections by mechanisms that may provide the basis for novel therapeutics.

Inhaled innate immune ligands to prevent pneumonia

Epithelial surfaces throughout the body continuously sample and respond to environmental stimuli. The accessibility of lung epithelium to inhaled therapies makes it possible to stimulate local antimicrobial defences with aerosolized innate immune ligands. This strategy has been shown to be effective in preclinical models, as delivery of innate immune ligands to the lungs of laboratory animals results in protection from subsequent challenge with microbial pathogens. Survival of the animal host in this setting correlates directly with killing of pathogens within the lungs, indicating the induction of a resistance mechanism. Resistance appears to be mediated primarily by activated epithelial cells rather than recruited leucocytes. Resistance reaches a peak within hours and persists for several days. Innate immune ligands can interact synergistically under some circumstances, and synergistic combinations of innate ligands delivered by aerosol are capable of inducing a high level of broad host resistance to bacteria, fungi and viruses. The induction of innate antimicrobial resistance within the lungs could have clinical applications in the prevention of lower respiratory tract infection in subjects transiently at high risk. These include cancer patients undergoing myeloablative chemotherapy, intubated patients being mechanically ventilated, vulnerable individuals during seasonal influenza epidemics, asthmatic subjects experiencing a respiratory viral infection, and healthy subjects exposed to virulent pathogens from a bioterror attack or emergent pandemic. In summary, stimulation of the lung epithelium to induce localized resistance to infection is a novel strategy whose clinical utility will be assessed in the near future.

Synergistic interactions of TLR2/6 and TLR9 induce a high level of resistance to lung infection in mice

Infectious pneumonias exact an unacceptable mortality burden worldwide. Efforts to protect populations from pneumonia have focused historically on antibiotic development and vaccine-enhanced adaptive immunity. However, we have reported recently that the lungs' innate defenses can be induced therapeutically by inhalation of a bacterial lysate that protects mice against otherwise lethal pneumonia. In this study, we tested in mice the hypothesis that TLRs are required for this antimicrobial phenomenon and found that resistance could not be induced in the absence of the TLR signaling adaptor protein MyD88. We then attempted to recapitulate the protection afforded by the bacterial lysate by stimulating the lung epithelium with aerosolized synthetic TLR ligands. Although most single or combination treatments yielded no protection, simultaneous treatment with ligands for TLR2/6 and TLR9 conferred robust, synergistic protection against virulent gram-positive and gram-negative pathogens. Protection was associated with rapid pathogen killing in the lungs, and pathogen killing could be induced from lung epithelial cells in isolation. Taken together, these data demonstrate the requirement for TLRs in inducible resistance against pneumonia, reveal a remarkable, unanticipated synergistic interaction of TLR2/6 and TLR9, reinforce the emerging evidence supporting the antimicrobial capacity of the lung epithelium, and may provide the basis for a novel clinical therapeutic that can protect patients against pneumonia during periods of peak vulnerability.

Host lung gene expression patterns predict infectious etiology in a mouse model of pneumonia

Background

Lower respiratory tract infections continue to exact unacceptable worldwide mortality, often because the infecting pathogen cannot be identified. The respiratory epithelia provide protection from pneumonias through organism-specific generation of antimicrobial products, offering potential insight into the identity of infecting pathogens. This study assesses the capacity of the host gene expression response to infection to predict the presence and identity of lower respiratory pathogens without reliance on culture data.

Methods

Mice were inhalationally challenged with S. pneumoniae, P. aeruginosa, A. fumigatus or saline prior to whole genome gene expression microarray analysis of their pulmonary parenchyma. Characteristic gene expression patterns for each condition were identified, allowing the derivation of prediction rules for each pathogen. After confirming the predictive capacity of gene expression data in blinded challenges, a computerized algorithm was devised to predict the infectious conditions of subsequent subjects.

Results

We observed robust, pathogen-specific gene expression patterns as early as 2 h after infection. Use of an algorithmic decision tree revealed 94.4% diagnostic accuracy when discerning the presence of bacterial infection. The model subsequently differentiated between bacterial pathogens with 71.4% accuracy and between non-bacterial conditions with 70.0% accuracy, both far exceeding the expected diagnostic yield of standard culture-based bronchoscopy with bronchoalveolar lavage.

Conclusions

These data substantiate the specificity of the pulmonary innate immune response and support the feasibility of a gene expression-based clinical tool for pneumonia diagnosis.