Various adhesion molecules impair microvascular leukocyte kinetics in ventilator-induced lung injury

Although the endothelial expression of various adhesion molecules substantially differs between pulmonary microvessels, their importance for neutrophil and lymphocyte sequestration in ventilator-induced lung injury (VILI) has not been systematically analyzed. We investigated the kinetics of polymorphonuclear cells (PMN) and mononuclear cells (MN) in the acinar microcirculation of the isolated rat lung with VILI by real-time confocal laser fluorescence microscopy, with or without inhibition of ICAM-1, VCAM-1, or P-selectin by monoclonal antibodies (MAb). Adhesion molecules in each microvessel were estimated by intravital fluorescence microscopy or immunohistochemical staining. In high tidal volume-ventilated lungs, 1) ICAM-1, VCAM-1, and P-selectin were differently upregulated in venules, arterioles, and capillaries; 2) venular PMN rolling was improved by inhibition of ICAM-1, VCAM-1, or P-selectin, whereas arteriolar PMN rolling was improved by ICAM-1 or VCAM-1 inhibition; 3) capillary PMN entrapment was ameliorated only by anti-ICAM-1 MAb; and 4) MN rolling in venules and arterioles and MN entrapment in capillaries were improved by ICAM-1 and VCAM-1 inhibition. In conclusion, the contribution of endothelial adhesion molecules to abnormal leukocyte behavior in VILI-injured microcirculation is microvessel and leukocyte specific. ICAM-1- and VCAM-1-dependent, but P-selectin-independent, arteriolar PMN rolling, which is expected to reflect the initial stage of tissue injury, should be taken as a phenomenon unique to ventilator-associated lung injury.     

Angiotensin-converting enzyme inhibitor captopril attenuates ventilator-induced lung injury in rats.

We hypothesized that lung inflammation and parenchymal apoptosis in ventilator-induced lung injury (VILI) are related to ANG II and assessed the ability of the angiotensin-converting enzyme inhibitor captopril to attenuate VILI in rats. Adult male Sprague-Dawley rats were randomized to receive two ventilation strategies for 2 h: 1) tidal volume of 40 ml/kg, respiratory rate of 25 breaths/min, and inspiratory O2 fraction of 0.21 [high-volume, 0 positive end-expiratory pressure (HVZP) group] and 2) injection of captopril (100 mg/kg ip) 30 min before HVZP ventilation (HVZP+CAP group). Another group, which did not receive ventilation, served as the control. Mean arterial pressure was significantly lower in the HVZP+CAP group than in the HVZP group at 2 h of ventilation. Total protein levels were significantly higher in bronchoalveolar lavage fluid (BALF) recovered from HVZP-ventilated rats than from controls. BALF macrophage inflammatory protein-2 and lung ANG II were significantly higher in the HVZP group than in the control and HVZP+CAP groups. Lung ANG II levels correlated positively with BALF protein and macrophage inflammatory protein-2. The number of apoptotic airway and alveolar wall cells was significantly higher in the HVZP and HVZP+CAP groups than in the control group and significantly lower in the HVZP+CAP group than in the HVZP group. These results suggest that the efficiency of captopril to attenuate VILI is related to reduction of inflammatory cytokines and inhibition of apoptosis and indicate that VILI is partly mediated by the local angiotensin system.     

Intratracheal anti-tumor necrosis factor-alpha antibody attenuates ventilator-induced lung injury in rabbits.

To evaluate the role of tumor necrosis factor (TNF)-alpha in the pathogenesis of ventilator-induced lung injury, we 1) measured TNF-alpha production in the lung caused by conventional mechanical ventilation (CMV) and 2) evaluated the protective effect of anti-TNF-alpha antibody (Ab) in saline-lavaged rabbit lungs. After they received saline lung lavage, rabbits were intratracheally instilled with 1 mg/kg of polyclonal anti-TNF-alpha Ab in the high-dose group (n = 6), 0.2 mg/kg of anti-TNF-alpha Ab in the low-dose group (n = 6), serum IgG fraction in the Ab control group (n = 6), and saline in the saline control group (n = 7). Animals then underwent CMV for 4 h. Levels of TNF-alpha in lung lavage fluid were significantly higher after CMV than before in both control groups. Pretreatment with intratracheal instillation of high and low doses of anti-TNF-alpha Ab improved oxygenation and respiratory compliance, reduced the infiltration of leukocytes, and ameliorated pathological findings. CMV led to TNF-alpha production in the lungs, and intratracheal instillation of anti-TNF-alpha Ab attenuated CMV-induced lung injury in this model.     

Soluble guanylyl cyclase contributes to ventilator-induced lung injury in mice

High tidal volume (HV(T)) ventilation causes pulmonary endothelial barrier dysfunction. HV(T) ventilation also increases lung nitric oxide (NO) and cGMP. NO contributes to HV(T) lung injury, but the role of cGMP is unknown. In the current study, ventilation of isolated C57BL/6 mouse lungs increased perfusate cGMP as a function of V(T). Ventilation with 20 ml/kg V(T) for 80 min increased the filtration coefficient (K(f)), an index of vascular permeability. The increased cGMP and K(f) caused by HV(T) were attenuated by nitric oxide synthase (NOS) inhibition and in lungs from endothelial NOS knockout mice. Inhibition of soluble guanylyl cyclase (sGC) in wild-type lungs (10 muM ODQ) also blocked cGMP generation and inhibited the increase in K(f), suggesting an injurious role for sGC-derived cGMP. sGC inhibition also attenuated lung Evans blue dye albumin extravasation and wet-to-dry weight ratio in an anesthetized mouse model of HV(T) injury. Additional activation of sGC (1.5 muM BAY 41-2272) in isolated lungs at 40 min increased cGMP production and K(f) in lungs ventilated with 15 ml/kg V(T). HV(T) endothelial barrier dysfunction was attenuated with a nonspecific phosphodiesterase (PDE) inhibitor (100 muM IBMX) as well as an inhibitor (10 muM BAY 60-7550) specific for the cGMP-stimulated PDE2A. Concordantly, we found a V(T)-dependent increase in lung cAMP hydrolytic activity and PDE2A protein expression with a decrease in lung cAMP concentration that was blocked by BAY 60-7550. We conclude that HV(T)-induced endothelial barrier dysfunction resulted from a simultaneous increase in NO/sGC-derived cGMP and PDE2A expression causing decreased cAMP.     

Mechanisms of early pulmonary neutrophil sequestration in ventilator-induced lung injury in mice

Polymorphonuclear leukocytes (PMN) play an important role in ventilator-induced lung injury (VILI), but the mechanisms of pulmonary PMN recruitment, particularly early intravascular PMN sequestration during VILI, have not been elucidated. We investigated the physiological and molecular mechanisms of pulmonary PMN sequestration in an in vivo mouse model of VILI. Anesthetized C57/BL6 mice were ventilated for 1 h with high tidal volume (injurious ventilation), low tidal volume and high positive end-expiratory pressure (protective ventilation), or normal tidal volume (control ventilation). Pulmonary PMN sequestration analyzed by flow cytometry of lung cell suspensions was substantially enhanced in injurious ventilation compared with protective and control ventilation, preceding development of physiological signs of lung injury. Anesthetized, spontaneously breathing mice with continuous positive airway pressure demonstrated that raised alveolar pressure alone does not induce PMN entrapment. In vitro leukocyte deformability assay indicated stiffening of circulating leukocytes in injurious ventilation compared with control ventilation. PMN sequestration in injurious ventilation was markedly inhibited by administration of anti-L-selectin antibody, but not by anti-CD18 antibody. These results suggest that mechanical ventilatory stress initiates pulmonary PMN sequestration early in the course of VILI, and this phenomenon is associated with stretch-induced inflammatory events leading to PMN stiffening and mediated by L-selectin-dependent but CD18-independent mechanisms.     

高密度脂蛋白减轻小鼠内毒素性急性肺损伤

高密度脂蛋白(high density lipoprotein, HDL)是一种血浆含量丰富的脂蛋白,通常认为它可在体内发挥抗炎作用,能够与内毒素结合而抑制其生物活性.为探讨人HDL对内毒素性急性肺损伤的影响,将昆明小鼠分为假手术对照组、脂多糖(lipopolysaccharide, LPS)组、HDL组和LPS HDL组,腹腔注射LPS(10mg/kg体重)复制内毒素性急性肺损伤模型,于腹腔注射LPS 30min后经尾静脉给予人血浆HDL(70mg/kg体重),6h后结束实验.处死动物前抽取动脉血测定血气变化(PaO2, pH, PaCO2).处死后行支气管肺泡灌洗,计数灌洗液中白细胞(white blood cell, WBC)数量,测定蛋白含量和乳酸脱氢酶(lactate dehydrogenase, LDH)活性,并取肺组织进行病理学观察,测定肺组织湿/干重比值、丙二醛(malondialdehyde, MDA)含量、髓过氧化酶(myeloperoxidase, MPO)活性和肿瘤坏死因子-α(tumor necrosis factor α, TNF-α)含量.结果显示:(1)HDL改善小鼠肺换气功能,显著降低LPS所致的PaO2、pH的降低和PaCO2的增高(P<0.01);(2)HDL显著抑制LPS所致的肺泡灌洗液中WBC数量、总蛋白浓度和LDH活性的增高(P<0.01),降低肺组织湿/干重比值、MPO活性、MDA和TNF-α含量(P<0.05, P<0.01);(3)病理形态学分析及评分显示,HDL治疗组小鼠在出血、肺水肿及肺组织内中性粒细胞浸润的程度均低于LPS所致肺损伤组(P<0.01).结果提示,HDL可减轻小鼠内毒素性急性肺损伤.     

TRPV4 initiates the acute calcium-dependent permeability increase during ventilator-induced lung injury in isolated mouse lungs

We have previously implicated calcium entry through stretch-activated cation channels in initiating the acute pulmonary vascular permeability increase in response to high peak inflation pressure (PIP) ventilation. However, the molecular identity of the channel is not known. We hypothesized that the transient receptor potential vanilloid-4 (TRPV4) channel may initiate this acute permeability increase because endothelial calcium entry through TRPV4 channels occurs in response to hypotonic mechanical stress, heat, and P-450 epoxygenase metabolites of arachidonic acid. Therefore, permeability was assessed by measuring the filtration coefficient (K(f)) in isolated perfused lungs of C57BL/6 mice after 30-min ventilation periods of 9, 25, and 35 cmH(2)O PIP at both 35 degrees C and 40 degrees C. Ventilation with 35 cmH(2)O PIP increased K(f) by 2.2-fold at 35 degrees C and 3.3-fold at 40 degrees C compared with baseline, but K(f) increased significantly with time at 40 degrees C with 9 cmH(2)O PIP. Pretreatment with inhibitors of TRPV4 (ruthenium red), arachidonic acid production (methanandamide), or P-450 epoxygenases (miconazole) prevented the increases in K(f). In TRPV4(-/-) knockout mice, the high PIP ventilation protocol did not increase K(f) at either temperature. We have also found that lung distention caused Ca(2+) entry in isolated mouse lungs, as measured by ratiometric fluorescence microscopy, which was absent in TRPV4(-/-) and ruthenium red-treated lungs. Alveolar and perivascular edema was significantly reduced in TRPV4(-/-) lungs. We conclude that rapid calcium entry through TRPV4 channels is a major determinant of the acute vascular permeability increase in lungs following high PIP ventilation.     

Linking lung function and inflammatory responses in ventilator-induced lung injury

Despite decades of research, the mechanisms of ventilator-induced lung injury are poorly understood. We used strain-dependent responses to mechanical ventilation in mice to identify associations between mechanical and inflammatory responses in the lung. BALB/c, C57BL/6, and 129/Sv mice were ventilated using a protective [low tidal volume and moderate positive end-expiratory pressure (PEEP) and recruitment maneuvers] or injurious (high tidal volume and zero PEEP) ventilation strategy. Lung mechanics and lung volume were monitored using the forced oscillation technique and plethysmography, respectively. Inflammation was assessed by measuring numbers of inflammatory cells, cytokine (IL-6, IL-1β, and TNF-α) levels, and protein content of the BAL. Principal components factor analysis was used to identify independent associations between lung function and inflammation. Mechanical and inflammatory responses in the lung were dependent on ventilation strategy and mouse strain. Three factors were identified linking 1) pulmonary edema, protein leak, and macrophages, 2) atelectasis, IL-6, and TNF-α, and 3) IL-1β and neutrophils, which were independent of responses in lung mechanics. This approach has allowed us to identify specific inflammatory responses that are independently associated with overstretch of the lung parenchyma and loss of lung volume. These data provide critical insight into the mechanical responses in the lung that drive local inflammation in ventilator-induced lung injury and the basis for future mechanistic studies in this field.     

Anti-inflammatory effects of clarithromycin in ventilator-induced lung injury

Background

Mechanical ventilation can promote lung injury by triggering a pro-inflammatory response. Macrolides may exert some immunomodulatory effects and have shown significant benefits over other antibiotics in ventilated patients. We hypothesized that macrolides could decrease ventilator-induced lung injury.

Methods

Adult mice were treated with vehicle, clarithromycin or levofloxacin, and randomized to receive mechanical ventilation with low (12 cmH₂O, PEEP 2 cmH₂O) or high (20 cmH₂O, ZEEP) inspiratory pressures for 150 minutes. Histological lung injury, neutrophil infiltration, inflammatory mediators (NFκB activation, Cxcl2, IL-10) and levels of adhesion molecules (E-selectin, ICAM) and proteases (MMP-9 and MMP-2) were analyzed.

Results

There were no differences among groups after low-pressure ventilation. Clarithromycin significantly decreased lung injury score and neutrophil count, compared to vehicle or levofloxacin, after high-pressure ventilation. Cxcl2 expression and MMP-2 and MMP-9 levels increased and IL-10 decreased after injurious ventilation, with no significant differences among treatment groups. Both clarithromycin and levofloxacin dampened the increase in NFκB activation observed in non-treated animals submitted to injurious ventilation. E-selectin levels increased after high pressure ventilation in vehicle- and levofloxacin-treated mice, but not in those receiving clarithromycin.

Conclusions

Clarithromycin ameliorates ventilator-induced lung injury and decreases neutrophil recruitment into the alveolar spaces. This could explain the advantages of macrolides in patients with acute lung injury and mechanical ventilation.     

Choosing the frequency of deep inflation in mice: balancing recruitment against ventilator-induced lung injury

Low tidal volume (Vt) ventilation is protective against ventilator-induced lung injury but can promote development of atelectasis. Periodic deep inflation (DI) can open the lung, but if delivered too frequently may cause damage via repeated overdistention. We therefore examined the effects of varying DI frequency on lung mechanics, gas exchange, and biomarkers of injury in mice. C57BL/6 males were mechanically ventilated with positive end-expiratory pressure (PEEP) of 2 cmH2O for 2 h. One high Vt group received a DI with each breath (HV). Low Vt groups received 2 DIs after each hour of ventilation (LV) or 2 DIs every minute (LVDI). Control groups included a nonventilated surgical sham and a group receiving high Vt with zero PEEP (HVZP). Respiratory impedance was measured every 4 min, from which tissue elastance (H) and damping (G) were derived. G and H rose progressively during LV and HVZP, but returned to baseline after hourly DI during LV. During LVDI and HV, G and H remained low and gas exchange was superior to that of LV. Bronchoalveolar lavage fluid protein was elevated in HV and HVZP but was not different between LV and LVDI. Lung tissue IL-6 and IL-1beta levels were elevated in HVZP and lower in LVDI compared with LV. We conclude that frequent DI can safely improve gas exchange and lung mechanics and may confer protection from biotrauma. Differences between LVDI and HV suggest that an optimal frequency range of DI exists, within which the benefits of maintaining an open lung outweigh injury incurred from overdistention.     

Retinoic acid attenuates the mild hyperoxic lung injury in newborn mice

Neonatal exposure to hyperoxia alters lung development in mice. We tested if retinoic acid (RA) treatment is capable to affect lung development after hyperoxic injury and to maintain structural integrity of lung. The gene of vascular endothelial growth factor A (VEGF-A) is one of the RA-responsive genes. Newborn BALB/c mice were exposed to room air, 40 % or 80 % hyperoxia for 7 days. One half of animals in each group received 500 mg/kg retinoic acid from day 3 to day 7 of the experiment. At the end of experiment we assessed body weight (BW), lung wet weight (LW), the wet-to-dry lung weight ratio (W/D) and the expression of mRNA for VEGF-A and G3PDH genes. On day 7 the hyperoxia-exposed sham-treated mice (group 80) weighed 20 % less than the room air-exposed group, whereas the 80 % hyperoxic group treated with RA weighed only 13 % less than the normoxic group. W/D values in 80 and 80A groups did not differ, although they both differed from the control group and from 40 groups. There was a significant difference between 40 and 40A groups, but the control group was different from 40 group but not from 40A groups. The 80 and 80A groups had mRNA VEGF-A expression lowered to 64 % and 41 % of the control group. RA treatment of normoxic and mild hyperoxic groups increased mRNA VEGF-A expression by about 50 %. We conclude that the retinoic acid treatment of newborn BALB/c mice exposed for 7 days to 80 % hyperoxia reduced the growth retardation in the 80 % hyperoxic group, reduced the W/D ratio in the 40 % but not in the 80 % hyperoxic group. Higher VEGF-A mRNA expression in the 80 % hyperoxic group treated with RA was not significant compared to the 80 % hyperoxic group.     

Lung-derived soluble mediators are pathogenic in ventilator-induced lung injury

Ventilator-induced lung injury (VILI) due to high tidal volume (V(T)) is associated with increased levels of circulating factors that may contribute to, or be markers of, injury. This study investigated if exclusively lung-derived circulating factors produced during high V(T) ventilation can cause or worsen VILI. In isolated perfused mouse lungs, recirculation of perfusate worsened injury (compliance impairment, microvascular permeability, edema) induced by high V(T). Perfusate collected from lungs ventilated with high V(T) and used to perfuse lungs ventilated with low V(T) caused similar compliance impairment and permeability and caused a dose-dependent decrease in transepithelial electrical resistance (TER) across rat distal lung epithelial monolayers. Circulating soluble factors derived from the isolated lung thus contributed to VILI and had deleterious effects on the lung epithelial barrier. These data demonstrate transferability of an injury initially caused exclusively by mechanical ventilation and provides novel evidence for the biotrauma hypothesis in VILI. Mediators of the TER decrease were heat-sensitive, transferable via Folch extraction, and (following ultrafiltration, 3 kDa) comprised both smaller and larger molecules. Although several classes of candidate mediators, including protein cytokines (e.g., tumor necrosis factor-α, interleukin-6, macrophage inflammation protein-1α) and lipids (e.g., eicosanoids, ceramides, sphingolipids), have been implicated in VILI, only prostanoids accumulated in the perfusate in a pattern consistent with a pathogenic role, yet cyclooxygenase inhibition did not protect against injury. Although no single class of factor appears solely responsible for the decrease in barrier function, the current data implicate lipid-soluble protein-bound molecules as not just markers but pathogenic mediators in VILI.     

LPS induces permeability injury in lung microvascular endothelium via AT(1) receptor

Lipopolysaccharide (LPS) is known to stimulate the circulation and local production of angiotensin II (Ang II). To assess whether Ang II plays a role in LPS-induced acute lung injury, rats were injected with LPS, the microvascular endothelial permeability injury was evaluated by histological changes, increased pulmonary wet/dry weight ratio, and pulmonary microvascular protein leak. Besides, increased rat pulmonary microvascular endothelial cell monolayer permeability coefficient (K(f)) was measured after treatment with LPS and/or Ang II, respectively. LPS/Ang II, treatment resulted in a significant increase in K(f). Ang II cooperates with LPS to further increase K(f). Hence, LPS increases pulmonary microvascular endothelial permeability both in vitro and in vivo. Local lung Ang II was increased in response to LPS challenge, and elevated Ang II ulteriorly exacerbates LPS-induced endothelium injury. [Sar(1),Ile(8)]Ang II, a selective block of Ang II type 1 (AT(1)) receptors, eliminated these changes significantly. Our conclusion is that the LPS-induced lung injury may be mediated by the AT(1) receptor.     

Activation of calpains mediates early lung neutrophilic inflammation in ventilator-induced lung injury

Lung inflammatory responses in the absence of infection are considered to be one of primary mechanisms of ventilator-induced lung injury. Here, we determined the role of calpain in the pathogenesis of lung inflammation attributable to mechanical ventilation. Male C57BL/6J mice were subjected to high (28 ml/kg) tidal volume ventilation for 2 h in the absence and presence of calpain inhibitor I (10 mg/kg). To address the isoform-specific functions of calpain 1 and calpain 2 during mechanical ventilation, we utilized a liposome-based delivery system to introduce small interfering RNAs targeting each isoform in pulmonary vasculature in vivo. Mechanical ventilation with high tidal volume induced rapid (within minutes) and persistent calpain activation and lung inflammation as evidenced by neutrophil recruitment, production of TNF-α and IL-6, pulmonary vascular hyperpermeability, and lung edema formation. Pharmaceutical calpain inhibition significantly attenuated these inflammatory responses caused by lung hyperinflation. Depletion of calpain 1 or calpain 2 had a protective effect against ventilator-induced lung inflammatory responses. Inhibition of calpain activity by means of siRNA silencing or pharmacological inhibition also reduced endothelial nitric oxide (NO) synthase (NOS-3)-mediated NO production and subsequent ICAM-1 phosphorylation following high tidal volume ventilation. These results suggest that calpain activation mediates early lung inflammation during ventilator-induced lung injury via NOS-3/NO-dependent ICAM-1 phosphorylation and neutrophil recruitment. Inhibition of calpain activation may therefore provide a novel and promising strategy for the prevention and treatment of ventilator-induced lung injury.     

Phalloidin attenuates postischemic neutrophil infiltration and increased microvascular permeability.

The aim of this study was to determine whether phalloidin (1 microM) or antamanide (1 microM), cyclic peptides that stabilize dense peripheral band and stress fiber F-actin in endothelium, would attenuate the increase in microvascular permeability induced by 4 h of ischemia and 30 min of reperfusion (I/R) in the isolated canine gracilis muscle. Changes in microvascular permeability (1 - sigma) were assessed by determining the solvent drag reflection coefficient for total plasma proteins (sigma) in muscles subjected to 4.5 h of continuous perfusion (nonischemic controls), I/R alone, I/R + phalloidin, or I/R + antamanide. Muscle neutrophil content was assessed by determination of myeloperoxidase (MPO) activity in tissue samples obtained at the end of the experiments. Fluorescent detection of nitrobenzoxadiazole-phallicidin in endothelial cell monolayers confirmed that phalloidin enters these cells. I/R was associated with marked increases in microvascular permeability and muscle neutrophil content (1 - sigma = 0.45 +/- 0.07; MPO = 8.9 +/- 0.5 units/g) relative to control (4.5 h continuous perfusion) preparations (1 - sigma = 0.12 +/- 0.03; MPO = 0.5 +/- 0.8 unit/g). These I/R-induced changes were largely prevented by administration of phalloidin (1 - sigma = 0.19 +/- 0.02; MPO = 0.8 +/- 0.4 U/g) or antamanide (1 - sigma = 0.07 +/- 0.11; MPO = 0.9 +/- 0.3 unit/g) at reperfusion. Similar results were obtained when phalloidin was administered before ischemia (1 - sigma = 0.24 +/- 0.04; MPO = 1.2 +/- 1.0 units/g). Although antamanide decreased superoxide production (by approximately 60%) and adherence to plastic (by approximately 75%) by activated neutrophils in vitro, phalloidin failed to alter these aspects of granulocyte function.(ABSTRACT TRUNCATED AT 250 WORDS)     

Lack of matrix metalloproteinase-9 worsens ventilator-induced lung injury

Matrix metalloproteinase-9 (MMP-9) is released by neutrophils at the sites of acute inflammation. This enzyme modulates matrix turnover and inflammatory response, and its activity has been found to be increased after ventilator-induced lung injury. To clarify the role of MMP-9, mice lacking this enzyme and their wild-type counterparts were ventilated for 2 h with high- or low-peak inspiratory pressures (25 and 15 cmH2O, respectively). Lung injury was evaluated by gas exchange, respiratory mechanics, wet-to-dry weight ratio, and histological analysis. The activity of MMP-9 and levels of IL-1beta, IL-4, and macrophage inflammatory protein (MIP-2) were measured in lung tissue and bronchoalveolar lavage fluid (BALF). Cell count and myeloperoxidase activity were measured in BALF. There were no differences between wild-type and Mmp9-/- animals after low-pressure ventilation. After high-pressure ventilation, wild-type mice exhibited an increase in MMP-9 in tissue and BALF. Mice lacking MMP-9 developed more severe lung injury than wild-type mice, in terms of impaired oxygenation and lung mechanics, and higher damage in the histological study. These effects correlated with an increase in both cell count and myeloperoxidase activity in the BALF, suggesting an increased neutrophilic influx in response to ventilation. An increase in IL-1beta and IL-4 in the BALF only in knockout mice could be responsible for the differences. There were no differences between genotypes in MMP-2, MMP-8, or tissue inhibitors of metalloproteinases. These results show that MMP-9 protects against ventilator-induced lung injury by decreasing alveolar neutrophilic infiltration, probably by modulation of the cytokine response in the air spaces.