acute diffuse, inflammatory lung injury, leading to increased pulmonary vascular permeability, increased lung weight, and loss of aerated lung tissue

Transcription

1 acute diffuse, inflammatory lung injury, leading to increased pulmonary vascular permeability, increased lung weight, and loss of aerated lung tissue [with] hypoxemia and bilateral radiographic opacities, associated with increased venous admixture, increased physiological dead space, and decreased lung compliance.

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3 Initial "exudative" stage-diffuse alveolar damage within the first week Proliferative" stage-resolution of pulmonary edema, proliferation of type II alveolar cells, squamous metaplasia, interstitial infiltration by myofibroblasts, and early deposition of collagen. Some patients progress to a third "fibrotic" stage, characterized by obliteration of normal lung architecture, diffuse fibrosis, and cyst formation

4 ALI/ARDS is caused by an autodestructive inflammatory process characterized by the activation of intrapulmonary and circulating cells and by a tremendous influx of neutrophils in pulmonary endothelium and cytokine production, resulting in a breakdown of the lung barrier and gas exchange functions ARDS pathophysiology is characterized by complex mechanisms that involve cells of inflammation, lung tissue cells, cytokines, chemokines, as well as apoptosis activators and inhibitors.

5 ALI is associated with an intense pulmonary inflammatory response with accumulation of both pro- and anti-inflammatory mediators If the proinflammatory process dominates, endothelial activation is followed by functional and, at a second stage, structural endothelial injury, leading to alterations in all the above critical metabolic functions that contribute to ARDS pathogenesis.

6 Acute lung injury/ards results from direct or indirect injury to the vascular endothelium or alveolar epithelium. Leads to activation of the humoral and cellular components initially begins as a localized inflammatory response. this response expands to involve the entire pulmonary system. Activation of inflammatory mediators leads to increased permeability of the pulmonary vasculature and/or alveolar tissue, allowing the free movement of fluid and protein into the interstitium and/or alveolar spaces. As the edema increases, more fluid floods the alveoli. Hypoxemia refractory to increasing amounts of oxygen. Increase in fluid in the interstitial space acts to decrease the functional residual capacity of the lungs and leads to airway closure as well as distal lung atelectasis Such a large-scale inflammatory response then allows mediators to spill over into the systemic circulation, leading to activation of systemic inflammation.

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8 Pulmonary endothelium plays a major role by: altering its metabolic activity, thus affecting pulmonary and systemic homeostasis; mediating cell-cell adhesions, especially with neutrophils; changing its barrier permeability, thus promoting pulmonary oedema formation

9 Synthesis and release of several vasoactive compounds such as angiotensin II, prostacyclin, thromboxane A2, nitric oxide (NO), and endothelins; regulation of vascular tone Expression of enzymes such as angiotensin converting enzyme, endothelin converting enzyme, nucleotidases, NO synthase and lipoprotein lipase Expression of receptors and signal transduction molecules Cell surface redox activity (transplasma membrane electron transport systems) Removal and biotransformation of drugs Regulation of coagulation and thrombolysis; promotion of hemofluidity Participation in immune reactions Binding of immune complexes Interaction with bacteria (phagocytosis) and blood components such as leukocytes and platelets Expression of adhesion molecules Production of growth factors Production of cytokines and chemokines Production of reactive oxygen species Barrier function

10 Most of these functions are constitutive while others are induced upon endothelial activation after exposure to proinflammatory stimuli such as endotoxin and/ or cytokines. In this respect the activated pulmonary endothelium: (a) expresses leukocyte adhesion molecules, (b) produces cytokines, (c) induces changes in vascular integrity and tone, (d) becomes procoagulant, (e) upregulates HLA(human leucocyte Ag) molecules

11 Endotoxin Cytokines, chemokines Activated leukocytes Proteolytic enzymes Partially reduced O2 species Immune complexes Microbes Hyperoxia Radiation Drugs Ischemia/reperfusion Hyperlipidemia Fibrin split products Actin and actin complexes Toxins Mechanical stretch

12 A particular pro-inflammatory process of high clinical importance is the ventilator-induced lung injury (VILI). highly morbid clinical entity is caused by excessive mechanical stress that alters epithelial and endothelial barrier properties and stimulates proinflammatory responses of several cell types including macrophages and neutrophils Conventional mechanical ventilation in ARDS patients can induce ventilator-associated lung injury (VALI) that leads to pro-inflammatory cytokine production, attenuated by a protective ventilatory strategy In this respect protective low tidal volumes appear to attenuate epithelial and endothelial injury

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16 Inflammation is necessary for the development of a proper response to the insult, but it can damage the tissue where it takes place. Inflammation leads initially to an alteration in homeostasis, during which tissue partially sacrifices cellular and extracellular matrix integrity and tissue functionality for the benefit of a quick response. A good example of this compromise is the early recruitment of immune cells to the site of injury. The first cells arriving are neutrophils. Representing 70% of total circulating leukocytes, these cells are rapid responders to chemokines, appearing in the lungs a few minutes after initial injury. After migrating through endothelial cells and before arriving in the alveolar space, leukocytes find the basement membrane, a highly organized extracellular matrix mainly composed of four families of glycoproteins (laminin, collagen type IV isoforms, nidogen and heparan sulfate proteoglycans). The basement membrane is actively involved in leukocyte recruitment by its potential to bind and release cytokines and chemotactic factors. The only way neutrophils can cross this barrier is by proteolytic degradation contributing to further damage of lung structure. The release of a variety of alarmins, free oxygen radicals, leukotrienes, proteases and other pro-inflammatory molecules maintain the inflammatory state, thereby contributing to the ALI phase.

17 Cytokines are soluble polypeptides serving as chemical messengers between cells they are involved in processes such as cell growth and differentiation, tissue repair and remodeling, and regulation of the immune response ECs are both targets and cytokine producers. TNF a and interleukin IL 1 are produced mostly by mononuclear phagocytes. In the lung they are mainly produced by activated interstitial and alveolar cells (primarily macrophages), as well as ECs, and have a major role in the early ALI stage. TNF-a and IL-1 share a number of biological properties and markedly amplify each other s biological actions. They act on EC mainly by inducing a functional program that promotes thrombosis and inflammation they induce: prothrombotic EC phenotype, the production of several cytokines including chemokines, colony-stimulating factors, IL-6 which has both pro- and anti-inflammatory properties, and IL-1 itself, TNF-a induces IL-8 production by pulmonary EC the production of several autacoids such as prostanoids including prostacyclin (PGI2) and thromboxane A2, platelet-activating factor (PAF) and nitric oxide (NO) the upregulation of adhesion molecules All these functions, with the latter being the most important, contribute to ALI/ARDS development

18 Principal mediator of acute inflammation Major source: macrophages Major effect: activation of pro-inflamm NFkB transcription factor Major actions: increase endothelial expression of selectins and integrins increase chemokine production in endothelial cells, macrophages increases cycloxygenase and lipoxygenase increased arichodinate metabolism production of prostaglandins and leukotrienes (pyrogens, chemotactic) induces prostacyclin expression in endothelium increases local blood flow increase IL-1 production in macrophages induces apoptosis in some cells (through p55 receptor activates caspase) at high concentrations in blood, can induce septic shock, organ failure, thrombosis, high mortality

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20 Eicosanoids are the product of arachidonate metabolism Prostaglandins, including thromboxanes TXA2 and TXB2, prostacyclins,leukotrienes and epoxides Prostaglandins and leukotrienes modulate the inflammatory process via stimulation of cytokine synthesis and release. Eicosanoids also elicit potent tissue level responses (e.g., vasoconstriction, bronchoconstriction) that indirectly damage endothelial and epithelial tissue and further potentiate the pathophysiologic effects of ALI/ARDS

21 Nuclear factor-kappa B [NF-κB], activator protein Induces the synthesis of new molecules that ultimately mediate the inflammatory response to the aggression. Increases alveolocapillary permeability, which causes a protein-rich edema, the neutrophil infiltrate (recruited from the circulation in response to chemokines) Stimulates release of a wide variety of mediators, such as cyto- and chemokines, proteases, eicosanoids and growth factors, into the extracellular space.

22 Transcription factors are major components of the molecular mechanism underlying the cytokine-induced EC activation. Nuclear factor-kb is a crucial factor for the maximal expression of many cytokines involved in ALI pathogenesis. NF-kB enhances the transcription of several genes including cytokines, growth factors, vasoactive mediators, adhesion molecules, immunoreceptors, and acute-phase proteins NF-kB regulates the cytokine-mediated inducibility of adhesion molecules and cytokines in EC NF-kB activation is the final target of a signal transduction pathway that leads from the cell surface to the nucleous.

23 Inducers of NF-kB stimulation - proinflammatory cytokines (mainly TNF-a and IL-1), bacterial and viral products [lipopolysaccharide (LPS) and reactive oxygen species (ROS). ROS at low (subcytotoxic) concentrations function as important signaling molecules, while at higher concentrations they induce cell injury and death (see below) [25]. NF-kB activation is a major redox-sensitive transcription factor: Thiol antioxidants such as N-acetylcysteine abolish LPS-induced activation of NF-kB and improve lung function in ARDS patients high intracellular glutathione concentrations inhibit NF-kB activation Cathecholamines that are often administered in ALI/ARDS subjects also affect NF-kB activation via several mechanisms, including ROS generation. Activation of NF-kB is a critical step in the initiation of neutrophilic inflammation is linked to ALI/ARDS pathogenesis. NF-kB activation is inhibited in vivo by treatment with antioxidants, corticosteroids, and the induction of endotoxin tolerance specific NF-kB inhibition would contribute to ALI/ARDS treatment.

24 Patients with ARDS are subjected to an oxidant burden that results in molecular/cellular damage and arises from an increased generation of ROS and reactive nitrogen species and/or a deficiency of antioxidant defenses. ROS superoxide anion (O2), hydrogen peroxide (H2O2) and hydroxyl radical (OH). ROS sources mitochondrial respiratory chain, the protease-mediated enzyme xanthine oxidase, the metabolic cascade of arachidonic acid, the oxidative burst of activated neutrophils RNS NO, NO is highly reactive with free radicals; the reaction between NO and O2 produces the very powerful and cell toxic ONOO(peroxynitrite) nitrogen dioxide (NO2) exposure to various inflammatory stimuli, pulmonary endothelial, epithelial, and alveolar macrophages are among the lung cell types that contribute to the production of ROS and RNS, with deleterious effects on pulmonary endothelium oxidant stress alters endothelial barrier function and increases endothelial permeability through activations of protein kinase C, myosin light chain kinase and other signaling pathways

25 RNS NO, NO is highly reactive with free radicals; the reaction between NO and O2 produces the very powerful and cell toxic ONOO(peroxynitrite) nitrogen dioxide (NO2) exposure to various inflammatory stimuli, pulmonary endothelial, epithelial, and alveolar macrophages are among the lung cell types that contribute to the production of ROS and RNS, with deleterious effects on pulmonary endothelium oxidant stress alters endothelial barrier function and increases endothelial permeability through activations of protein kinace C, myosin light chain kinase and other signaling pathways

26 Pulmonary endothelial-leukocyte interaction is a key step in ALI/ARDS development Neutrophils are the key cell type related to pulmonary injury in ALI/ARDS eosinophils,macrophages( responsible for ALI occurring in neutropenic patients ) Neutrophil adhesion to EC is a multistage process First phase - Initial capture and rolling is mediated by cell adhesion molecules of the selectin family: l-selectin is constitutively expressed on neutrophils, P-selectin is found on platelets and EC E selectin is expressed solely on EC surface after EC activation by stimuli such as histamine, thrombin, bradykinin, leukotriene C4 or free radicals; E-selectin is rapidly synthesized by EC after cell activation by cytokines such as TNF-a and IL-1, or endotoxin. Second phase is firm neutrophil adhesion augmented by TNF-a, IL-1, g-interferon, and endotoxin. Oxidant stress promotes neutrophil adhesion. Once neutrophils firmly adhere on the pulmonary endothelial layer - increased production of proteases and ROS (i.e., oxidant burst) that induce cell injury and death. Third phase neutrophil migration through the endothelium.ros appear to increase endothelial permeability, facilitating leukocyte transmigration. Neutrophil interaction with pulmonary endothelium in ALI gives potential for therapeutic interventions

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29 Macrophages comprise more than 90% of the pulmonary leukocyte population in normal lungs and function to regulate the inflammatory and immune response in ALI/ARDS via the release of numerous mediators. TNF-α and IL-1, two of the most critical cytokines, are released from the macrophage along with IL-6 and IL The macrophage can also move into the interstitial and alveolar spaces and release oxygenderived free radicals, proteases, and metalloproteinases, further increasing the protease burden of the lungs. Recent evidence suggests that lymphocytes play a role in the evolution of ALI/ARDS.3 Lymphocytes undergo activation, sequestration, adhesion, and chemotaxis in much the same manner as the neutrophil and use many of the same chemotactants.

30 Pulmonary endothelium affects platelet function through the production of platelet aggregation inhibitors PGI2 and NO; the production of vwf; conversion of adenosine diphosphate (which can induce platelet aggregation) to adenosine monophosphate the removal of serotonin from the pulmonary circulation In ARDS all the above features may be altered leading to enhanced platelet aggregation. oxidative injury to EC generated from ROS pulmonary endothelium-mediated extraction of serotonin is decreased in ARDS patients. vwf is altered in ALI/ARDS, and vwf is a sensitive marker denoting the existence of EC injury or activation. vwf is synthesized predominantly by vascular ECs. Markedly elevated levels of plasma vwf were reported in patients ARDS

31 Platelets and Fibroblasts Platelet aggregation leads to microvascular embolization production of various cytokines and eicosanoids. These mediators result in vasoconstriction and bronchoconstriction further vascular and alveolar membrane damage. Fibroblasts and type II pneumocytes synthesize and release many complement components (e.g., C3, C5).

32 Activated neutrophils cause lung injuries in patients with ARDS by releasing oxidants, proteases, leukotrienes, and other proinflammatory molecules, such as platelet-activating factor. Neutrophils have a relatively short life span; therefore, the accumulation of neutrophils at inflammatory sites in the lung is due in part to delayed neutrophil apoptosis, while apoptosis of epithelial cells is enhanced at the same site. In light of their prolonged life span, the removal of neutrophils without the release of their cytotoxic contents becomes very important in the resolution of inflammation. Apoptosis of epithelial cells is enhanced at the same site Apoptosis of EC in ARDS is enhanced by same factors that delay the apoptosis of neutrophils Apoptosis of epithelial cells and neutrophils are interrelated events. In response TNF-, bronchiolar epithelial cells undergo apoptosis and secrete interleukin-8 and NF-kB, which in turn suppresses the apoptosis of neutrophils

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34 Increased pulmonary vascular permeability is a hallmark of ALI/ARDS pathogenesis since it is a sine qua non for noncardiogenic pulmonary edema formation. ARDS patients exhibit persistent increased pulmonary endothelial permeability. the severity of vascular permeability appeared related to lung injury and the number of neutrophils in bronchoalveolar lavage Increased pulmonary endothelial permeability may be induced by ALI-related cytokines, other agents, and via EC cytoskeletalrelated mechanisms in response to stimuli such as thrombin or mechanical stretch. Vascular endothelial growth factor (VEGF) is a potent vascular permeability inducer. VEGF was higher in the plasma of ARDS patients, VEGF may be another important factor in the pathogenesis of noncardiogenic pulmonary edema in ARDS

35 ECs possess a sophisticated metabolic machinery of interactive factors that modulates all three components of the hemostatic system: platelet aggregation, blood coagulation, and fibrinolysis. In the healthy lung the combined effect of these factors promotes hemofluidity, while under pathological conditions the injured pulmonary endothelium becomes thrombogenic. Platelets and pulmonary endothelium Pulmonary endothelium and coagulation Pulmonary endothelium and fibrinolysis

36 Pulmonary endothelium possesses both anticoagulant and procoagulant properties. Antithrombin III (AT III) is a major inhibitor of thrombin EC possess heparinlike glycosaminoglycans and sulfated proteoglycans on their surface that sequester AT III and thrombin from the circulation, facilitating their reaction Thrombomodulin (TM) is an anticoagulant proteoglycan located on the EC surface. TM reacts with thrombin producing a marked increase in the thrombin-catalyzed activation of protein C, which in turn inactivates coagulation factors VA and VIIIA Plasma TM is increased in ARDS patients, possibly through proteolytic release from the injured pulmonary endothelium, mediated by activated neutrophils. Endothelial procoagulant properties under normal conditions are covered by its predominant anticoagulant activity. In this respect the activity of thromboplastin, an EC-associated procoagulant factor, is normally low but can be induced by various ALI-related stimuli such as endotoxin, IL-1, and thrombin.

37 Pulmonary endothelium is actively involved in the fibrinolytic process, expressing tissue-type (t-pa) and urokinase-type (u-pa) plasminogen activators as well as plasminogen activator inhibitors. The EC fibrinolytic activity appears to be affected by several ALI-related mediators including endotoxin, IL-1, TNF-a, and thrombin. ARDS patients expressed higher procoagulant activity and plasminogen activator inhibitor (PAI) 1 as well as lower fibrinolytic potential confirming the procoagulant pulmonary endothelial profile in ARDS

38 Nitric oxide NO is a free radical (RNS) with a very short half-life and is very unstable in biological systems. NO is formed from l-arginine by NO synthase (NOS). NOS isoenzymes: (a) neuronal (n) NOS, also expressed in pulmonary arterial smooth muscle cells (SMC), (b) inducible (i) NOS, induced by several pro-inflammatory mediators, which upon expression produces NO at very high rates with profound effects on cardiovascular homeostasis. (c) endothelial (e) NOS, a constitutive isoenzyme expressed principally in EC. The latter is the main isoenzyme involved in vascular tone regulation. Deficiency of l-arginine result in enos-generated O2 instead of or along with NO, promoting the formation of highly reactive RNS such as ONOO(peroxynitrite).

39 NO activates soluble guanylate cyclase, thus producing 3,5- cyclic monophosphate (cgmp) and eliciting cgmp-mediated SMC relaxation In addition to vascular SMC relaxation, NO inhibits (a) platelet aggregation, (b) leukocyte adhesion, and (c) cellular proliferation. In the pulmonary circulation NO synthesis is reduced under hypoxia, and as such it may modulate HPV(hypoxic pulmonary vasoconstriction) a feature that is lost in ARDS. NO can in addition exert either pro- or anti-oxidative effects, depending on the type and the quantity of oxygen radicals present; NO can additionally attenuate ARDS-associated lung leak Therapeutic NO inhalation improves oxygenation in ARDS, while in addition it inhibits neutrophil activation, platelet adhesion, and the production of inflammatory mediators in the injured lungs.

40 Endothelins Endothelins (ETs) are the most potent naturally occurring vasoconstrictors. ET-1, ET-2, and ET-3 all formed by ET-converting enzyme ET-1 is produced mainly by EC, and its production is induced by several factors including hypoxia, endotoxin, TNF-a, interferon, and epinephrine pro-inflammatory stimuli increasing ET-1 and decreasing enos expression ( NO) The human lung is an important site for both ET-1 clearance and production: balance between pulmonary ET-1 clearance and release is decreased early in ALI, reversing in patients who subsequently recovered. Plasma ET-1 values are increased in septic patients with and without ARDS, possibly contributing to the ALI-associated pulmonary hypertension

41 Prostaglandins cyclo-oxygenase (COX) products PGI2 and thromboxane A2 are the most important in ALI. PGI2 is a potent vasodilator and an important inhibitor of platelet aggregation. Thromboxane A2 is a potent pulmonary vasoconstrictor secondary to endotoxin infusion; Thromboxane A2 also increases capillary permeability and platelet aggregation [7]. Prostaglandin E1 (PGE1) is another COX product with EC protective properties. PGE1 is a vasodilator and platelet aggregation inhibitor, also impairing neutrophil chemotaxis and macrophage activation. COX products contribute to HPV; however, their vasoactive action varies with the size of the artery and the species involved.

42 Platelet activating factor Pulmonary ECs release the phospholipid PAF, a highly reactive mediator that cause both vasodilation and vasoconstriction in vivo, depending on its concentration PAF has been additionally reported to increase lung permeability, to activate platelets, neutrophils, and macrophages and to cause EC release of tpa and PGI2. PAF synthesis by EC may be induced by H2O2 and other reactive oxygen species. PAF-mediates microvascular injury and leak as well as neutrophil activation-sequestration in the lungs. PAF receptor antagonist may block all the above events,suggesting a potential future therapeutic application for this compound.

43 Pulmonary endothelial angiotensin-converting enzyme(peace) Angiotensin-converting enzyme (ACE) hydrolyzes angiotensin I to angiotensin II and breaks down bradykinin. Pulmonary endothelium-bound (PE) ACE has a central role in maintaining adequate local and systemic homeostasis, bradykinin that escapes ACE inactivation exerts vasodilatory, antiinflammatory and antithrombotic actions through stimulation of endothelial B2 kinin receptors, causing the synthesis and release of substances such as NO and PGI2, generated by enos and constitutive COX (COX-1), respectively. PEACE activity reduction seen in ALI is related to enzyme downregulation, mediated by overproduction of ONOO and other ROS/RNS. Plasma soluble ACE (sace) activity is decreased in ARDS patients PCEB-ACE activity in mechanically ventilated patients belonging to high-risk groups for ARDS decreased early during the ALI/ARDS continuum and was inversely related to the lung injury score, suggesting that the clinical severity of the syndrome is related to the degree of PCEB-ACE activity depression (i.e., the underlying pulmonary endothelial dysfunction)

44 Pulmonary endothelial functional and structural alterations are key components of ALI pathogenesis. EC-related therapies may have beneficial effects in ALI/ ARDS. Such therapies should restore adequate endocrine and paracrine EC functions, and protect ECs against harmful insults as well as against pro-inflammatory cellcell interactions. Several therapeutic interventions, most of them related to endothelium-derived vasoactive (and anti-inflammatory) mediators are already in place, in an effort to improve arterial oxygenation and treat ARDS-related pulmonary hypertension.

45 In this respect inhaled or intravenous PGI2, inhaled or intravenous (in either native or liposomal form) PGE1, and, mainly, inhaled NO have been used, with mixed results. Additional agents include antioxidants such as Nacetylcysteine and hyperoncotic albumin, and the thromboxane synthetase inhibitor ketoconazole. Several agents already in clinical use for treating pathologies other than ALI, such as ET-1 receptor antagonists, phosphodiesterase inhibitors, and ACE inhibitors might have beneficial effects. aspirin may modulate to provide a beneficial effect in ARDS. Aspirin has significant anti-platelet properties through the inhibition of cyclo-oxygenase enzymes that prevent thromboxane A2 production, therefore suppressing platelet aggregation; Delayed neutrophil apoptosis is a prominent feature of ARDS, allowing inflammatory cells to remain within the alveolar space, prolonging the period of lung injury and hypoxia. Aspirin has previously been shown to preserve neutrophil apoptosis and in the setting of ARDS, experimental evidence suggests that aspirintriggered production of the anti-inflammatory lipid mediator 15-epi-lipoxin A4 restores neutrophil apoptosis and enhances the resolution of alveolar inflammation Statins (contraversial) are lipid-lowering agents which act by inhibiting 3-hydroxy-3- methylglutaryl coenzyme A reductase and are generally used for cardiovascular prevention. Statins, however, also present anti-inflammatory and immunomodulating effects which could be of interest in sepsis and acute respiratory distress syndrome (ARDS) Sildenafil NO inhibits NADPH oxidase expression through a cyclic GMP-mediated mechanism, sildenafil, a type V phosphodiesterase inhibitor, may be therapeutically effective in ARDS through an augmentation of NO-mediated inhibition of NADPH oxidase