Granulocyte-alveolar-macrophage interaction in the pulmonary clearance of Staphylococcus aureus

Toll-like receptor 2 has a prominent but nonessential role in innate immunity to Staphylococcus aureus pneumonia

Staphylococcus aureus is an important cause of acute bacterial pneumonia. Toll‐like receptor 2 (TLR2) recognizes multiple components of the bacterial cell wall and activates innate immune responses to gram‐positive bacteria. We hypothesized that TLR2 would have an important role in pulmonary host defense against S. aureus. TLR null (TLR2^−/−) mice and wild type (WT) C57BL/6 controls were challenged with aerosolized S. aureus at a range of inocula for kinetic studies of cytokine and antimicrobial peptide expression, lung inflammation, bacterial killing by alveolar macrophages, and bacterial clearance. Survival was measured after intranasal infection. Pulmonary induction of most pro‐inflammatory cytokines was significantly blunted in TLR2^−/− mice 4 and 24 h after infection in comparison with WT controls. Bronchoalveolar concentrations of cathelicidin‐related antimicrobial peptide also were reduced in TLR2^−/− mice. Lung inflammation, measured by enumeration of bronchoalveolar neutrophils and scoring of histological sections, was significantly blunted in TLR2^−/− mice. Phagocytosis of S. aureus by alveolar macrophages in vivo after low‐dose infection was unimpaired, but viability of ingested bacteria was significantly greater in TLR2^−/− mice. Bacterial clearance from the lungs was slightly impaired in TLR2^−/− mice after low‐dose infection only; bacterial elimination from the lungs was slightly accelerated in the TLR2^−/− mice after high‐dose infection. Survival after high‐dose intranasal challenge was 50–60% in both groups. TLR2 has a significant role in early innate immune responses to S. aureus in the lungs but is not required for bacterial clearance and survival from S. aureus pneumonia.

Mathematical model of a three-stage innate immune response to a pneumococcal lung infection

Pneumococcal pneumonia is a leading cause of death and a major source of human morbidity. The initial immune response plays a central role in determining the course and outcome of pneumococcal disease. We combine bacterial titer measurements from mice infected with Streptococcus pneumoniae with mathematical modeling to investigate the coordination of immune responses and the effects of initial inoculum on outcome. To evaluate the contributions of individual components, we systematically build a mathematical model from three subsystems that describe the succession of defensive cells in the lung: resident alveolar macrophages, neutrophils and monocyte-derived macrophages. The alveolar macrophage response, which can be modeled by a single differential equation, can by itself rapidly clear small initial numbers of pneumococci. Extending the model to include the neutrophil response required additional equations for recruitment cytokines and host cell status and damage. With these dynamics, two outcomes can be predicted: bacterial clearance or sustained bacterial growth. Finally, a model including monocyte-derived macrophage recruitment by neutrophils suggests that sustained bacterial growth is possible even in their presence. Our model quantifies the contributions of cytotoxicity and immune-mediated damage in pneumococcal pathogenesis.

Animal models of Streptococcus pneumoniae disease

SUMMARY

Streptococcus pneumoniae is a colonizer of human nasopharynx, but it is also an important pathogen responsible for high morbidity, high mortality, numerous disabilities, and high health costs throughout the world. Major diseases caused by S. pneumoniae are otitis media, pneumonia, sepsis, and meningitis. Despite the availability of antibiotics and vaccines, pneumococcal infections still have high mortality rates, especially in risk groups. For this reason, there is an exceptionally extensive research effort worldwide to better understand the diseases caused by the pneumococcus, with the aim of developing improved therapeutics and vaccines. Animal experimentation is an essential tool to study the pathogenesis of infectious diseases and test novel drugs and vaccines. This article reviews both historical and innovative laboratory pneumococcal animal models that have vastly added to knowledge of (i) mechanisms of infection, pathogenesis, and immunity; (ii) efficacies of antimicrobials; and (iii) screening of vaccine candidates. A comprehensive description of the techniques applied to induce disease is provided, the advantages and limitations of mouse, rat, and rabbit models used to mimic pneumonia, sepsis, and meningitis are discussed, and a section on otitis media models is also included. The choice of appropriate animal models for in vivo studies is a key element for improved understanding of pneumococcal disease.

Staphylococcus aureus elicits marked alterations in the airway proteome during early pneumonia

Pneumonia caused by Staphylococcus aureus is a growing concern in the health care community. We hypothesized that characterization of the early innate immune response to bacteria in the lungs would provide insight into the mechanisms used by the host to protect itself from infection. An adult mouse model of Staphylococcus aureus pneumonia was utilized to define the early events in the innate immune response and to assess the changes in the airway proteome during the first 6 h of pneumonia. S. aureus actively replicated in the lungs of mice inoculated intranasally under anesthesia to cause significant morbidity and mortality. By 6 h postinoculation, the release of proinflammatory cytokines caused effective recruitment of neutrophils to the airway. Neutrophil influx, loss of alveolar architecture, and consolidated pneumonia were observed histologically 6 h postinoculation. Bronchoalveolar lavage fluids from mice inoculated with phosphate-buffered saline (PBS) or S. aureus were depleted of overabundant proteins and subjected to strong cation exchange fractionation followed by liquid chromatography and tandem mass spectrometry to identify the proteins present in the airway. No significant changes in response to PBS inoculation or 30 min following S. aureus inoculation were observed. However, a dramatic increase in extracellular proteins was observed 6 h postinoculation with S. aureus, with the increase dominated by inflammatory and coagulation proteins. The data presented here provide a comprehensive evaluation of the rapid and vigorous innate immune response mounted in the host airway during the earliest stages of S. aureus pneumonia.

Role of interleukin-8 in community-acquired pneumonia: relation to microbial load and pulmonary function

In pneumonia local phagocyte activation is crucial for clearing of pathogenic microorganisms. In this context alveolar macrophage interleukin-8 secretion, phagocyte oxidative response and concentrations of lavage proteins were quantified, including interleukin-8, in 31 patients with pneumonia, 13 age matched patients with peripheral lung consolidation and six healthy volunteers; these findings were related to the impairment of gas exchange and the bacterial load in the alveolar space. Increased interleukin-8 levels were found in bronchoalveolar lavage fluid (BALF) and in alveolar macrophage supernatants from patients with pneumonia (214 ng/10(5) AM +/- 121 vs 71 ng/10(5) AM +/- 35 and 66 ng/10(5) AM +/- 30, p < 0.05). Interleukin-8 release from alveolar macrophages correlated with the upregulated spontaneous luminol enhanced oxidative response of pulmonary phagocytes but not with the neutrophil count in BALF. In pneumonia patients a significant difference was found between patients with 10(4) or more colony forming units (CFU)/ml BALF of one pathogen and patients with less CFU or nonspecific microbiological results (261 ng/10(5) AM +/- 89 vs 179 ng/10(5) AM +/- 81 and 7.5 ng/ml BALF +/- 17 vs 0.44 ng/ml BALF +/- 1, p < 0.05). Further, a negative correlation between interleukin-8 release of alveolar macrophages and the arterial pO2 at the time of BALF could be demonstrated (r = -0.47, p < 0.05). The results demonstrate local cellular activation in community-acquired pneumonia, which is related to the bacterial load in the alveolar space and to impairment of gas exchange. This is consistent with the hypothesis that pulmonary phagocytes play a central role in the pathogenesis of bacterial pneumonia, contributing not only to bacterial clearing but also to local tissue damage.

Effect of presensitization of Nocardia asteroides with specific antibody on the viability of the organism

Nocardia asteroides from various growth phases was treated in vitro with normal rabbit sera, immune rabbit sera containing nocardial polyclonal antibodies and a monoclonal antibody. At intervals, samples were grown in broth or on blood agar plates to determine their viability. Log and stationary phase cells were injected intra-peritoneally into female BALB/c mice and their survival rates in the liver and spleen were determined. Presensitization with antibodies reduced the viability of the log phase cells by 48% and that of the late stationary phase by 4%. The antibody-treated log phase organisms were less viable on the blood agar medium and in the spleen and liver than the control organisms. This indicates that pretreatment with antibody has a lethal effect on N. asteroides and affects its survival in vivo.

Differing roles for platelet-activating factor during inflammation of the lung and subarachnoid space. The special case of Streptococcus pneumoniae

Although well-characterized in the lung, the role of platelet-activating factor (PAF) in inflammation in the central nervous system is undefined. Using rabbit models of meningitis and pneumonia, PAF was found to induce significant blood-brain barrier permeability and brain edema at doses five times lower than those required to generate leukocyte recruitment to the subarachnoid space. Both leukocytosis and increased vascular permeability occurred in response to PAF in the lung. Antibody to the CD-18 family of leukocyte adhesion molecules inhibited leukocyte recruitment in response to PAF in the brain (greater than 80%); a similar level of inhibition in the lung required treatment with a combination of a PAF receptor antagonist (L-659,989) and anti-CD18 antibody. Treatment with L-659,989 decreased abnormal cerebrospinal fluid cytochemical values induced by intracisternal challenge with pneumococci but not Haemophilus influenzae, indicating a special role for PAF in pneumococcal disease. Antibodies directed at phosphorylcholine, a unique, shared determinant of bioactivity of PAF and pneumococcal cell wall, obviated the inflammatory potential of both agents. However, no evidence for a direct PAF-like activity of pneumococcal cell wall components was detected in vitro by bioassay using platelets or neutrophils. It is concluded that PAF can induce inflammation in the subarachnoid space. In brain, PAF effects appear to be mediated through CD-18-dependent events, while in lung, PAF effects independent of CD-18 are also evident. At both sites, PAF is of particular clinical importance during inflammation induced by pneumococci apparently due to a unique proinflammatory relationship between the pneumococcal cell wall teichoic acid and PAF.

Immune enhancement of pulmonary clearance of nontypable Haemophilus influenzae

BALB/c mice systemically immunized by intraperitoneal injection with whole, viable cells of two different strains of nontypable Haemophilus influenzae (NTHI) exhibited a markedly enhanced ability to clear the homologous strain of NTHI from the lower respiratory tract. Immunization did not influence the number of phagocytic cells recovered by bronchoalveolar lavage from mice before or after intrapulmonary challenge with NTHI. Immunization also induced the synthesis of relatively large quantities of NTHI-directed antibodies which were detectable in both the bloodstream and the alveolar spaces of the lung. Radioimmunoprecipitation and Western blot (immunoblot) analyses indicated that these antibodies were directed against both the proteins and lipooligosaccharide (LOS) in the NTHI outer membrane. Bactericidal and opsonophagocytic assays determined that the NTHI-directed antibodies in the serum were functional and able to kill or opsonize the homologous NTHI strain. Mice immunized with an NTHI major outer membrane protein-LOS complex also had an increased ability to effect pulmonary clearance of NTHI. Serum and bronchoalveolar lavage fluid collected from these animals immunized with the outer membrane protein-LOS complex contained relatively high levels of antibodies to both of these antigens. The serum from these animals also possessed bactericidal and opsonic activity against the homologous NTHI strain. These results indicate that systemic immunization can enhance the ability of experimental animals to clear NTHI from the lower respiratory tract and suggest that immunoprophylaxis of NTHI pulmonary disease may be feasible.

Pulmonary clearance of Mycoplasma pulmonis in C57BL/6N and C3H/HeN mice

In C57BL/6N and C3H/HeN mice known to be free of all murine pathogens and matched for age, sex, and environmental factors, pulmonary clearance was measured over a 72-h time period after exposure to infectious aerosols of 35S-labeled Mycoplasma pulmonis. Reduced clearance of M. pulmonis in C3H/HeN mice relative to C57BL/6N mice was primarily due to impaired mycoplasmacidal activity in the lungs of the C3H/HeN mice. The C3H/HeN mice also had a slightly slower rate of mechanical transport of radiolabel from the lungs in the first 4 h after infection relative to the C57BL/6N mice but not at any later times. By 72 h after infection (relative to 0 h, C3H/HeN mice had an over 4,000% (1.75 X 10(7) versus 4.30 X 10(5] increase in neutrophils and an over 18,000% (more than 2 orders of magnitude) increase in numbers of M. pulmonis recovered from mechanically disaggregated lungs. In contrast, C57BL/6N mice reduced the number of M. pulmonis present by over 83% (nearly 2 orders of magnitude) before any increase in inflammatory cells, which was only a slight increase in lymphocytes and macrophages at 24 h after infection. These results directly link decreased mycoplasmal pulmonary clearance in C3H/HeN mice with the increased susceptibility to, and severity of, murine respiratory mycoplasmosis observed in this strain. The resistance of C57BL/6N mice appears to be related to nonspecific host defense mechanisms responsible for limiting the extent of infection.

Effect of systemic immunization on pulmonary clearance of Haemophilus influenzae type b

The effect of systemic immunization on pulmonary clearance of Haemophilus influenzae type b (Hib) was studied in a mouse model system. Immunization of mice by intraperitoneal injection of viable Hib cells resulted in the appearance of Hib-directed antibodies in both serum and bronchoalveolar lavage fluid. The development of this Hib-directed antibody activity was associated with significant enhancement of early pulmonary clearance of Hib. Systemic immunization did not affect the recruitment of polymorphonuclear leukocytes to the alveoli, suggesting that the enhanced clearance of Hib observed in immunized animals was due to specific antibodies which promote either phagocytosis or extracellular killing of Hib. The spectrum of Hib-directed antibody specificities detected in sera from immunized animals was essentially identical to that detected in bronchoalveolar lavage fluids from these same animals. Similarly, intravenous administration of an immunoglobulin G monoclonal antibody specific for Hib lipopolysaccharide resulted in the subsequent appearance of this antibody in the alveolar spaces where it enhanced pulmonary clearance of Hib. This study shows that this mouse model system can be used to measure the effect of both active and passive immunization on the clearance of Hib from the lower respiratory tract.

Interaction between the legionnaires' disease bacterium (Legionella pneumophila) and human alveolar macrophages. Influence of antibody, lymphokines, and hydrocortisone

We have studied the interaction between virulent Legionella pneumophila and human alveolar macrophages, the resident phagocytes at the site of infection in Legionnaires' disease. L. pneumophila multiplied 2.5-5 logs within 3 d, as measured by colony forming units, when incubated with freshly explanted alveolar macrophages in monolayer culture. At the peak of bacterial multiplication, the alveolar macrophage monolayers were destroyed. L. pneumophila multiplied more rapidly in 4-d-old than in freshly explanted alveolar macrophages. Inside alveolar macrophages, L. pneumophila were located within membrane-bound vacuoles whose cytoplasmic sides were studded with ribosomes. Alveolar macrophages that were incubated with concanavalin A (Con A) stimulated human mononuclear cell supernatants (cytokines), inhibited L. pneumophila multiplication, and the degree of inhibition was proportional to the concentration of Con A supernatant added. Anti-L. pneumophila antibody in conjunction with complement promoted phagocytosis of L. pneumophila by alveolar macrophages. By electron microscopy, most (75%) of the phagocytized L. pneumophila were intracellular. However, freshly explanted alveolar macrophages were able to kill only 0-10% of an innoculum of L. pneumophila even in the presence of antibody and complement. At the same time, alveolar macrophages also killed opsonized Escherichia coli poorly. Increasing the ratio of macrophages to bacteria, adhering the macrophages to microcarrier beads, or preincubating the macrophages for 24 or 48 h with Con A supernatants failed to augment alveolar macrophage killing of opsonized E. coli. Corticosteroids appear to increase patient susceptibility to Legionnaires' disease. However, pretreatment of alveolar macrophages and monocytes with hydrocortisone had no influence on intracellular multiplication of L. pneumophila or on the inhibition of that multiplication by activated alveolar macrophages or monocytes. Hydrocortisone did impair cytokine-induced aggregation of alveolar macrophages. These findings demonstrate that L. pneumophila multiplies in human alveolar macrophages and that they do so within a ribosome-lined phagosome; that freshly explanted alveolar macrophages kill few L. pneumophila even in the presence of antibody and complement; that activated alveolar macrophages inhibit L. pneumophila multiplication; and that steroids do not exert a direct suppressive effect on the anti-L. pneumophila activity of activated or nonactivated alveolar macrophages. Our findings indicate that alveolar macrophages may play a central role in both the pathogenesis of Legionnaires' disease and in host defense against it. This paper shows that human resident macrophage can be activated to a higher state of antimicrobial capacity and that the human alveolar macrophage can serve as an effector call in call-mediated immunity.

Pulmonary clearance of encapsulated and unencapsulated Haemophilus influenzae strains

A mouse model system was employed to investigate the temporal pattern of pulmonary clearance of Haemophilus influenzae and to evaluate the effect of the type b polysaccharide capsule on this clearance pattern. The lungs of BALB/c mice were inoculated with boluses of several different H. influenzae strains via an endobronchial catheter. A fully encapsulated H. influenzae type b strain multiplied readily in the lungs for at least 6 h and then was eventually cleared from the lungs over the next 18 h. The pulmonary clearance pattern obtained with an unencapsulated variant of this H. influenzae type b strain was identical to that obtained with the fully encapsulated parent strain. Two nontypable H. influenzae strains isolated by transtracheal aspiration of patients with acute H. influenzae pneumonia also multiplied in the lung and resisted significant clearance for at least 6 h after inoculation. Bolus deposition of either H. influenzae type b or nontypable H. influenzae in the lungs resulted in an eventual influx of polymorphonuclear leukocytes into the alveoli. The observed delay in clearance of all these strains suggests that resident host defense mechanisms must be augmented for clearance to occur. Furthermore, these data indicate that one or more factors other than the Haemophilus capsule are important bacterial determinants of pulmonary clearance of H. influenzae.

Tracheal function during influenza infections

Studies with animal models have demonstrated that viral respiratory tract infections suppress bacterial clearance processes in the lung and that this alteration in host defenses appears to explain the excessive mortality from bacterial pneumonia during influenza epidemics. However, since the pathogenesis of postinfluenza pneumonia and other pneumonias probably involves the aspiration of normal nasopharyngeal flora, injury to major airways associated with influenza infections could also contribute to the development of bacterial pneumonia by increasing bacterial deposition in the peripheral lung. We investigated this possibility by evaluating tracheal clearance processes and spontaneous changes in the tracheal flora in a murine model for acute influenza. During fine-particle aerosol exposures to Staphylococcus aureus, influenza-infected mice retained the same number of bacteria on their proximal tracheal surfaces as did control mice, and the relative adherence of the staphylococci to the trachea was similar in both groups of mice. However, the clearance of viable staphylococci from the trachea was significantly delayed in influenza-infected mice. Control and influenza-infected mice were also evaluated for changes in their normal tracheal flora. Mice with established influenza infections had more frequent spontaneous colonization with gram-negative bacteria, more bacterial isolates per animal, and higher bacterial concentrations in tracheal homogenates than control mice. These changes in tracheal flora were most pronounced on day 7 after virus inoculation and persisted after virus titers were undetectable, but eventually resolved by day 14 after virus infection. Tetracycline therapy started 2 days after virus inoculation prevented the increased colonization. This impaired clearance function and increased spontaneous colonization were associated with morphological evidence of mucosal regeneration. We conclude that spontaneous changes in tracheal flora occur during influenza infections, that these changes reflect, in part, impaired clearance functions, and that such changes could contribute to the development of pneumonia regardless of the clearance capacity of the lung parenchyma.