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MEDIATORS AND MECHANISMS OF ACUTE LUNG INJURY
ACUTE RESPIRATORY DISTRESS SYNDROME
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MEDIATORS AND MECHANISMS OF ACUTE LUNG INJURY Polly E. Parsons, MD
The saying “history repeats itself” applies well to the topic of this article. The subject of this article was reviewed in Clinics in Chest Medicine in 1990, in an article entitled ”Mechanisms and Mediators of the Adult Respiratory Distress Syndrome,” authored by Rinaldo and Chri~trnan.5~ Those authors noted that the last review on the same topic had been in the early 1980s and they reflected back on the accomplishments that had occurred in the intervening 8 years. In their introduction, they concluded that important advances had been made in investigations into the mechanisms of acute lung injury (ALI) but they stated: “Despite years of research, no other specific pharmacologic therapies have proved useful, and we are less confident in 1990 that we understand the pathogenesis of ARDS than we were in 1982.” In many ways, that statement is as true in 2000 as it was in 1990. There is still no specific pharmacologic intervention for the treatment of acute respiratory distress syndrome (ARDS), although new studies have indicated that mortality can be influenced by modes of supportive care-specifically, mechanical ventilation strategies. Furthermore, although tremendous advances have been made in our understanding of the mechanisms of in-
flammation, how the process actually evolves in patients at risk for and with ALI is still confounding. Just as it was not possible for Rinaldo and Christman to include all the accomplishments and reports in the 8 years since the review before theirs, it is again not possible to delineate all the accomplishments of the past 10 years. Many of the investigations, however, have focused on specific themes that have been reiterated and developed over the 10 years. These serve as the focus of this review. THE IMPACT OF DEFINITIONS AND EPIDEMIOLOGIC DATA
It may seem odd to begin a discussion of mechanisms and mediators with a review of the evolution of clinical definitions and the epidemiology of the clinical syndrome, but developments in these areas have enhanced our understanding of and our ability to further investigate the pathogenesis of ALI. The inflammatory processes that lead to the development of ALI are well described and defined in vitro and in animal models. Confirming that those processes occur in humans with ALI, however, has been, at times, frustrating.
From the Division of Pulmonary and Critical Care Medicine, Department of Medicine, University of Colorado Health Sciences Center; and the Medical Intensive Care Unit, Denver Health Medical Center, Denver, Colorado
CLINICS IN CHEST MEDICINE VOLUME 21 * NUMBER 3 SEPTEMBER 2000
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Studies frequently have been done to confirm that a specific mediator is important in the development of lung injury, only to have conflicting results reported by different investigators from different institutions. Early studies of the role of tumor necrosis factor (TNF) in sepsis and ARDS are a good example. In patients with sepsis, the percentage of patients with measurable TNF in their circulation ranges from 14% to 54%,43leading investigators to conclude that there may or may not be an association between TNF and sepsis. Results were similarly confusing from studies evaluating the measurement of TNF as a predictive marker for the development of either ALI or multiple organ failure. In one study, a clear association between TNF and the development of ARDS was demonstrated30;in two subsequent studies no association was found between TNF measurements and the development of -either ARDS*‘jor multiple organ failure.” In reviewing these and other studies, the
Soluble ICAM-1
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author has asked herself the question, “Why are the results so different?” innumerable times. Although there is no definitive answer to that question, several factors recently have been identified that could contribute to the answer. The first factor is that the patients included in clinical studies of ALI are often heterogeneous. Since ARDS was first described in the 1970s, the definition of the syndrome has been in evolution. A recent focus on refining the definitions for both ARDS4 and the major risk factor for the s y n d r o m e sepsiss-will likely increase the homogeneity of patients in clinical studies in the future. Patient heterogeneity is further confounded by the recognition that all patients at risk for ARDS may not be the same. Recent studies suggest that measurements of inflammatory mediators and response are different for patients with sepsis versus trauma. As shown in Figure 1, for example, circulating levels of ICAM and E-selectin are increased in septic patients at risk for ARDS, whereas they are
Soluble E-Selectin
Figure 1. Both soluble CAM-1 and E-selectin levels are significantly greater in septic patients (solid bar) at risk for acute respiratoty distress syndrome (ARDS) than in at-risk trauma patients (hatched bars).* P < .001. (Data from Moss M, Gillespie M, Ackerson L, et al: Endothelial cell activity varies in patients at risk for the adult respiratory distress syndrome. Crit Care Med 24:1782-1786, 1996.)
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Figure 2. The incidence of ARDS is greater in those patients at risk for ARDS who abuse alcohol than in those patients who are not identified as alcoholics. (Data from Moss M, Bucher B, Moore F, et al: Predicting the development of the adult respiratory distress syndrome: The role of chronic alcohol abuse. JAMA 27550-54, 1996.)
not increased in trauma patients at risk.39 These differences persist even after the patients have developed ARDS. Similarly, patients with direct lung injury (pneumonia, aspiration) respond differently to mechanical ventilation strategies than patients with indirect injury (distal sepsis or trauma).18In light of recent data suggesting that changes in mechanical ventilatory strategy can decrease circulating cytokine levels, it is possible that the pathogenesis of direct lung injury is different from indirect injury. An additional contribution to patient heterogeneity in clinical trials is the existence of co-morbid conditions. As shown in Figure 2, patients at risk for ARDS who abuse alcohol have recently been shown to have an increased incidence of ARDS and an increased mortality from the syndrome.38In contrast, patients with diabetes who are septic have a decreased incidence of ARDS compared with nondiabetic septic patients.40Both alcohol abuse and diabetes impact several factors in
the inflammatory response and could thereby influence the pathogenesis of ARDS. The timing and site of measurements are also now recognized as important contributors to study results and interpretation. The focus on the role of the neutrophil in ALI arose from early observations in histologic specimens that neutrophils accumulate in the air spaces and interstitium of the lungs of patients with ARDS,’ and the bronchoalveolar lavage fluid (BAL) from these patients is characterized by ne~trophilia.4~ Subsequent studies of serial BAL from ARDS patients have further contributed to the hypothesis that the neutrophil is important. Using serial BAL, Steinberg and colleagues showed that patients with ARDS requiring prolonged ventilatory support had persistent neutrophilia in BAL and that there was an association between the degree of neutrophilia and mortality,59whereas the rate of mortality was decreased in patients who had an increase in macrophages in sequential BAL. Serial analy
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cantly to the variability in study results, additional reasons for the discrepancies in the results from these clinical studies were outlined in a review by Pittet and colleague^.^^ They included the fact that TNF is released within an hour after the administration of endotoxin in normal volunteers and declines rapidly, so detection in patients could be difficult. To address this issue, at least one group of investigators has identified patients early in the course of their at-risk illness and performed serial measurements of TNF to determine whether the absolute level of TNF and the persistence of TNF in the circulation were associated with the development of ARDS.46 There was no difference in either parameter between patients who did and did not develop the syndrome. Other factors discussed by Pittet et a150that could have contributed to the variability in THE ROLE OF INFLAMMATORY the clinical study results include the recogniMEDIATORS tion that many different TNF assays were used for the clinical studies and only recently Early studies in ARDS focused on the role was the importance of avoiding endotoxin of complement and then endotoxin in the decontamination during the collection and analvelopment of lung injury.53In the past decade, ysis of samples recognized. the importance of inflammatory cytokines One criticism of the studies that measured and the recognition of new cytokines have TNF only in the circulation of critically ill led to the development of more detailed and patients was that TNF could be produced complex hypotheses. The inflammatory cylocally within tissue such as the lung, and tokines that have received the most attention that the levels there would be a more relevant are TNF and interleukin (1L)-1, -6, and -8. marker. Although direct measurement of TNF These mediators share several characteristics in the lungs from humans would be difficult, that make their role in the development of TNF has been measured in BAL fluid and ALI highly plausible. They are all proinpulmonary edema fluid from patients with flammatory cytokines. Their production and ARDS. TNF levels are increased in these flurelease are stimulated by multiple relevant ids but are not specific markers for ARDS and mediators, including endotoxin,42and are regulated by nuclear factor kappaB (NF-KB).~ do not correlate with morbidity and mortalit^.^^, 32 Also, TNF-a in pulmonary edema They have all been shown to be implicated in fluid may not be biologically active.52The the development of ALI in animal models likely source for the TNF in the lung is alveoand they are all present in patients at risk for lar macrophages, although there could be leak and with ARDS. from the circulation as well. TNF was the first cytokine to be extensively IL-1, -6, and -8 also have been measured studied in patients at risk for and with ARDS. in both the circulation and lung fluid from As previously discussed, many studies have patients at risk for and with ARDS. Many of investigated the role of circulating TNF in the the problems identified in the studies of TNF pathogenesis of ARDS in humans, and the have been encountered with these other proresults have been confounding. TNF is variinflammatory cytokines as well. Plasma IL-1 ably present in patients at risk for and with levels are increased in the circulation of some ARDS, and whether TNF levels predict morcritically ill patients, but predict neither the bidity or mortality is not clear. Although padevelopment of ARDS or other organ failure, tient heterogeneity may contribute signifi-
sis of patients therefore was more helpful for prognosis than a single early analysis and the data from serial analyses contributed to the understanding of the pathogenesis of both the development and resolution of ALI. In critically ill patients with ARDS, pathogenesis studies are generally limited to analysis of peripheral blood, BAL, or pulmonary edema fluid, and, to a lesser extent, urine. Data accumulated from studies performed using material from these different sites have enhanced the awareness of the potential importance of studying the inflammatory process both systemically and within the lung and have led to an increased interest and awareness of the involvement of other organs in the process.
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nor mortality.50IL-1 levels are increased in the BAL and pulmonary edema obtained from patients with ARDS, and alveolar macrophages from these patients release more IL-1 both at baseline and following lipopolysaccharide (LPS) stimulation than alveolar macrophages from either normal subjects or patients with other lung diseases.25Of interest, two studies have now demonstrated that IL-1 likely accounts for the majority of proinflammatory cytokine activity in the lung in ARDS.51,52 Both TNF and IL-1 stimulate the production of IL-8, a proinflammatory cytokine that is a potent neutrophil chemoattractant. As with TNF and IL-1, studies of IL-8 in plasma from critically ill patients have yielded conflicting results.31,52 In light of its chemoattractant activity, it is likely that IL-8 contributes more to acute inflammation within the lung than in the circulation. IL-8 levels are increased in the BAL fluid and pulmonary edema fluid in ARDS13,33, 34, 52 and correlate with neutrophil concentrations within the lung, lending plausibility to the hypothesis that IL-8 contributes to ALI, at least in part, through its neutrophil chemoattractant capacity. IL-8 levels are higher in the lungs of patients at risk who ultimately develop ARDSI3,26 and higher IL-8 levels may be associated with mortality from ARDS,’” 34, 51, 52 although, again, there is no absolute level of IL-8 that predicts either morbidity or mortality. Plasma levels of IL-6 have been touted as being the most predictive of morbidity and mortality of all the pro-inflammatory cytokines measured to date. Several studies have found that IL-6 levels are significantly increased in patients at risk for ARDS and are higher and persist longer in patients who die.9,20, 49 As pointed out in previous reviews, however, there is no specific IL-6 level that predicts which patients will die,44so the measurement is more likely to be useful in identifying groups of patients at high risk for mortality rather than in using the data for any individual patient. With the accumulated evidence suggesting that multiple proinflammatory cytokines may be important in the development of ARDS,
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some recent investigations have focused on the role of a more proximal common regulatory factor, NF-KB. NF-KB is the regulatory transcription factor for TNF, IL-1, IL-6, and IL-8 as well as many other proteins that could be involved in the development of ARDS.3Of course, no regulatory mechanism is simple. Whereas NF-KBregulates IL-1 and TNF, both IL-1 and TNF also activate NF-KB.In animal models, regulation of NF-KB activation can alter inflammatory processes, including neutrophil-mediated lung injury.6In humans, NFKB activation is significantly increased in alveolar macrophages obtained from patients with ARDS compared with patients without lung injury.55Although this association requires further study, if NF-KB regulation is important in the development of ALI, it may allow for the consideration of novel therapeutic options. Numerous inhibitors of NF-KB activation, including IL-10 and glucocorticoids, have been identified3 and, although complete and prolonged inactivation of NFKB could interfere with host defense, selective inhibition could be helpful in the abrogation of lung injury. THE ROLE OF INFLAMMATORY MODULATORS
Because no one cytokine (or any other proinflammatory mediator) has been shown to predict who develops ARDS, be specific for the diagnosis of ARDS, or consistently reflect the pathogenesis of ARDS, investigators are becoming less enamored with studying single proinflammatory mediators in isolation. Furthermore, it has become apparent that modulators of inflammation are also important in the development of ALI. The concept that injury could result from an alteration in the balance between pro- and anti-inflammatory mediators was probably first introduced when activated oxygen species were found to be associated with ARDS. It was recognized early on that there was an innate antioxidant defense that could be upregulated, such that oxidants and antioxidants could not be studied in isolation, but, rather, the relationship between the two needs to be considered. This
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concept of a balance between inflammatory mediators and modulators can now be extended to cytokines. Specific naturally occurring antagonists for TNF-a and IL-1 have been identified, and it is likely that antagonists for other cytokines exist as well. The identified antagonists for TNF are two soluble TNF receptors (sTNFR) that are released when cell surface-associated TNF receptors undergo proteolytic cleavage. sTNFR are found in the circulation of normal volunteers following the administration of endotoxin58and, of interest, in vitro, neutrophils release substantial quantities of both of these receptors following IL-1-stimulated adherence to endothelium.5 Furthermore, sTNFR protect against TNF-induced inflammation in animal models.z0This implies sTNFR could be protective in the development of ARDS. sTNFR are present in the circulation of patients with sepsiszoand, at least in one study, were higher in patients who died. The sTNFR levels were also significantly greater than the TNF levels in the circulation, however, so their protective effect in sepsis and ARDS remains to be studied.2O The natural antagonist for IL-1 is IL-lral2 which inhibits IL-1 activity in vitro and abrogates mortality in multiple animal models of sepsis.43IL-lra levels are significantly increased following the administration of endotoxin to normal volunteers and are elevated in the circulation of critically ill patients.15,29 The administration of IL-lra to patients with sepsis, however, failed to decrease morbidity or mortality.16 Furthermore, although IL-lra levels are increased in the circulation in patients at risk for ARDS, they do not predict the development of the syndrome and the levels are inversely correlated with mortality." In contrast, low levels of IL-lra and IL10 in BAL fluid from patients with ARDS correlate with rn0rta1ity.l~Again, therefore, the role of this anti-inflammatory mediator in the pathogenesis of ARDS remains unclear. IL-10 can also modulate the effects of IL-1. IL-10 inhibits cytokine production, including IL-1, from multiple cell types and potentiates the release of IL-lra following stimulation with LPS." Like IL-lra, IL-10 levels are increased following the administration of endo-
toxin and IL-10 has been shown to decrease mortality from endotoxemia in animal models." Furthermore, IL-lra levels are increased in patients at risk for ARDS but, again, they do not predict the development of ARDS and are higher in patients who die.47 CALCULATING THE BALANCE
As more and more inflammatory mediators and modulators are discovered, investigators have attempted to quantify their relative contribution to the development of ALI by developing "scoring systems" that incorporate multiple mediator measurements. A LPScytokine score incorporating TNF, IL-1, IL-6, and LPS measurements was evaluated in patients with sepsis by Casey et a19 and found to predict mortality more accurately than any one of the individual mediators measured. Other investigators have combined pro- and anti-inflammatory mediator measurements such as TNF and IL-10 in both the circulation and the lung and found differences in the ratios of these mediators between patients at risk and patients with ARDS.' This concept, that combined measurements may be more relevant than any single measurement, is important. These studies are confounded by our incomplete understanding of the pathogenesis of ALI, however. It is difficult to know what measurements to consider when developing these scoring systems. It is likely that measurements of systemic inflammation need to be considered in tandem with markers of lung response, as well as physiologic parameters. THE ROLE OF GENETIC SUSCEPTIBILITY
The role of genetic susceptibility in ALI is beginning to receive substantial attention. It is clear that not all patients identified as being at risk for ARDS develop the syndrome and that, to date, there is no marker measured in the circulation or the lung that clearly differentiates those patients with and without lung injury. With rapid progress in mapping of the
MEDIATORS AND MECHANISMS OF ACUTE LUNG INJURY
human genome and the explosion in knowledge of the mechanisms of genetic regulation, it is now possible to begin to consider evaluating the impact of genetic susceptibility to lung injury. There are several likely targets for these investigations, encompassing both the initial host response to inflammatory stimuli as well as the lung response to systemic and local inflammation. To date, the role of genetic regulation of TNF production has received the most attention in studies of patients at risk for and with ARDS. Early studies showed that stimulated TNF production from human monocytes varied significantly and consistently among normal suggesting that individuals could have inherent differences in susceptibility to disease processes that involved stimulated TNF.production. Furthermore, in a subsequent study, relatives of patients with meningococcal disease were found to vary in their capacity for TNF production. Interestingly, those families that had low TNF production had the highest mortality rates.62 Polymorphisms of the TNF-a gene that are associated with enhanced production of TNF have now been identified and found to be associated with morbidity from some severe infections, including meningococcal disease4I and malaria. Early studies of patients with sepsis found no difference in incidence of TNF polymorphisms in patients with sepsis compared with normal controls,60although one study suggested an association between polymorphisms for TNF and morbidity.60A subsequent study compared 89 patients with septic shock with 87 normal volunteer blood donors and found that the TNF polymorphism TNF2 occurred more frequently in the patients with sepsis than in the normal volunteers and that there was a strong association between this polymorphism and susceptibility to both the development of septic shock and mortality, although there was not an association between the polymorphism and measured levels of TNF?5 These studies are intriguing and suggest that further investigation into the role of TNF genomic polymorphisms in the development of ARDS could contribute to the understanding of the pathogenesis of the syndrome as well as to the
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ability to identify patients for targeted therapeutic interventions. Genomic polymorphisms have been identified for other relevant cytokines, including IL-1, IL-lO,' and IL-lra.= Although not yet evaluated in patients specifically at risk for and with ARDS, these polymorphisms are likely to contribute to the susceptibility to develop ALI. As shown for TNF, there is significant intrasubject variability in the stimulated production of these cytokines, as well and, in the study of first degree relatives of patients with meningococcus discussed previously, there was also a strong genetic determination of IL-10 production in addition to the genetic determination of TNF.62IL-10 production and mortality were associated but, interestingly, the association was with high IL-10 production, which would have likely been predicted to be protective. As these, and other, polymorphisms are evaluated in ARDS, many issues that have been addressed in studies of circulating markers will have to be considered, including patient homogeneity, pre-existing and comorbid conditions, and the interplay or balance between the various polymorphisms and their effects. Genetic determinants of LPS response are also likely to be important in the development of ARDS. A group of proteins known as toll-like receptors (TLR) recently were recognized as being a key element in the signaling pathway for LPS.36To date, five of these proteins have been identified and at least two of them, TLR-2 and TLR-4, could influence the susceptibility of a host to develop ARDS. TLR-2 enhances LPS response by augmenting cellular signaling." It seems that LPS directly binds to TLR-2 and initiates a signaling cascade that leads through NF-KBto the release of cytokines as well as other molecules. TLR4 can also induce the expression of NF-KB, and appears to be responsible for the regulation of LPS response in mice.I9 The potential role for genomic polymorphisms of TLR in ARDS is now being explored. Schwartz et alZ8 have analyzed the response to inhaled LPS and and demonstrated that there are reproducible LPS-sensitive and hyporesponsive phenotypes characterized by their pulmonary response to inhaled LPS and by the LPS-stim-
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ulated release of IL-6 and IL-8 from their peripheral blood monocytes and alveolar macrophages. The authors speculate that differences in genetic susceptibility could account, at least in part, for their results. Several potential genetic factors could contribute to the susceptibility of the pulmonary vasculature and the alveoli to systemic and local inflammation, as well. One of the characteristic features of ALI is the derangement of the pulmonary endothelium, which has been hypothesized to occur, at least in part, secondary to an interaction between inflammatory cells (specifically neutrophils) and the vascular endotheli~m.~ One of the purported mediators for this injury is active oxygen species that have been clearly associated with the development of ALI in animal models. One of the potential defense mechanisms for this injury is the production or presence of superoxide dismutases (SOD). Two of the SODsmanganese SOD (MnSOD) and extracellular SOD (ECSODkare inducible by cytokines, so the genetic regulation of cytokines could contribute to variations in their expression. In addition, however, genetic variants of MnSOD and ECSOD have been describedI7,54 and have been reported to be associated with human disease,56,63 although the association with ALI is not known. An additional feature of ALI is the derangement of both the composition and function of alveolar surfactant proteins (SP)?O Allelic variants of SP-A and SP-B have been described and found to be associated with the development of neonatal respiratory distress syndr0me.2~Genetic polymorphism of SP-B has also been associated with ARDS.48 Because the SPs are important in the function of surfactant and they contribute to both host defense and inflammatory response, genetic variations in the regulation of either production or function could contribute to the susceptibility of an individual to the development of ALI. Of interest, another contributor to the heterogeneity of the patient population at risk for and with ARDS has been uncovered in the studies of SP genetic polymorphisms. There are significant differences in the distribution of the alleles of SP-B among racial groups,27, suggesting that future stud-
ies of patients should consider this variable as well.
SUMMARY
Since last reviewed in this forum, there have been remarkable advances in our understanding of the acute inflammatory process and how it contributes to the development of ALI. As stated in the beginning of this article, it is not possible to even begin to review all the specific advances that have been made. Instead, the author has focused on concepts that have emerged and improved our ability to study the pathogenesis of ARDS. These include the recognition that patients at risk for and with ARDS represent a heterogeneous population, that mediators or markers of inflammation cannot be considered in isolation, that a balance between proinflammatory mediators and inflammatory modulators may be important, and that there are several genetic factors that could contribute to the susceptibility for the development of ARDS. Hopefully these concepts can be expanded and clarified so that the next review of this topic can report on successful therapeutic interventions for the prevention and the treatment of ARDS.
References 1. Armstrong L, Millar AB: Relative production of tumor necrosis factor-alpha and interleukin-10 in adult respiratory distress. Thorax 5242-446, 1997 2. Bachofen M, Weibel E: Structural alterations of lung parenchyma in the adult respiratory distress syndrome. Clin Chest Med 3:35-56, 1982 3. Barnes P, Karlin M Nuclear Factor-kB-A pivotal transcription factor in chronic inflammation diseases. N Engl J Med 336106f+1072, 1997 4. Bernard G, Brigham K, Carlet J, et al: The AmericanEuropean Consensus Conference on ARDS Definitions, mechanisms, relevant outcomes, and clinical trial coordination. Am J Respir Ciit Care Med 14981S824, 1994 5. Bjornberg F, Kantz M Adherence to endothelial cells induce release of soluble tumor necrosis factor (TNF) receptor forms from neutrophil granulocytes. Biochem Biophys Res Comm 244:594-598,1998 6. Blackwell TS, Christman JW: The role of nuclear factor-K B in cytokine gene regulation. Am J Respir Cell Mol Biol 173-9, 1997 7. Bone R The pathogenesis of sepsis. Ann Intern Med 115457-469, 1991
MEDIATORS AND MECHANISMS OF ACUTE LUNG INJURY 8. Bone R, Balk R, Cerra F, et al: Definitions for sepsis
and organ failure and guidelines for the use of innovative therapies in sepsis. Chest 101:1644-1655, 1992 9. Casey LC, Balk R, Bone R Plasma cytokine and endotoxin levels correlate with survival in patients with sepsis syndrome. Ann Intern Med 119:771-778, 1993 10. Chollet-Martin S, Montravers P, Gilbert C, et al: High levels of interleukin-8 in the blood and alveolar spaces of patients with pneumonia and adult respiratory distress syndrome. Infect Immun 61:4553-4559, 1993 11. Degroote MA, Martin MA, Densen P, et al: Plasma tumor necrosis factor levels in patients with presumed sepsis: Results in those treated with antilipid A antibody versus placebo. JAMA 262:249-251, 1989 12. Dinarello CA: Interleukin-1 and interleukin-1 antagonism. Blood 771627-1652, 1991 13. Donnelly SC, Strieter RM, Kunkel SL, et al: Interleukin-8 and development of adult respiratory distress syndrome in at-risk patient groups. Lancet 341:643647, 1993 14. Donnelly S, Strieter R, Reid P, et al: The association between mortality rates and decreased concentrations of interleukin-10 and interleukin-1-receptor antagonist in the lung fluids of patients with the adult respiratory distress syndrome. Ann Intern Med 125~191-196,1996 15. Fischer E, Van Zee KJ, Marano MA, et al: Interleukin1-receptor antagonist circulates in experimental inflammation and in human disease. Blood 79:21962199, 1992 16. Fisher C, Dhainaut J, Opal S, et al: Recombinant human interleukin-1 receptor antagonist in the treatment of patients with sepsis syndrome. JAMA 271:1836-1843, 1994 Identification of a 17. Folz RJ, Peno-Green L, Crapo JD: homozygous missense mutation (Arg to Gly) in the critical building region of the human EC-SOD gene (SOD3) and its association with dramatically increased serum enzyme levels. Hum Mol Genet 3:2251-2254, 1994 18. Gattinoni L, Pelosi P, Suter PM, et al: Acute respiratory distress syndrome caused by pulmonary and extrapulmonary disease: Different syndromes? Am J Respir Crit Care Med 158:3-11, 1998 19. Gerard C: Bacterial infection: For whom the bell tolls. Nature 395:217-219, 1998 20. Goldie AS, Fearon K, Ross J, et al: Natural cytokine antagonists and endogenous antiendotoxin core antibodies in sepsis syndrome. JAMA 274:172-177, 1995 21. Hack C, DeGroote E, Felt-Bersma R, et al: Increased plasma levels of IL-6 in sepsis. Blood 74:704, 1989 22. Hiroshi S, Yoshiki M, Heihachiro K Interleukin-lreceptor antagonist gene polymorphism in Japanese patients with systemic lupus erythematosus. Arthritis Rheum 40:389-390, 1997 23. Hurme M, Lahdenpohja N, Santtila S Gene polymorphisms of the interleukins 1 and 10 in infectious and autoimmune disease. AM Med 30:469473, 1998 24. Hyers T, Tricomi SM, Dettenmeier P, et al: Tumor necrosis factor levels in serum and bronchoalveolar lavage fluid of patients with the adult respiratory distress syndrome. Am Rev Respir Dis 144:268-271, 1991 25. Jacobs R, Tabor DR, Burks W, et al: Elevated interleukin-1 release by human alveolar macrophages during the adult respiratory distress syndrome. Am Rev Respir Dis 140:1686-1692, 1989 26. Jorens PG, Van Damme J, De Backer W, et al: Interleu-
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kin-8 (IL-8) in the bronchoalveolar lavage fluid from patients with the adult respiratory distress syndrome. Cytokine 4:592-597, 1992 27. Kala P, Ten Have T, Nielsen H, et al: Association of pulmonary surfactant protein A (SP-A) gene and respiratory distress syndrome: Interaction with SP-B. Pediatr Res 43:169-177, 1998 28. Kline JN, Cowden JD, Hunninghake GW, et al: Variable airway responsiveness to inhaled lipopolysaccharide. Am J Respir Crit Care Med 160:297-303,1999 29. Mandrup-Paulsen T, Wogensen LD, Jensen M, et al: Circulating interleukin receptor and antagonist concentrations are increased in adult patients with thermal injury. Crit Care Med 3:26-33, 1995 30. Marks J, Marks CB, Luce JM, et al: Plasma tumor necrosis factor in patients with sepsis shock: Mortality rate, incidence of adult respiratory distress syndrome, and effects of methylprednisolone administration. Am Rev Respir Dis 141:94-97, 1990 31. Marty C, Misset B, Tamlon F, et al: Circulating interleukin-8 concentrations in patients with multiple organ failure of septic and non-septic origin. Crit Care Med 22673, 1994 32. Millar AB, Foley NM, Singer N, et al: Tumour necrosis factor in bronchopulmonary secretions of patients with adult respiratory distress syndrome. Lancet 2:712-714, 1989 33. Miller E, Cohen A, Matthay M: Elevated interleukin8 levels in the pulmonary edema fluid of patients with ARDS from sepsis. Crit Care Med 24:14481454., 1996 ~- -
34. Miller E, Cohen A, Nagao S, et al: Elevated levels of NAP-l/interleukin-8 are present in the air space of patients with the adult respiratory distress syndrome and are associated with increased mortality. Am Rev Respir Dis 146:427432, 1992 35. Mira JP, Cariou A, Grall F, et al: Association of TNF, a TNF promotor polymorphism, with septic shock susceptibility and mortality. JAMA 282:561-568, 1999 36. Modlin RL, Brightbill HD, Godowski PJ: The toll of innate immunity of microbial pathogens. N Engl J Med 3403833-1836, 1999 37. Molvig J, Baek L, Christensen P, et al: Endotoxinstimulated human monocyte secretion of interleukin1, tumor necrosis factor-cx and prostaglandin shows stable interindividual differences. Scand J Immunol 27705-716, 1988 38. Moss M, Bucher B, Moore F, et al: Predicting the development of the adult respiratory distress syndrome: The role of chronic alcohol abuse. JAMA 275:50-54, 1996 39. Moss M, Gillespie M, Ackerson L, et al: Endothelial cell activity varies in patients at risk for the adult respiratory distress syndrome. Crit Care Med 24:1782-1786, 1996 40. Moss M, Steinberg KP, Guidot DM, et al: Diabetic patients with septic shock have a decreased incidence of the acute respiratory distress syndrome (ARDS). Am J Respir Crit Care Med 155:503, 1997 41. Nadel S, Newport M, Booy R, et al: Variation in the tumor necrosis factor: A gene promoter region may be associated with death from meningcoccal disease. J Infect Dis 1742378-880, 1996 42. Parsons P: Complement endotoxin and ARDS. In Haslett C, Evans T (eds): The Acute Respiratory Distress Syndrome. London, Chapman and Hall, 1996 43. Parsons P, Moss M: Circulating markers of sepsis and acute lung injury. In Sepsis and Multiorgan Failure: Mechanisms for Treating Strategies. Media, PA, Williams & Wilkins, 1998
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44. Parsons P, Moss M: Early detection and markers of sepsis. Clin Chest Med 17192-196, 1996 45. Parsons P, Fowler A, Hyers T, et al: Chemotactic activity in bronchodalveolar lavage fluid from patients with adult respiratory distress syndrome. Am Rev Respir Dis 132:490493, 1985 46. Parsons P, Moore E, Moore F, et al: Studies on the role of tumor necrosis factor in ARDS The lack of an independent standard. Am Rev Respir Dis 146694700, 1992 47. Parsons P, Moss M, Vannice J, et al: Circulating ILIra and IL-10 levels are increased but do not predict the development of acute respiratory distress syndrome in at-risk patients. Am J Respir Crit Care Med 155:1469-1473, 1997 48. Pearson C, Pison U, Lin Z, et al: SP-B Gene polymorphisms and acute respiratory distress syndrome (ARDS) in a German population. Am J Respir Crit Care Med 159:206, 1999 49. Pinsky M, Vincent JL, Daviere J, et al: Serum cytokine in human septic shock: Relation to multiple-system organ failure and mortality. Chest 103:565, 1993 50. Pittet J, Mackersie R, Martin T, et al: Biological markers of acute lung injury: Prognostic and pathogenetic significance. Am J Respir Crit Care Med 155:11871205, 1997 51. Pugin J, Ricou B, Steinberg KP, et al: Proinflammatory activity in bronchoalveolar lavage fluids from patients with ARDS, a prominent role for interleukin-2. Am J Respir Crit Care Med 153:1850-1856, 1996 52. Pugin J, Verghese G, Widmer M, et al: The alveolar space is the site of intense inflammatory and profibrotic reactions in the early phase of acute respiratory distress syndrome. Crit Care Med 27304-312, 1999 53. Rinaldo J, Cristman J: Mechanisms and mediators of the adult respiratory distress syndrome. Clin Chest Med 11:621-632, 1990 54. Sandstrom J, Nilsson P, Karlsson K, et al: 10-Fold increase in human plasma extracellular superoxide dismutase content caused by a mutation in heparinbinding domain. J Biol Chem 269:19163-19166, 1994 55. Schwartz MD, Moore E, Shenkar R, et al: Nuclear
56.
57.
58.
59.
60.
61.
62. 63.
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factor-tcB is activated in alveolar macrophages from patients with acute respiratory distress syndrome. Crit Care Med 241285-1292, 1996 Shimoda-Matsubayashi S, Matsumine H, Kobayashi T, et al: Structural dimorphism in the mitochondrial targeting sequence in the human manganese superoxide dismutase gene: A predictive evidence for conformational change to influence mitochondrial transport and a study of allelic association in Parkinson's disease. Biochem Biophys Res Commun 226:561565, 1996 Siler TM, Swierkosz JE, Hyers T, et al: Immunoreactive interleukin-1 in bronchoaveolar lavage fluid of high-risk patients and patients with the adult respiratory distress syndrome. Exp Lung Res 15:881-884, 1989 Spinas G, Keller U, Brockhaus M: Release of soluble receptors for tumor necrosis factor (TNF) in relation to circulating TNF during experimental endotoxinemia. J Clin Invest 90:533-536, 1992 Steinberg KF', Milberg JA, Martin TR, et al: Evolution of bronchoalveolar cell populations in the adult respiratory distress syndrome. Am J Respir Crit Care Med 150:113-122, 1994 Stuber F, Udalova IA, Book M, et al: -308 Tumor necrosis factor (TNF) polymorphism is not associated with survival in severe sepsis and is unrelated to lipopolysaccharide inducibility of the human TNF promotor. J Inflamm 46:42-50, 1995 Veletza S, Rogan P, TenHave T, et al: Reference manager: Racial differences in allelic distribution at the human pulmonary surfactant protein B gene locus (SP-B). Exp Lung Res 22:489495, 1996 Westendorp RGJ, Langermans JAM, Huizinga TWJ, et al: Genetic influence on cytokine production and fatal meningococcal disease. Lancet 349:170-173,1997 Yamada H, Yamada Y, Adachi T, et al: Molecular analysis of extracellular superoxide dismutase gene associated with high level in serum. Jpn J Hum Genet 40~177-184,1995 Yang RB, Mark MR, Gray A, et al: Toll-like receptor2 mediates lipopolysaccharide-induced cellular signaling. Nature 395:284-248, 1998
Address reprint requests to Polly E. Parsons, MD #4000-Department of Medicine Denver Health Medical Center 777 Bannock Street Denver, CO 80204
0272-5231 /00 $15.00
+ .OO
MEDIATORS AND MECHANISMS OF ACUTE LUNG INJURY Polly E. Parsons, MD
The saying “history repeats itself” applies well to the topic of this article. The subject of this article was reviewed in Clinics in Chest Medicine in 1990, in an article entitled ”Mechanisms and Mediators of the Adult Respiratory Distress Syndrome,” authored by Rinaldo and Chri~trnan.5~ Those authors noted that the last review on the same topic had been in the early 1980s and they reflected back on the accomplishments that had occurred in the intervening 8 years. In their introduction, they concluded that important advances had been made in investigations into the mechanisms of acute lung injury (ALI) but they stated: “Despite years of research, no other specific pharmacologic therapies have proved useful, and we are less confident in 1990 that we understand the pathogenesis of ARDS than we were in 1982.” In many ways, that statement is as true in 2000 as it was in 1990. There is still no specific pharmacologic intervention for the treatment of acute respiratory distress syndrome (ARDS), although new studies have indicated that mortality can be influenced by modes of supportive care-specifically, mechanical ventilation strategies. Furthermore, although tremendous advances have been made in our understanding of the mechanisms of in-
flammation, how the process actually evolves in patients at risk for and with ALI is still confounding. Just as it was not possible for Rinaldo and Christman to include all the accomplishments and reports in the 8 years since the review before theirs, it is again not possible to delineate all the accomplishments of the past 10 years. Many of the investigations, however, have focused on specific themes that have been reiterated and developed over the 10 years. These serve as the focus of this review. THE IMPACT OF DEFINITIONS AND EPIDEMIOLOGIC DATA
It may seem odd to begin a discussion of mechanisms and mediators with a review of the evolution of clinical definitions and the epidemiology of the clinical syndrome, but developments in these areas have enhanced our understanding of and our ability to further investigate the pathogenesis of ALI. The inflammatory processes that lead to the development of ALI are well described and defined in vitro and in animal models. Confirming that those processes occur in humans with ALI, however, has been, at times, frustrating.
From the Division of Pulmonary and Critical Care Medicine, Department of Medicine, University of Colorado Health Sciences Center; and the Medical Intensive Care Unit, Denver Health Medical Center, Denver, Colorado
CLINICS IN CHEST MEDICINE VOLUME 21 * NUMBER 3 SEPTEMBER 2000
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Studies frequently have been done to confirm that a specific mediator is important in the development of lung injury, only to have conflicting results reported by different investigators from different institutions. Early studies of the role of tumor necrosis factor (TNF) in sepsis and ARDS are a good example. In patients with sepsis, the percentage of patients with measurable TNF in their circulation ranges from 14% to 54%,43leading investigators to conclude that there may or may not be an association between TNF and sepsis. Results were similarly confusing from studies evaluating the measurement of TNF as a predictive marker for the development of either ALI or multiple organ failure. In one study, a clear association between TNF and the development of ARDS was demonstrated30;in two subsequent studies no association was found between TNF measurements and the development of -either ARDS*‘jor multiple organ failure.” In reviewing these and other studies, the
Soluble ICAM-1
,
author has asked herself the question, “Why are the results so different?” innumerable times. Although there is no definitive answer to that question, several factors recently have been identified that could contribute to the answer. The first factor is that the patients included in clinical studies of ALI are often heterogeneous. Since ARDS was first described in the 1970s, the definition of the syndrome has been in evolution. A recent focus on refining the definitions for both ARDS4 and the major risk factor for the s y n d r o m e sepsiss-will likely increase the homogeneity of patients in clinical studies in the future. Patient heterogeneity is further confounded by the recognition that all patients at risk for ARDS may not be the same. Recent studies suggest that measurements of inflammatory mediators and response are different for patients with sepsis versus trauma. As shown in Figure 1, for example, circulating levels of ICAM and E-selectin are increased in septic patients at risk for ARDS, whereas they are
Soluble E-Selectin
Figure 1. Both soluble CAM-1 and E-selectin levels are significantly greater in septic patients (solid bar) at risk for acute respiratoty distress syndrome (ARDS) than in at-risk trauma patients (hatched bars).* P < .001. (Data from Moss M, Gillespie M, Ackerson L, et al: Endothelial cell activity varies in patients at risk for the adult respiratory distress syndrome. Crit Care Med 24:1782-1786, 1996.)
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/
0
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Figure 2. The incidence of ARDS is greater in those patients at risk for ARDS who abuse alcohol than in those patients who are not identified as alcoholics. (Data from Moss M, Bucher B, Moore F, et al: Predicting the development of the adult respiratory distress syndrome: The role of chronic alcohol abuse. JAMA 27550-54, 1996.)
not increased in trauma patients at risk.39 These differences persist even after the patients have developed ARDS. Similarly, patients with direct lung injury (pneumonia, aspiration) respond differently to mechanical ventilation strategies than patients with indirect injury (distal sepsis or trauma).18In light of recent data suggesting that changes in mechanical ventilatory strategy can decrease circulating cytokine levels, it is possible that the pathogenesis of direct lung injury is different from indirect injury. An additional contribution to patient heterogeneity in clinical trials is the existence of co-morbid conditions. As shown in Figure 2, patients at risk for ARDS who abuse alcohol have recently been shown to have an increased incidence of ARDS and an increased mortality from the syndrome.38In contrast, patients with diabetes who are septic have a decreased incidence of ARDS compared with nondiabetic septic patients.40Both alcohol abuse and diabetes impact several factors in
the inflammatory response and could thereby influence the pathogenesis of ARDS. The timing and site of measurements are also now recognized as important contributors to study results and interpretation. The focus on the role of the neutrophil in ALI arose from early observations in histologic specimens that neutrophils accumulate in the air spaces and interstitium of the lungs of patients with ARDS,’ and the bronchoalveolar lavage fluid (BAL) from these patients is characterized by ne~trophilia.4~ Subsequent studies of serial BAL from ARDS patients have further contributed to the hypothesis that the neutrophil is important. Using serial BAL, Steinberg and colleagues showed that patients with ARDS requiring prolonged ventilatory support had persistent neutrophilia in BAL and that there was an association between the degree of neutrophilia and mortality,59whereas the rate of mortality was decreased in patients who had an increase in macrophages in sequential BAL. Serial analy
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cantly to the variability in study results, additional reasons for the discrepancies in the results from these clinical studies were outlined in a review by Pittet and colleague^.^^ They included the fact that TNF is released within an hour after the administration of endotoxin in normal volunteers and declines rapidly, so detection in patients could be difficult. To address this issue, at least one group of investigators has identified patients early in the course of their at-risk illness and performed serial measurements of TNF to determine whether the absolute level of TNF and the persistence of TNF in the circulation were associated with the development of ARDS.46 There was no difference in either parameter between patients who did and did not develop the syndrome. Other factors discussed by Pittet et a150that could have contributed to the variability in THE ROLE OF INFLAMMATORY the clinical study results include the recogniMEDIATORS tion that many different TNF assays were used for the clinical studies and only recently Early studies in ARDS focused on the role was the importance of avoiding endotoxin of complement and then endotoxin in the decontamination during the collection and analvelopment of lung injury.53In the past decade, ysis of samples recognized. the importance of inflammatory cytokines One criticism of the studies that measured and the recognition of new cytokines have TNF only in the circulation of critically ill led to the development of more detailed and patients was that TNF could be produced complex hypotheses. The inflammatory cylocally within tissue such as the lung, and tokines that have received the most attention that the levels there would be a more relevant are TNF and interleukin (1L)-1, -6, and -8. marker. Although direct measurement of TNF These mediators share several characteristics in the lungs from humans would be difficult, that make their role in the development of TNF has been measured in BAL fluid and ALI highly plausible. They are all proinpulmonary edema fluid from patients with flammatory cytokines. Their production and ARDS. TNF levels are increased in these flurelease are stimulated by multiple relevant ids but are not specific markers for ARDS and mediators, including endotoxin,42and are regulated by nuclear factor kappaB (NF-KB).~ do not correlate with morbidity and mortalit^.^^, 32 Also, TNF-a in pulmonary edema They have all been shown to be implicated in fluid may not be biologically active.52The the development of ALI in animal models likely source for the TNF in the lung is alveoand they are all present in patients at risk for lar macrophages, although there could be leak and with ARDS. from the circulation as well. TNF was the first cytokine to be extensively IL-1, -6, and -8 also have been measured studied in patients at risk for and with ARDS. in both the circulation and lung fluid from As previously discussed, many studies have patients at risk for and with ARDS. Many of investigated the role of circulating TNF in the the problems identified in the studies of TNF pathogenesis of ARDS in humans, and the have been encountered with these other proresults have been confounding. TNF is variinflammatory cytokines as well. Plasma IL-1 ably present in patients at risk for and with levels are increased in the circulation of some ARDS, and whether TNF levels predict morcritically ill patients, but predict neither the bidity or mortality is not clear. Although padevelopment of ARDS or other organ failure, tient heterogeneity may contribute signifi-
sis of patients therefore was more helpful for prognosis than a single early analysis and the data from serial analyses contributed to the understanding of the pathogenesis of both the development and resolution of ALI. In critically ill patients with ARDS, pathogenesis studies are generally limited to analysis of peripheral blood, BAL, or pulmonary edema fluid, and, to a lesser extent, urine. Data accumulated from studies performed using material from these different sites have enhanced the awareness of the potential importance of studying the inflammatory process both systemically and within the lung and have led to an increased interest and awareness of the involvement of other organs in the process.
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nor mortality.50IL-1 levels are increased in the BAL and pulmonary edema obtained from patients with ARDS, and alveolar macrophages from these patients release more IL-1 both at baseline and following lipopolysaccharide (LPS) stimulation than alveolar macrophages from either normal subjects or patients with other lung diseases.25Of interest, two studies have now demonstrated that IL-1 likely accounts for the majority of proinflammatory cytokine activity in the lung in ARDS.51,52 Both TNF and IL-1 stimulate the production of IL-8, a proinflammatory cytokine that is a potent neutrophil chemoattractant. As with TNF and IL-1, studies of IL-8 in plasma from critically ill patients have yielded conflicting results.31,52 In light of its chemoattractant activity, it is likely that IL-8 contributes more to acute inflammation within the lung than in the circulation. IL-8 levels are increased in the BAL fluid and pulmonary edema fluid in ARDS13,33, 34, 52 and correlate with neutrophil concentrations within the lung, lending plausibility to the hypothesis that IL-8 contributes to ALI, at least in part, through its neutrophil chemoattractant capacity. IL-8 levels are higher in the lungs of patients at risk who ultimately develop ARDSI3,26 and higher IL-8 levels may be associated with mortality from ARDS,’” 34, 51, 52 although, again, there is no absolute level of IL-8 that predicts either morbidity or mortality. Plasma levels of IL-6 have been touted as being the most predictive of morbidity and mortality of all the pro-inflammatory cytokines measured to date. Several studies have found that IL-6 levels are significantly increased in patients at risk for ARDS and are higher and persist longer in patients who die.9,20, 49 As pointed out in previous reviews, however, there is no specific IL-6 level that predicts which patients will die,44so the measurement is more likely to be useful in identifying groups of patients at high risk for mortality rather than in using the data for any individual patient. With the accumulated evidence suggesting that multiple proinflammatory cytokines may be important in the development of ARDS,
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some recent investigations have focused on the role of a more proximal common regulatory factor, NF-KB. NF-KB is the regulatory transcription factor for TNF, IL-1, IL-6, and IL-8 as well as many other proteins that could be involved in the development of ARDS.3Of course, no regulatory mechanism is simple. Whereas NF-KBregulates IL-1 and TNF, both IL-1 and TNF also activate NF-KB.In animal models, regulation of NF-KB activation can alter inflammatory processes, including neutrophil-mediated lung injury.6In humans, NFKB activation is significantly increased in alveolar macrophages obtained from patients with ARDS compared with patients without lung injury.55Although this association requires further study, if NF-KB regulation is important in the development of ALI, it may allow for the consideration of novel therapeutic options. Numerous inhibitors of NF-KB activation, including IL-10 and glucocorticoids, have been identified3 and, although complete and prolonged inactivation of NFKB could interfere with host defense, selective inhibition could be helpful in the abrogation of lung injury. THE ROLE OF INFLAMMATORY MODULATORS
Because no one cytokine (or any other proinflammatory mediator) has been shown to predict who develops ARDS, be specific for the diagnosis of ARDS, or consistently reflect the pathogenesis of ARDS, investigators are becoming less enamored with studying single proinflammatory mediators in isolation. Furthermore, it has become apparent that modulators of inflammation are also important in the development of ALI. The concept that injury could result from an alteration in the balance between pro- and anti-inflammatory mediators was probably first introduced when activated oxygen species were found to be associated with ARDS. It was recognized early on that there was an innate antioxidant defense that could be upregulated, such that oxidants and antioxidants could not be studied in isolation, but, rather, the relationship between the two needs to be considered. This
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concept of a balance between inflammatory mediators and modulators can now be extended to cytokines. Specific naturally occurring antagonists for TNF-a and IL-1 have been identified, and it is likely that antagonists for other cytokines exist as well. The identified antagonists for TNF are two soluble TNF receptors (sTNFR) that are released when cell surface-associated TNF receptors undergo proteolytic cleavage. sTNFR are found in the circulation of normal volunteers following the administration of endotoxin58and, of interest, in vitro, neutrophils release substantial quantities of both of these receptors following IL-1-stimulated adherence to endothelium.5 Furthermore, sTNFR protect against TNF-induced inflammation in animal models.z0This implies sTNFR could be protective in the development of ARDS. sTNFR are present in the circulation of patients with sepsiszoand, at least in one study, were higher in patients who died. The sTNFR levels were also significantly greater than the TNF levels in the circulation, however, so their protective effect in sepsis and ARDS remains to be studied.2O The natural antagonist for IL-1 is IL-lral2 which inhibits IL-1 activity in vitro and abrogates mortality in multiple animal models of sepsis.43IL-lra levels are significantly increased following the administration of endotoxin to normal volunteers and are elevated in the circulation of critically ill patients.15,29 The administration of IL-lra to patients with sepsis, however, failed to decrease morbidity or mortality.16 Furthermore, although IL-lra levels are increased in the circulation in patients at risk for ARDS, they do not predict the development of the syndrome and the levels are inversely correlated with mortality." In contrast, low levels of IL-lra and IL10 in BAL fluid from patients with ARDS correlate with rn0rta1ity.l~Again, therefore, the role of this anti-inflammatory mediator in the pathogenesis of ARDS remains unclear. IL-10 can also modulate the effects of IL-1. IL-10 inhibits cytokine production, including IL-1, from multiple cell types and potentiates the release of IL-lra following stimulation with LPS." Like IL-lra, IL-10 levels are increased following the administration of endo-
toxin and IL-10 has been shown to decrease mortality from endotoxemia in animal models." Furthermore, IL-lra levels are increased in patients at risk for ARDS but, again, they do not predict the development of ARDS and are higher in patients who die.47 CALCULATING THE BALANCE
As more and more inflammatory mediators and modulators are discovered, investigators have attempted to quantify their relative contribution to the development of ALI by developing "scoring systems" that incorporate multiple mediator measurements. A LPScytokine score incorporating TNF, IL-1, IL-6, and LPS measurements was evaluated in patients with sepsis by Casey et a19 and found to predict mortality more accurately than any one of the individual mediators measured. Other investigators have combined pro- and anti-inflammatory mediator measurements such as TNF and IL-10 in both the circulation and the lung and found differences in the ratios of these mediators between patients at risk and patients with ARDS.' This concept, that combined measurements may be more relevant than any single measurement, is important. These studies are confounded by our incomplete understanding of the pathogenesis of ALI, however. It is difficult to know what measurements to consider when developing these scoring systems. It is likely that measurements of systemic inflammation need to be considered in tandem with markers of lung response, as well as physiologic parameters. THE ROLE OF GENETIC SUSCEPTIBILITY
The role of genetic susceptibility in ALI is beginning to receive substantial attention. It is clear that not all patients identified as being at risk for ARDS develop the syndrome and that, to date, there is no marker measured in the circulation or the lung that clearly differentiates those patients with and without lung injury. With rapid progress in mapping of the
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human genome and the explosion in knowledge of the mechanisms of genetic regulation, it is now possible to begin to consider evaluating the impact of genetic susceptibility to lung injury. There are several likely targets for these investigations, encompassing both the initial host response to inflammatory stimuli as well as the lung response to systemic and local inflammation. To date, the role of genetic regulation of TNF production has received the most attention in studies of patients at risk for and with ARDS. Early studies showed that stimulated TNF production from human monocytes varied significantly and consistently among normal suggesting that individuals could have inherent differences in susceptibility to disease processes that involved stimulated TNF.production. Furthermore, in a subsequent study, relatives of patients with meningococcal disease were found to vary in their capacity for TNF production. Interestingly, those families that had low TNF production had the highest mortality rates.62 Polymorphisms of the TNF-a gene that are associated with enhanced production of TNF have now been identified and found to be associated with morbidity from some severe infections, including meningococcal disease4I and malaria. Early studies of patients with sepsis found no difference in incidence of TNF polymorphisms in patients with sepsis compared with normal controls,60although one study suggested an association between polymorphisms for TNF and morbidity.60A subsequent study compared 89 patients with septic shock with 87 normal volunteer blood donors and found that the TNF polymorphism TNF2 occurred more frequently in the patients with sepsis than in the normal volunteers and that there was a strong association between this polymorphism and susceptibility to both the development of septic shock and mortality, although there was not an association between the polymorphism and measured levels of TNF?5 These studies are intriguing and suggest that further investigation into the role of TNF genomic polymorphisms in the development of ARDS could contribute to the understanding of the pathogenesis of the syndrome as well as to the
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ability to identify patients for targeted therapeutic interventions. Genomic polymorphisms have been identified for other relevant cytokines, including IL-1, IL-lO,' and IL-lra.= Although not yet evaluated in patients specifically at risk for and with ARDS, these polymorphisms are likely to contribute to the susceptibility to develop ALI. As shown for TNF, there is significant intrasubject variability in the stimulated production of these cytokines, as well and, in the study of first degree relatives of patients with meningococcus discussed previously, there was also a strong genetic determination of IL-10 production in addition to the genetic determination of TNF.62IL-10 production and mortality were associated but, interestingly, the association was with high IL-10 production, which would have likely been predicted to be protective. As these, and other, polymorphisms are evaluated in ARDS, many issues that have been addressed in studies of circulating markers will have to be considered, including patient homogeneity, pre-existing and comorbid conditions, and the interplay or balance between the various polymorphisms and their effects. Genetic determinants of LPS response are also likely to be important in the development of ARDS. A group of proteins known as toll-like receptors (TLR) recently were recognized as being a key element in the signaling pathway for LPS.36To date, five of these proteins have been identified and at least two of them, TLR-2 and TLR-4, could influence the susceptibility of a host to develop ARDS. TLR-2 enhances LPS response by augmenting cellular signaling." It seems that LPS directly binds to TLR-2 and initiates a signaling cascade that leads through NF-KBto the release of cytokines as well as other molecules. TLR4 can also induce the expression of NF-KB, and appears to be responsible for the regulation of LPS response in mice.I9 The potential role for genomic polymorphisms of TLR in ARDS is now being explored. Schwartz et alZ8 have analyzed the response to inhaled LPS and and demonstrated that there are reproducible LPS-sensitive and hyporesponsive phenotypes characterized by their pulmonary response to inhaled LPS and by the LPS-stim-
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ulated release of IL-6 and IL-8 from their peripheral blood monocytes and alveolar macrophages. The authors speculate that differences in genetic susceptibility could account, at least in part, for their results. Several potential genetic factors could contribute to the susceptibility of the pulmonary vasculature and the alveoli to systemic and local inflammation, as well. One of the characteristic features of ALI is the derangement of the pulmonary endothelium, which has been hypothesized to occur, at least in part, secondary to an interaction between inflammatory cells (specifically neutrophils) and the vascular endotheli~m.~ One of the purported mediators for this injury is active oxygen species that have been clearly associated with the development of ALI in animal models. One of the potential defense mechanisms for this injury is the production or presence of superoxide dismutases (SOD). Two of the SODsmanganese SOD (MnSOD) and extracellular SOD (ECSODkare inducible by cytokines, so the genetic regulation of cytokines could contribute to variations in their expression. In addition, however, genetic variants of MnSOD and ECSOD have been describedI7,54 and have been reported to be associated with human disease,56,63 although the association with ALI is not known. An additional feature of ALI is the derangement of both the composition and function of alveolar surfactant proteins (SP)?O Allelic variants of SP-A and SP-B have been described and found to be associated with the development of neonatal respiratory distress syndr0me.2~Genetic polymorphism of SP-B has also been associated with ARDS.48 Because the SPs are important in the function of surfactant and they contribute to both host defense and inflammatory response, genetic variations in the regulation of either production or function could contribute to the susceptibility of an individual to the development of ALI. Of interest, another contributor to the heterogeneity of the patient population at risk for and with ARDS has been uncovered in the studies of SP genetic polymorphisms. There are significant differences in the distribution of the alleles of SP-B among racial groups,27, suggesting that future stud-
ies of patients should consider this variable as well.
SUMMARY
Since last reviewed in this forum, there have been remarkable advances in our understanding of the acute inflammatory process and how it contributes to the development of ALI. As stated in the beginning of this article, it is not possible to even begin to review all the specific advances that have been made. Instead, the author has focused on concepts that have emerged and improved our ability to study the pathogenesis of ARDS. These include the recognition that patients at risk for and with ARDS represent a heterogeneous population, that mediators or markers of inflammation cannot be considered in isolation, that a balance between proinflammatory mediators and inflammatory modulators may be important, and that there are several genetic factors that could contribute to the susceptibility for the development of ARDS. Hopefully these concepts can be expanded and clarified so that the next review of this topic can report on successful therapeutic interventions for the prevention and the treatment of ARDS.
References 1. Armstrong L, Millar AB: Relative production of tumor necrosis factor-alpha and interleukin-10 in adult respiratory distress. Thorax 5242-446, 1997 2. Bachofen M, Weibel E: Structural alterations of lung parenchyma in the adult respiratory distress syndrome. Clin Chest Med 3:35-56, 1982 3. Barnes P, Karlin M Nuclear Factor-kB-A pivotal transcription factor in chronic inflammation diseases. N Engl J Med 336106f+1072, 1997 4. Bernard G, Brigham K, Carlet J, et al: The AmericanEuropean Consensus Conference on ARDS Definitions, mechanisms, relevant outcomes, and clinical trial coordination. Am J Respir Ciit Care Med 14981S824, 1994 5. Bjornberg F, Kantz M Adherence to endothelial cells induce release of soluble tumor necrosis factor (TNF) receptor forms from neutrophil granulocytes. Biochem Biophys Res Comm 244:594-598,1998 6. Blackwell TS, Christman JW: The role of nuclear factor-K B in cytokine gene regulation. Am J Respir Cell Mol Biol 173-9, 1997 7. Bone R The pathogenesis of sepsis. Ann Intern Med 115457-469, 1991
MEDIATORS AND MECHANISMS OF ACUTE LUNG INJURY 8. Bone R, Balk R, Cerra F, et al: Definitions for sepsis
and organ failure and guidelines for the use of innovative therapies in sepsis. Chest 101:1644-1655, 1992 9. Casey LC, Balk R, Bone R Plasma cytokine and endotoxin levels correlate with survival in patients with sepsis syndrome. Ann Intern Med 119:771-778, 1993 10. Chollet-Martin S, Montravers P, Gilbert C, et al: High levels of interleukin-8 in the blood and alveolar spaces of patients with pneumonia and adult respiratory distress syndrome. Infect Immun 61:4553-4559, 1993 11. Degroote MA, Martin MA, Densen P, et al: Plasma tumor necrosis factor levels in patients with presumed sepsis: Results in those treated with antilipid A antibody versus placebo. JAMA 262:249-251, 1989 12. Dinarello CA: Interleukin-1 and interleukin-1 antagonism. Blood 771627-1652, 1991 13. Donnelly SC, Strieter RM, Kunkel SL, et al: Interleukin-8 and development of adult respiratory distress syndrome in at-risk patient groups. Lancet 341:643647, 1993 14. Donnelly S, Strieter R, Reid P, et al: The association between mortality rates and decreased concentrations of interleukin-10 and interleukin-1-receptor antagonist in the lung fluids of patients with the adult respiratory distress syndrome. Ann Intern Med 125~191-196,1996 15. Fischer E, Van Zee KJ, Marano MA, et al: Interleukin1-receptor antagonist circulates in experimental inflammation and in human disease. Blood 79:21962199, 1992 16. Fisher C, Dhainaut J, Opal S, et al: Recombinant human interleukin-1 receptor antagonist in the treatment of patients with sepsis syndrome. JAMA 271:1836-1843, 1994 Identification of a 17. Folz RJ, Peno-Green L, Crapo JD: homozygous missense mutation (Arg to Gly) in the critical building region of the human EC-SOD gene (SOD3) and its association with dramatically increased serum enzyme levels. Hum Mol Genet 3:2251-2254, 1994 18. Gattinoni L, Pelosi P, Suter PM, et al: Acute respiratory distress syndrome caused by pulmonary and extrapulmonary disease: Different syndromes? Am J Respir Crit Care Med 158:3-11, 1998 19. Gerard C: Bacterial infection: For whom the bell tolls. Nature 395:217-219, 1998 20. Goldie AS, Fearon K, Ross J, et al: Natural cytokine antagonists and endogenous antiendotoxin core antibodies in sepsis syndrome. JAMA 274:172-177, 1995 21. Hack C, DeGroote E, Felt-Bersma R, et al: Increased plasma levels of IL-6 in sepsis. Blood 74:704, 1989 22. Hiroshi S, Yoshiki M, Heihachiro K Interleukin-lreceptor antagonist gene polymorphism in Japanese patients with systemic lupus erythematosus. Arthritis Rheum 40:389-390, 1997 23. Hurme M, Lahdenpohja N, Santtila S Gene polymorphisms of the interleukins 1 and 10 in infectious and autoimmune disease. AM Med 30:469473, 1998 24. Hyers T, Tricomi SM, Dettenmeier P, et al: Tumor necrosis factor levels in serum and bronchoalveolar lavage fluid of patients with the adult respiratory distress syndrome. Am Rev Respir Dis 144:268-271, 1991 25. Jacobs R, Tabor DR, Burks W, et al: Elevated interleukin-1 release by human alveolar macrophages during the adult respiratory distress syndrome. Am Rev Respir Dis 140:1686-1692, 1989 26. Jorens PG, Van Damme J, De Backer W, et al: Interleu-
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kin-8 (IL-8) in the bronchoalveolar lavage fluid from patients with the adult respiratory distress syndrome. Cytokine 4:592-597, 1992 27. Kala P, Ten Have T, Nielsen H, et al: Association of pulmonary surfactant protein A (SP-A) gene and respiratory distress syndrome: Interaction with SP-B. Pediatr Res 43:169-177, 1998 28. Kline JN, Cowden JD, Hunninghake GW, et al: Variable airway responsiveness to inhaled lipopolysaccharide. Am J Respir Crit Care Med 160:297-303,1999 29. Mandrup-Paulsen T, Wogensen LD, Jensen M, et al: Circulating interleukin receptor and antagonist concentrations are increased in adult patients with thermal injury. Crit Care Med 3:26-33, 1995 30. Marks J, Marks CB, Luce JM, et al: Plasma tumor necrosis factor in patients with sepsis shock: Mortality rate, incidence of adult respiratory distress syndrome, and effects of methylprednisolone administration. Am Rev Respir Dis 141:94-97, 1990 31. Marty C, Misset B, Tamlon F, et al: Circulating interleukin-8 concentrations in patients with multiple organ failure of septic and non-septic origin. Crit Care Med 22673, 1994 32. Millar AB, Foley NM, Singer N, et al: Tumour necrosis factor in bronchopulmonary secretions of patients with adult respiratory distress syndrome. Lancet 2:712-714, 1989 33. Miller E, Cohen A, Matthay M: Elevated interleukin8 levels in the pulmonary edema fluid of patients with ARDS from sepsis. Crit Care Med 24:14481454., 1996 ~- -
34. Miller E, Cohen A, Nagao S, et al: Elevated levels of NAP-l/interleukin-8 are present in the air space of patients with the adult respiratory distress syndrome and are associated with increased mortality. Am Rev Respir Dis 146:427432, 1992 35. Mira JP, Cariou A, Grall F, et al: Association of TNF, a TNF promotor polymorphism, with septic shock susceptibility and mortality. JAMA 282:561-568, 1999 36. Modlin RL, Brightbill HD, Godowski PJ: The toll of innate immunity of microbial pathogens. N Engl J Med 3403833-1836, 1999 37. Molvig J, Baek L, Christensen P, et al: Endotoxinstimulated human monocyte secretion of interleukin1, tumor necrosis factor-cx and prostaglandin shows stable interindividual differences. Scand J Immunol 27705-716, 1988 38. Moss M, Bucher B, Moore F, et al: Predicting the development of the adult respiratory distress syndrome: The role of chronic alcohol abuse. JAMA 275:50-54, 1996 39. Moss M, Gillespie M, Ackerson L, et al: Endothelial cell activity varies in patients at risk for the adult respiratory distress syndrome. Crit Care Med 24:1782-1786, 1996 40. Moss M, Steinberg KP, Guidot DM, et al: Diabetic patients with septic shock have a decreased incidence of the acute respiratory distress syndrome (ARDS). Am J Respir Crit Care Med 155:503, 1997 41. Nadel S, Newport M, Booy R, et al: Variation in the tumor necrosis factor: A gene promoter region may be associated with death from meningcoccal disease. J Infect Dis 1742378-880, 1996 42. Parsons P: Complement endotoxin and ARDS. In Haslett C, Evans T (eds): The Acute Respiratory Distress Syndrome. London, Chapman and Hall, 1996 43. Parsons P, Moss M: Circulating markers of sepsis and acute lung injury. In Sepsis and Multiorgan Failure: Mechanisms for Treating Strategies. Media, PA, Williams & Wilkins, 1998
476
PARSONS
44. Parsons P, Moss M: Early detection and markers of sepsis. Clin Chest Med 17192-196, 1996 45. Parsons P, Fowler A, Hyers T, et al: Chemotactic activity in bronchodalveolar lavage fluid from patients with adult respiratory distress syndrome. Am Rev Respir Dis 132:490493, 1985 46. Parsons P, Moore E, Moore F, et al: Studies on the role of tumor necrosis factor in ARDS The lack of an independent standard. Am Rev Respir Dis 146694700, 1992 47. Parsons P, Moss M, Vannice J, et al: Circulating ILIra and IL-10 levels are increased but do not predict the development of acute respiratory distress syndrome in at-risk patients. Am J Respir Crit Care Med 155:1469-1473, 1997 48. Pearson C, Pison U, Lin Z, et al: SP-B Gene polymorphisms and acute respiratory distress syndrome (ARDS) in a German population. Am J Respir Crit Care Med 159:206, 1999 49. Pinsky M, Vincent JL, Daviere J, et al: Serum cytokine in human septic shock: Relation to multiple-system organ failure and mortality. Chest 103:565, 1993 50. Pittet J, Mackersie R, Martin T, et al: Biological markers of acute lung injury: Prognostic and pathogenetic significance. Am J Respir Crit Care Med 155:11871205, 1997 51. Pugin J, Ricou B, Steinberg KP, et al: Proinflammatory activity in bronchoalveolar lavage fluids from patients with ARDS, a prominent role for interleukin-2. Am J Respir Crit Care Med 153:1850-1856, 1996 52. Pugin J, Verghese G, Widmer M, et al: The alveolar space is the site of intense inflammatory and profibrotic reactions in the early phase of acute respiratory distress syndrome. Crit Care Med 27304-312, 1999 53. Rinaldo J, Cristman J: Mechanisms and mediators of the adult respiratory distress syndrome. Clin Chest Med 11:621-632, 1990 54. Sandstrom J, Nilsson P, Karlsson K, et al: 10-Fold increase in human plasma extracellular superoxide dismutase content caused by a mutation in heparinbinding domain. J Biol Chem 269:19163-19166, 1994 55. Schwartz MD, Moore E, Shenkar R, et al: Nuclear
56.
57.
58.
59.
60.
61.
62. 63.
64.
factor-tcB is activated in alveolar macrophages from patients with acute respiratory distress syndrome. Crit Care Med 241285-1292, 1996 Shimoda-Matsubayashi S, Matsumine H, Kobayashi T, et al: Structural dimorphism in the mitochondrial targeting sequence in the human manganese superoxide dismutase gene: A predictive evidence for conformational change to influence mitochondrial transport and a study of allelic association in Parkinson's disease. Biochem Biophys Res Commun 226:561565, 1996 Siler TM, Swierkosz JE, Hyers T, et al: Immunoreactive interleukin-1 in bronchoaveolar lavage fluid of high-risk patients and patients with the adult respiratory distress syndrome. Exp Lung Res 15:881-884, 1989 Spinas G, Keller U, Brockhaus M: Release of soluble receptors for tumor necrosis factor (TNF) in relation to circulating TNF during experimental endotoxinemia. J Clin Invest 90:533-536, 1992 Steinberg KF', Milberg JA, Martin TR, et al: Evolution of bronchoalveolar cell populations in the adult respiratory distress syndrome. Am J Respir Crit Care Med 150:113-122, 1994 Stuber F, Udalova IA, Book M, et al: -308 Tumor necrosis factor (TNF) polymorphism is not associated with survival in severe sepsis and is unrelated to lipopolysaccharide inducibility of the human TNF promotor. J Inflamm 46:42-50, 1995 Veletza S, Rogan P, TenHave T, et al: Reference manager: Racial differences in allelic distribution at the human pulmonary surfactant protein B gene locus (SP-B). Exp Lung Res 22:489495, 1996 Westendorp RGJ, Langermans JAM, Huizinga TWJ, et al: Genetic influence on cytokine production and fatal meningococcal disease. Lancet 349:170-173,1997 Yamada H, Yamada Y, Adachi T, et al: Molecular analysis of extracellular superoxide dismutase gene associated with high level in serum. Jpn J Hum Genet 40~177-184,1995 Yang RB, Mark MR, Gray A, et al: Toll-like receptor2 mediates lipopolysaccharide-induced cellular signaling. Nature 395:284-248, 1998
Address reprint requests to Polly E. Parsons, MD #4000-Department of Medicine Denver Health Medical Center 777 Bannock Street Denver, CO 80204