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Proposed Classification and Pathologic Mechanisms of Purpura Fulminans and Skin Necrosis
Purpura (Latin for purple) refers to the extravasation of formed blood elements into the skin.1 Purpura fulminans, coined by Henoch in 1887, was used to describe an entity characterized by the sudden onset of purpura with rapid progression to death in individuals with essentially negative autopsy findings.2 In 1964, Hjort and coworkers3 reviewed the available literature on purpura fulminans and found 50 cases they believed defined a specific disease entity. Based on this review, they defined purpura fulminans as a disease of children that is preceded by a benign disorder, usually an infection, associated with bleeding into the skin, which progresses rapidly and results in severe anemia, and is followed by extensive skin necrosis if the child lives. Also noted were two other common features that were later included in the definition, specifically: coagulation abnormalities compatible with disseminated intravascular coagulation (DIC) and widespread thrombosis of the capillaries and venules within the affected dermal sites.3 They observed that many cases in the literature called purpura fulminans did not fit these criteria but included other types of fulminant purpura. Contemporary reviews of the purpura fulminans literature have displayed a similar tendency to incorporate a variety of cases of hemorrhagic skin necrosis under the distinction "purpura fulminans."4-8
From the Department of Pathology, University of Colorado Health Sciences Center, and the Colorado Permanente Medical Group, Denver, Colorado; Department of Pathology and Microbiology, University of Nebraska School of Medicine, Omaha, Nebraska; and the Departments of Pathology, Pediatrics, and Biochemistry, University of Colorado Health Sciences Center, and Laboratory Services, Denver VA Medical Center, Denver, Colorado. Reprint requests: Dr. Marlar, Laboratory Services (#113), Denver VA Medical Center, 1055 Clermont St., Denver, CO 80220.
Most notable have been the inclusion of both severe purpura accompanying bacterial sepsis and neonatal purpura fulminans within this category.7,8 In addition, cases identical to those described by Hjort et al, but occurring in adults, have also been called "purpura fulminans."4 Although these reports do not accurately fit Hjort et al's original definition, many have important common features. A number of these cases have in common: the sudden onset of massive bleeding into the skin, followed by extensive skin necrosis and/or death associated with signs of DIC. Based on these unifying characteristics and an enhanced appreciation of the pathophysiologic basis underlying hemorrhagic skin necrosis (as described in the previous articles in this issue of Seminars), a revised classification of purpura fulminans is proposed. Also briefly reviewed are possible initiating and mechanistic aspects of the various types of purpura fulminans.
VASCULAR ENDOTHELIUM AND LEUKOCYTES IN HEMOSTASIS AND THROMBOSIS The vascular endothelium provides a dynamic interface between the blood and surrounding tissue compartments. The endothelial cell functions in a number of diverse physiologic systems including coagulation, inflammation, immune regulation, modulation of vascular tone, angiogenesis, and synthesis of mesenchymal components.9 This biologically active role of the endothelial cell has only been appreciated in recent years. Contemporary development of human endothelial cell cultures has been critical to the investigation and enhanced understanding of the varied functions of the
Copyright © 1990 by Thieme Medical Publishers, Inc., 381 Park Avenue South, New York, NY 10016. All rights reserved.
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DOROTHY M. ADCOCK, M.D., JOHN BROZNA, M.D., Ph.D., and RICHARD A. MARLAR, Ph.D.
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endothelial cell. Recent elaboration of cytokine function and interaction with the endothelium has also been integral to this goal. Although much has been learned in recent years, our knowledge of endothelial physiology is still primitive. One important function of the vascular endothelium is its participation in the complex scheme of hemostasis and thrombosis. The endothelial cell has the capability of expressing both procoagulant and anticoagulant properties.9-13 The balance between these overtly antagonistic functions is dependent on a variety of factors that influence the endothelial cell, such as activation of the hemostatic system and production of inflammatory mediators or perturbation of the environment surrounding the endothelial cell. In the quiescent state the vascular endothelium serves predominantly an anticoagulant role. 12,13 Modulation (activation or perturbation) leads to a shift toward enhanced procoagulant activity and diminished anticoagulant function.9-14 This dynamic alteration is a result of the endothelial cell's active metabolism and physiologic responsiveness.
Anticoagulant Functions of the Endothelial Cell in the Quiescent State The endothelial cell associated anticoagulant properties include the protein C (PC)-thrombomodulin system, the antithrombin III (AT III)-heparin system, and the profibrinolytic mechanisms. 10-13 The endothelial cell also functions as an inhibitor of platelet aggregation through its production and release of prostacyclin.10'13 Thrombomodulin is a constitutively expressed endothelial cell surface protein, serving as a high-affinity receptor for thrombin.11 Binding of thrombin to thrombomodulin greatly facilitates the activation of PC. PC and its cofactor protein S (PS) are potent, naturally occurring, vitamin K-dependent anticoagulants synthesized by the liver.15 PS is also synthesized by the endothelial cell.16 Activated PC in the presence of PS, calcium ions, and cell membrane surface impairs fibrin formation through its ability to proteolytically degrade Factors Va and Villa. 17 In addition to the anticoagulant function of PC, the binding of thrombin to thrombomodulin impairs thrombin's procoagulant functions.11 AT III is another important naturally occurring anticoagulant.18 In the presence of heparin or heparinlike molecules, AT III rapidly inactivates thrombin and Factors Xa, XIIa, XIa, and IXa. The endothelial cell produces heparin-like molecules that coat the surface of the vascular lumen.19 These heparin-like species are glycosaminoglycans that contain an AT III binding region similar to that present on heparin. These heparinlike molecules allow the AT III molecule to bind and inactivate selective procoagulant enzymes rapidly.
Lysis of thrombi in the microcirculation is dependent on localized fibrinolysis, which is mediated by the endothelial cell. 10,20-22 The endothelial cell synthesizes multiple but distinct molecular forms of plasminogen activators (PA). 21,22 One of the most physiologically important PA is tissue-type plasminogen activator (t-PA). Release of t-PA from the endothelial cell is effected by a variety of physiologic stimuli, such as stress, hypoxia, epinephrine, thrombin, and certain drugs. 10,21,23 These PA molecules cleave plasminogen to plasmin, a trypsinlike enzyme central to the fibrinolytic system. The fibrinolytic response of the endothelial cell is further regulated by the release of plasminogen activator inhibitor, type 1 (PAI-1) from the endothelial cell.23
Procoagulant Functions of the Endothelial Cell in the Perturbed State Under appropriate circumstances, the endothelium can promote a procoagulant response. 9-13,24 These procoagulant activities enhance both platelet aggregation and fibrin formation. Procoagulant functions include decreased synthesis of prostacyclin, increased synthesis and expression of procoagulant substances on the luminal surface of the endothelial cell, and modification of the basic physical properties of the endothelial cell surface.10'24 The endothelial cell manufactures a number of procoagulant substances, including: tissue factor, von Willebrand factor, Factor V, Factor VIIIC, thrombospondin, fibronectin, and others. 10,24-26 These substances function in the promotion of platelet plug formation, cell adhesion, and generation of the fibrin clot. The most important procoagulant synthesized and expressed by the endothelial cell is tissue factor.27,28 Tissue factor binds Factor VII/VIIa in the presence of calcium ions, promoting the activation of Factors IX and X and ultimately fibrin formation. Minimal to no tissue factor is expressed in the quiescent state of the endothelial cell. 24,28 Expression of tissue factor activity is mediated by a variety of stimuli, including gram-negative endotoxin, thrombin, interleukin-1 (IL-1), and tumor necrosis factor alpha (TNF).28 The endothelium provides an important surface on which the procoagulants can assemble, be activated, and function, since all major coagulation reactions require a phospholipid surface for maximal activity.29
Interaction of Leukocytes and Endothelial Cells The role of leukocytes in mediating thrombosis is a relatively recent concept. Evidence to support this inter-
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action is extensive and beyond the scope of this review. Pertinent to this discussion is the role of leukocytes in the mediation of the generalized Shwartzman reaction. As has been summarized by Brozna in this issue of Seminars, the generalized Shwartzman reaction is a systemic response to the intravenous injection of endotoxin. Two doses of intravenous endotoxin given 18 to 24 hours apart induce DIC and widespread microvascular thrombosis. Leukocytes are necessary for this response.30 When rabbits are rendered leukopenic with the administration of nitrogen mustard, thrombosis does not develop following intravenous injections of endotoxin.30 Cytokines provide an important link between the inflammatory response and the coagulation system. Cytokines are important mediators of the inflammatory response which also have a significant effect on the hemostatic mechanism.14,30-34 Cytokines are produced by a variety of cells, including monocytes/macrophages, lymphocytes, endothelial cells, and keratinocytes30-34 (see also Brozna in this issue of Seminars). Cytokine production is mediated through exposure to specific primers, such as endotoxin, antigens, toxins, injury, and activated lymphocytes.32,33 These cytokines function by modifying or amplifying immunologic and inflammatory responses. IL-1 and TNF are two of a number of these immunoregulatory peptides.32,33 IL-1 is one of the key mediators of the body's response to inflammation, tissue injury, immunologic reactions and microbial infections. 31 ' 32 ' 34 Many of the general manifestations of the acute and chronic inflammatory response, such as fever, production of acute-phase reactant proteins, fibroblast proliferation, and chemotaxis are mediated by IL-1. 32 ' 34 TNF is identical to cachectin and functions as one of the principal mediators of lethal shock induced by endotoxin.33 Both IL-1 and TNF induce neutrophil adhesion to endothelium and mediate endotoxin-induced emigration and accumulation of neutrophils. These cytokines function synergistically.35 Endotoxin induces the synthesis of IL-1 and TNF primarily by the monocyte/macrophage.14,32-34 IL-1 and TNF in turn mediate endotoxin-induced endothelial cell injury. These peptides have similar effects on the hemostatic functions of the endothelial cell, modifying the endothelial cell surface characteristics in a similar way. Exposure of the endothelial cell to IL-1 and TNF promotes a unified procoagulant response: concomitant expression of tissue factor activity, suppression of thrombomodulin activity, and increased secretion of PAI-1 1 4 , 3 6 - 3 8 This induces the endothelial cell to switch from an anticoagulant role to one that promotes coagulation. This procoagulant response occurs in the absence of physical endothelial cell damage. In addition, endothelial cells exposed to endotoxin, TNF, or thrombin treatment can synthesize and release IL-1. 3 2 - 3 6
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Endothelial cell expression of thrombomodulin is influenced by IL-1, TNF, or indirectly by endotoxin.37 Following exposure to any of these molecules, surface thrombomodulin is internalized and degraded by a lysosomal dependent pathway. Suppression of thrombomodulin activity impairs not only activation of the PC anticoagulant system, but also increases thrombin procoagulant activity.17'37 Exposure of the endothelium to IL-1 or TNF results in rapid activation of the fibrinolytic pathway.38 Plasma t-PA levels increase within 1 hour of exposure; resulting in the generation of plasmin. PA activity terminates approximately 3 hours after exposure concomitant with increased plasma levels of PAI-1. 38 This diminished fibrinolytic activity impairs localized clot lysis, resulting in persistence of microvascular thrombosis. Movat has demonstrated the vital role that both IL-1 and TNF play in mediating dermal hemorrhage and microvascular thrombosis.35 In elaborate studies, a Shwartzman-like reaction was elicited in rabbits by preparing the skin with intradermal injections of recombinant human IL-1 and recombinant human TNF, followed by the intravenous injection of endotoxin. The dermal sites showed diffuse hemorrhage into the skin, leukocyte infiltration, and microvascular thrombosis. IL-1 and TNF are therefore able to substitute for endotoxin in eliciting the local Shwartzman reaction.35
Role of the Protein C System in the Development of DIC In addition to its anticoagulant function, PC may play an important role in the pathophysiology of intravascular coagulation.17,39,40 In cases of DIC associated with sepsis, a decrease in plasma PC antigen and activity has been demonstrated.39 Furthermore, the exogenous infusion of activated PC prevented the development of DIC in baboons infused with lethal doses of Escherichia coli.41 It is not known whether this was a function of the anticoagulant properties of PC or whether PC functioned in down-regulating the effects of endotoxin-induced inflammatory mediators. In addition, in vitro activated PC and thrombin decreased PAI activity and therefore increasing fibrinolytic activity.42
CLASSIFICATION OF THROMBOTICINITIATED PURPURIC LESIONS The term "purpura fulminans" should be used only when rapidly progressive skin necrosis with massive hemorrhage into the skin of a catastrophic nature is found, associated with signs of DIC. 4,5 (see also Adcock and Hicks in this issue of Seminars). Based on this
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definition, purpura fulminans can be classified into three distinct categories (Table I): (1) occurring in individuals with hereditary or acquired dysfunction of the PC system or other coagulation regulatory mechanisms; (2) occurring in patients with acute, current infection, and (3) occurring in individuals without acute infection or known abnormalities of the coagulation regulatory mechanisms. The first category, which includes patients with a dysfunctional PC anticoagulant system, likely represents a rather diverse group. This category of purpura fulminans includes skin necrosis associated with heparin therapy and a variety of clinical situations associated with the PC antithrombotic system, such as homozygous PC or PS deficiencies, warfarin-induced skin necrosis, and antiphospholipid antibodies. This broad category will be referred to as "hemostasis-initiated purpura fulminans." The second category, which occurs in association with acute infection, will be called "acute infectious purpura fulminans." The third group of purpura fulminans, occurring in individuals without known abnormalities of the hemostatic system or concomitant infection has been coined "idiopathic purpura fulminans." Francis, in this issue of Seminars, notes that this final category will likely be subject to further characterization as this idiopathic form of hemorrhagic skin necrosis is better delineated.
been described in the preceding articles of this issue of Seminars. Skin necrosis represents the common end that interrelates a number of different but interactive pathophysiologic processes. The main differences between these processes are the initiating events. The two major mediating pathways include the hemostatic system (Fig. 1) and the inflammatory system (Fig. 2). Each system modulates the endothelial cell and surrounding milieu inducing changes that ultimately lead to skin necrosis. These two systems have many pathophysiologic mechanisms in common and show considerable overlap. Dermal vascular thrombosis and hemorrhagic skin necrosis represent the common endpoint. To put into perspective the similarities and differences between the hemostatic and inflammatory initiating pathways, each of these pathways and their proposed mechanisms as they pertain to skin necrosis will be briefly discussed. The event central to the development of skin necrosis as it involves the hemostatic pathway is microvascular thrombosis (Fig. 1). The vast array of initiating stimuli, and hence variety of pathways that can lead to the development of microvascular thrombosis, makes this pathway difficult to study. In general, thrombosis will not develop in individuals with a functional hemostatic system unless the regulatory anticoagulant and/or fibrinolytic systems are overwhelmed, such as in situations of direct vascular damage or heparin-induced platelet ag-
BASIC MECHANISMS OF THE THROMBOTIC-INDUCED PURPURIC LESIONS There are a number of different clinical situations associated with dermal vascular skin necrosis that have
TABLE 1. Classification of the Thrombotic Purpuric Lesions 1. Acute infectious purpura fulminans 2. Hemostasis-initiated purpura fulminans A. Dysfunction of the protein C system Homozygous protein C or protein S deficiency Acquired protein C or protein S deficiency Cholestasis Warfarin-induced skin necrosis Antiphospholipid syndrome (lupus anticoagulant) B. Platelet mediated Heparin-induced thrombocytopenia C. Hereditary or acquired dysfunction of other hemostasis regulatory systems 3. Idiopathic purpura fulminans A. Postinfection purpura fulminans B. Purpura fulminans of unknown etiology
FIG. 1. Schematic representation of the general proposed mechanism of skin necrosis and purpura fulminans as initiated and propagated by the hemostatic system. The central component is the endothelial cell (E.C.). Both the coagulation and fibrinolytic systems contribute to the thrombotic and ischemic components of these types of thrombohemorrhagic purpuric lesions. The inflammatory component (cytokines and leukocytes [PMN]) may or may not contribute to the initial lesion development, but may play a role in the propagation of the irreversible types of purpuric lesions. See text for details.
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FIG. 2. Schematic representation of the general proposed mechanism of skin necrosis and purpura fulminans as initiated and propagated by the inflammatory response. The central component is the endothelial cell (E.C.). The interrelationship between the leukocytes (in particular the PMNs) and the cytokines (IL-1 and TNF) are multiple and complex (arrows). The coagulation component (thrombosis) plays a definite but minor role in these types of necrotic lesions. See text for details.
glutination. Vascular thrombosis can, however, develop after relatively minor stimuli in patients with aberrant coagulation regulatory mechanisms, such as homozygous PC or PS deficiencies. Thrombosis may develop in these patients following stimuli that would lead to no thrombosis or clinically insignificant thrombosis in the normal individual. Dermal vascular thrombosis occurring in individuals with a functional hemostatic system includes a variety of causes beyond the scope of this review. Atherosclerosis and other entities that induce vascular damage are common causes. Heparin-induced thrombosis is unique in that it occurs in the presence of a normal endothelium and underlying hemostatic system.43 The vast majority of cases of skin necrosis initiated through the hemostatic pathway occur in patients with hereditary or acquired dysfunction of the PC system or other coagulation regulatory mechanisms. The naturally occurring anticoagulant systems and in particular the PC antithrombotic system play a vital role in preventing the development of microthrombi in the dermal vessels and lysing small thrombi once they develop. For this reason, a dysfunctional PC system may play a major role in the development of microvascular dermal thrombosis. Pathophysiologically, relatively minor stimuli such as venipuncture, local hypoxia, and pressure-induced ischemia may initiate microvascular thrombosis in these impaired patients.47 (see Marlar and Neumann in this issue of Seminars). While a functioning hemostatic system would rapidly lyse such small clots that form, a dysfunctional PC system would allow
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persistence and propagation of the clot, causing irreversible skin damage and necrosis (Fig. 1). Loss of a major antithrombotic regulatory mechanism therefore may alter the balance of the hemostatic system, allowing development of microvascular thrombosis in response to minor stimuli. Ischemic vascular damage to the dermis may in some patients incite a mild perivascular inflammatory response (Fig. 1). Inflammation, however, is secondary and is not necessary for skin necrosis to occur. The cytokine mediators (discussed later) are associated with the inflammatory response and may perpetuate thrombosis; but the role of cytokines is minor in hemostasisinitiated dermal thrombosis. The inflammatory system is the other of the two major mediating pathways of dermal vascular necrosis (Fig. 2). The inflammatory-initiated pathway of skin necrosis is centered around the generation and action of cytokines (Fig. 2). Endotoxin is an important initiator of cytokine production leading to dermal necrosis.31'35 In response to endotoxin, cytokines (IL-1 and TNF) are produced by the leukocytes, endothelial cells, and keratinocytes.32-34 Endotoxin also results in leukocyte emigration and activation.35 Cytokines cause leukocyte adherence to the endothelial cell. 32 ' 34 Neutrophil activation and release of proteases induces vascular damage, cellular destruction, and possibly fibrinolysis35 (see also Brozna in this issue of Seminars). IL-1 and TNF promote procoagulant activity while causing diminished anticoagulant function as the cytokines incite increased tissue factor exposure and decreased thrombomodulin expression.24 The chemotactic activity of cytokines promotes continued influx of inflammatory cells propagating this response.32,33 The inflammatory-induced vascular damage and procoagulant expression of the endothelial cell induces microvascular thrombosis.35 The naturally occurring anticoagulant mechanisms are locally overwhelmed. With prolonged ischemia, skin necrosis occurs. The possibility exists that systemic dysfunction of hemostasis, such as in patients with DIC or a mild acquired deficiency of PC/PS, may augment microvascular thrombosis. Although dermal thrombosis is a component of the inflammatory-mediated pathway of skin necrosis, its role is minor compared with the role that cytokines play (Fig. 2). Although hemostasis-associated and inflammatorymediated skin necroses have different initiating events, these pathways have a number of commonalities. The endothelial cell is the common and central factor to these pathways which lead to the development of skin necrosis (see Fig 1,2). Both pathways cause the endothelial cell to switch from an anticoagulant state to one that promotes coagulation.10,12,13 Three important pathophysiologic responses involving the endothelial cell that are common to both pathways include increased tissue factor
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expression/exposure,28 down-regulation of thrombomodulin expression and PC antithrombotic system activity (possibly other regulatory antithrombotic mechanisms as well 17,37 ), and impaired fibrinolysis.23,38 These three physiologic responses promote microvascular thrombosis and hence thrombohemorrhagic skin necrosis. Vascular damage, as induced by venipuncture, ischemia, or inflammation, results in exposure of tissue factor. 10,12,24,28 Tissue factor may be present either within the perivascular tissue or expressed on the luminal surface of the endothelial cell. 24,27,28 In addition, in the absence of direct vascular damage, endotoxin and cytokines directly promote expression of tissue factor activity by the endothelial cell as well as the monocyte/ macrophage.24,28,45 Furthermore, IL-1 and TNF cause the down-regulation of thrombomodulin activity on the endothelial cell surface.37 These cytokines also promote increased secretion of PAI-1. 23,38 These effects of the inflammatory response induce microvascular thrombosis. Likewise, dysfunction of the PC antithrombotic system concomitant with tissue factor exposure results in an overall procoagulant effect of the endothelial cell. 41 This occurs predominantly through impaired degradation of activated Factors V and VIII and enhanced availability of thrombin.11,17 In addition, dysfunction of the PC anticoagulant system is associated with impaired fibrinolysis through an unknown mechanism.17,40 The propensity for microvascular thrombosis to involve the dermal vasculature with a predilection for areas rich in subcutaneous fat is unknown. This may represent variability in endothelial cell expression of procoagulant and anticoagulant factors between different organs. Preferential dermal involvement could also reflect differences in cytokine attraction, activity, and function. Physical properties unique to the skin, such as variations in temperature, predisposition to trauma, and variable blood flow, may also play a role in the development of skin necrosis. Preferential involvement of fat-rich areas could be a function of the generous supply of small vessels in this tissue. Mechanistic studies to describe these variables better, as they pertain to dermal vascular necrosis, are proposed.
FUTURE MECHANISTIC STUDIES OF SKIN NECROSIS AND PURPURA FULMINANS Information obtained from the studies of hemostasis-initiated purpura fulminans, acute infectious purpura fulminans, idiopathic purpura fulminans, and the Shwartzman reaction in animals has suggested several possible common etiologic mechanisms. Since these processes of
hemorrhagic skin necrosis are centered around the endothelial cell, endothelial cell biology, as it relates to the development of purpura fulminans and hemorrhagic skin necrosis, must be studied in greater detail. Further investigations of cytokine function and the hemostatic proteins are also vital to enhanced elucidation of the pathophysiology of these devastating syndromes. The fundamental changes in endothelial cell function at prepared skin sites in animal models of the Shwartzman reaction and at sites of early purpura fulminans lesions in patients with PC deficiency need to be defined before the pathogenesis of purpura fulminans will be understood. Specifically, what initiates the sudden development of sharply demarcated thrombohemorrhagic lesions in patients receiving coumarin or having abnormally low levels of PC? Are endothelial cells in sites of early purpuric lesions activated by previous infectious agents such as bacteria or viruses? Do these endothelial cells have a decreased capacity to bind AT III, activate PC, promote fibrinolysis, or have increased tissue factor procoagulant expression? Using immunohistologic techniques, immunoassays, and functional assays, endothelial cell associated tissue factor, thrombomodulin, and fibrinolytic proteins can be measured in early lesions of purpura fulminans and compared with expression of these coagulation factors on endothelial cells from areas of skin not involved by purpuric changes and other organs as well. Alterations in expression of tissue factor, thrombomodulin, fibrinolytic proteins, and binding of AT III could explain the predilection for dermal involvement. In idiopathic purpura fulminans it is thought that an antecedent bacterial or viral infection involving the skin is a prerequisite for the development of the dermal thrombohemorrhagic lesion. Identification of infectious agents as the initiators of the purpuric lesions will be difficult to document. Possibly, using polymerase chain reaction techniques with tissue sections could lead to the identification of virus-infected endothelial or epithelial cells in sites predisposed to the development of purpuric lesions. If infectious agents cannot be found in skin sites predisposed to developing thrombosis in patients with abnormal coagulation regulatory factor levels, evidence for the presence of cytokines such as IL-1, TNT, or interferon might be obtained by using polymerase chain reaction to identify increased levels of cytokine mRNA in either endothelial or epithelial cells, or in transient inflammatory cell infiltrates in early lesions of purpura fulminans. The presence of increased levels of mRNA for these inflammatory cytokines would suggest that cytokines prepare the skin for developing thrombohemorrhagic and necrotic lesions. Once the fundamental changes in endothelial cells
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SUMMARY The syndromes of purpuric lesions associated with a thrombotic mechanism are very rare in the general population. Dermal vascular thrombosis, however, can be devastating and associated with significant morbidity and mortality. These syndromes share common features in their clinical course, pathogenesis, and histology. Although these syndromes can be initiated by either the hemostatic or inflammatory pathways, both pathways center around perturbations of the endothelial cell, which promote thrombosis. If animal models or better testing can be developed, enhanced appreciation of mechanisms underlying purpura fulminans may be deduced. Characterization of the pathophysiology may then allow directed treatment modalities that could limit the course of these syndromes and reduce morbidity and mortality. Acknowledgements. The authors would like to thank Dr. Lou Fink for his help and discussions on the topic of the mechanism or mechanisms of skin necrosis. This work was supported in part by a grant to R.A.M. from the Veterans Administration.
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responsible for initiating the thrombogenic state at sites of purpura are known, the mechanisms involved in the development of progressive thrombohemorrhagic lesions can be defined. Clearly, an impaired PC antithrombotic system (inherited or acquired) contributes to microvascular thrombosis in some patients with purpura fulminans. To what extent abnormal plasma levels of procoagulant, anticoagulant, and fibrinolytic proteins contribute to dermal thrombosis is unclear. Measurement of circulating levels of these proteins and the various activated factors (such as thrombin-AT III complexes, D-dimer) during the early stages of purpura fulminans and the following convalescent period might help to explain why and how thrombosis and ischemic necrosis develop. Since patients with warfarin-induced skin necrosis or homozygous PC deficiency associated skin necrosis are not routinely available for prospective studies, progress toward understanding the mechanisms involved in this type of purpura fulminans has been lagging. The development of an animal model of warfarin-induced skin necrosis or homozygous PC deficiency could be useful for characterizing prerequisite changes in endothelial cell biology and circulating coagulation factors leading to the development of purpura fulminans. With a well-defined model, initiating factors of localized skin necrosis associated with abnormally low levels of PC or other regulatory factors could be identified.
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Willebrand protein in Weibel-Pelade bodies of human endothelial cells. J Cell Biol 95:355-360, 1982. Cerveny TJ, DN Fass, KG Mann: Synthesis of coagulation factor V by cultured aortic endothelium. Blood 63:1467-1474, 1984. Nemerson Y: Tissue factor and hemostasis. Blood 71:1-8, 1988. Nawroth PP, D Handley, CT Esmon, DM Stern: Interleukin 1 induces endothelial cell procoagulant activity while suppressing cell surface anticoagulant activity. Proc Natl Acad Sci USA 83:3460-3464, 1986. Zwaal RFA, HC Hemker: Blood cell membranes and hemostasis. Haemostasis 11:12-39, 1982. Stetson CA, RA Good: Studies on the mechanisms of the Shwartzman phenomenon. Evidence for participation of polymorphonuclear leukocytes in the phenomenon. J Exp Med 93:49-64, 1951. Lisby G, C Avnstorp, GL Wantzin: Interleukin-1. A new mediator in dermatology. Int J Dermatol 26:8-13, 1987. Dinarello CA: Biology of interleukin 1. FASEB J 2:100-115, 1988. Beutler B, A Cerami: Cachectin: More than a tumor necrosis factor. N Engl J Med 316:379-385, 1987. Bevilacqua MP, JS Pober, ME Wheeler, D Mendrick, RS Cotran, MA Gimbrone: Interleukin-1 acts on cultured human vascular endothelial cells to increase the adhesion of polymorphonuclear leukocytes, monocytes and related leukocyte cell lines. J Clin Invest 76:2003-2011, 1985. Movat HZ, CE Burrows, MI Cybulsky, CA Dinarello: Acute inflammation and a Shwartzman-like reaction induced by interleukin-1 and tumor necrosis factor. Am J Pathol 129:463-476, 1987.
36. Dinarello CA: Interleukin-1: Amino acid sequences, multiple biologic activities and comparison with tumor necrosis factor (cachectin). Year Immunol 2:68-89, 1986. 37. Moore KL, CT Esmon, NL Esmon: Tumor necrosis factor leads to the internalization and degradation of thrombomodulin from the surface of bovine aortic endothelial cells in culture. Blood 73:159165, 1989. 38. Suffredini AF, PC Harpel, JE Parrillo: Promotion and subsequent inhibition of plasminogen activator after administration of intravenous endotoxin to normal subjects. N Engl J Med 320:11651172, 1989. 39. Marlar RA, J Endres-Brooks, C Miller: Serial studies of protein C and its plasma inhibitor in patients with disseminated intravascular coagulation. Blood 66:59-63, 1985. 40. Clouse LH, PC Comp. The regulation of hemostasis: The protein C system. N Engl J Med 314:1298-1304, 1986. 41. Taylor FB, A Chang, CT Esmon, A D'Angelo, S ViganoD'Angelo, KE Blick: Protein C prevents the coagulopathic and lethal effects of escherichia coli infusion in the baboon. J Clin Invest 233:918-925, 1987. 42. Van Hinsbergh VVM, RM Bertina, A van Wijngaarden, NH van Tilburg, JJ Emeis, F Haverkate: Activated protein C decreases plasminogen activator-inhibitor activity in endothelial cell-conditioned medium. Blood 65:444-451, 1985. 43. Hartman AR, RM Hood, CE Anagnostopoulos: Phenomenon of heparin-induced thrombocytopenia associated with skin necrosis. J Vase Surg 7:781-784, 1988. 44. Brozna JP, SD Carson: Monocyte-associated tissue factor is suppressed by phorbol myristate acetate. Blood 72:456-462, 1988.
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Proposed Classification and Pathologic Mechanisms of Purpura Fulminans and Skin Necrosis
Purpura (Latin for purple) refers to the extravasation of formed blood elements into the skin.1 Purpura fulminans, coined by Henoch in 1887, was used to describe an entity characterized by the sudden onset of purpura with rapid progression to death in individuals with essentially negative autopsy findings.2 In 1964, Hjort and coworkers3 reviewed the available literature on purpura fulminans and found 50 cases they believed defined a specific disease entity. Based on this review, they defined purpura fulminans as a disease of children that is preceded by a benign disorder, usually an infection, associated with bleeding into the skin, which progresses rapidly and results in severe anemia, and is followed by extensive skin necrosis if the child lives. Also noted were two other common features that were later included in the definition, specifically: coagulation abnormalities compatible with disseminated intravascular coagulation (DIC) and widespread thrombosis of the capillaries and venules within the affected dermal sites.3 They observed that many cases in the literature called purpura fulminans did not fit these criteria but included other types of fulminant purpura. Contemporary reviews of the purpura fulminans literature have displayed a similar tendency to incorporate a variety of cases of hemorrhagic skin necrosis under the distinction "purpura fulminans."4-8
From the Department of Pathology, University of Colorado Health Sciences Center, and the Colorado Permanente Medical Group, Denver, Colorado; Department of Pathology and Microbiology, University of Nebraska School of Medicine, Omaha, Nebraska; and the Departments of Pathology, Pediatrics, and Biochemistry, University of Colorado Health Sciences Center, and Laboratory Services, Denver VA Medical Center, Denver, Colorado. Reprint requests: Dr. Marlar, Laboratory Services (#113), Denver VA Medical Center, 1055 Clermont St., Denver, CO 80220.
Most notable have been the inclusion of both severe purpura accompanying bacterial sepsis and neonatal purpura fulminans within this category.7,8 In addition, cases identical to those described by Hjort et al, but occurring in adults, have also been called "purpura fulminans."4 Although these reports do not accurately fit Hjort et al's original definition, many have important common features. A number of these cases have in common: the sudden onset of massive bleeding into the skin, followed by extensive skin necrosis and/or death associated with signs of DIC. Based on these unifying characteristics and an enhanced appreciation of the pathophysiologic basis underlying hemorrhagic skin necrosis (as described in the previous articles in this issue of Seminars), a revised classification of purpura fulminans is proposed. Also briefly reviewed are possible initiating and mechanistic aspects of the various types of purpura fulminans.
VASCULAR ENDOTHELIUM AND LEUKOCYTES IN HEMOSTASIS AND THROMBOSIS The vascular endothelium provides a dynamic interface between the blood and surrounding tissue compartments. The endothelial cell functions in a number of diverse physiologic systems including coagulation, inflammation, immune regulation, modulation of vascular tone, angiogenesis, and synthesis of mesenchymal components.9 This biologically active role of the endothelial cell has only been appreciated in recent years. Contemporary development of human endothelial cell cultures has been critical to the investigation and enhanced understanding of the varied functions of the
Copyright © 1990 by Thieme Medical Publishers, Inc., 381 Park Avenue South, New York, NY 10016. All rights reserved.
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endothelial cell. Recent elaboration of cytokine function and interaction with the endothelium has also been integral to this goal. Although much has been learned in recent years, our knowledge of endothelial physiology is still primitive. One important function of the vascular endothelium is its participation in the complex scheme of hemostasis and thrombosis. The endothelial cell has the capability of expressing both procoagulant and anticoagulant properties.9-13 The balance between these overtly antagonistic functions is dependent on a variety of factors that influence the endothelial cell, such as activation of the hemostatic system and production of inflammatory mediators or perturbation of the environment surrounding the endothelial cell. In the quiescent state the vascular endothelium serves predominantly an anticoagulant role. 12,13 Modulation (activation or perturbation) leads to a shift toward enhanced procoagulant activity and diminished anticoagulant function.9-14 This dynamic alteration is a result of the endothelial cell's active metabolism and physiologic responsiveness.
Anticoagulant Functions of the Endothelial Cell in the Quiescent State The endothelial cell associated anticoagulant properties include the protein C (PC)-thrombomodulin system, the antithrombin III (AT III)-heparin system, and the profibrinolytic mechanisms. 10-13 The endothelial cell also functions as an inhibitor of platelet aggregation through its production and release of prostacyclin.10'13 Thrombomodulin is a constitutively expressed endothelial cell surface protein, serving as a high-affinity receptor for thrombin.11 Binding of thrombin to thrombomodulin greatly facilitates the activation of PC. PC and its cofactor protein S (PS) are potent, naturally occurring, vitamin K-dependent anticoagulants synthesized by the liver.15 PS is also synthesized by the endothelial cell.16 Activated PC in the presence of PS, calcium ions, and cell membrane surface impairs fibrin formation through its ability to proteolytically degrade Factors Va and Villa. 17 In addition to the anticoagulant function of PC, the binding of thrombin to thrombomodulin impairs thrombin's procoagulant functions.11 AT III is another important naturally occurring anticoagulant.18 In the presence of heparin or heparinlike molecules, AT III rapidly inactivates thrombin and Factors Xa, XIIa, XIa, and IXa. The endothelial cell produces heparin-like molecules that coat the surface of the vascular lumen.19 These heparin-like species are glycosaminoglycans that contain an AT III binding region similar to that present on heparin. These heparinlike molecules allow the AT III molecule to bind and inactivate selective procoagulant enzymes rapidly.
Lysis of thrombi in the microcirculation is dependent on localized fibrinolysis, which is mediated by the endothelial cell. 10,20-22 The endothelial cell synthesizes multiple but distinct molecular forms of plasminogen activators (PA). 21,22 One of the most physiologically important PA is tissue-type plasminogen activator (t-PA). Release of t-PA from the endothelial cell is effected by a variety of physiologic stimuli, such as stress, hypoxia, epinephrine, thrombin, and certain drugs. 10,21,23 These PA molecules cleave plasminogen to plasmin, a trypsinlike enzyme central to the fibrinolytic system. The fibrinolytic response of the endothelial cell is further regulated by the release of plasminogen activator inhibitor, type 1 (PAI-1) from the endothelial cell.23
Procoagulant Functions of the Endothelial Cell in the Perturbed State Under appropriate circumstances, the endothelium can promote a procoagulant response. 9-13,24 These procoagulant activities enhance both platelet aggregation and fibrin formation. Procoagulant functions include decreased synthesis of prostacyclin, increased synthesis and expression of procoagulant substances on the luminal surface of the endothelial cell, and modification of the basic physical properties of the endothelial cell surface.10'24 The endothelial cell manufactures a number of procoagulant substances, including: tissue factor, von Willebrand factor, Factor V, Factor VIIIC, thrombospondin, fibronectin, and others. 10,24-26 These substances function in the promotion of platelet plug formation, cell adhesion, and generation of the fibrin clot. The most important procoagulant synthesized and expressed by the endothelial cell is tissue factor.27,28 Tissue factor binds Factor VII/VIIa in the presence of calcium ions, promoting the activation of Factors IX and X and ultimately fibrin formation. Minimal to no tissue factor is expressed in the quiescent state of the endothelial cell. 24,28 Expression of tissue factor activity is mediated by a variety of stimuli, including gram-negative endotoxin, thrombin, interleukin-1 (IL-1), and tumor necrosis factor alpha (TNF).28 The endothelium provides an important surface on which the procoagulants can assemble, be activated, and function, since all major coagulation reactions require a phospholipid surface for maximal activity.29
Interaction of Leukocytes and Endothelial Cells The role of leukocytes in mediating thrombosis is a relatively recent concept. Evidence to support this inter-
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action is extensive and beyond the scope of this review. Pertinent to this discussion is the role of leukocytes in the mediation of the generalized Shwartzman reaction. As has been summarized by Brozna in this issue of Seminars, the generalized Shwartzman reaction is a systemic response to the intravenous injection of endotoxin. Two doses of intravenous endotoxin given 18 to 24 hours apart induce DIC and widespread microvascular thrombosis. Leukocytes are necessary for this response.30 When rabbits are rendered leukopenic with the administration of nitrogen mustard, thrombosis does not develop following intravenous injections of endotoxin.30 Cytokines provide an important link between the inflammatory response and the coagulation system. Cytokines are important mediators of the inflammatory response which also have a significant effect on the hemostatic mechanism.14,30-34 Cytokines are produced by a variety of cells, including monocytes/macrophages, lymphocytes, endothelial cells, and keratinocytes30-34 (see also Brozna in this issue of Seminars). Cytokine production is mediated through exposure to specific primers, such as endotoxin, antigens, toxins, injury, and activated lymphocytes.32,33 These cytokines function by modifying or amplifying immunologic and inflammatory responses. IL-1 and TNF are two of a number of these immunoregulatory peptides.32,33 IL-1 is one of the key mediators of the body's response to inflammation, tissue injury, immunologic reactions and microbial infections. 31 ' 32 ' 34 Many of the general manifestations of the acute and chronic inflammatory response, such as fever, production of acute-phase reactant proteins, fibroblast proliferation, and chemotaxis are mediated by IL-1. 32 ' 34 TNF is identical to cachectin and functions as one of the principal mediators of lethal shock induced by endotoxin.33 Both IL-1 and TNF induce neutrophil adhesion to endothelium and mediate endotoxin-induced emigration and accumulation of neutrophils. These cytokines function synergistically.35 Endotoxin induces the synthesis of IL-1 and TNF primarily by the monocyte/macrophage.14,32-34 IL-1 and TNF in turn mediate endotoxin-induced endothelial cell injury. These peptides have similar effects on the hemostatic functions of the endothelial cell, modifying the endothelial cell surface characteristics in a similar way. Exposure of the endothelial cell to IL-1 and TNF promotes a unified procoagulant response: concomitant expression of tissue factor activity, suppression of thrombomodulin activity, and increased secretion of PAI-1 1 4 , 3 6 - 3 8 This induces the endothelial cell to switch from an anticoagulant role to one that promotes coagulation. This procoagulant response occurs in the absence of physical endothelial cell damage. In addition, endothelial cells exposed to endotoxin, TNF, or thrombin treatment can synthesize and release IL-1. 3 2 - 3 6
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Endothelial cell expression of thrombomodulin is influenced by IL-1, TNF, or indirectly by endotoxin.37 Following exposure to any of these molecules, surface thrombomodulin is internalized and degraded by a lysosomal dependent pathway. Suppression of thrombomodulin activity impairs not only activation of the PC anticoagulant system, but also increases thrombin procoagulant activity.17'37 Exposure of the endothelium to IL-1 or TNF results in rapid activation of the fibrinolytic pathway.38 Plasma t-PA levels increase within 1 hour of exposure; resulting in the generation of plasmin. PA activity terminates approximately 3 hours after exposure concomitant with increased plasma levels of PAI-1. 38 This diminished fibrinolytic activity impairs localized clot lysis, resulting in persistence of microvascular thrombosis. Movat has demonstrated the vital role that both IL-1 and TNF play in mediating dermal hemorrhage and microvascular thrombosis.35 In elaborate studies, a Shwartzman-like reaction was elicited in rabbits by preparing the skin with intradermal injections of recombinant human IL-1 and recombinant human TNF, followed by the intravenous injection of endotoxin. The dermal sites showed diffuse hemorrhage into the skin, leukocyte infiltration, and microvascular thrombosis. IL-1 and TNF are therefore able to substitute for endotoxin in eliciting the local Shwartzman reaction.35
Role of the Protein C System in the Development of DIC In addition to its anticoagulant function, PC may play an important role in the pathophysiology of intravascular coagulation.17,39,40 In cases of DIC associated with sepsis, a decrease in plasma PC antigen and activity has been demonstrated.39 Furthermore, the exogenous infusion of activated PC prevented the development of DIC in baboons infused with lethal doses of Escherichia coli.41 It is not known whether this was a function of the anticoagulant properties of PC or whether PC functioned in down-regulating the effects of endotoxin-induced inflammatory mediators. In addition, in vitro activated PC and thrombin decreased PAI activity and therefore increasing fibrinolytic activity.42
CLASSIFICATION OF THROMBOTICINITIATED PURPURIC LESIONS The term "purpura fulminans" should be used only when rapidly progressive skin necrosis with massive hemorrhage into the skin of a catastrophic nature is found, associated with signs of DIC. 4,5 (see also Adcock and Hicks in this issue of Seminars). Based on this
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definition, purpura fulminans can be classified into three distinct categories (Table I): (1) occurring in individuals with hereditary or acquired dysfunction of the PC system or other coagulation regulatory mechanisms; (2) occurring in patients with acute, current infection, and (3) occurring in individuals without acute infection or known abnormalities of the coagulation regulatory mechanisms. The first category, which includes patients with a dysfunctional PC anticoagulant system, likely represents a rather diverse group. This category of purpura fulminans includes skin necrosis associated with heparin therapy and a variety of clinical situations associated with the PC antithrombotic system, such as homozygous PC or PS deficiencies, warfarin-induced skin necrosis, and antiphospholipid antibodies. This broad category will be referred to as "hemostasis-initiated purpura fulminans." The second category, which occurs in association with acute infection, will be called "acute infectious purpura fulminans." The third group of purpura fulminans, occurring in individuals without known abnormalities of the hemostatic system or concomitant infection has been coined "idiopathic purpura fulminans." Francis, in this issue of Seminars, notes that this final category will likely be subject to further characterization as this idiopathic form of hemorrhagic skin necrosis is better delineated.
been described in the preceding articles of this issue of Seminars. Skin necrosis represents the common end that interrelates a number of different but interactive pathophysiologic processes. The main differences between these processes are the initiating events. The two major mediating pathways include the hemostatic system (Fig. 1) and the inflammatory system (Fig. 2). Each system modulates the endothelial cell and surrounding milieu inducing changes that ultimately lead to skin necrosis. These two systems have many pathophysiologic mechanisms in common and show considerable overlap. Dermal vascular thrombosis and hemorrhagic skin necrosis represent the common endpoint. To put into perspective the similarities and differences between the hemostatic and inflammatory initiating pathways, each of these pathways and their proposed mechanisms as they pertain to skin necrosis will be briefly discussed. The event central to the development of skin necrosis as it involves the hemostatic pathway is microvascular thrombosis (Fig. 1). The vast array of initiating stimuli, and hence variety of pathways that can lead to the development of microvascular thrombosis, makes this pathway difficult to study. In general, thrombosis will not develop in individuals with a functional hemostatic system unless the regulatory anticoagulant and/or fibrinolytic systems are overwhelmed, such as in situations of direct vascular damage or heparin-induced platelet ag-
BASIC MECHANISMS OF THE THROMBOTIC-INDUCED PURPURIC LESIONS There are a number of different clinical situations associated with dermal vascular skin necrosis that have
TABLE 1. Classification of the Thrombotic Purpuric Lesions 1. Acute infectious purpura fulminans 2. Hemostasis-initiated purpura fulminans A. Dysfunction of the protein C system Homozygous protein C or protein S deficiency Acquired protein C or protein S deficiency Cholestasis Warfarin-induced skin necrosis Antiphospholipid syndrome (lupus anticoagulant) B. Platelet mediated Heparin-induced thrombocytopenia C. Hereditary or acquired dysfunction of other hemostasis regulatory systems 3. Idiopathic purpura fulminans A. Postinfection purpura fulminans B. Purpura fulminans of unknown etiology
FIG. 1. Schematic representation of the general proposed mechanism of skin necrosis and purpura fulminans as initiated and propagated by the hemostatic system. The central component is the endothelial cell (E.C.). Both the coagulation and fibrinolytic systems contribute to the thrombotic and ischemic components of these types of thrombohemorrhagic purpuric lesions. The inflammatory component (cytokines and leukocytes [PMN]) may or may not contribute to the initial lesion development, but may play a role in the propagation of the irreversible types of purpuric lesions. See text for details.
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FIG. 2. Schematic representation of the general proposed mechanism of skin necrosis and purpura fulminans as initiated and propagated by the inflammatory response. The central component is the endothelial cell (E.C.). The interrelationship between the leukocytes (in particular the PMNs) and the cytokines (IL-1 and TNF) are multiple and complex (arrows). The coagulation component (thrombosis) plays a definite but minor role in these types of necrotic lesions. See text for details.
glutination. Vascular thrombosis can, however, develop after relatively minor stimuli in patients with aberrant coagulation regulatory mechanisms, such as homozygous PC or PS deficiencies. Thrombosis may develop in these patients following stimuli that would lead to no thrombosis or clinically insignificant thrombosis in the normal individual. Dermal vascular thrombosis occurring in individuals with a functional hemostatic system includes a variety of causes beyond the scope of this review. Atherosclerosis and other entities that induce vascular damage are common causes. Heparin-induced thrombosis is unique in that it occurs in the presence of a normal endothelium and underlying hemostatic system.43 The vast majority of cases of skin necrosis initiated through the hemostatic pathway occur in patients with hereditary or acquired dysfunction of the PC system or other coagulation regulatory mechanisms. The naturally occurring anticoagulant systems and in particular the PC antithrombotic system play a vital role in preventing the development of microthrombi in the dermal vessels and lysing small thrombi once they develop. For this reason, a dysfunctional PC system may play a major role in the development of microvascular dermal thrombosis. Pathophysiologically, relatively minor stimuli such as venipuncture, local hypoxia, and pressure-induced ischemia may initiate microvascular thrombosis in these impaired patients.47 (see Marlar and Neumann in this issue of Seminars). While a functioning hemostatic system would rapidly lyse such small clots that form, a dysfunctional PC system would allow
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persistence and propagation of the clot, causing irreversible skin damage and necrosis (Fig. 1). Loss of a major antithrombotic regulatory mechanism therefore may alter the balance of the hemostatic system, allowing development of microvascular thrombosis in response to minor stimuli. Ischemic vascular damage to the dermis may in some patients incite a mild perivascular inflammatory response (Fig. 1). Inflammation, however, is secondary and is not necessary for skin necrosis to occur. The cytokine mediators (discussed later) are associated with the inflammatory response and may perpetuate thrombosis; but the role of cytokines is minor in hemostasisinitiated dermal thrombosis. The inflammatory system is the other of the two major mediating pathways of dermal vascular necrosis (Fig. 2). The inflammatory-initiated pathway of skin necrosis is centered around the generation and action of cytokines (Fig. 2). Endotoxin is an important initiator of cytokine production leading to dermal necrosis.31'35 In response to endotoxin, cytokines (IL-1 and TNF) are produced by the leukocytes, endothelial cells, and keratinocytes.32-34 Endotoxin also results in leukocyte emigration and activation.35 Cytokines cause leukocyte adherence to the endothelial cell. 32 ' 34 Neutrophil activation and release of proteases induces vascular damage, cellular destruction, and possibly fibrinolysis35 (see also Brozna in this issue of Seminars). IL-1 and TNF promote procoagulant activity while causing diminished anticoagulant function as the cytokines incite increased tissue factor exposure and decreased thrombomodulin expression.24 The chemotactic activity of cytokines promotes continued influx of inflammatory cells propagating this response.32,33 The inflammatory-induced vascular damage and procoagulant expression of the endothelial cell induces microvascular thrombosis.35 The naturally occurring anticoagulant mechanisms are locally overwhelmed. With prolonged ischemia, skin necrosis occurs. The possibility exists that systemic dysfunction of hemostasis, such as in patients with DIC or a mild acquired deficiency of PC/PS, may augment microvascular thrombosis. Although dermal thrombosis is a component of the inflammatory-mediated pathway of skin necrosis, its role is minor compared with the role that cytokines play (Fig. 2). Although hemostasis-associated and inflammatorymediated skin necroses have different initiating events, these pathways have a number of commonalities. The endothelial cell is the common and central factor to these pathways which lead to the development of skin necrosis (see Fig 1,2). Both pathways cause the endothelial cell to switch from an anticoagulant state to one that promotes coagulation.10,12,13 Three important pathophysiologic responses involving the endothelial cell that are common to both pathways include increased tissue factor
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expression/exposure,28 down-regulation of thrombomodulin expression and PC antithrombotic system activity (possibly other regulatory antithrombotic mechanisms as well 17,37 ), and impaired fibrinolysis.23,38 These three physiologic responses promote microvascular thrombosis and hence thrombohemorrhagic skin necrosis. Vascular damage, as induced by venipuncture, ischemia, or inflammation, results in exposure of tissue factor. 10,12,24,28 Tissue factor may be present either within the perivascular tissue or expressed on the luminal surface of the endothelial cell. 24,27,28 In addition, in the absence of direct vascular damage, endotoxin and cytokines directly promote expression of tissue factor activity by the endothelial cell as well as the monocyte/ macrophage.24,28,45 Furthermore, IL-1 and TNF cause the down-regulation of thrombomodulin activity on the endothelial cell surface.37 These cytokines also promote increased secretion of PAI-1. 23,38 These effects of the inflammatory response induce microvascular thrombosis. Likewise, dysfunction of the PC antithrombotic system concomitant with tissue factor exposure results in an overall procoagulant effect of the endothelial cell. 41 This occurs predominantly through impaired degradation of activated Factors V and VIII and enhanced availability of thrombin.11,17 In addition, dysfunction of the PC anticoagulant system is associated with impaired fibrinolysis through an unknown mechanism.17,40 The propensity for microvascular thrombosis to involve the dermal vasculature with a predilection for areas rich in subcutaneous fat is unknown. This may represent variability in endothelial cell expression of procoagulant and anticoagulant factors between different organs. Preferential dermal involvement could also reflect differences in cytokine attraction, activity, and function. Physical properties unique to the skin, such as variations in temperature, predisposition to trauma, and variable blood flow, may also play a role in the development of skin necrosis. Preferential involvement of fat-rich areas could be a function of the generous supply of small vessels in this tissue. Mechanistic studies to describe these variables better, as they pertain to dermal vascular necrosis, are proposed.
FUTURE MECHANISTIC STUDIES OF SKIN NECROSIS AND PURPURA FULMINANS Information obtained from the studies of hemostasis-initiated purpura fulminans, acute infectious purpura fulminans, idiopathic purpura fulminans, and the Shwartzman reaction in animals has suggested several possible common etiologic mechanisms. Since these processes of
hemorrhagic skin necrosis are centered around the endothelial cell, endothelial cell biology, as it relates to the development of purpura fulminans and hemorrhagic skin necrosis, must be studied in greater detail. Further investigations of cytokine function and the hemostatic proteins are also vital to enhanced elucidation of the pathophysiology of these devastating syndromes. The fundamental changes in endothelial cell function at prepared skin sites in animal models of the Shwartzman reaction and at sites of early purpura fulminans lesions in patients with PC deficiency need to be defined before the pathogenesis of purpura fulminans will be understood. Specifically, what initiates the sudden development of sharply demarcated thrombohemorrhagic lesions in patients receiving coumarin or having abnormally low levels of PC? Are endothelial cells in sites of early purpuric lesions activated by previous infectious agents such as bacteria or viruses? Do these endothelial cells have a decreased capacity to bind AT III, activate PC, promote fibrinolysis, or have increased tissue factor procoagulant expression? Using immunohistologic techniques, immunoassays, and functional assays, endothelial cell associated tissue factor, thrombomodulin, and fibrinolytic proteins can be measured in early lesions of purpura fulminans and compared with expression of these coagulation factors on endothelial cells from areas of skin not involved by purpuric changes and other organs as well. Alterations in expression of tissue factor, thrombomodulin, fibrinolytic proteins, and binding of AT III could explain the predilection for dermal involvement. In idiopathic purpura fulminans it is thought that an antecedent bacterial or viral infection involving the skin is a prerequisite for the development of the dermal thrombohemorrhagic lesion. Identification of infectious agents as the initiators of the purpuric lesions will be difficult to document. Possibly, using polymerase chain reaction techniques with tissue sections could lead to the identification of virus-infected endothelial or epithelial cells in sites predisposed to the development of purpuric lesions. If infectious agents cannot be found in skin sites predisposed to developing thrombosis in patients with abnormal coagulation regulatory factor levels, evidence for the presence of cytokines such as IL-1, TNT, or interferon might be obtained by using polymerase chain reaction to identify increased levels of cytokine mRNA in either endothelial or epithelial cells, or in transient inflammatory cell infiltrates in early lesions of purpura fulminans. The presence of increased levels of mRNA for these inflammatory cytokines would suggest that cytokines prepare the skin for developing thrombohemorrhagic and necrotic lesions. Once the fundamental changes in endothelial cells
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SUMMARY The syndromes of purpuric lesions associated with a thrombotic mechanism are very rare in the general population. Dermal vascular thrombosis, however, can be devastating and associated with significant morbidity and mortality. These syndromes share common features in their clinical course, pathogenesis, and histology. Although these syndromes can be initiated by either the hemostatic or inflammatory pathways, both pathways center around perturbations of the endothelial cell, which promote thrombosis. If animal models or better testing can be developed, enhanced appreciation of mechanisms underlying purpura fulminans may be deduced. Characterization of the pathophysiology may then allow directed treatment modalities that could limit the course of these syndromes and reduce morbidity and mortality. Acknowledgements. The authors would like to thank Dr. Lou Fink for his help and discussions on the topic of the mechanism or mechanisms of skin necrosis. This work was supported in part by a grant to R.A.M. from the Veterans Administration.
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responsible for initiating the thrombogenic state at sites of purpura are known, the mechanisms involved in the development of progressive thrombohemorrhagic lesions can be defined. Clearly, an impaired PC antithrombotic system (inherited or acquired) contributes to microvascular thrombosis in some patients with purpura fulminans. To what extent abnormal plasma levels of procoagulant, anticoagulant, and fibrinolytic proteins contribute to dermal thrombosis is unclear. Measurement of circulating levels of these proteins and the various activated factors (such as thrombin-AT III complexes, D-dimer) during the early stages of purpura fulminans and the following convalescent period might help to explain why and how thrombosis and ischemic necrosis develop. Since patients with warfarin-induced skin necrosis or homozygous PC deficiency associated skin necrosis are not routinely available for prospective studies, progress toward understanding the mechanisms involved in this type of purpura fulminans has been lagging. The development of an animal model of warfarin-induced skin necrosis or homozygous PC deficiency could be useful for characterizing prerequisite changes in endothelial cell biology and circulating coagulation factors leading to the development of purpura fulminans. With a well-defined model, initiating factors of localized skin necrosis associated with abnormally low levels of PC or other regulatory factors could be identified.
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