INTRODUCTION
Blunt chest trauma resulting in pulmonary contusion is a common injury, affecting 10% to 17% of all trauma admissions, with estimates of mortality at 10% to 25% (1, 2). Sequelae of pulmonary contusion vary widely and include infection (pneumonia), local organ failure (acute respiratory distress syndrome [ARDS]), and remote organ failure (multiple organ dysfunction/failure) (3). In both animal and human studies, lung contusion results in activation of innate immunity with the localized production of a variety of inflammatory cytokines and chemokines, followed by neutrophil infiltration into the areas of injury (4-6). The magnitude of this response has been associated with both ARDS and infection (7, 8).
The innate inflammatory response to noninfectious tissue injury is, in many ways, indistinguishable from that initiated by infection. Activation of innate immunity during infection is mediated by activation of the Toll-like receptors (TLRs) (9, 10). At least 13 TLRs have been identified (11), recognizing conserved components of infectious bacteria [flagella, LPS, peptidoglycan (PGN)] and viruses (e.g., double-stranded RNA). All but TLR-3 use the intracellular adapter protein MyD88 to induce nuclear factor-κB (NF-κB) activation and subsequent inflammatory mediator expression (12). An evolving paradigm has suggested that endogenous TLR ligands are liberated after cell injury or necrosis, and the resulting "danger signal" serves to activate innate immune mechanisms warning the host of tissue injury (13). Indeed, evidence is accumulating that supports a role for TLR signaling after noninfectious lung injury including identification of a number of endogenous TLR ligands such as hyaluronan, heparin sulfate, heat shock protein 60 (HSP-60), HSP-70, high-mobility group box 1 (HMGB-1) protein, and surfactant protein-A, which are potentially liberated after lung trauma (14-19). We have previously demonstrated a role for TLR-2-induced activation of innate immunity after lung contusion involving TLR-2-dependent chemokine (C-X-C motif) ligand 1 (CXCL-1) expression and neutrophil influx to the injured lung (20). Complementary findings were seen after bleomycin-induced and lethal oxidant lung injury, in which TLR-2 and TLR-4 deficiency resulted in reduced pulmonary neutrophilia and CXC chemokine expression (14, 21, 22). In addition, exogenous expression of the TLR-2 and TLR-4 ligand, HSP-70, in pulmonary epithelial cells has been associated with improvement of ARDS and reduced pulmonary neutrophilia (23). These data clearly indicate that TLR-mediated events play an important part in the initial innate immune response to noninfectious lung injury.
Little is known about the role of TLR-4 and MyD88 in the inflammatory response to pulmonary contusion. The purpose of this study was to determine, using clinically relevant criteria, if TLR-4 and MyD88 participate in the response to blunt chest injury. We delivered a blunt chest injury to wild-type (WT) and mice deficient in TLR-4 (TLR-4−/−) or MyD88 (MyD88−/−) expressions and evaluated systemic inflammatory mediator expression, pulmonary neutrophil recruitment, and postinjury lung function. We found a TLR-4-dependent inflammatory response to pulmonary contusion that is characterized by edema, neutrophil infiltration, and increased expression of the innate immunity proinflammatory mediator IL-6 and the CXCL-1. We further demonstrate that these responses share a common signal transduction pathway that uses the TLR adapter protein, MyD88. Our results indicate that lung inflammation and systemic innate immune responses are dependent on TLR activation after pulmonary contusion.
METHODS
Animals
Male age-matched (8-9 weeks) WT (C57/BL6), TLR-4−/−, and MyD88−/− mice were used in this study. The MyD88−/− mice were provided by S. Akira (Osaka University, Osaka, Japan) and backcrossed onto a C57/BL6 background for at least eight generations. The TLR-4−/− mice were obtained from Jackson Laboratories (Bar Harbor, Maine), and the allele was introgressed into the C57/BL6 background for at least five generations. All animals were bred and maintained under specific pathogen-free conditions at the animal facility at Wake Forest University School of Medicine. The protocol used in this study was approved by the Animal Care and Use Committee (no. A06-068).
Blunt chest injury model
Injury was induced using the cortical contusion impactor (CCI) as described previously (20). Briefly, mice were anesthetized with 2% isoflurane at a flow rate of 1 L/min. The mouse is positioned left lateral decubitus, and during inspiration, the right side of the chest was struck with the CCI along the posterior axillary line, 1 cm above the costal margin. Control animals received anesthetic alone. Mice were followed for various times after injury as specified in the figure legends. Serum and tissue samples were collected at the time of death by isoflurane overdose and cervical dislocation.
Arterial blood gas
Arterial blood gas samples were obtained from the iliac artery. The animals were induced with isoflurane anesthesia (3%) and then maintained on an admixture of 1.5% isoflurane with 100% oxygen at a flow rate of 1 L/min. After 5 min, a midline laparotomy incision was created from just above the pubis to just below the sternum. The animal was eviscerated, and the iliac artery was exposed. A 26-gauge needle was then used to cannulate the iliac artery to obtain the blood. The partial pressure of oxygen (PO2) was measured in each sample using a Stat Profile pHOx Blood gas Analyzer (Nova Biomedical, Waltham, Mass) according to the manufacturer's instructions.
Histopathology
Lung specimens harvested at time of death were fixed in 10% formalin, sectioned, and stained with hematoxylin and eosin. Slides were evaluated by an experienced pathologist (N.A.B.) and graded for the presence of interstitial neutrophilic infiltrate, intra-alveolar hemorrhage, and pulmonary septal edema as described previously (20).
Bronchoalveolar lavage
After sacrifice, bronchoalveolar lavage (BAL) was performed by cannulation of the trachea, and lavage was performed using 4 mL of phosphate buffered saline (Sigma-Aldrich, St Louis, Mo) at 4°C. The BAL was centrifuged at 300g, 4°C for 10 min, and the supernatant was collected and stored at −70°C until use. The cell pellet was counted and differentiated as previously described (20).
Lung wet-dry weights
Whole lung specimens were removed, weighed, and placed in an oven at 37°C for 24 h to dry and then reweighed. Wet weight-dry weight ratios were calculated.
Cytokine and chemokine expression
The BAL and/or serum levels of IL-6 and CXCL-1 were measured using commercially available enzyme-linked immunosorbent assay (ELISA) kits (R&D Systems, Minneapolis, Minn) according to the manufacturer's instructions. Samples were assayed in duplicate.
Immunoblot (Western blot) analysis
Lung tissue samples were thawed and suspended in homogenization buffer (25 mM Tris-HCl pH 7.6, 150 mM NaCl, 1% NP-40, 0.1% sodium dodecyl sulfate, protease inhibitors) and homogenized (TissueMiser, Fisher Scientific, Pittsburgh, Pa). The homogenate was centrifuged at 3,000g, 4°C for 10 min, and the supernatant was centrifuged again at 10,000g, 4°C for 10 min. Solubilized protein concentrations were determined using Coomassie protein assay reagent according to the manufacturer's instructions (Pierce, Rockford, Ill). Proteins were separated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and transferred to an Immuno-Blot polyvinylidene difluoride membrane (Bio-Rad Laboratories, Hercules, Calif). Blots were blocked for 1 h with Odyssey blocking buffer (Li-Cor Biosciences, Lincoln, Nebr). Anti-neutrophil elastase (Santa Cruz Biotechnology, Santa Cruz, Calif) and anti-β-actin (Sigma-Aldrich, St Louis, Mo) antibodies were diluted in Odyssey blocking buffer with 0.1% Tween-20, and the membranes were incubated overnight at 4°C. Antibody-antigen complexes were detected using appropriately matched IRDye secondary antibodies (Li-Cor Biosciences) and visualized using the Odyssey infrared imaging system (Li-Cor Biosciences). Relative protein levels were quantitated using Odyssey v1.2 software (Li-Cor Biosciences) and reported as integrated intensity.
Immunohistochemistry
Lung sections were deparaffinized with three changes of xylenes and were hydrated through a series of graded alcohol changes (three washes of 100% EtOH for 5 min, 95% EtOH for 5 min, and 75% EtOH for 5 min). Endogenous peroxidase was quenched with 0.3% H2O2 in MeOH for 30 min. An antigen retrieval step was performed by heating sections in 10 mM citrate buffer pH 6.0 for 45 min. Sections were incubated with a 1:50 dilution of anti-neutrophil elastase antibody (Santa Cruz Biotechnology) and visualized using VECTASTAIN ABC and diaminobenzidine peroxidase substrate kits (Vector Laboratories, Burlingame, Calif) according to the manufacturer's instructions. Sections were counterstained with Mayer hematoxylin solution (Sigma-Aldrich, St Louis, Mo) and mounted using VectaMount mounting medium (Vector Laboratories).
Statistical analysis
Data are reported using GraphPad Prism (v4.03, San Diego, Calif) and expressed as the mean ±SE of independent observations as indicated in the figure legends. One-way ANOVA with multiple-comparison post-test (Bonferroni) was used to compare the means between injury groups. A value of P < 0.05 was considered significant.
RESULTS
Lung dysfunction after pulmonary contusion was dependent upon the TLR-4 pathway
We used the CCI to induce a pulmonary contusion in our mouse model of blunt chest injury. Blood gas measurements showed that hypoxia induced by contusion was significant at 3 h after injury and continued to increase for at least 24 h (Fig. 1A). We found significantly less hypoxia in TLR-4−/− animals, suggesting that TLR-4 participates in the response to acute lung injury. Consistent with this observation, we further found that MyD88 deficiency also reduced hypoxia in injured animals (compare TLR-4−/− and MyD88−/− with WT, Fig. 1B). As a further indicator of acute lung injury, pulmonary edema was significantly elevated with blunt chest injury, but significantly less edema was found in MyD88−/− mice (Fig. 1C). These results showed that acute lung injury activated TLR-4 and intracellular signaling through MyD88.
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Fig. 1: Lung dysfunction after pulmonary contusion was dependent on the TLR-4 pathway. Arterial blood from uninjured (open) and injured animals (closed) was isolated, and blood gas analysis was performed as described in the Methods. A, PaO2:FiO2 ratios were calculated and were shown for WT animals at 3, 6, and 24 h after injury (n = 5 per time). There was a significant decrease in PaO2:FiO2 in WT animals after injury (*P ≤ 0.01), with the most significant decrease at 24 h (**P < 0.01 relative to other injury times). B, Blood gas analysis on injured TLR-4−/− and MyD88−/− animals (gray) at 24 h shows significantly increased PaO2:FiO2 ratios when compared with WT injured animals (#P ≤ 0.05, n = 7 per genotype). C, Wild-type (black), TLR-4−/−, and MyD88−/− (gray) injured lungs were isolated at 24 h after injury and weighed before (wet weight) and after drying (dry weight) as described in the Methods (n = 5 per genotype). Pulmonary edema (wet weight/dry weight) was significant (*P ≤ 05) in all animals and was significantly decreased in MyD88−/− mice (**P < 0.05 compared with WT).
Lung pathology after pulmonary contusion was dependent on the TLR-4 pathway
Acute lung injury is often correlated with the presence of neutrophils in the BAL of patients with pulmonary contusion. In our animal model, we evaluated pulmonary histology and BAL specimens for the presence of a neutrophilic infiltrate after injury. Histological examination of injured lungs from WT mice showed evidence of intra-alveolar hemorrhage, alveolar septal edema, and neutrophilic infiltrate as early as 3 h, with maximum pathologies observed at 24 h after injury (Fig. 2). Injured lungs from TLR-4−/− and MyD88−/− mice also showed evidence of hemorrhage, edema, and neutrophils. Pathological evaluation (blinded) of lung injury in knockout (KO) animals reported similar intra-alveolar hemorrhage to WT mice but reduced alveolar septal edema and neutrophil infiltration.
Fig. 2: Lung pathology after pulmonary contusion was dependent on the TLR-4 pathway. Injured lungs were formalin-fixed, stained, and analyzed by an experienced pathologist. Lung sections of the contusion were shown (hematoxylin-eosin, original magnification Ă—100) at 24 h after injury. Hemorrhage was evident in all injuries (left panels) and masked by red filter (right panels) to highlight neutrophil staining. Septal edema and neutrophil infiltration was reduced in TLR-4−/− and MyD88−/− mice when compared with WT. Specimens shown were representative of lung injuries in at least three animals per genotype.
Neutrophil recruitment to the lung after pulmonary contusion was dependent on the TLR-4 pathway
To quantitate neutrophil recruitment after injury, we used BAL to measure alveolar neutrophil infiltration. As found in our previous studies (4, 20), we observed a significant increase in neutrophils in the BAL from injured mice (Fig. 3A). The number of BAL neutrophils was significantly decreased in TLR-4−/− and MyD88−/− mice, supporting that the local inflammatory response is dependent on TLR activation. To further assess the role of neutrophils in our model of pulmonary contusion, we used immunoblot to measure neutrophil elastase levels in injured tissue. As shown in Figure 3B, we found low levels of elastase in tissue homogenates from uninjured lungs. Consistent with increased neutrophil recruitment and activation by pulmonary contusion, we found increased levels of elastase in injured lung tissue. Elastase levels were decreased in TLR-4−/− and MyD88−/− (KO) mice. Immunohistochemical staining of lung tissue for neutrophil elastase (Fig. 3C) was consistent with these results. Taken together, these results showed that blunt chest injury and pulmonary contusion significantly increase neutrophil infiltration and activation. The recruitment of neutrophils in this model of acute lung injury was dependent on the TLR-4 pathway and intracellular signaling through MyD88.
Fig. 3: Neutrophil recruitment to the lung after pulmonary contusion was dependent on the TLR-4 pathway. A, The BAL from WT (closed) and TLR-4−/− and MyD88−/− (gray, KO) injured lungs were analyzed for neutrophil infiltration. Compared with uninjured mice (n = 5), a significant increase in neutrophils in the BAL from WT mice was observed (**P ≤ 0.001) that was significantly decreased in KO mice (*P ≤ 0.001, n = 6 per genotype). B, Lung homogenates from uninjured (open), and WT (closed), TLR-4−/−, and MyD88−/− (gray) injured lungs were analyzed by immunoblot for neutrophil elastase (n=4). A significant increase in elastase in WT lung extracts was observed (**P ≤ 0.001) that was significantly decreased in KO mice (* P ≤ 0.01). C, Immunostaining for neutrophil elastase in lungs from WT, TLR-4−/−, or MyD88−/− animals (hematoxylin-eosin, original magnification Ă—100). Specimens were representative of immunohistochemistry for least three animals per genotype. Data shown were obtained at 24 h after injury.
The TLR-4 pathway was required for increased pulmonary and systemic IL-6 and CXCL-1 levels after pulmonary contusion
Elaboration of cytokines and chemokines was characteristic of inflammatory responses observed in patients with pulmonary contusion and in experimental models of acute lung injury. We measured IL-6, CXCL-1, CXCL-2/3, and macrophage inflammatory protein-1α levels in the BAL from mice at various times after injury. Overall, we found the only significant and measurable increases in IL-6 and CXCL-1 in the BAL at 3 h after injury (Fig. 4). We observed significant increases in these mediators in the BAL from WT mice when compared with KO mice, supporting that local inflammatory responses are dependent on the TLR-4 pathway and MyD88 intracellular signaling. Finally, we measured levels of inflammatory mediators in the blood after pulmonary contusion. In the serum of injured animals, we found a rapid, robust, and transient increase in both IL-6 and CXCL-1 (Fig. 5). These levels were significantly decreased in TLR-4−/− and MyD88−/− mice. Levels of CXCL-2/3 and macrophage inflammatory protein-1α were not elevated in the BAL or the serum (data not shown), suggesting that these chemokines are not involved in the inflammatory response to lung contusion. These results show that systemic mediators of inflammation are increased in blunt chest injury with pulmonary contusion. Taken together, our results show that local and systemic responses to injury are dependent, at least in part, to TLR-4 activation and are mediated by the intracellular adapter MyD88.
Fig. 4: The TLR-4 pathway was required for pulmonary inflammatory response to contusion. IL-6 and CXCL-1 in the BAL from uninjured (open) and injured (filled) animals (n = 5 per genotype) were measured by ELISA as described in the Methods. Levels in all samples were low, and results were reported if the difference between uninjured and injured was statistically significant (**P ≤ 0.001). A, Levels of IL-6 in the BAL from injured MyD88−/− animals (gray) at 3 h show a significant decrease when compared with WT injured animals (*P ≤ 0.01). B, Levels of CXCL-1 in the BAL from injured TLR-4−/− and MyD88−/− animals (gray) at 3 h show a significant decrease when compared with WT injured animals (*P ≤ 0.001). Uninjured (IL-6 and CXCL-1) and CXCL-1 in MyD88−/− animals' BAL was below sensitivity (2 pg/mL) of the assay.
Fig. 5: The TLR-4 pathway was required for systemic innate immune responses to pulmonary contusion. Innate immune proteins in the serum of uninjured (open) and after injury (closed) were measured by ELISA as described in the Methods. A rapid and significant (**P ≤ 0.001) increase in IL-6 (A) and CXCL-1 (B) levels in the serum from injured WT mice at 3 h was observed. Levels of IL-6 and CXCL-1 in the serum from KO animals at 3 h were significantly less (*P ≤ 0.001). A, Three hours, n = 9 per genotype; 24 h, n = 5 per genotype. B, Three hours, n = 8 per genotype; 24 h, n = 6 per genotype.
DISCUSSION
Mounting evidence supports a role for TLR-mediated signaling in the initial inflammatory response to noninfectious tissue injury, with numerous potential endogenous TLR ligands being identified (14-19). A paradigm is evolving in which these endogenous ligands, when liberated from injured or necrotic tissue, act as danger signals warning the host of tissue injury and initiating the ensuing inflammatory response (13). We have used a mouse model of blunt chest trauma to study the initial innate inflammatory response as might be observed in patients with pulmonary contusion. We found that our model approximates the human injury using pathophysiological and biochemical markers. Furthermore, our previous studies showed that the innate inflammatory response to pulmonary contusion depends on TLR-2-induced signaling (20). We found that TLR-2−/− animals have a less severe lung injury. The reduction in neutrophil chemotaxis to the contused lung correlated with decreased CXCL-1 expression and seemed to depend, at least in part, on TLR-2 activation.
Having previously identified a role for TLR-2-dependent signaling, in this report, we have extended our investigations into the mechanisms regulating the response to traumatic lung injury and pulmonary contusion. As the TLR-4 pathway shares many components with TLR-2 signaling, we hypothesized that TLR-4 may also participate in this injury response. We tested for TLR-4-dependent responses in our injury model and found a clinically less severe lung injury, reduced pulmonary neutrophilia, and a diminished innate response in TLR-4−/− mice. In a manner similar to TLR-2, TLR-4 activation by pulmonary contusion seems to direct neutrophil migration to the lung through the expression of the chemokine CXCL-1 and independent of CXCL-2/3. This is consistent with TLR-4-mediated signaling that has been implicated in several other models of noninfectious tissue injury and systemic inflammation (17, 18, 24-29). Specifically, in bleomycin-induced and hyperoxic models of lung injury, TLR-4 deficiency was associated with reduced pulmonary neutrophilia, diminished chemokine expression, and increased mortality. However, our study supports a model where TLR-4 deficiency is protective in a primary acute lung injury model, as demonstrated by better arterial blood gas seen after lung contusion, with no differences seen in mortality over the study period. Others have reported findings similar to ours in models of I/R and sterile tissue injury, where the absence of TLR-4 is associated with a reduced innate inflammatory response and improved organ function. One explanation for the divergent findings seen in noninfectious lung models involves differing mechanisms of injury with a greater dependence upon neutrophil-mediated tissue injury after contusion.
Our results support the concept that neutrophil recruitment to the lung after blunt chest injury is dependent on CXCL-1 and independent of CXCL-2/3. There are four major chemokine families, and in general, neutrophil chemotaxis is highly dependent upon CXC chemokine expression (30). The CXCL-1 and CXCL-2/3 are the murine equivalents of IL-8, the principal chemokine involved in neutrophil chemotaxis after traumatic lung injury in humans (31). Alveolar macrophages and, to a lesser extent, alveolar epithelial cells have been demonstrated to produce CXC chemokines after lung injury (32). Both CXCL-1 and CXCL-2/3 have been demonstrated to be elevated in other models of infectious and noninfectious lung injury, and divergent roles for both have been described (14, 22, 33-35). Our results showed that TLR-4- and MyD88-induced neutrophil recruitment to the lung after contusion is dependent on CXCL-1, raising the possibility that differing pharmacokinetics, ligand specificity, cellular involvement, and/or disruption of normal cell-cell interactions occur after pulmonary contusion when compared with other lung injury models. The involvement of other chemokines such as leukotriene B4 or C5a cannot be excluded based upon our results.
Activation of TLR-4 initiates intracellular signaling by recruitment of one or more adaptor proteins. MyD88 was the first adapter protein identified as involved in TLR-4 signal transduction and NF-κB activation (9, 10). Toll-like receptor 4 uses both MyD88-dependent and MyD88-independent signaling pathways that result in activation of NF-κB and subsequent inflammatory cytokine expression. MyD88 also participates in TLR-2 signaling. In our injury model, MyD88−/− animals showed improved oxygenation as well as reduced pulmonary neutrophil influx and innate response when compared with animals deficient in TLR-4. These findings support that TLR-4 uses the MyD88-dependent signaling pathway when activated after lung contusion; however, the MyD88-independent pathway using the adapter protein, TRIF, cannot be completely excluded. Our study also supports that the response to lung contusion is not entirely dependent on TLR-4 signaling, but other receptors are likely involved in mediating this response. MyD88 is a component of the intracellular signaling pathways for IL-1, IL-18, and all known TLRs, with the exception of TLR-3. We have previously shown that the innate immune response to pulmonary contusion is also TLR-2 dependent, and the more protective effect of MyD88 deficiency may reflect the sharing of this intracellular mediator by TLR-2 and TLR-4.
We are uncertain how TLR signaling is initiated in the lung after contusion and what, if any, coreceptors are used. Thus far, HMGB-1 and hyaluronan are potential TLR-4 activators in models of soft tissue injury; however, a large number of endogenous TLR-4 ligands exist, including surfactant, heat shock, heme, and other matrix proteins, and HMGB-1 (14-19). Many of these mediators are likely present in the lung after contusion, and some of these ligands have specificity for more than one TLR, likely with differing affinities. Thus, it is possible that several different ligands act in concert through different TLRs to generate a response. Ligand gradients and localization may also contribute to injury responses. Participation of coreceptors such as CD14 in the TLR-4 response to pulmonary contusion also remains a possibility. For example, surfactant proteins are endogenous TLR-4 ligands that require CD14 to activate TLR signaling (36, 37). In contrast, other TLR-4 ligands such as HMGB-1 and hyaluronan are CD14 independent after sterile tissue injury (17, 18). Low-molecular weight hyaluronan fragments are thought to promote formation of a TLR-4 complex with MD-2 and CD44 (17) for signal transduction.
Cell type specificity of the TLR response to noninfectious injury is also not known. Alveolar macrophages, type II alveolar epithelial cells, pulmonary endothelial cells, and tracheobronchial cells participate in the lung's innate response to both infectious and noninfectious lung injury (14, 32, 34, 38, 39). Germane to this report, Mollen et al. (13) used chimeric mice in a model of hemorrhagic shock and bilateral femur fracture to address the question of cell type-specific responses. They found that both bone marrow-derived and parenchymal cells are required to generate the ensuing TLR-dependent inflammatory response, suggesting that multiple cell types are involved in the generation of the innate response to noninfectious lung injury. Similar findings were reported by Wu et al. (25) in a model of renal I/R; however, it was suggested that renal parenchymal cells, rather than immune cells, initiated the inflammatory response. Thus, as there are likely multiple endogenous ligands and TLRs involved in the response to pulmonary contusion, there seem to be multiple cell types acting in concert that are responsible for the inflammatory response to noninfectious lung injury.
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