the initiation of coagulation , especially in severe sepsis , is probably mediated by the induction of tissue factor ( tf ) expression by endotoxin . this leads to the activation of factor x , which then combines with factor va to convert prothrombin to thrombin . although thrombin is usually considered to have purely procoagulant activity , it is in fact a multifunctional protein that has some important homeostatic anticoagulant effects ( fig . one of its main functions is to bind to thrombomodulin , which is expressed on the endothelial cell surface . thrombomodulin is the major physiologic buffer for the procoagulant effects of thrombin in normal vessels . because thrombomodulin binds to the same site on thrombin that would normally bind to fibrinogen , platelets or factor v , instead , the thrombin - thrombomodulin complex activates protein c ( through a different site on the thrombin molecule ) , resulting in initiation of the activated protein c pathway . activated protein c must dissociate from epcr before it can bind to protein s and function as an effective anticoagulant through the inactivation of factor va . ec , endothelial cell ; paf , platelet - activating factor ; pdgf , platelet - derived growth factor ; pmn , polymorphonuclear leukocyte ; tgf , transforming growth factor although the number of thrombomodulin molecules per endothelial cell is approximately constant , the local concentration of thrombomodulin is determined by the number of endothelial cells that are in contact with the blood . because the endothelial cell surface area per unit blood volume is much greater in the microcirculation ( approximately 3000 - 5000 cm of endothelium / ml blood ) than in larger blood vessels ( approximately 1.5 cm / ml ) ( fig . 2 ) , there is a correspondingly high concentration of thrombomodulin in the normal microvasculature ( approximately 500 nmol / l ) as compared with the larger vessels ( approximately 0.1 - 0.2 nmol / l ) . as a result , thrombin is rapidly removed from the microcirculation as it is bound to thrombomodulin . this suggests that the activated protein c system is uniquely poised to regulate coagulopathies in the microcirculation , and this has been confirmed in clinical studies . importantly , at least in some forms of sepsis , thrombomodulin can be downregulated , with resulting loss of ability to regulate thrombin , and hence clotting , in the microcirculation . homozygous protein c deficiency has a similar result , such that affected babies develop microvascular thrombosis ( purpura fulminans ) almost immediately after birth . in addition to its procoagulant and anticoagulant effects , thrombin can also be involved in the process of inflammation ( fig . , it can activate endothelial cells to express p - selectin , causing neutrophil and monocyte adhesion . it is also chemotactic for polymorphonuclear leukocytes and is a potent inducer of endothelial platelet - activating factor formation . platelet - activating factor is a major activator of neutrophils , which have been demonstrated to play an important role in the pathogenesis of sepsis . the inflammatory response also modulates the coagulation system , including downregulation of the expression of thrombomodulin ( fig . 3 ) . inflammation also increases levels of 1-antitrypsin , which is an acute - phase response protein and an important inhibitor of the protein c pathway . acute inflammatory mediators such as tumor necrosis factor ( tnf)- and endotoxin amplify the production of tf , particularly by monocytes , triggering further coagulation . complement activation by endotoxin increases exposure of clot - promoting membrane phospholipids to blood , and thereby propagates the coagulation response . in certain forms of inflammation , levels of both fibrinogen and plasminogen activator inhibitor ( pai)-1 ( an inhibitor of fibrinolysis ) are increased . if there were no mechanisms to disrupt the amplification of coagulation by inflammation , and vice versa , then the ensuing vicious cycle would have a catastrophic effect , leading ultimately to death ( fig . it was hypothesized , therefore , that endogenous activated protein c interferes with these amplification loops by modulating both coagulation and inflammation , which differentiates it from other types of anticoagulants , and that protein c depletion may occur in severe sepsis . the inflammation - coagulation autoamplification loop . adapted from esmon et al . a primate model was used in the initial study undertaken to investigate this possibility . in animals infused with a lethal escherichia coli dose alone , 5a ) , and the animals died within 24 - 32 h. in contrast , organ damage was prevented in animals given a lethal dose of e. coli plus an infusion of activated protein c ( fig . importantly , it was shown that animals could still be rescued if activated protein c was given 2 - 3 h after the lethal e. coli dose was administered . similarly , when animals were given a sublethal dose of e. coli and protein c activation was blocked with an antibody , mimicking the clinical situation in a patient with sepsis and low protein c levels , the sublethal bacterial dose became lethal , with complete fibrinogen consumption , a disseminated intravascular coagulation - like syndrome , organ failure , and a full - blown septic shock - like response ( fig . sgpt , serum glutamic pyruvic transaminase ( indicator of liver damage ) . adapted from taylor et al . hpc4 , murine monoclonal antibody to human protein c ; ig , immunoglobulin ; sgpt , serum glutamic pyruvic transaminase ( indicator of liver damage ) . adapted from taylor et al . c4-binding protein ( c4bp ) is an acute - phase protein that is known to bind and inhibit protein s , a vital cofactor for the activated protein c system . in another primate study , it was shown that administration of c4bp together with a normally sublethal dose of e. coli resulted in complete fibrinogen consumption and death . in the same study , it was shown that , whereas either sublethal e. coli doses or c4bp alone had no appreciable effect on tnf- production , administration of both substances led to a circulating tnf- level that approached that seen with a lethal dose of e. coli ( fig . 7 ) , with rapid consumption of fibrinogen , systemic organ damage , and eventual death . with co - administration of protein s , however , tnf- levels remained similar to those seen in animals administered a sublethal dose of e. coli alone ( fig . this suggests that reversal of the inhibitory effects of inflammatory cytokines on the activated protein c pathway can control the inflammatory response . tnf- production in response to sublethal or lethal e. coli infusion , with or without c4bp and protein s ( ps ) supplementation . c4bp and ps were administered at doses of 20 mg / kg and 3 mg / kg , respectively . slec , 10% of a lethal dose of e. coli ; lec , 100% lethal dose dose of e. coli . adapted from taylor et al . one involves the ability of activated protein c to form a tight complex with pai-1 . once the complex with activated protein c forms , pai-1 ( the major inhibitor of tissue plasminogen activator ) can no longer inhibit tissue plasminogen activator . because of the ability of activated protein c to limit thrombin generation , it can also reduce the activation of a precarboxypeptidase b commonly referred to as thrombin activatable fibrinolysis inhibitor . thrombin activatable fibrinolysis inhibitor functions by removing lysine residues from the fibrin clot that would normally play a role in augmenting plasminogen activation and the fibrinolytic activity of plasmin . the potentially protective effect of activated protein c has also been shown in two rat studies , both of which suggested inhibition of leukocyte activation as an important mechanism . in one of these studies , activated protein c was shown to reduce motor disturbance resulting from spinal cord injury by reducing local levels of tnf- , thereby inhibiting neutrophil accumulation . in the other study , animals treated with activated protein c were found to have a reduction in ischemia / reperfusion - induced renal injury , which was associated with reductions in renal levels of tnf- , interleukin-8 and myeloperoxidase , with resulting inhibition of leukocyte activation . activated protein c has a number of properties that differentiate it from other anticoagulants , such as heparin or antithrombin . first , the activated protein c system is geared to modulate coagulopathy within the microvasculature , and it is microvascular disease that lies at the root of sepsis . second , the activated protein c system involves the epcr , which was identified during research to discover possible receptors and pathways linked to the system . the epcr was found to be homologous to the major histocompatibility complex class i family of molecules , all of which are involved in the regulation of inflammation . the epcr is synthesized on the endothelial cell surface and moves to specialized regions ( caveolae ) in the endothelium , through which it is able to enter the cell and then the cell nucleus . once inside the nucleus , it is able to redirect the gene expression profiles within the cell . unlike other plasma membrane receptors , epcr is also capable of carrying activated protein c into the nucleus , with subsequent alterations to some of the gene products . activated protein c / epcr - mediated anticoagulant pathway . apc , activated protein c ; mp , metalloproteinase ; palm , palmitate ; pc , protein c ; pma , phorbol myristate acetate ; tm , thrombomodulin ; rrc , arg , arg , lys . adapted from esmon . furthermore , when thrombin or inflammatory cytokines come into contact with the endothelium , they induce shedding of epcr from the cell surface , a process that is mediated by a metalloproteinase . soluble epcr binds to proteinase 3 , a serine proteinase that is found in neutrophils . this soluble epcr - proteinase 3 complex , either alone or in a complex with protein c , appears to interact with neutrophils via an integrin ( mac-1 ) to inhibit their adhesion to the endothelium , and as a result prevents the migration of the neutrophils into the tissues . several unique mechanisms by which activated protein c can affect inflammation and coagulation have been demonstrated in various in vitro studies . in one study , it was shown that incubation of monocytes with activated protein c leads to a decrease in tnf- production and blockade of the translocation and activation of the central proinflammatory transcription factor nuclear factor-b in response to lipopolysaccharide exposure . the binding of activated protein c to a specific receptor on mononuclear phagocytes has also been shown to inhibit intracellular calcium signaling and monocyte - dependent t - cell proliferative responses . finally , epcr - dependent tf suppression by activated protein c has been demonstrated in a human monoblastic leukemia cell line . the sepsis cascade , which is associated with simultaneous activation of systemic inflammation and coagulation , along with altered fibrinolysis , leads to microvascular endothelial injury , acute organ dysfunction , and possibly death . thrombin , with its procoagulant and anticoagulant effects , as well as its involvement in the process of inflammation , is thought to be central to the sepsis cascade . in vitro and animal models suggest that activated protein c , a protein that has antithrombotic , anti - inflammatory and profibrinolytic activities , may be an important modulator of the vicious cycle of coagulation and inflammation associated with the sepsis cascade , which if unchecked may ultimately lead to death . several properties of activated protein c , including its unique positioning to regulate coagulopathies in the microvasculature and the ability of epcr to facilitate protein c activation , support the continued development of activated protein c - mediated strategies that are aimed at disrupting the process whereby inflammation initiates coagulation and coagulation amplifies inflammation . c4bp = c4-binding protein ; epcr = endothelial cell protein c receptor ; pai = plasminogen activator inhibitor ; tf = tissue factor ; tnf = tumor necrosis factor .