The contact system

Development of new anticoagulants targeting coagulation factor XI and prospects for clinical use

Thrombosis is a potentially fatal condition for which various anticoagulant therapies have been used for prevention and treatment. However, bleeding events remain a concern with all anticoagulant drugs. Recent evidence suggests that inhibiting coagulation factor XI (FXI) and activated FXI (FXIa) plays a greater role in the formation of pathological thrombi in thrombosis than in normal hemostatic thrombi, allowing for the potential to address these two events separately. Consequently, FXI/XIa inhibition has become the focus of anticoagulant drug research, leading to the development of numerous FXI-targeting compounds with diverse mechanisms of action. Herein, we aimed to review FXI/FXIa inhibitors under development, discussing the role of FXI in the coagulation reaction and the advantages and disadvantages associated with its deficiency. The results of a Phase II study showed that FXI/XIa inhibitors provide efficacy comparable to that of low molecular weight heparin therapy while reducing clinically significant bleeding events. Additionally, in a study of patients with atrial fibrillation, FXI/XIa inhibitors reduced bleeding events compared to those with direct oral anticoagulants. Furthermore, when combined with antiplatelet therapy, FXI/XIa inhibitors did not significantly increase bleeding risk in non-cardioembolic stroke or acute coronary syndrome. However, conflicting trial results have also been reported, highlighting the difficulty in assessing the clinical benefit of FXI/XIa inhibitors in different clinical settings, such as atrial fibrillation and acute myocardial infarction. Future large, well-designed Phase III studies are needed to evaluate the safety and efficacy of FXI/XIa inhibitors across diverse populations requiring antithrombotic therapy.

A Rare Cause of Isolated Prolonged Activated Partial Thromboplastin Time: An Overview of Prekallikrein Deficiency and the Contact System

Prekallikrein (PK) deficiency, also known as Fletcher factor deficiency, is a very rare disorder inherited as an autosomal recessive trait. It is usually identified incidentally in asymptomatic patients with a prolonged activated partial thromboplastin time (aPTT). In this article, we present the case of a 52-year-old woman, with no prior personal or family history of thrombotic or hemorrhagic disorders, who was noted to have substantial protracted aPTT through the routine coagulation assessment before a kidney biopsy. The patient had an uneventful biopsy course after receiving fresh frozen plasma (FFP). Laboratory investigations performed before the biopsy indicated normal activity for factors VIII, IX, XI, XII, and von Willebrand factor (vWF) as well as negative lupus anticoagulant (LA) screen. The plasma PK assay revealed low activity at 15% consistent with mild PK deficiency. The deficit of PK is characterized by a severely prolonged aPTT and normal prothrombin time (PT) in the absence of bleeding tendency. PK plays a role in the contact-activated coagulation pathway and the inflammatory response. Thus, other differential diagnoses of isolated prolonged aPTT include intrinsic pathway factor deficiencies and nonspecific inhibitors such as LA. We concluded that the initial evaluation of a prolonged aPTT with normal PT should appraise the measurement of contact activation factors and factor inhibitors. PK deficiency should be considered in asymptomatic patients with isolated aPTT prolongation, which corrects on incubation, with normal levels of the contact activation factors and factor inhibitors.

Donor-to-recipient transmission of factor XII deficiency by orthotopic liver transplantation

Transmission of congenital clotting factor deficiencies following orthotopic liver transplantation is rare. There has been one reported case of donor-to-recipient transmission of factor XII deficiency in a transplant, and we report the second case.

Myocardial Infarction and Arterial Thrombosis in Severe (Homozygous) FXII Deficiency: No Apparent Causative Relation

Twenty-one patients (12 female and 9 male) with severe (homozygous) factor XII (FXII) deficiency and 58 (32 female and 26 male) with heterozygous FXII deficiency were observed for an average 16.2 years. No patient with homozygous FXII deficiency experienced myocardial infarction or any other arterial thrombosis. The same was true for heterozygotes. The cases of FXII deficiency and arterial thrombosis reported in the literature were evaluated. In every instance, associated risk factors were present that could justify the arterial thrombosis. Dyslipidemia, hypertension, smoking, and diabetes mellitus were the most frequent findings. The examination of the few papers that dealt with the prevalence of arterial thrombosis in patients with severe FXII deficiency showed that only 1 patient of 61 experienced myocardial infarction. In conclusion, it seems that the role of FXII deficiency in the pathogenesis of arterial thrombosis is minor.

Modulation of Hemostatic and Inflammatory Responses by Leptospira Spp

Leptospirosis is a worldwide spread zoonotic and neglected infectious disease of human and veterinary concern that is caused by pathogenic Leptospira species. In severe infections, hemostatic impairments such as coagulation/fibrinolysis dysfunction are frequently observed. These complications often occur when the host response is controlled and/or modulated by the bacterial pathogen. In the present investigation, we aimed to analyze the modulation of the hemostatic and inflammatory host responses by the bacterial pathogen Leptospira. The effects of leptospires and their secreted products on stimulation of human intrinsic and extrinsic pathways of coagulation were investigated by means of altered clotting times, assembly and activation of contact system and induction of tissue factor. We show that both extrinsic and intrinsic coagulation cascades are modulated in response to Leptospira or leptospiral secreted proteins. We further find that the pro-inflammatory mediator bradykinin is released following contact activation at the bacterial surface and that pro-coagulant microvesicles are shed from monocytes in response to infection. Also, we show that human leptospirosis patients present higher levels of circulating pro-coagulant microvesicles than healthy individuals. Here we show that both pathways of the coagulation system are modulated by leptospires, possibly leading to altered hemostatic and inflammatory responses during the disease. Our results contribute to the understanding of the leptospirosis pathophysiological mechanisms and may open new routes for the discovery of novel treatments for the severe manifestations of the disease.

Leptospirosis is one of the most relevant and spread zoonotic and neglected infectious diseases affecting humans and other mammals, and is caused by pathogenic bacteria of the genus Leptospira. During infectious diseases, when bacterial pathogens control and/or modulate the host response, impaired hemostasis and inflammation are frequently observed. Here we studied the effects of leptospires and their secreted products on stimulation of human intrinsic and extrinsic pathways of coagulation, showing that both coagulation cascades are modulated in response to Leptospira or leptospiral secreted proteins. We further find that activation of the coagulation cascades culminates in the release of the pro-inflammatory mediator bradykinin and noted an induction of pro-coagulant microvesicles. These findings contribute to a better understanding of the local and systemic hemostastic complications during leptospirosis. Collectively, our results show how leptospires can affect host responses, possibly leading to altered host responses during the disease and giving rise to the leptospirosis symptomatology.

Hypercoagulability and the risk of myocardial infarction and ischemic stroke in young women

BACKGROUND

Myocardial infarction (MI) and ischemic stroke (IS) are acute forms of arterial thrombosis and share some, but not all, risk factors, indicating different pathophysiological mechanisms.

OBJECTIVE

This study aims to determine if hypercoagulability has a differential effect on the risk of MI and IS.

PATIENTS AND METHODS

We reviewed the results from the Risk of Arterial Thrombosis in Relation to Oral Contraceptives study, a population-based case-control study involving young women (< 50 years) with MI, non-cardioembolic IS and healthy controls. From these data, relative odds ratios (ORIS /ORMI ) and their corresponding confidence intervals for all prothrombotic factors that were studied in both subgroups were calculated.

RESULTS

Twenty-nine prothrombotic risk factors were identified as measures of hypercoagulability. Twenty-two of these risk factors (21/29, 72%) had a relative odds ratios > 1; for 12 (41%), it was > 2; and for 5 (17%), it was > 2.75. The five risk factors with the largest differences in associations were high levels of activated factor XI (FXI) and FXII, kallikrein, the presence of lupus anticoagulans, and a genetic variation in the FXIII gene.

CONCLUSION

In young women, prothrombotic factors are associated more with the risk of IS than with MI risk, suggesting a different role of hypercoagulability in the mechanism leading to these two diseases.

Antigen levels of coagulation factor XII, coagulation factor XI and prekallikrein, and the risk of myocardial infarction and ischemic stroke in young women

BACKGROUND

High levels of activated protein–inhibitor complexes of the intrinsic coagulation proteins are associated with ischemic stroke (IS) but not with myocardial infarction (MI). This study was aimed at determining whether the antigen levels of coagulation factors(factor XII, FXII, and FXI and prekallikrein (PK)are associated with MI and IS, and whether this association is independent of levels of activated protein–inhibitor complexes.

PATIENTS AND METHODS

The RATIO study included young women (< 50 years) with MI (N = 205)and IS (N = 175), and 638 healthy controls. Antigen levels of FXII, FXI and PK were measured and expressed as percentages of of those in pooled normal plasmas. Odds ratios (ORs) and corresponding 99% confidence intervals (CIs) were calculated for high levels (i.e. ≥ 90th percentile of controls) as measures of rate ratios.

RESULTS

After adjustment for potential confounders, high levels of FXII antigen were not associated with MI risk or IS risk(OR(MI) 1.18, 99% CI 0.51–2.74; ORIS 1.03, 9% CI 0.41–2.55). High levels of FXI antigen were slightly associated with an increase in MI risk (OR(MI) 1.55, 9% CI 0.74–3.21), whereas there was a substantial association with IS risk (ORIS 2.65, 9% CI 1.27–5.56). PK antigen was slightly associated with MI risk but not with IS risk(ORMI 1.54, 9% CI 0.67–3.52; ORIS 0.90, 9% CI 0.35–2.33). All associations remained similar after adjustment for levels of protein–inhibitor complexes.

CONCLUSION

Increased levels of FXI antigen were associated with an increase in IS risk, whereas they showed only a marginal association with MI risk. FXII antigen and PK antigen levels were not substantially associated with MI risk and IS risk.

A novel frameshift mutation in exon 4 causing a deficiency of high-molecular-weight kininogen in a patient with splenic infarction

High-molecular-weight kininogen (HMWK) deficiency is a very rare hereditary disorder. We herein report a case of HMWK deficiency with splenic infarction. The HMWK activity of the proband was markedly decreased (0.9%). Direct sequencing of his HMWK gene showed a homozygous "TC" insertion at c523-524 in exon 4. This insertion led to an amino acid substitution, Ser175Ser, resulting in a frameshift mutation and a premature stop codon in amino acid 183. Furthermore, the HMWK activity was also reduced in the patient's three children, who exhibited the heterozygous "TC" insertion at c523-524 in exon 4. This is the first report of this gene alteration in a patient with HMWK deficiency.

Prekallikrein deficiency presenting as recurrent cerebrovascular accident: case report and review of the literature

We report the case of a woman with history of hypertension and hyperlipidemia presenting with recurrent episodes consistent clinically with cerebrovascular accidents (CVA), and MRI changes suggestive of ischemia versus vasculitis as their cause. No anatomical neurological, rheumatic, cardioembolic, or arteriosclerotic etiologies could be determined by extensive workup. Incidentally, the patient was found to have prolonged activated Partial Thromboplastin Time (aPTT) and a normal Prothrombin Time (PT); further testing revealed a prekallikrein deficiency. Since no other cause for the CVAs was established, and other prothrombotic states were ruled out, it is proposed that they are clinical manifestations derived from the prekallikrein deficiency, which in a patient with known cardiovascular risk factors could lead to thrombotic complications such as stroke.

Congenital prekallikrein deficiency

The congenital deficiency of prekallikrein (PK) is a rare condition in which there is a peculiar discrepancy between a severe in vitro defect and absence of bleeding. The gene controlling PK synthesis is located on chromosome 4 and consists of 14 exons and 15 introns. Only approximately 80 cases of PK deficiency have been described in the literature. Owing to the lack of bleeding, most cases go undetected or, if detected, go unreported. Occasional bleeding or thrombosis have been reported in a few patients but this was only due to the presence of associated risk factors. It is certain that the defect does not protect from thrombosis. Diagnosis is based on the presence of a great prolongation of partial thromboplastin time and normal prothrombin time and thrombin time. The long partial thromboplastin time is fully corrected by the addition of normal plasma or normal serum and presents the unusual feature of shortening on long incubation times. Platelet and vascular tests are normal. Immunological studies allow differentiation into two types, namely cases of true deficiency, which are approximately 70% of the total, and cases with abnormal forms. PK is a glycoprotein synthesized in the liver as a single-chain peptide of 88000 Da. It mostly circulates (∼75%) as a complex with high-molecular-weight kininogen. It is cleaved by FXIIa into a heavy chain and a light chain (catalytic domain), held together by disulfide bonds. Molecular biology techniques have so far only been applied to eleven families, and these studies do not yet allow definite phenotype/genotype conclusions. The exons involved are 5, 8, 11, 14 and 15. The noncoagulative effects of PK, mainly based on the effect of kallikrein, have been studied less, since they appear to be the result of the involvement of other components of the contact phase. Kallikrein can mainly affect the formation of bradykinin from high-molecular-weight kininogen and the activation of pro-urokinase to urokinase. Bradykinin causes inflammation, vasodilatation and an increase in vessel permeability. The activation of pro-urokinase results in enhanced fibrinolysis. However, fibrinolysis has been reported to be normal or defective in these patients.

Comparative incidence of thrombosis in reported cases of deficiencies of factors of the contact phase of blood coagulation

Thrombotic manifestations occurring in patients with coagulation defects have drawn considerable attention during the last decade. It concerned mainly patients with hemophilia, vW disease or FVII deficiency. Occasional reports involved also the deficiencies of the contact phase of blood coagulation, mainly FXII deficiency. The purpose of the present study was to evaluate the comparative incidence of thrombosis in all reported patients with FXII, Prekallikrein and Kininogens deficiencies. Out of the reported 341 cases with these conditions that could be tracked there were 43 cases with thrombosis. More specifically, there were 32 patients with FXII deficiency who also had a thrombotic event (16 arterial and 16 venous). As far as Prekallikrein deficiency is concerned, there were nine cases with thrombosis (five arterial and four venous). Finally, two patients with Total or High molecular weight Kininogen deficiencies had also a thrombotic manifestation (one arterial and one venous). The thrombotic manifestations were M.I. 11 cases; ischemic stroke 9 cases; peripheral arteries 3 cases; deep vein thrombosis with or without pulmonary embolism 17 cases; thrombosis in other veins 3 cases. Congenital or acquired associated prothrombotic risk factors were present in 33 out of 36 cases. In three cases the existence of associated risk factors was excluded whereas in the remaining seven patients no mention is made in this regard. This study clearly indicates that the severe in vitro coagulation defect seen in these conditions does not protect from thrombosis.

Contact Activation by Pathogenic Bacteria: A Virulence Mechanism Contributing to the Pathophysiology of Sepsis

Activation of the so-called contact system has 2 major consequences; initiation of the intrinsic pathway of coagulation and the release of bradykinin, a highly potent proinflammatory peptide inducing vascular permeability and capillary leakage. Several significant human pathogens have the ability to activate the contact system, and the potential significance of this mechanism in bacterial virulence, including its role in sepsis, is discussed in this review.

Rosai-Dorfman disease with factor XII deficiency

A 17-year-old female patient presented with chronic symmetrical oligoarthritis of both knees and ankles, xerostomia, xerophthalmia, multiple bilateral lymphadenopathies in the cervical region, and bilateral parotid enlargement with the histological finding of chronic sialoadenitis. She had been already given methotrexate, chloroquine, and corticosteroids with the diagnosis of rheumatoid arthritis (RA) before referral to our outpatient clinic. Because her complaints and the lumps did not remit and she could be classified as neither RA nor primary Sjögren's syndrome (SS) according to 1987 ACR RA criteria or European preliminary criteria for SS, lymph node biopsy was repeated and revealed the diagnosis of Rosai-Dorfman disease (RDD) with the histological findings of histiocytes, phagocyting lymphocytes in enlarged sinuses, and mature plasma cells infiltrating the pulpa. All the medications were stopped after the pathological diagnosis of RDD and consulting with the Division of Hematology. She was reevaluated with magnetic resonance imaging, which showed dense infiltrative areas around knee and ankle joints, and computed tomography that showed a soft tissue mass surrounding the descending aorta and upper part of the abdominal aorta. Activated partial thromboplastin time was found to be prolonged in prebiopsy examinations, and factor XII deficiency was detected after detailed hematological evaluation. The symptoms of joint involvement were relieved with nonsteroidal antiinflammatory drugs. She has been followed-up without medication without obvious clinical or laboratory change. We herein report a patient with RDD mimicking RA and SS. We consider that RDD should be kept in mind especially in patients with resistant symptoms to conventional therapies, younger disease onset, and predominant parotid and lymph node enlargement.

Antiphospholipid antibodies: laboratory and pathogenetic aspects

Antiphospholipid antibodies (aPL) constitute a heterogeneous group of autoantibodies that share the ability to bind phospholipids (PL) alone, protein-PL complexes, or PL-binding proteins. They have been detected in isolation, in association with autoimmune diseases such as systemic lupus erythematosus (SLE), and during the course of different infections. aPL have been associated with an array of clinical manifestations in virtually every organ, although deep vein and arterial thrombosis as well as pregnancy morbidity are predominant. The co-occurrence of these clinical findings with aPL constitutes the so-called antiphospholipid syndrome (APS). aPL can be detected by immunological methods [e.g., anticardiolipin antibodies (aCL)] or by functional methods that exploit the effect of aPL on blood coagulation [lupus anticoagulant (LA)]. Since aPL are heterogeneous, numerous immunological and coagulation assays have been developed. These assays have not been fully standardized, and, therefore, problems such as high interlaboratory variation are relatively frequent. Recently, recommendations have been published regarding LA and aCL testing. Not all aPL are pathogenic. However, when they are not associated with infections, they have a role in the pathogenesis of APS. Clinical and experimental data have shown that aPL exert their pathogenic activity by interfering with the function of coagulation factors, such as thrombin and factors X, XI and XII, and with the function of anticoagulant proteins of the protein C system. In addition, aPL interaction with platelets and endothelial cells induces a pro-adhesive activated phenotype.

The Recalcified Coagulation Activity

Thrombin activity generated after plasma recalcification is of analytical and clinical interest. Fifty microliters of citrated plasma was recalcified with 5 µL of 250 mM CaCl2. After 0 to 90 minutes (37°C) 50 µl 2.5 M arginine, pH 8.6, was added. After 20 minutes, thrombin was chromogenically quantified. In normal recalcified plasma, the generated thrombin activity is about 0.1-0.2 IU/ml (37°C) when fibrin generation starts. Pooling of normal plasmas increases the generated thrombin activity about 3-fold. Plasmas of patients on heparin or coumarin generate about 10-fold less thrombin activity. Freezing of pooled plasma at −20°C and thawing at room temperature or 37°C increases thrombin generation approximately 1.5- or 2-fold, respectively. Only thrombin activities in the ascending part of the thrombin generation curve (RECA-t2/RECAt1>1) are valid. So a prothrombotic state in blood or plasma can be diagnosed.

The plasma kallikrein-kinin system and risk of cardiovascular disease in men

BACKGROUND

The plasma kallikrein-kinin system (PKKS) has been implicated in cardiovascular disease, but activation of the PKKS has not been directly probed in individuals at risk of coronary heart disease (CHD) or stroke.

OBJECTIVE

To determine the involvement of the PKKS, including factor XI, in cardiovascular disease occurring in a nested case-control study from the Second Northwick Park Heart Study (NPHS-II).

METHODS AND RESULTS

After a median follow-up of 10.7 years, 287 cases of CHD and stroke had been recorded and 542 age-matched controls were selected. When FXIIa-C1 esterase inhibitor (C1-inhibitor) concentrations were divided into tertiles (lowest tertile as reference), the odds ratios (ORs) at 95% CIs for CHD were 0.52 (0.34-0.80) in the middle tertile and 0.73 (0.49-1.09) in the highest tertile (P = 0.01 for the overall difference; P = 0.01 for CHD and stroke combined). For kallikrein-C1-inhibitor complexes, the ORs for stroke were 0.29 (0.12-0.72) and 0.67 (0.30-1.52) in the middle and high tertiles, respectively (P = 0.02). FXIIa-C1-inhibitor and kallikrein-C1-inhibitor complexes were negatively related to smoking and fibrinogen (P < 0.005). FXIa-inhibitor complexes correlated strongly with FXIIa-inhibitor complexes.

CONCLUSIONS

Lower levels of inhibitory complexes of the PKKS enzymes and particularly of FXIIa contribute to the risk of CHD and stroke in middle-aged men. This observation supports the involvement of the PKKS in atherothrombosis.

Extracellular RNA constitutes a natural procoagulant cofactor in blood coagulation

Upon vascular injury, locally controlled haemostasis prevents life-threatening blood loss and ensures wound healing. Intracellular material derived from damaged cells at these sites will become exposed to blood components and could contribute to blood coagulation and pathological thrombus formation. So far, the functional and mechanistic consequences of this concept are not understood. Here, we present in vivo and in vitro evidence that different forms of eukaryotic and prokaryotic RNA serve as promoters of blood coagulation. Extracellular RNA was found to augment (auto-)activation of proteases of the contact phase pathway of blood coagulation such as factors XII and XI, both exhibiting strong RNA binding. Moreover, administration of exogenous RNA provoked a significant procoagulant response in rabbits. In mice that underwent an arterial thrombosis model, extracellular RNA was found associated with fibrin-rich thrombi, and pretreatment with RNase (but not DNase) significantly delayed occlusive thrombus formation. Thus, extracellular RNA derived from damaged or necrotic cells particularly under pathological conditions or severe tissue damage represents the long sought natural "foreign surface" and provides a procoagulant cofactor template for the factors XII/XI-induced contact activation/amplification of blood coagulation. Extracellular RNA thereby reveals a yet unrecognized target for antithrombotic intervention, using RNase or related therapeutic strategies.

Factor XII autoantibodies as a novel marker for thrombosis and adverse obstetric history in patients with systemic lupus erythematosus

AIM

To investigate the clinical significance of anti-factor XII (FXII) in a large cohort of patients with systemic lupus erythematosus (SLE).

PATIENTS AND METHODS

This study comprised 127 patients with SLE. IgG and IgM anti-FXII were tested by an in-house ELISA. 123 healthy donors comprised the control group.

RESULTS

51 (40%) patients with SLE and 9 (7%) healthy controls were positive for anti-FXII. IgG and IgM anti-FXII were frequently found in patients with thrombosis (28% and 13%, respectively). Levels of IgG and IgM anti-FXII were higher in patients with thrombosis than in the control group (p<0.001 and p=0.005, respectively). Anti-FXII was more frequent in patients with arterial thrombosis (31% vs 4% for IgG and 14% vs 3% for IgM, respectively) and venous thrombosis than in controls (37% vs 4% for IgG). IgG anti-FXII were more frequent in patients with miscarriages and fetal death (35% and 40% vs 4% for IgM). The prevalence of IgM anti-FXII was not different between groups.

CONCLUSION

Anti-FXII are frequent in patients with SLE. Their presence is associated with thrombosis and adverse obstetric history, making these antibodies a novel marker for the antiphospholipid syndrome.

Factor XII deficiency acquired by orthotopic liver transplantation: case report and review of the literature

Transmission of congenital clotting factor deficiencies after orthotopic liver transplantation is rare. There are published reports of liver donor-to-recipient transmission of protein C deficiency with dysfibrinogenemia, protein S, factor VII and factor XI deficiencies. We report a case of transmission of factor XII deficiency with liver transplantation in a patient with Budd-Chiari syndrome. There was a persistent elevation of the activated partial thromboplastin time (aPTT), but no evidence of bleeding while the patient was maintained on warfarin. The presence of a persistently abnormal aPTT may raise suspicion for the presence of a clotting factor deficiency; however, deficiencies of other clotting factors may not be readily apparent on routine blood tests performed in a donor. Being aware of the possibilities of transmission of these inherited deficiencies of coagulation factors will aid in their early detection and management in the transplant donor and recipient.

The intrinsic coagulation activity assay

A new assay for the contact-phase-mediated generation of thrombin activity has been developed - the intrinsic coagulation activity assay (INCA). Citrated plasma (50 microl) is incubated with 5 microl SiO2, 250 mmol/l CaCl2 in polystyrole flat-bottom wells. After exactly 4 and 5 min (37 degrees C) coagulation reaction times (INCA-4 and INCA-5), 100 microl of 2.5 mol/l arginine, pH 8.6, is added to inhibit hemostasis activation in the important ascending part of the thrombin generation curve and to depolymerize fibrin. After 20 min, 50 microl of 1 mmol/l (final concentration 0.24 mmol/l) chromogenic thrombin substrate CHG-Ala-Arg-pNA in 1.25 mol/l arginine, pH 8.7, is added. The increase in absorbance is determined at 405 nm using a microtiterplate photometer. The assay is calibrated against 1 IU/ml thrombin. The normal thrombin activity range of INCA-4 (main value) or INCA-5 (control value) is 100 +/- 30% of normal (mean value +/- 1 SD; 100% = 0.5 IU/ml for INCA-4 and 1.9 IU/ml for INCA-5). With the INCA the normal range of intrinsic hemostasis is reflected, low-molecular-weight heparins can be monitored, the plasma matrix is not changed significantly, and the assay results are a percentage of normal generated thrombin activity and not coagulation seconds.

Unique donor-suitability issues

On occasion, there arise questions or situations involving blood-donor eligibility determination, which are not adequately addressed by the existing regulations and standards. In such instances, even the most experienced blood collector may be uncertain regarding the best course of action and unable to find adequate guidance in the standard blood banking references, regulations and literature. In order to examine this area in greater depth, the American Association of Blood Banks (AABB) sponsored a short topic session on 'Unique Donor Suitability Issues' at their 2004 annual meeting. The invited speakers were four seasoned physician medical directors, with a combined experience of over 40 years in blood collection at both regional and national levels. They were tasked with identifying and researching problematic areas in donor-suitability determination, and suggesting an overall approach to dealing with such issues. They determined that three of the most problematic areas of eligibility evaluation included donors with: (1) disabilities, (2) disorders of haemostasis, and (3) trans-sexual, homosexual and other unusual gender-related issues. Each of these topics was presented in a 10-min lecture, followed by an open format consisting of audience participation and panel discussion by the speakers. The session was additionally enhanced by a representative of the United States Food and Drug Administration (FDA) who participated as a member of the audience. This review presents the contents of the short topic session in an expanded form.

Small molecule inhibitors of plasma kallikrein

Plasma kallikrein is a serine protease that is involved in pathways of inflammation, complement fixation, coagulation, and fibrinolysis. Herein, we describe the SAR and structural binding modes of a series of inhibitors of plasma kallikrein as well as the pharmacokinetics of a lead analog 11 in rat.

Expression, crystallization, and three-dimensional structure of the catalytic domain of human plasma kallikrein

Plasma kallikrein is a serine protease that has many important functions, including modulation of blood pressure, complement activation, and mediation and maintenance of inflammatory responses. Although plasma kallikrein has been purified for 40 years, its structure has not been elucidated. In this report, we described two systems (Pichia pastoris and baculovirus/Sf9 cells) for expression of the protease domain of plasma kallikrein, along with the purification and high resolution crystal structures of the two recombinant forms. In the Pichia pastoris system, the protease domain was expressed as a heterogeneously glycosylated zymogen that was activated by limited trypsin digestion and treated with endoglycosidase H deglycosidase to reduce heterogeneity from the glycosylation. The resulting protein was chromatographically resolved into four components, one of which was crystallized. In the baculovirus/Sf9 system, homogeneous, crystallizable, and nonglycosylated protein was expressed after mutagenizing three asparagines (the glycosylation sites) to glutamates. When assayed against the peptide substrates, pefachrome-PK and oxidized insulin B chain, both forms of the protease domain were found to have catalytic activity similar to that of the full-length protein. Crystallization and x-ray crystal structure determination of both forms have yielded the first three-dimensional views of the catalytic domain of plasma kallikrein. The structures, determined at 1.85 A for the endoglycosidase H-deglycosylated protease domain produced from P. pastoris and at 1.40 A for the mutagenically deglycosylated form produced from Sf9 cells, show that the protease domain adopts a typical chymotrypsin-like serine protease conformation. The structural information provides insights into the biochemical and enzymatic properties of plasma kallikrein and paves the way for structure-based design of protease inhibitors that are selective either for or against plasma kallikrein.

Comparative aspects of blood coagulation

Blood coagulation is a basic physiological defense mechanism that occurs in all vertebrates to prevent blood loss following vascular injury. In all species the basic mechanism of clot formation is similar; when endothelium is damaged a complex sequence of enzymatic reactions occurs that is localized to the site of trauma and involves both activated cells and plasma proteins. The reaction sequence is initiated by the expression of tissue factor on the surface of activated cells and results in the generation of thrombin, the most important enzyme in blood clot formation. Thrombin converts soluble fibrinogen, via soluble fibrin monomers, into the insoluble fibrin that forms the matrix of a blood clot as well as exerting positive-feedback regulation that effectively promotes additional thrombin generation that facilitates the rapid development of a thrombus. Both spontaneous and trauma-induced haemorrhagic episodes can develop in all mammals with inherited or acquired abnormalities in one or more of the coagulant proteins. Experimental studies with plasma from a wide range of species have led to the conclusion that there are extensive differences in the rates of thrombin generation and fibrin formation among species. However, current evidence suggests that at least some of these quantitative differences are likely due to the use of non-species specific laboratory reagents. Although the individual proteins involved in the procoagulant pathways exhibit similar functions in all animals, differences in amino acid sequence cause incomplete homology and varying degrees of immunological cross-reactivity for the same protein across species.

Increased activity of plasma and tissue kallikreins, plasma kininase II and salivary kallikrein in pemphigus foliaceus (fogo selvagem)

BACKGROUND

Pemphigus foliaceus (PF) is an autoimmune blistering disease of unknown aetiology, which is endemic in Brazil. Although the pathogenesis of PF is still unknown, proteins of the contact system have been implicated.

OBJECTIVES

As the components of the kinin system may interact with those of the contact system, in this study we evaluated the plasma levels of high-molecular-weight kininogen (HK) and low-molecular-weight kininogen (LK), and the activity of plasma kallikrein, tissue kallikrein and kininase II in plasma of patients with PF presenting with Nikolsky's sign. As kidneys and salivary glands are relevant sources of tissue kallikrein for plasma, we also evaluated urinary/salivary kallikrein and urinary kininase II activities.

METHODS

Fifteen patients and 15 age- and sex-matched controls were studied. Kininogen levels were determined by enzyme-linked immunosorbent assay, and the activities of kallikreins and kininase II were determined using selective chromogenic substrates.

RESULTS

Compared with controls, plasma HK levels were decreased (P = 0.031), whereas the activities of plasma kallikrein, tissue kallikrein and kininase II in plasma, and the activity of salivary kallikrein, were increased in patients (P < 0.001 for each comparison). Plasma levels of LK and the activities of urinary kallikrein and urinary kininase II were not significantly different from controls.

CONCLUSIONS

Diminished levels of HK associated with increased activities of plasma kallikrein and kininase II indicate that the kinin system is activated at the systemic level in PF. As active plasma kallikreins may act on some proteins of the contact system, it is possible that the enzyme may contribute to blister formation. The further observation of an increased tissue kallikrein activity at the systemic and saliva levels may be interpreted as a systemic reflex of skin inflammation. Whether the activation of the kinin system is a cause or a consequence of blister formation needs further clarification.

Plasma levels of factor XII, prekallikrein and high molecular weight kininogen in normal blood donors and patients having suffered venous thrombosis

INTRODUCTION

The contact system proteins factor XII (FXII), prekallikrein (PK) and high molecular weight kininogen (HK) have roles in coagulation, fibrinolysis, thrombin-induced platelet activation, cell adhesion and angeogenisis. It has been suggested that inherited or acquired deficiencies of these proteins may be risk factors for thrombosis. Studies on the levels of FXII in plasma from normal and thrombotic patient populations have been reported, to our knowledge however, no systematic study on plasma levels of PK and HK in large populations of normal blood donors and patients having had venous thrombotic events has been performed.

MATERIALS AND METHODS

Chromogenic substrate assays were used to measure plasma levels of FXII, PK and HK in 300 normal blood donors (ND) and 300 patients attending our anticoagulant clinic who had a history of venous thrombosis (deep vein thrombosis or pulmonary embolism [VT]). All subjects were Caucasian, antiphospholipid antibody negative and had normal liver function.

RESULTS

Mean values +/- SD were: FXII: ND 99.4 +/- 26.7%: VT 91.0 +/- 27.2%: PKK: ND 99.7 +/- 19.8%: VT 99.1 +/- 21.2%: HK: ND 101.0 +/- 20.5%: VT 110.7 +/- 32.3%. Statistical analysis of the data revealed significantly lower (p< or =0.001) mean values for FXII and significantly higher (p< or =0.001) mean values for HK in the VT group. Calculated lower limits of normal for each parameter were: FXII: 49.1%, PKK: 66.8%, HK: 63.4%. The prevalence of values below the lower limit of normal were FXII-ND 2.3%: FXII-VT 8.0%, PKK-ND 3.0%: PKK-VT 4.7%, HK-ND 2.3%: HK-VT 5.0%. No homozygous deficiency patients were found for any parameter. One VT patient had combined FXII and HK deficiency and one ND and two VT patients had combined PK and HK deficiency.

CONCLUSIONS

FXII levels were lower and HK levels and the prevalence of FXII, PK and HK deficiency higher in a population of patients with a history of VT than in a population of healthy blood donors.

College of American Pathologists Consensus Conference XXXVI: Diagnostic Issues in Thrombophilia

OBJECTIVES

To review the state of the art relating to laboratory testing for thrombophilia, as reflected by the medical literature and the consensus opinion of recognized experts in the field, and to make recommendations regarding laboratory testing (whom to test, when to test, what tests to perform, rationale for testing, and other issues) in the assessment of thrombotic risk in individual patients and their family members.

DATA SOURCES

Review of the medical literature (primarily from the last 10 years) and the experience and opinions of experts in the field were used as data sources.

DATA EXTRACTION AND SYNTHESIS

Participating authors evaluated the medical literature and prepared manuscripts with specific proposed recommendations. Drafts of all of the manuscripts were prepared and circulated to every participant in the College of American Pathologists Conference XXXVI: Diagnostic Issues in Thrombophilia prior to the conference. Each of the conclusions and associated recommendations was then presented for discussion. Recommendations were accepted if a consensus of 70% or more of the 27 experts attending the conference was reached. The results of the discussion were then used to revise the manuscripts and recommendations into final form.

CONCLUSIONS

Consensus was reached on 179 recommendations, all of which are presented in articles in this issue of the Archives. Detailed discussion of the rationale for each of these recommendations is found in the text of the respective articles, along with citations to justify the level of evidence for the recommendations. This is an evolving area of research, and it is certain that further clinical studies will change many of the recommendations, cause some to be deleted, and add others in the future.