- Email: [email protected]
Interaction of Coagulation and Inflammation
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KEY POINTS • Despite new information about the pathophysiology and treatment of severe sepsis, it continues to be associated with an unacceptably high mortality rate. • The endothelium is key in initiating, perpetuating, and modulating the host response to infection. • Endothelial dysfunction occurs in the absence of reliable circulating markers, and the endothelium is a highly complex organ; therefore isolated endothelial cells cannot be relied on for a full understanding of the endothelium in health and disease. The endothelium is largely overlooked in clinical practice, a bench-to-bedside gap. • Enormous resources have been expended on sepsis trials; most therapies failed to reduce mortality in patients with severe sepsis, but at least five phase 3 clinical trials demonstrated improved survival in critically ill patients or patients with severe sepsis.
The innate immune response involves the concomitant activation of inflammation and coagulation (see Chapter 50 for an overview of coagulation). Indeed, it may be argued that inflammation and coagulation always occur together. Although usually an adaptive mechanism, the innate immune response may lead to disease. Severe sepsis is the leading cause of death among hospitalized patients in noncoronary intensive care units. An important goal is to develop improved therapeutic strategies that will have a favorable impact on patient outcome. Sepsis is invariably associated with activation of coagulation and inflammation. Recent studies have pointed to a critical role for the endothelium in orchestrating the host response in severe sepsis. This chapter provides a conceptual framework for understanding the pathophysiology of sepsis, emphasizes the importance of the cross-talk between inflammation and coagulation, and underscores the potential value of the endothelium as a target for sepsis therapy.
SEPSIS PATHOPHYSIOLOGY Sepsis pathophysiology may be described according to the following themes (Box 51.1). First, the host response—rather than the identity of the pathogen—is the primary determinant of
patient outcome. Second, monocytes and endothelial cells serve to initiate and perpetuate the host response to infection. Third, sepsis is characterized by systemic activation of the inflammatory and coagulation cascades.1 Fourth, the inflammatory and coagulation pathways interact with one another to further amplify the host response. Finally, the host response inflicts collateral damage on normal tissues, resulting in pathology that is not diffuse but remarkably focal in its distribution. Each of these themes has been previously reviewed in detail (Aird, 2003). The pathophysiology of sepsis may be simplified according to the scheme shown in Figure 51.1. Monocytes and tissue macrophages engage pathogens through pattern recognition receptors (e.g., toll-like receptors). Ligand-receptor interactions result in the activation of both inflammatory and coagulation pathways. On the inflammatory side, the monocyte releases many inflammatory mediators—including tumor necrosis factor (TNF)–a and interleukin (IL)-1—which then bind to receptors in the presence of monocytes and endothelial cells, resulting in autocrine and paracrine activation, respectively. On the coagulation side, activated monocytes and macrophages express tissue factor (TF) on their cell surface, which in turn triggers the clotting cascade.
Box 51.1 Themes in Sepsis Pathophysiology 1. 2. 3. 4.
Host response is a primary determinant of pathology Monocytes initiate host response to infection Endothelial cells perpetuate the host response Inflammatory and coagulation pathways are always activated 5. There is cross-talk between inflammatory and coagulation pathways 6. Host response may inflict focal “collateral damage” (organ dysfunction) on the host
1 On the inflammatory side, IL-6 levels are increased in virtually every patient with severe sepsis, and TNF-a levels are increased in the majority of patients. On the coagulation side, D-dimers are elevated in all patients with severe sepsis, protein C levels are decreased in up to 90% of such patients, and antithrombin III levels are below 60% in more than half of patients. Although the operational definition varies among studies, disseminated intravascular coagulation (DIC) is estimated to occur in 15% to 30% of patients with severe sepsis, including those with septic shock.
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Hematologic Problems Box 51.2 Bench-to-Bedside Gap in Endothelial Biomedicine
Innate immune response
INFLAMMATION TOCRINE AU
1. The endothelium is inaccessible 2. The endothelium displays emergent properties 3. The endothelium is tightly coupled to native microenvironment
COAGULATION LPS
C3a, C5a
AUTOCRINE
TLR4
Cytokines: IL-6 IL-1 TNFa IFNg
TF
IXa
Xa
Chemokines: MCP-1 IL-8 MIP-1a RANTES IP-10 Other: O 2– NO VEGF CD40L PAF
VIIa IX
Monocyte/macrophage
Prothrombin
Thrombin
Fibrinogen PARACRINE
Fibrin
PARACRINE
LPS TLR4
PAR
Endothelium
Figure 51.1. Innate immune response. Circulating monocytes or tissue macrophages bind to lipopolysaccharide (LPS) via toll-like receptor 4 (TLR4), resulting in activation of inflammatory and coagulation pathways. Components of each pathway participate in autocrine and paracrine loops, which in turn lead to additional activation of monocytes and endothelial cells, respectively. At the level of the endothelium, input may arrive directly via LPS or indirectly via monocyte-derived inflammatory/coagulation mediators. The innate immune response may also be activated in nonsepsis states such as trauma and surgery.
Importantly, the inflammatory and coagulation cascades engage in significant cross-talk. For example, TNF-a and IL-1 (inflammatory cytokines) induce expression of TF on monocytes and possibly subsets of endothelial cells. In the other direction, serine proteases in the clotting cascade bind to proteaseactivated receptors on the surface of several cell types (including monocytes and endothelial cells), resulting in a proinflammatory phenotype.
ENDOTHELIUM IN SEPSIS Bench-to-Bedside Gap in Endothelial Biomedicine The endothelium, which lines the inside of all blood vessels, is a highly metabolically active organ. Despite a robust literature on endothelial cells (tens of thousands of published articles), the endothelium is largely overlooked in clinical practice. There are several possible explanations for this bench-to-bedside gap (Box 556 51.2). First, the endothelium is hidden from view and is poorly
accessible in the patient. Indeed, the endothelium does not lend itself to inspection, palpation, percussion, or auscultation. Although certain other organs such as the pancreas and kidney are also difficult to examine at the bedside, they are spatially confined and thus amenable to diagnostic imaging. Moreover, whereas disease of these latter organs is associated with changes in blood chemistry (e.g., amylase, blood urea nitrogen, and creatinine), endothelial dysfunction occurs in the absence of reliable circulating markers. Second, the endothelium—like other organs in the body—is highly complex. Most endothelial cell biologists study specific aspects of endothelial cell function in tissue culture and in doing so tend to overlook critical levels of organization that are essential to a full understanding of the system. Just as one could never predict the behavior of an ant colony by studying an individual ant in isolation, one cannot rely solely on isolated endothelial cells to fully understand the endothelium in health and disease. Third, the endothelium, more so than most other tissues in the human body, is extraordinarily adaptive and flexible. It is like a chameleon, “marching to the tune” of the local microenvironment. Indeed, so tightly coupled is the endothelium to the extracellular milieu that when it is removed from its native environment and grown in tissue culture, it undergoes phenotypic drift. Therefore, any results from in vitro studies must be interpreted with caution and ultimately validated in vivo.
Endothelial Cell Function and Dysfunction The two most commonly used terms or descriptors in endothelium-based diseases are endothelial cell activation and endothelial cell dysfunction (Box 51.3). Both terms were coined in the 1980s, and their meaning has changed over the years. Endothelial cell activation was originally used to describe the proadhesive function of cultured endothelial cells treated with
Box 51.3 Definitions Endothelial cell activation: Phenotypic response of the endothelium to inflammatory mediators (as occurs in sepsis, trauma, and surgery), usually consisting of some combination of proadhesive surface, procoagulant activity, altered vasomotor tone, increased apoptosis, and change in barrier function. Note: the phenotype may be functional (adaptive) or dysfunctional (nonadaptive) Endothelial cell dysfunction: Phenotypic response of the endothelium that poses a net liability to the host. Note: the phenotype may be activated or nonactivated.
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Interaction of Coagulation and Inflammation inflammatory mediators. Today, the term most commonly refers to the phenotypic response of the endothelium to inflammation or infection. An important caveat is that the so-called “activation phenotype” differs between different sites of the vasculature. For example, activated endothelial cells in culture have been shown to express decreased levels of thrombomodulin (TM) on their cell surface (in theory, resulting in reduced capacity to activate protein C). However, the human brain expresses very little, if any, TM. That does not mean that the blood-brain barrier is in a chronic state of activation. Rather, the brain must rely on other natural anticoagulants to balance local hemostasis. As another example, P-selectin (a cell adhesion molecule) is considered an activation marker for endothelial cells. However, endothelial cells in uninflamed skin constitutively express Pselectin. In summary, the state of activation must be judged in an appropriate spatial context. Endothelial cell dysfunction is most often used to describe the abnormal endothelial phenotype in atherosclerosis (most notably, changes in endothelium-mediated vasomotor tone). However, the term may be applied more broadly to other disease states. Indeed, endothelial cell dysfunction occurs when the endothelial phenotype—whether or not it meets a definition for activation—represents a net liability to the host. This may occur locally (e.g., coronary artery disease) or systemically (e.g., sepsis).
Endothelial Response in Severe Sepsis Each endothelial cell in the body is analogous to a miniature adaptive input-output device (Fig. 51.2). Input is derived from the extracellular environment and may include biochemical or biomechanical forces. Output represents the phenotype of the endothelium and may include changes in proliferation, cell sur-
Endothelial cells as an input-output device
Soluble mediators: Growth factors Cytokines Chemokines LPS ROS
INPUT
Endothelial cell
Hypoxia
vival, vasomotor tone, permeability, hemostatic balance, and/or release of inflammatory mediators. In sepsis, there are many changes in the input signals, including components of the bacterial wall, complement, cytokines, chemokines, serine proteases, fibrin, activated platelets and leukocytes, hyperglycemia, and/or changes in oxygenation or blood flow. Endothelial phenotypes in sepsis include both structural alterations (e.g., nuclear vacuolization, cytoplasmic swelling, cytoplasmic fragmentation, denudation, and/or detachment) and functional changes (e.g., shifts in the hemostatic balance, increased cell adhesion and leukocyte trafficking, altered vasomotor tone, loss of barrier function, and programmed cell death). Finally, the “set point” of the endothelium—as determined by the influence of epigenetic processes, age, comorbidity, and genetic polymorphisms—may alter the phenotype and/or transduction capacity (input-output coupling) of the endothelial cell.
Endothelial Dysfunction in Severe Sepsis The host response to sepsis involves a complex orchestra of cells (e.g., leukocytes, platelets, and endothelial cells) and soluble mediators (e.g., components of the inflammatory and coagulation cascades). Normally, these mechanisms are highly coordinated with one another to defend host against pathogen. However, if the host response is disproportionate to the nature of the threat, i.e., it is excessive, sustained, or poorly localized, then the balance of power shifts in favor of the pathogen, resulting in the sepsis phenotype—namely, dysfunction of subsets of organ systems. Many hypotheses have been proposed to explain the cause of organ dysfunction. However, at this time, our understanding of the precise pathophysiologic processes is unclear. Available evidence suggests that severe sepsis is associated with excessive, sustained, and generalized activation of the endothelium. Without artificial organ support, virtually all patients with severe sepsis would die of their disease. In other words, most of these individuals have crossed the threshold from an adaptive to a maladaptive response. Insofar as the endothelium contributes to the severe sepsis phenotype, its behavior may be characterized as dysfunctional. An important goal for the future is to learn how to identify the transition from function to dysfunction, before the onset of significant (and perhaps irreversible) organ damage. Importantly, the endothelium is an attractive therapeutic target.
SEPSIS THERAPY
Shear stress/strain Temperature pH
OUTPUT
Hemostatic balance Vasomotor tone Leukocyte trafficking Migration Proliferation Barrier function
Figure 51.2. Endothelial cell as an input-output device. Each endothelial cell in the body (there are about 60 trillion of them) behaves like a miniature adaptive input-output device sensing changes in the extracellular environment and responding via altered phenotype. LPS, lipopolysaccharide; ROS, reactive oxygen species.
Enormous resources have been expended on sepsis trials, with more than 10,000 patients enrolled in over 20 placebocontrolled, randomized phase 3 clinical trials. Most of these therapies have failed to reduce mortality in patients with severe sepsis, including antiendotoxin, anticytokine, antiprostaglandin, antibradykinin and antiplatelet activating factor (PAF) strategies, antithrombin III (ATIII), and tissue factor pathway inhibitor (TFPI). At the time of this writing, a total of five phase 3 clinical trials have demonstrated improved survival in critically ill patients or patients with severe sepsis. These include the use of low tidal volume ventilation, activated protein C, low-dose glucocorticoids, intensive insulin therapy, and early goal-directed therapy. 557
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Sepsis therapy
LPS
A e.g., anti-IL-1
antibodies
INFLAMMATION
B e.g., hirudin
Monocyte
COAGULATION
C e.g., rhAPC
Endothelium
Figure 51.3. Sepsis therapy. Shown are components of the innate immune response. Circulating monocytes or tissue macrophages bind to lipopolysaccharide (LPS), resulting in activation of inflammatory and coagulation cascades (compare Figure 51.1). These pathways then operate in autocrine and paracrine pathways to activate monocytes and endothelial cells, respectively. As a general rule, neither the selective targeting of an inflammatory mediator (A) nor thrombin generation (B) improves survival in severe sepsis. In contrast, therapies that target both inflammatory and coagulation pathways and/or attenuate endothelial cell dysfunction (C) show more promise.
may reduce the activity of proinflammatory transcription factors in endothelial cells, while intensive insulin therapy may reduce the deleterious effects of high glucose on the endothelium. Finally, early goal-directed therapy is predicted to maintain flow and hence shear stress at the level of the blood vessel wall. The extent to which these therapies exert their benefit through the endothelium remains unknown. There are many other possible strategies for attenuating endothelial cell dysfunction (Aird, 2003). One may target the input signals, the coupling mechanism inside the cell, or the cellular phenotype (output). Examples of extracellular signals as targets include endotoxin, TNF-a, IL-1, and PAF. Examples of coupling mechanisms include receptors, such as cellular adhesion molecules and protease-activated receptors; signaling pathways such as p38 MAPK or novel/atypical PKC isoforms; and transcription factors, including NF-kB, GATA-2, and the Ets family of transacting proteins. Potential target outputs include endothelial control of hemostasis, inflammation, vasomotor tone, permeability, and leukocyte trafficking. Although the failure of antiendotoxin and TNF-a antibodies to improve mortality in patients with severe sepsis is consistent with the complexity of the host response, these findings do not exclude a role for single target (“smart bomb”) therapy. The host response, while unquestionably redundant and pleiotropic, is likely to contain certain component parts—whether an extracellular mediator, a cell surface receptor, a signal intermediate, or a transcription factor—that are so highly connected as to render that component (and the entire system) vulnerable to therapeutic targeting. A key challenge is to identify these socalled hubs in the sepsis cascade and to target those factors accordingly.
CONCLUSIONS
There are several lessons to be learned from these clinical trials. First, the use of single modality therapy aimed at inhibiting endotoxin or inflammation is ineffective (Fig. 51.3). These data are consistent with the notion that the inflammatory cascade, while certainly an important contributor to sepsis morbidity and mortality, is sufficiently redundant, pleiotropic, and interdependent as to preclude single modality therapy. Second, preclinical studies with selective thrombin inhibitors suggest that fibrin ablation has little or no effect on organ dysfunction or mortality. One interpretation of these findings is that clotting in and of itself is nonlethal in sepsis. Third, the combination of antiinflammatory and anticoagulant appears more promising. For example, TFPI, ATIII, and recombinant human activated protein C (rhAPC) have each been shown to inhibit inflammation and coagulation in vitro and in vivo and to yield improved survival in nonhuman primate models of sepsis and phase 2 clinical trials of severe sepsis. Of these three agents, only rhAPC reduced mortality in phase 3 clinical studies. A common theme of the five successful treatments in severe sepsis is their capacity to attenuate endothelial cell dysfunction. The effect of rhAPC on the endothelium was discussed above. 558 Low-volume ventilation would be expected to reduce barotraumas to the pulmonary endothelium. Low-dose glucocorticoids
Despite new information about the pathophysiology and treatment of severe sepsis, this disorder continues to be associated with an unacceptably high mortality rate. Future breakthroughs will require a conceptual shift that emphasizes relationships between the various mediators (inflammatory and coagulation) and cells involved in host response. The endothelium is key in initiating, perpetuating, and modulating the host response to infection. Additional studies promise to provide new insight into the endothelium, not as an isolated mechanism of sepsis pathophysiology but rather as the coordinator of a far more expansive, spatially and temporally orchestrated response.
SUGGESTED READING Aird WC: The role of the endothelium in severe sepsis and the multiple organ dysfunction syndrome. Blood 2003;101;3765–3777. Annane D, Sebille V, Charpentier C, et al: Effect of treatment with low doses of hydrocortisone and fludrocortisone on mortality in patients with septic shock. JAMA 2002;288:862–871. ARDSNET: Ventilation with lower tidal volumes as compared with traditional tidal volumes for acute lung injury and the acute respiratory distress syndrome. The Acute Respiratory Distress Syndrome Network. N Engl J Med 2000;342:1301–1308. Bernard GR, Vincent JL, Laterre PF, et al: Efficacy and safety of recombinant human activated protein C for severe sepsis. N Engl J Med 2001; 344:699–709.
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Rivers E, Nguyen B, Havstad S, et al: Early goal-directed therapy in the treatment of severe sepsis and septic shock. N Engl J Med 2001; 345:1368–1377. Van den Berghe G, Wouters P, Weekers F, et al: Intensive insulin therapy in the critically ill patients. N Engl J Med 2001;345:1359–1367.
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Interaction of Coagulation and Inflammation William C. Aird
KEY POINTS • Despite new information about the pathophysiology and treatment of severe sepsis, it continues to be associated with an unacceptably high mortality rate. • The endothelium is key in initiating, perpetuating, and modulating the host response to infection. • Endothelial dysfunction occurs in the absence of reliable circulating markers, and the endothelium is a highly complex organ; therefore isolated endothelial cells cannot be relied on for a full understanding of the endothelium in health and disease. The endothelium is largely overlooked in clinical practice, a bench-to-bedside gap. • Enormous resources have been expended on sepsis trials; most therapies failed to reduce mortality in patients with severe sepsis, but at least five phase 3 clinical trials demonstrated improved survival in critically ill patients or patients with severe sepsis.
The innate immune response involves the concomitant activation of inflammation and coagulation (see Chapter 50 for an overview of coagulation). Indeed, it may be argued that inflammation and coagulation always occur together. Although usually an adaptive mechanism, the innate immune response may lead to disease. Severe sepsis is the leading cause of death among hospitalized patients in noncoronary intensive care units. An important goal is to develop improved therapeutic strategies that will have a favorable impact on patient outcome. Sepsis is invariably associated with activation of coagulation and inflammation. Recent studies have pointed to a critical role for the endothelium in orchestrating the host response in severe sepsis. This chapter provides a conceptual framework for understanding the pathophysiology of sepsis, emphasizes the importance of the cross-talk between inflammation and coagulation, and underscores the potential value of the endothelium as a target for sepsis therapy.
SEPSIS PATHOPHYSIOLOGY Sepsis pathophysiology may be described according to the following themes (Box 51.1). First, the host response—rather than the identity of the pathogen—is the primary determinant of
patient outcome. Second, monocytes and endothelial cells serve to initiate and perpetuate the host response to infection. Third, sepsis is characterized by systemic activation of the inflammatory and coagulation cascades.1 Fourth, the inflammatory and coagulation pathways interact with one another to further amplify the host response. Finally, the host response inflicts collateral damage on normal tissues, resulting in pathology that is not diffuse but remarkably focal in its distribution. Each of these themes has been previously reviewed in detail (Aird, 2003). The pathophysiology of sepsis may be simplified according to the scheme shown in Figure 51.1. Monocytes and tissue macrophages engage pathogens through pattern recognition receptors (e.g., toll-like receptors). Ligand-receptor interactions result in the activation of both inflammatory and coagulation pathways. On the inflammatory side, the monocyte releases many inflammatory mediators—including tumor necrosis factor (TNF)–a and interleukin (IL)-1—which then bind to receptors in the presence of monocytes and endothelial cells, resulting in autocrine and paracrine activation, respectively. On the coagulation side, activated monocytes and macrophages express tissue factor (TF) on their cell surface, which in turn triggers the clotting cascade.
Box 51.1 Themes in Sepsis Pathophysiology 1. 2. 3. 4.
Host response is a primary determinant of pathology Monocytes initiate host response to infection Endothelial cells perpetuate the host response Inflammatory and coagulation pathways are always activated 5. There is cross-talk between inflammatory and coagulation pathways 6. Host response may inflict focal “collateral damage” (organ dysfunction) on the host
1 On the inflammatory side, IL-6 levels are increased in virtually every patient with severe sepsis, and TNF-a levels are increased in the majority of patients. On the coagulation side, D-dimers are elevated in all patients with severe sepsis, protein C levels are decreased in up to 90% of such patients, and antithrombin III levels are below 60% in more than half of patients. Although the operational definition varies among studies, disseminated intravascular coagulation (DIC) is estimated to occur in 15% to 30% of patients with severe sepsis, including those with septic shock.
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Hematologic Problems Box 51.2 Bench-to-Bedside Gap in Endothelial Biomedicine
Innate immune response
INFLAMMATION TOCRINE AU
1. The endothelium is inaccessible 2. The endothelium displays emergent properties 3. The endothelium is tightly coupled to native microenvironment
COAGULATION LPS
C3a, C5a
AUTOCRINE
TLR4
Cytokines: IL-6 IL-1 TNFa IFNg
TF
IXa
Xa
Chemokines: MCP-1 IL-8 MIP-1a RANTES IP-10 Other: O 2– NO VEGF CD40L PAF
VIIa IX
Monocyte/macrophage
Prothrombin
Thrombin
Fibrinogen PARACRINE
Fibrin
PARACRINE
LPS TLR4
PAR
Endothelium
Figure 51.1. Innate immune response. Circulating monocytes or tissue macrophages bind to lipopolysaccharide (LPS) via toll-like receptor 4 (TLR4), resulting in activation of inflammatory and coagulation pathways. Components of each pathway participate in autocrine and paracrine loops, which in turn lead to additional activation of monocytes and endothelial cells, respectively. At the level of the endothelium, input may arrive directly via LPS or indirectly via monocyte-derived inflammatory/coagulation mediators. The innate immune response may also be activated in nonsepsis states such as trauma and surgery.
Importantly, the inflammatory and coagulation cascades engage in significant cross-talk. For example, TNF-a and IL-1 (inflammatory cytokines) induce expression of TF on monocytes and possibly subsets of endothelial cells. In the other direction, serine proteases in the clotting cascade bind to proteaseactivated receptors on the surface of several cell types (including monocytes and endothelial cells), resulting in a proinflammatory phenotype.
ENDOTHELIUM IN SEPSIS Bench-to-Bedside Gap in Endothelial Biomedicine The endothelium, which lines the inside of all blood vessels, is a highly metabolically active organ. Despite a robust literature on endothelial cells (tens of thousands of published articles), the endothelium is largely overlooked in clinical practice. There are several possible explanations for this bench-to-bedside gap (Box 556 51.2). First, the endothelium is hidden from view and is poorly
accessible in the patient. Indeed, the endothelium does not lend itself to inspection, palpation, percussion, or auscultation. Although certain other organs such as the pancreas and kidney are also difficult to examine at the bedside, they are spatially confined and thus amenable to diagnostic imaging. Moreover, whereas disease of these latter organs is associated with changes in blood chemistry (e.g., amylase, blood urea nitrogen, and creatinine), endothelial dysfunction occurs in the absence of reliable circulating markers. Second, the endothelium—like other organs in the body—is highly complex. Most endothelial cell biologists study specific aspects of endothelial cell function in tissue culture and in doing so tend to overlook critical levels of organization that are essential to a full understanding of the system. Just as one could never predict the behavior of an ant colony by studying an individual ant in isolation, one cannot rely solely on isolated endothelial cells to fully understand the endothelium in health and disease. Third, the endothelium, more so than most other tissues in the human body, is extraordinarily adaptive and flexible. It is like a chameleon, “marching to the tune” of the local microenvironment. Indeed, so tightly coupled is the endothelium to the extracellular milieu that when it is removed from its native environment and grown in tissue culture, it undergoes phenotypic drift. Therefore, any results from in vitro studies must be interpreted with caution and ultimately validated in vivo.
Endothelial Cell Function and Dysfunction The two most commonly used terms or descriptors in endothelium-based diseases are endothelial cell activation and endothelial cell dysfunction (Box 51.3). Both terms were coined in the 1980s, and their meaning has changed over the years. Endothelial cell activation was originally used to describe the proadhesive function of cultured endothelial cells treated with
Box 51.3 Definitions Endothelial cell activation: Phenotypic response of the endothelium to inflammatory mediators (as occurs in sepsis, trauma, and surgery), usually consisting of some combination of proadhesive surface, procoagulant activity, altered vasomotor tone, increased apoptosis, and change in barrier function. Note: the phenotype may be functional (adaptive) or dysfunctional (nonadaptive) Endothelial cell dysfunction: Phenotypic response of the endothelium that poses a net liability to the host. Note: the phenotype may be activated or nonactivated.
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Interaction of Coagulation and Inflammation inflammatory mediators. Today, the term most commonly refers to the phenotypic response of the endothelium to inflammation or infection. An important caveat is that the so-called “activation phenotype” differs between different sites of the vasculature. For example, activated endothelial cells in culture have been shown to express decreased levels of thrombomodulin (TM) on their cell surface (in theory, resulting in reduced capacity to activate protein C). However, the human brain expresses very little, if any, TM. That does not mean that the blood-brain barrier is in a chronic state of activation. Rather, the brain must rely on other natural anticoagulants to balance local hemostasis. As another example, P-selectin (a cell adhesion molecule) is considered an activation marker for endothelial cells. However, endothelial cells in uninflamed skin constitutively express Pselectin. In summary, the state of activation must be judged in an appropriate spatial context. Endothelial cell dysfunction is most often used to describe the abnormal endothelial phenotype in atherosclerosis (most notably, changes in endothelium-mediated vasomotor tone). However, the term may be applied more broadly to other disease states. Indeed, endothelial cell dysfunction occurs when the endothelial phenotype—whether or not it meets a definition for activation—represents a net liability to the host. This may occur locally (e.g., coronary artery disease) or systemically (e.g., sepsis).
Endothelial Response in Severe Sepsis Each endothelial cell in the body is analogous to a miniature adaptive input-output device (Fig. 51.2). Input is derived from the extracellular environment and may include biochemical or biomechanical forces. Output represents the phenotype of the endothelium and may include changes in proliferation, cell sur-
Endothelial cells as an input-output device
Soluble mediators: Growth factors Cytokines Chemokines LPS ROS
INPUT
Endothelial cell
Hypoxia
vival, vasomotor tone, permeability, hemostatic balance, and/or release of inflammatory mediators. In sepsis, there are many changes in the input signals, including components of the bacterial wall, complement, cytokines, chemokines, serine proteases, fibrin, activated platelets and leukocytes, hyperglycemia, and/or changes in oxygenation or blood flow. Endothelial phenotypes in sepsis include both structural alterations (e.g., nuclear vacuolization, cytoplasmic swelling, cytoplasmic fragmentation, denudation, and/or detachment) and functional changes (e.g., shifts in the hemostatic balance, increased cell adhesion and leukocyte trafficking, altered vasomotor tone, loss of barrier function, and programmed cell death). Finally, the “set point” of the endothelium—as determined by the influence of epigenetic processes, age, comorbidity, and genetic polymorphisms—may alter the phenotype and/or transduction capacity (input-output coupling) of the endothelial cell.
Endothelial Dysfunction in Severe Sepsis The host response to sepsis involves a complex orchestra of cells (e.g., leukocytes, platelets, and endothelial cells) and soluble mediators (e.g., components of the inflammatory and coagulation cascades). Normally, these mechanisms are highly coordinated with one another to defend host against pathogen. However, if the host response is disproportionate to the nature of the threat, i.e., it is excessive, sustained, or poorly localized, then the balance of power shifts in favor of the pathogen, resulting in the sepsis phenotype—namely, dysfunction of subsets of organ systems. Many hypotheses have been proposed to explain the cause of organ dysfunction. However, at this time, our understanding of the precise pathophysiologic processes is unclear. Available evidence suggests that severe sepsis is associated with excessive, sustained, and generalized activation of the endothelium. Without artificial organ support, virtually all patients with severe sepsis would die of their disease. In other words, most of these individuals have crossed the threshold from an adaptive to a maladaptive response. Insofar as the endothelium contributes to the severe sepsis phenotype, its behavior may be characterized as dysfunctional. An important goal for the future is to learn how to identify the transition from function to dysfunction, before the onset of significant (and perhaps irreversible) organ damage. Importantly, the endothelium is an attractive therapeutic target.
SEPSIS THERAPY
Shear stress/strain Temperature pH
OUTPUT
Hemostatic balance Vasomotor tone Leukocyte trafficking Migration Proliferation Barrier function
Figure 51.2. Endothelial cell as an input-output device. Each endothelial cell in the body (there are about 60 trillion of them) behaves like a miniature adaptive input-output device sensing changes in the extracellular environment and responding via altered phenotype. LPS, lipopolysaccharide; ROS, reactive oxygen species.
Enormous resources have been expended on sepsis trials, with more than 10,000 patients enrolled in over 20 placebocontrolled, randomized phase 3 clinical trials. Most of these therapies have failed to reduce mortality in patients with severe sepsis, including antiendotoxin, anticytokine, antiprostaglandin, antibradykinin and antiplatelet activating factor (PAF) strategies, antithrombin III (ATIII), and tissue factor pathway inhibitor (TFPI). At the time of this writing, a total of five phase 3 clinical trials have demonstrated improved survival in critically ill patients or patients with severe sepsis. These include the use of low tidal volume ventilation, activated protein C, low-dose glucocorticoids, intensive insulin therapy, and early goal-directed therapy. 557
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Hematologic Problems
Sepsis therapy
LPS
A e.g., anti-IL-1
antibodies
INFLAMMATION
B e.g., hirudin
Monocyte
COAGULATION
C e.g., rhAPC
Endothelium
Figure 51.3. Sepsis therapy. Shown are components of the innate immune response. Circulating monocytes or tissue macrophages bind to lipopolysaccharide (LPS), resulting in activation of inflammatory and coagulation cascades (compare Figure 51.1). These pathways then operate in autocrine and paracrine pathways to activate monocytes and endothelial cells, respectively. As a general rule, neither the selective targeting of an inflammatory mediator (A) nor thrombin generation (B) improves survival in severe sepsis. In contrast, therapies that target both inflammatory and coagulation pathways and/or attenuate endothelial cell dysfunction (C) show more promise.
may reduce the activity of proinflammatory transcription factors in endothelial cells, while intensive insulin therapy may reduce the deleterious effects of high glucose on the endothelium. Finally, early goal-directed therapy is predicted to maintain flow and hence shear stress at the level of the blood vessel wall. The extent to which these therapies exert their benefit through the endothelium remains unknown. There are many other possible strategies for attenuating endothelial cell dysfunction (Aird, 2003). One may target the input signals, the coupling mechanism inside the cell, or the cellular phenotype (output). Examples of extracellular signals as targets include endotoxin, TNF-a, IL-1, and PAF. Examples of coupling mechanisms include receptors, such as cellular adhesion molecules and protease-activated receptors; signaling pathways such as p38 MAPK or novel/atypical PKC isoforms; and transcription factors, including NF-kB, GATA-2, and the Ets family of transacting proteins. Potential target outputs include endothelial control of hemostasis, inflammation, vasomotor tone, permeability, and leukocyte trafficking. Although the failure of antiendotoxin and TNF-a antibodies to improve mortality in patients with severe sepsis is consistent with the complexity of the host response, these findings do not exclude a role for single target (“smart bomb”) therapy. The host response, while unquestionably redundant and pleiotropic, is likely to contain certain component parts—whether an extracellular mediator, a cell surface receptor, a signal intermediate, or a transcription factor—that are so highly connected as to render that component (and the entire system) vulnerable to therapeutic targeting. A key challenge is to identify these socalled hubs in the sepsis cascade and to target those factors accordingly.
CONCLUSIONS
There are several lessons to be learned from these clinical trials. First, the use of single modality therapy aimed at inhibiting endotoxin or inflammation is ineffective (Fig. 51.3). These data are consistent with the notion that the inflammatory cascade, while certainly an important contributor to sepsis morbidity and mortality, is sufficiently redundant, pleiotropic, and interdependent as to preclude single modality therapy. Second, preclinical studies with selective thrombin inhibitors suggest that fibrin ablation has little or no effect on organ dysfunction or mortality. One interpretation of these findings is that clotting in and of itself is nonlethal in sepsis. Third, the combination of antiinflammatory and anticoagulant appears more promising. For example, TFPI, ATIII, and recombinant human activated protein C (rhAPC) have each been shown to inhibit inflammation and coagulation in vitro and in vivo and to yield improved survival in nonhuman primate models of sepsis and phase 2 clinical trials of severe sepsis. Of these three agents, only rhAPC reduced mortality in phase 3 clinical studies. A common theme of the five successful treatments in severe sepsis is their capacity to attenuate endothelial cell dysfunction. The effect of rhAPC on the endothelium was discussed above. 558 Low-volume ventilation would be expected to reduce barotraumas to the pulmonary endothelium. Low-dose glucocorticoids
Despite new information about the pathophysiology and treatment of severe sepsis, this disorder continues to be associated with an unacceptably high mortality rate. Future breakthroughs will require a conceptual shift that emphasizes relationships between the various mediators (inflammatory and coagulation) and cells involved in host response. The endothelium is key in initiating, perpetuating, and modulating the host response to infection. Additional studies promise to provide new insight into the endothelium, not as an isolated mechanism of sepsis pathophysiology but rather as the coordinator of a far more expansive, spatially and temporally orchestrated response.
SUGGESTED READING Aird WC: The role of the endothelium in severe sepsis and the multiple organ dysfunction syndrome. Blood 2003;101;3765–3777. Annane D, Sebille V, Charpentier C, et al: Effect of treatment with low doses of hydrocortisone and fludrocortisone on mortality in patients with septic shock. JAMA 2002;288:862–871. ARDSNET: Ventilation with lower tidal volumes as compared with traditional tidal volumes for acute lung injury and the acute respiratory distress syndrome. The Acute Respiratory Distress Syndrome Network. N Engl J Med 2000;342:1301–1308. Bernard GR, Vincent JL, Laterre PF, et al: Efficacy and safety of recombinant human activated protein C for severe sepsis. N Engl J Med 2001; 344:699–709.
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Interaction of Coagulation and Inflammation Cohen J, Guyatt G, Bernard GR, et al: New strategies for clinical trials in patients with sepsis and septic shock. Crit Care Med 2001;29:880– 886. Eichacker PQ, Parent C, Kalil A, et al: Risk and the efficacy of antiinflammatory agents: retrospective and confirmatory studies of sepsis. Am J Respir Crit Care Med 2002;166:1197–1205.
Rivers E, Nguyen B, Havstad S, et al: Early goal-directed therapy in the treatment of severe sepsis and septic shock. N Engl J Med 2001; 345:1368–1377. Van den Berghe G, Wouters P, Weekers F, et al: Intensive insulin therapy in the critically ill patients. N Engl J Med 2001;345:1359–1367.
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