Characterization of a murine model of endotoxin-induced acute lung injury

Physiologic, biochemical, and imaging characterization of acute lung injury in mice

RATIONALE

Most models of acute lung injury in mice have yet to be fully characterized.

OBJECTIVES

To directly compare and contrast endotoxin and oleic acid models of acute lung injury in mice in terms of their physiologic, biochemical, histopathologic, and imaging manifestations.

METHODS

Survival studies, lung weights, x-ray computed tomographic scanning, light and electron microscopy, bronchoalveolar lavage, lung uptake of ((18)F)fluorodeoxyglucose, tissue myeloperoxidase, arterial blood gases, mean arterial pressure, and lung tissue prostanoids were measured in separate groups of C57Bl/6 mice (normal animals, endotoxin only [20 microg/g], oleic acid only [0.15 microl/g], or endotoxin + oleic acid).

RESULTS

Endotoxin alone caused only mild pulmonary neutrophilic inflammation with little functional or structural damage to the alveolar architecture. In contrast, oleic acid caused severe alveolar damage with the development of alveolar edema of the increased-permeability type with associated abnormalities in gas exchange. When given together, endotoxin and oleic acid acted synergistically to increase pulmonary edema and to worsen gas exchange and hemodynamics, thereby increasing mortality. This synergism was significantly attenuated by the prior administration of the endotoxin antagonist E5564 (eritoran).

CONCLUSIONS

Under the conditions of these studies, only mice exposed to oleic acid showed both structural and functional characteristics of acute lung injury. Nevertheless, endotoxin had potent synergistic physiologic effects that increased mortality. Overall, these models, which can be translated to genetically altered mice, are amenable to study with state-of-the-art imaging techniques, and with experimental interventions that can probe the underlying mechanisms of injury.

Aerosol-administered alpha-tocopherol attenuates lung inflammation in rats given lipopolysaccharide intratracheally

Intrapulmonary administration of bacterial lipopolysaccharide (LPS) induces a well-characterized lung inflammatory response involving alveolar macrophage activation, proinflammatory cytokine elaboration, and neutrophil influx. Vitamin E, a lipophilic antioxidant consisting of a family that includes tocopherols and tocotrienols, has previously been shown to have a variety of anti-inflammatory effects, raising interest in its possible uses in disease prevention or therapy. Because aerosol delivery is a specific and rapid way to administer agents to the lungs, the authors undertook to determine whether inhaled vitamin E aerosols would have an anti-inflammatory effect in the lungs. Using a rat model of acute lung inflammation caused by intratracheally administered LPS (10 microg Pseudomonas aeruginosa LPS), the authors examined the effect of aerosol-administered vitamin E, in this case alpha-tocopherol, on several indices of lung inflammation which are increased by LPS treatment. It was found that inhaled alpha-tocopherol aerosol, but not inhaled alpha-tocopherol acetate aerosol, decreased tumor necrosis factor alpha (TNFalpha) and cytokine-induced neutrophil chemoattractant-1 (CINC-1) mRNA levels in lung tissue, TNFalpha and CINC-1 immunoreactive protein levels in lung lavage, and the number of neutrophils recoverable by lung lavage from rats given LPS intratracheally. These results contribute to the increasing body of work describing immunomodulatory functions of alpha-tocopherol, and support the idea that direct aerosol administration of alpha-tocopherol may play a beneficial role in strategies to control inflammatory lung illnesses.

Circulating levels of macrophage migration inhibitory factor are associated with mild pulmonary dysfunction after cardiopulmonary bypass

Macrophage migration inhibitory factor (MIF) is a central mediator of inflammatory response and acute lung injury that is secreted in response to corticosteroids. A rise in systemic MIF levels was described after cardiac surgery in steroid-treated patients. This study aimed to investigate the circulating levels of MIF and the possible relationship of this cytokine to pulmonary dysfunction after cardiopulmonary bypass (CPB). We included 74 patients without previous organ dysfunction undergoing elective coronary artery bypass surgery (CABS). The same team performed all CABS via a standard technique adding methylprednisolone (15 mg/kg) to the CPB priming solution (Group MP, n = 37). In the remaining patients (Group NS, n = 37), methylprednisolone was withdrawn from the CPB priming. MIF, C-reactive protein (CRP), and total C3 were assayed in peripheral blood sampled immediately before anesthesia induction and 3, 6, and 24 h post-CPB. Preoperative risk scores and peri- and postoperative variables were documented. Postoperative kinetics of MIF and C3 were similar for both groups. Levels of CRP 24 h post-CPB were higher in Group MP (P = 0.003). Higher MIF levels were detected 6 h post-CPB, and returned to preoperative levels 24 h after CPB. MIF levels 6 h post-CPB were inversely related to the postoperative PaO2/FiO2 ratio (P = 0.0021) and were directly related to the duration of mechanical ventilation (P = 0.014). Perioperative use of methylprednisolone did not modify the MIF response to CPB, but it was related to an enhanced acute phase response. Higher circulating MIF levels 6 h post-CPB were associated with worse postoperative pulmonary short-course outcome.

Impaired induction of IL-10 expression in the lung following hemorrhagic shock

The balance between pro- and anti-inflammatory cytokines is considered to be an important determinant of the magnitude of inflammation in a number of disease states. We previously showed that resuscitated hemorrhagic shock augmented LPS-induced release of proinflammatory molecules by alveolar macrophages (AM). In the present studies, we evaluated the expression and regulation of the counter inflammatory cytokine IL-10 in the lung using this model. We hypothesized that impaired up-regulation of IL-10 in shock/resuscitated animals might serve as a mechanism contributing to accentuated lung inflammation. In a rodent model, animals exposed to LPS alone exhibited enhanced IL-10 mRNA levels in lung tissue as well as in AM, but antecedent shock/resuscitation delayed and attenuated the LPS-induced IL-10 mRNA levels. The ability of shock to attenuate LPS-stimulated IL-10 was also seen in the protein levels. This effect correlated with an augmented expression of cytokine-induced neutrophil chemoattractant (CINC) mRNA. Shock/resuscitated animals given exogenous IL-10 had reduced proinflammatory response, as shown by decreased expression of CINC mRNA and decreased neutrophil sequestration in the lung. Shock/resuscitation plus LPS markedly reduced the transcription rate of IL-10 mRNA compared to LPS alone but did not affect IL-10 mRNA stability. Reduced IL-10 transcription was not caused solely by impaired nuclear translocation of STAT3 and Sp1/Sp3 transcription factors because LPS-induced nuclear translocation of these factors was augmented by antecedent shock. Considered together, these findings show that shock/resuscitation suppresses LPS-induced IL-10 expression by AM in the lung by inhibiting IL-10 gene transcription. Failed up-regulation of counter inflammatory cytokines may contribute to augmented organ dysfunction in trauma patients.

Lung effects during a generalized Shwartzman reaction and therapeutic intervention with dexamethasone or vitamin E

We investigated if a two-hit shock model, commonly referred to as generalized Shwartzman reaction (GSR), can prime for indirect acute respiratory distress syndrome (ARDS) in mice. The GSR was provoked in C57BL/6 mice by two consecutive i.p. injections of 100 microg lipopolysaccharide (LPS) at t = 0 and t = 20 h. These mice demonstrated a dramatic decrease in respiratory capacity and 80% mortality after the second injection. No such effect was observed when LPS was given as a single 200 microg dose at t = 0. Increased expression of proinflammatory cytokines in serum (interleukin-1beta, interleukin-6 and interferon-gamma), lung neutrophilia, and edema formation were observed in mice injected with one dose of LPS, but notably, mice exposed twice did not further increase their inflammatory response. Early treatment 1 h after the first LPS injection (t = 1 h) with either dexamethasone (10 mg/kg) or vitamin E (50 mg/kg) improved respiratory function and down-modulated the induction of proinflammatory cytokines in serum. In conclusion, mice with a generalized Shwartzman reaction exhibited features resembling some aspects of the pathophysiology in septic ARDS, i.e., neutrophilic inflammation, edema formation, impaired respiratory capacity, and mortality. Our data indicate that a systemic cytokine response and lung neutrophilia may prime for the GSR but that other mechanisms account for the rapid decline in lung function after the second challenge. We suggest that this model can be used for studies of pathogenesis and therapeutic prevention of acute respiratory failure.

Endotoxin-induced lung injury in mice: structural, functional, and biochemical responses

Acute lung injury is usually a complication of sepsis, and endotoxin treatment of mice is a frequently used experimental model. To define this model and to clarify pathogenesis of the lung injury, we injected with 1 mg/kg endotoxin ip and measured pulmonary function, pulmonary edema, serum concentrations of cytokines and growth factors, and lung histology over 48 h. During the first 6 h, tidal volume and minute volume increased and respiratory frequency decreased. Serum concentrations of cytokines showed three patterns: 10 cytokines peaked at 2 h and declined rapidly, two peaked at 6 h and declined, and two had biphasic peaks at 2 and 24 h. Growth factors increased later and remained elevated longer. Both collagen and fibronectin were deposited in the lungs beginning within hours of endotoxin and resolving over 48 h. Histologically, lungs showed increased cellularity at 6 h with minimal persistent inflammation at 48 h. Lung water peaked at 6 h and gradually decreased over 48 h. We conclude that intraperitoneal administration of endotoxin to mice causes a transient systemic inflammatory response and transient lung injury and dysfunction. The response is characterized by successive waves of cytokine release into the circulation, early evidence of lung fibrogenesis, and prolonged increases in growth factors that may participate in lung repair.

Intercomparison of recruitment maneuver efficacy in three models of acute lung injury*

OBJECTIVE

To compare the relative efficacy of three forms of recruitment maneuvers in diverse models of acute lung injury characterized by differing pathoanatomy.

DESIGN

We compared three recruiting maneuver (RM) techniques at three levels of post-RM positive end-expiratory pressure in three distinct porcine models of acute lung injury: oleic acid injury; injury induced purely by the mechanical stress of high-tidal airway pressures; and pneumococcal pneumonia.

SETTING

Laboratory in a clinical research facility.

SUBJECTS

Twenty-eight anesthetized mixed-breed pigs (23.8 +/- 2.6 kg).

INTERVENTIONS

The RM techniques tested were sustained inflation, extended sigh or incremental positive end-expiratory pressure, and pressure-controlled ventilation.

PRIMARY MEASUREMENTS

Oxygenation and end-expiratory lung volume.

MAIN RESULTS

The post-RM positive end-expiratory pressure level was the major determinant of post-maneuver PaO2, independent of the RM technique. The pressure-controlled ventilation RM caused a lasting increase of PaO2 in the ventilator-induced lung injury model, but in oleic acid injury and pneumococcal pneumonia, there were no sustained oxygenation differences for any RM technique (sustained inflation, incremental positive end-expiratory pressure, or pressure-controlled ventilation) that differed from raising positive end-expiratory pressure without RM.

CONCLUSIONS

Recruitment by pressure-controlled ventilation is equivalent or superior to sustained inflation, with the same peak pressure in all tested models of acute lung injury, despite its lower mean airway pressure and reduced risk for hemodynamic compromise. Although RM may improve PaO2 in certain injury settings when traditional tidal volumes are used, sustained improvement depends on the post-RM positive end-expiratory pressure value.

Acute Rat Lung Injury: Feasibility of Assessment with Micro-CT

PURPOSE

To evaluate the feasibility of micro-computed tomography (CT) for analysis of the lung fine structure and its alterations during endotoxin-induced lung injury.

MATERIALS AND METHODS

Intravital perfusion-fixed rat lungs with (n = 5) and without (n = 5) endotoxin perfusion were scanned with micro-CT. Three imaging modalities (conventional histology, intravital microscopy, and electron microscopy) were used to document the effect of endotoxin and the in vivo application of contrast agent (a mixture of barium sulfate, gelatin, and thymol). The effect of endotoxin on structural changes of the lung was evaluated with analysis of variance.

RESULTS

Intravital microscopy, conventional histology, and electron microscopy demonstrated capillary perfusion of contrast agent, inflated alveoli, and no extravasation of barium sulfate in the extravascular space. Systemic application of endotoxin led to a significant increase in the soft-tissue volume of the lungs (ie, tissue edema) (58.09 microm(3)+/- 4.6 [standard error of the mean] vs 8.31 microm(3)+/- 1.63, P <.001) and significant thickening of the alveolar walls (34.01 microm +/- 4.5 vs 14.83 microm +/- 2.5, P <.001) at micro-CT. Simultaneously, endotoxin-treated rat lungs showed a significant reduction in total air space (49.74 microm(3)+/- 1.72 vs 100.99 microm(3)+/- 1.16, P <.001).

CONCLUSION

These findings indicate that micro-CT is feasible for structural evaluation of the lung fine structure and its alterations during endotoxin-induced lung injury.

[Pathophysiological role of endotoxins, a common denominator to various diseases]

A growing number of investigations point to endotoxin or lipopolysaccharide as a central player in many pathophysiological states and diseases. Endotoxins are one of the most toxic biological contaminants continuously shed by both dead and live Gram negative bacteria. Endotoxins induce the primitive form of defense called innate immunity. Endotoxins have been related to inflammatory reactions observed in patients suffering from respiratory distress syndrome, multiorgan failure and septic shock, hepatic diseases, or in subjects affected by graft versus host disease after allogeneic transplantation. As our understanding of the molecular mechanisms underlying pathologies progresses, more diseases involving endotoxins emerge. Although these illnesses are multifactorial, the objective of this article is to review some of the common and distinct processes involving endotoxins in various disease states.

TEZOSENTAN COUNTERACTS ENDOTOXIN-INDUCED PULMONARY EDEMA AND IMPROVES GAS EXCHANGE

Sepsis-induced acute lung injury is still associated with high morbidity and mortality. The pathophysiology is complex, and markers of injury include increased extravascular lung water. To evaluate the effects of the novel dual endothelin receptor antagonist tezosentan on endotoxin-induced changes in extravascular lung water and gas exchange, 16 pigs were anaesthetized and catheterized. Twelve animals were subjected to 5 h of endotoxemia. After 2 h, six of these animals received a bolus of tezosentan 1 mg kg(-1) followed by a continuous infusion of 1 mg kg(-1) h(-1) to the end of the experiment at 5 h. Conventional pulmonary and hemodynamic parameters were measured. Extravascular lung water was determined in these pigs after 5 h of endotoxemia, as well as in the four additional nonendotoxemic sham animals. Tezosentan in the current dosage counteracted the deterioration of lung function caused by endotoxin, as measured by dead space, venous admixture, and compliance. In addition, pulmonary hypertension was attenuated. Tezosentan had a marked effect on the endotoxin-induced increase in extravascular lung water that was reduced to levels observed in nonendotoxemic sham animals. These results suggest that endothelin is involved in endotoxin-induced lung injury and the development of pulmonary edema. Dual endothelin receptor antagonism may be of value in the treatment of sepsis-related acute lung injury.

Exhaled Nitric Oxide and Acute Lung Injury in a Rat Model of Extracorporeal Circulation

Exhaled nitric oxide (NO) concentration, a marker of pulmonary inflammation, has been shown to be elevated in various models of acute lung injury (ALI). This study was undertaken to evaluate the pulmonary NO production in a rat model of postextracorporeal circulation (ECC) ALI. Wistar rats underwent either a partial femorofemoral ECC in normothermia for 3 h (n = 10) or a sham procedure (n = 10). The extracorporeal circuit consisted of a roller pump and a membrane oxygenator. Exhaled NO concentration was monitored with a chemiluminescence analyzer. After sacrifice, lungs were harvested for microscopic studies and to analyze the inducible nitric oxide synthase (iNOS) activity and expression (Western blot). ECC was responsible for an ALI characterized by a decreased arterial blood oxygen saturation (88.9% [51.7-94.2] vs. 93.7% [91.4-98.6] P = 0.005) and pulmonary histological changes (marked alveolar neutrophil infiltration; interstitial edema; intraalveolar hemorrhage). The lung injury score was significantly higher in the ECC group (n = 5; 3.0 [2-4]) in comparison to the sham group (n = 5; 1.0 [0-2]). Exhaled NO concentration remained stable throughout the experiment in all sham rats whereas it significantly increased in the ECC group from baseline (2 ppb [1-5]) until the end of experiment (33.5 ppb [1-47]). Lung iNOS activity and expression were also significantly increased in the ECC group. An increase in exhaled NO, however, did not correlate with the decrease in arterial oxygen pressure. ECC was responsible for an ALI in rats and for an elevated pulmonary NO production. Determination of the relationship between exhaled NO and the severity of the inflammatory process in ALI will require further studies.

Intracellular localization of differentially regulated RNA-specific adenosine deaminase isoforms in inflammation

Adenosine-to-inosine (A-to-I) RNA editing is a post-transcriptional process that amplifies the repertoire of protein production. Recently, the induction of this process through up-regulation of the editing enzyme RNA-specific adenosine deaminase 1 (ADAR1) was documented during acute inflammation. Here we report that the inflammation-induced up-regulation of ADAR1 involves differential production and intracellular localization of several isoforms with distinct RNA-binding domains and localization signals. These include the full-length ADAR1 (p150) and two functionally active short isoforms (p80 and p110). ADAR1 p80 starts at a methionine 519 (M519) due to alternative splicing in exon 2, which deletes the putative nuclear localization signal, the Z-DNA binding domain, and the entire RNA binding domain I. ADAR1 p110 is the mouse homologue of the human ADAR1 110-kDa variant (M246), which retains the second half of the Z-DNA binding domain, all RNA binding domains, and the deaminase domain. Additional variations are found in the third RNA binding domain of ADAR1; they are differentially regulated during inflammation, generating isoforms with different levels of activities. Studies in several cell types transfected with ADAR1-EGFP chimeras demonstrated that the p150 and p80 variants are localized in the cytoplasm and nucleolus, respectively. In agreement with this observation, endogenous ADAR1 was identified in the cytoplasm and nucleolus of mouse splenocytes and HeLa cells. Since the ADAR1 variants are differentially regulated during acute inflammation, it suggests that the localization of these variants and of A-to-I RNA editing in the cytoplasm, nucleus, and nucleolus is intracellularly reorganized in response to inflammatory stimulation.

LPS induces pulmonary intravascular macrophages producing inflammatory mediators via activating NF-kappaB

Pulmonary intravascular macrophages (PIMs) are often responsible for the clearance of blood-borne pathogens, including endotoxin, lipopolysaccharide of Gram-negative bacteria. It is well accepted that PIMs play a pivotal role in the pathogenesis of endotoxin-induced acute lung injury. However, the mechanisms by which PIMs are involved in the lipopolysaccharide-induced inflammatory responses remain unclear. Through the present study the following results were found: (1) When challenged with lipopolysaccharide (10 micrograms/ml), PIMs underwent marked cellular enlargement, intercellular adhesion plaques became longer, and some particulates were enwrapped in the pseudopods. (2) Lipopolysaccharide could up-regulate the expression of some inflammatory mediators in PIMs, including TNF-alpha, IL-1beta, IL-6, IL-8, and COX-2, and these up-regulated expression of inflammatory mediators correlated with NF-kappaB activation. (3) Dexamethasone as well as acetylsalicylic acid reduced the expression of TNF-alpha in lipopolysaccharide-challenged PIMs, and the decreased expression of TNF-alpha was also consistent with decreased NF-kappaB activation. Our results suggest that NF-kappaB activation in PIMs followed by phagocytizing lipopolysaccharide resulted in the up-regulation of TNF-alpha, IL-1beta, IL-6, IL-8, and COX-2, which could be alleviated by dexamethasone.

Enhancement of acute lung injury related to bacterial endotoxin by components of diesel exhaust particles

BACKGROUND

Diesel exhaust particles (DEP) synergistically aggravate acute lung injury related to lipopolysaccharide (LPS) in mice, but the components in DEP responsible for this have not been identified. A study was undertaken to examine the effects of the organic chemicals (DEP-OC) and residual carbonaceous nuclei (washed DEP) derived from DEP on LPS related lung injury.

METHODS

ICR mice were divided into experimental groups and vehicle, LPS, washed DEP, DEP-OC, washed DEP+LPS, and DEP-OC+LPS were administered intratracheally. The cellular profile of the bronchoalveolar lavage (BAL) fluid, pulmonary oedema, lung histology, and expression of proinflammatory molecules and Toll-like receptors in the lung were evaluated.

RESULTS

Both DEP-OC and washed DEP enhanced the infiltration of neutrophils into BAL fluid in the presence of LPS. Washed DEP combined with LPS synergistically exacerbated pulmonary oedema and induced alveolar haemorrhage, which was concomitant with the enhanced lung expression of interleukin-1beta, macrophage inflammatory protein-1alpha, macrophage chemoattractant protein-1, and keratinocyte chemoattractant, whereas DEP-OC combined with LPS did not. Gene expression of Toll-like receptors 2 and 4 was increased by combined treatment with washed DEP and LPS. The enhancement effects of washed DEP on LPS related changes were comparable to those of whole DEP.

CONCLUSIONS

These results suggest that the residual carbonaceous nuclei of DEP rather than the extracted organic chemicals predominantly contribute to the aggravation of LPS related lung injury. This may be mediated through the expression of proinflammatory cytokines, chemokines, and Toll-like receptors.

Influence of endotoxin-induced acute lung injury on pulmonary innate and adaptive immunity

OBJECTIVE

The aim of our study was to analyze pulmonary innate and adaptive immune responses during endotoxemia-induced acute lung injury (ALI).

EXPERIMENTAL DESIGN

Female BALB/c mice were challenged by endotoxin given intraperitoneally and followed for 24 h by unrestrained plethysmographic analysis. After this period, the mice were sacrificed by CO2 anesthesia and lung histopathology, pulmonary and serum levels of pro-inflammatory cytokines, numbers of lymphocyte subsets in blood and lung, and lung-derived macrophages (Mphi) were analyzed.

MAIN RESULTS

Animals with endotoxemia demonstrated significant depression of tidal volumes indicating respiratory failure compared to control mice. Lung histopathology of endotoxin-exposed animals revealed alveolar leakage characterizing ALI. Pulmonary levels of interleukin (IL)-1beta, IL-6 and interferon (IFN)-gamma in animals with endotoxemia were significantly elevated, whereas serum levels of IL-6 only were increased. IFN-gamma was strongly expressed by lung-derived Mphi with high CD11b expression, and this subset significantly increased in the lungs after endotoxin challenge. Additionally, the numbers of lung-resident CD4+ and total T-lymphocytes were significantly reduced after challenge.

CONCLUSION

These data suggest that endotoxemia-induced ALI is associated with exaggerated and sustained pulmonary innate immune responses partly mediated by activated Mphi, whereas adaptive immunity in the lungs is compromised.

Widespread inosine-containing mRNA in lymphocytes regulated by ADAR1 in response to inflammation

Adenosine-to-inosine (A-to-I) RNA editing is a post-transcriptional modification of pre-mRNA catalysed by an RNA-specific adenosine deaminase (ADAR). A-to-I RNA editing has been previously reported in the pre-mRNAs of brain glutamate and serotonin receptors and in lung tissue during inflammation. Here we report that systemic inflammation markedly induces inosine-containing mRNA to approximately 5% of adenosine in total mRNA. Induction was the result of up-regulation of A-to-I RNA editing as both dsRNA editing activity and ADAR1 expression were increased in the spleen, thymus and peripheral lymphocytes from endotoxin-treated mice. Up-regulation of ADAR1 was confirmed in vitro in T lymphocytes and macrophages stimulated with a variety of inflammatory mediators including tumour necrosis factor-alpha and interferon-gamma. A late induction of RNA editing was detected in concanavalin A-activated splenocytes stimulated with interleukin-2 in vitro. Taken together, these data suggest that a large number of inosine-containing mRNAs are produced during acute inflammation via up-regulation of ADAR1-mediated RNA editing. These events may affect the inflammatory and immune response through modulation of protein production.