by

Kourosh Moaref

Milwaukee School of Engineering

Graduate Perfusion Student

The blood coagulation cascade is an important and complicated set of events. It is a part of the systemic inflammatory response utilized by the body to maintain hemostasis. The detailed understanding of the mechanisms involved is vital to the prediction and treatment of several blood disorders and surgical operations. Such applications include different forms of hemophilia, thrombosis, disseminated intravascular coagulation (DIC), and the induction of anticoagulation during invasive surgical procedures. The coagulation cascade has been studied and analyzed for the past four decades. The classical view has long been toward the well-accepted waterfall cascade hypothesis described in the early 60s. However, recent examinations of the cascade have produced new perspectives and theories on the coagulation mechanism and sequence of events. Thus, an attempt is made to examine and address the collective issues involved in past and current status of the coagulation cascade.

The cascade/waterfall hypothesis organized the events of hemostasis into three distinct but related phases: extrinsic, intrinsic, and common pathway. The traditional theories of coagulation explain its initiation or activation by two separate methods, extrinsic or intrinsic, which converge into one final common pathway to achieve coagulation. The activating event distinguishes between the two initiating pathways: damaged tissue or vessel activates the extrinsic pathway, whereas a foreign non-endothelial surface activates the intrinsic pathway. The terminology intrinsic and extrinsic is based on the concept of availability of factors in each pathway [2]. Intrinsic pathway is named so because all the factors are considered to be present in blood in an inactivated form, whereas the extrinsic pathway is based on the fact that the subendothelial tissue factor is required in addition to the circulating factors [2].

The initiation of coagulation in response to infection or "contact" with foreign surfaces, and the activation of factor XII, has traditionally been considered as the intrinsic pathway of the coagulation cascade. However, several clinical and experimental issues have sparkled the re-examination of the coagulation cascade as described in the previous section. Increasing amount of evidence suggest a more central initiation of the coagulation pathway with a more horizontal cross-link between coagulation and inflammation as part of a unique, defensive host response. Thus, in this section an attempt is made to address these issues in an effort to shed light on the reasons behind the advent of new and revised hypotheses of the coagulation cascade.

The notion of intrinsic pathway has been brought into question due to studies indicating failure of the "contact system" activation in humans with sublethal endotoxemia despite marked activation of the coagulation system [1]. It has been shown that the blockade of factor XII has failed to prevent the development of disseminated intravascular coagulation (DIC) but decreased hypotension has been noted. Hence, the possible role of factor XII may be to react or be activated by lipopolysaccharides (LPS) or bacterial proteinases contributing to the Kallikrein-Kinin inflammatory axis and to activation of the classical C1-esterase-dependent complement cascade suggesting that it may contribute more to septic hypotension [1].

Clinical data has also raised valid questions about the validity of the intrinsic pathway concept. Patients with genetic deficiencies of either factor XI, IX, or VIII, all factors from the intrinsic pathway, have normal fragment factor 1 and 2 plasma levels [4]. Furthermore, individuals with abnormal partial thromboplastin time due to a deficiency in one of the contact factors are entirely asymptomatic. Patients with factor XI deficiency (hemophiliac C) have much milder general states than those with factor VIII ( hemophiliac A) or factor IX (hemophiliac B). Finally, patients with isolated, factor VII deficiency bleed excessively [2].

In lue of such new discrepancies, one may wonder then about the true initiating mechanism of the coagulation cascade. Significant evidence exists suggesting tissue factor (TF or factor III) as the central initiating mechanism of coagulation cascade during inflammation states.

Tissue factor is tightly regulated and is absent from the blood stream in non-inflammatory conditions. Thus, it can be considered as the triggering mechanism of coagulation whenever the vascular endothelial integrity is breached. Within the blood stream, only monocytes, macrophages and endothelium can express TF and have been demonstrated to do so in response to E.Coli, LPS, and various cytokin mediators. Virtually every microbial species tested and their surface components have been shown to induce monocyte/macrophage tissue factor activity [1]. Furthermore, a multitude of cytokines and inflammatory responses including tissue necrotic factor (TNF(), Interleukin 1 (IL1), IL2, C5a, IL6, and platelet activating factor (PAF) have been shown to upregulate TF expression [1].

Different strategies towards preventing the coagulation related endorgan damage has demonstrated efficacy when TF or TF-VIIa complex have been neutralized. The induction of anti-factor VIIa, anti-TF antibody, and tissue factor pathway inhibitor (TFPI) have been shown to reduce endorgan damage associated with endotoxemia or bacteremia in experimental models [1].

The coagulation system, as a part of the defensive host response, responds to events that cause vascular endothelial injury. Both inflammation and coagulation are activated by the damaged or injured endothelium. Thus, the goal of this section is to provide coverage of the new revised concepts in the coagulation cascade. In the previous section the initiation mechanism was discussed where the TF-VIIa complex was considered. In this section coagulation is presented from a new approach by considering, the regulatory mechanisms involved in both the coagulation and inflammation and their linkage affecting the coagulation cascade and mechanism. Hence an attempt is made to present a revised hypothesis of blood coagulation.

Under physiological conditions, vascular endothelium prevents the coagulation process, however when damaged this state is disturbed and the antithrombotic properties are significantly diminished. The endothelium represents the interface between inflammation and coagulation: it is a surface where coagulation is activated, but at the same time, under cytokin stimulation during an inflammatory state such as sepsis, it provides a site of attachment for inflammatory effector cells [5]. Thrombomodulin (TM), a high affinity receptor protein for thrombin, is expressed on the endothelial surface and when complexed with thrombin activates protein C which inhibits coagulation and activates the fibrinolytic pathway [5]. The presence of TM on the endothelial surface and the integrity of protein C are fundamental requirements for correct balance between coagulation and fibrinolysis. During inflammation, this balance between pro- and anticoagulant factors is strongly disturbed and impaired. Cytokines IL1 and TNF( induce the expression of TF and the down regulation of TM and protein C [5]. In doing so, anticoagulation is diminished by the lower levels of TM-PC complexes along with the procoagulatory effects of higher TF levels. Hence, the balance is shifted towards a procoagulatory state. This state is further reinforced by platelet-granulocyte interactions. Lipopolysaccharides induced increase of tissue factor activity in monocytes has been shown to be produced by platelet-granulocyte interactions [5]. Such interactions include the expression of P-selectins on activated platelets which mediates interaction between platelets, leukocytes and endothelial cells [5].

The initiation of blood coagulation response to vascular injury follows the formation of a catalytic complex composed of factor VIIa and tissue factor [3]. Activated TF-VIIa complex activates trace amounts of factor X into Xa, which branches off to several directions initiating a trickling effect in several directions [3]. The trace amounts of activated Xa catalyzes further activation of TF-VIIa complexes in positive feedback fashion and also primes the propagation of the coagulation response,, as depicted in figure 1, by proteolytically modifying factor IX to an intermediary form IX( by the factor VIIa-TF complex [3]. Once the propagation phase has begun the TF-VIIa complex converts factor IX( to its activated form IXa(. Activated factor Xa also takes part in the regulatory aspect of the cascade by binding to tissue factor pathway inhibitor (TFPI) to down regulate TF-VIIa complexes in a negative feedback manner [2]. TFPI, a Kunitz-type serine protease inhibitor, is basic pancreatic trypsin inhibitor (aprotinin), which directly inhibits factor Xa and VIIa/TF complex activation [2]. The propagation phase then continues by the activation of more factor X into Xa via factor IXa( and cofactor VIIIa followed by the activation of prothrombin into thrombin in the presence of platelet factor 3, calcium, and activated factor Va. Thrombin forms fibrin monomer and activates the stabilizing factor XIIIa to form the fibrin polymer producing the clot or thrombus. It also acts as a positive feedback pathway to regenerate factors VIIIa and Va which are involved in factor Xa and thrombin generation respectively [3].

As mentioned earlier in this report a formed clot is the result of an orchestrated collaborative effort between the inflammatory (cytokin mediated), immune (monocyte/phagocyte interaction) and coagulation system. The complete revised cascade and all its intricacies depicted in figure 1, reflect this multifaceted approach.

Conclusion and Discussion

The coagulation hypothesis has evolved significantly over the past decade since its initial description in the early 1960s. During the course of this report an effort has been made to cover the various strides made in understanding this complicated and intricate control system so vital to the many aspect of medicine. It must be noted that the understanding of the coagulation process is under ongoing research and still remains the topic of several controversies among hematologist, immunologist, and physiology researchers. The reader must take note of and appreciate the level of complexity involved in this process due to the multifaceted activities involved during the total defensive host response initiated by the human body. Thus, this report has been written to provide a reflection of the most agreed upon concepts in the current thinking of researchers and their published literature.

Bibliography

[1] McGilvray, Ian and Rotstein, Ori: Role of the coagulation System in the Local and Systemic Inflammatory Response, World Journal of Surgery, Vol. 22, No2, Feb 1998.

[2] Broze, George and Luchtman, Lori: The Current States of Coagulation, Annals of Medicine, Vol 27, pp 47-52, 1995.

[3]Vlasuk, George: The New Anticoagulants: New Opportunities, New Issues, Archives of Pathology and Laboratory Medicine, Vol 122, No 9, Sep 1998.

[4]Buller, H.R., and Cate, J.W. : Coagulation and Platelet Activation Pathways, European Heart Journal, Vol 16 ( Supplement L ), pp 8-10, 1995.

[5]Cicala, Carla, and Cirino, Giuseppe: Linkage Between Inflammation and Coagulation: An Update on the Molecular Basis of the Crosstalk, Life Sciences, Vol. 62, No 20, pp 1817-1824, 1998.

Fragment factor 1 + 2 is used to analyze coagulation activation under basal and activated conditions. It represents the small peptides split from the native coagulation proteins during activation.