Influence of Lupus Anticoagulant on a Commercially Available Kit for APC-Resistance

Interaction between Antiphospholipid Antibodies and Protein C Anticoagulant Pathway: A Narrative Review

Thrombotic antiphospholipid syndrome (APS) is a condition in which thrombosis in venous, arterial, and/or small vessels is ascribed to the presence of antiphospholipid antibodies (aPL). Among the various proposed pathogenic theories to explain thrombotic APS, those involving the interaction between aPL and the protein C system have gained much consensus. Indeed, robust data show an acquired activated protein C resistance (APC-R) in these patients. The role of aPL in this impairment is clear, but the mechanism of action is uncertain, as the type of aPL and to what extent aPL are involved remains a gray area. Lupus anticoagulant (LA) is often associated with APC-R, but antibodies generating LA comprise those directed to β2-glycoprotein I and antiphosphatidylserine/prothrombin. Moreover, the induction of APC-R by aPL requires the presence of phospholipids and is suppressed by the presence of an excess of phospholipids. How phospholipids exposed on the cell membranes work in the system in vivo is unknown. Interestingly, acquired APC-R due to aPL might explain the clinical phenotypes of thrombotic APS. Indeed, the literature reports cases of both venous and arterial thromboembolism as well as skin necrosis, the latter observed in the severe form of protein C deficiency and in catastrophic APS.

Hypercoagulable State in Patients with Advanced Gastrointestinal Cancer: Evidence for an Acquired Resistance to Activated Protein C

Aims and background

Thromboembolic complications are common in patients with cancer and represent the second cause of death in patients with overt malignant disease. The aim of this study was to investigate the activated protein C pathway in cancer.

Methods

We studied the coagulation cascade, natural clotting inhibitors, fibrinolytic proteins and resistance to activated protein C in 20 patients with advanced gastrointestinal cancer and 84 volunteers by measuring PT, APTT, fibrinogen, AT III, PC, PS, APC resistance, fibrinolytic system (PLG, ANPL, PAI-1 and t-PA) and activation peptides (D-Dimers, prothrombin 0 fragment 1+2/F1+2).

Results

Laboratory tests confirmed coagulation abnormalities in cancer patients. Fibrinogen, D-Dimers and F1+2 were increased, while t-PA activity was significantly lower than that of controls. APC resistance was higher in cancer patients compared to the control group (55% vs 2%; P < 0.0001). Excess thrombin generation was manifested by increased F1+2 plasma levels in APC-resistant cancer patients. Genetic analyses showed that only one patient with a poor response to APC carried a factor V R506Q mutation in exon 10.

Conclusions

Our findings show a high prevalence of APC resistance in cancer, compatible with an acquired defect in the APC pathway.

The Activated Protein C Resistance Phenotype of the Antiphospholipid Syndrome May Follow a Relapsing Course

Activated protein C resistance, the most frequent thrombophilic condition may be inherited or acquired; the an tiphospholipid syndrome, either primary or secondary is one of the causes of the acquired form. The case of a patient with a primary form of antiphospholipid syndrome in whom the acti vated protein C resistance phenotype was assessed in four in stances during a 3-year period is presented. In two instances the abnormal phenotype of the activated protein C resistance was present, whereas in the other two it was absent. The factor V gene (G→A, 1691) leading to the (R 506 Q) factor V mutation, typical of most inherited forms of the activated protein C re sistance, was not found. The relapsing behavior of this phe nomenon has not been previously shown. The changing pattern of this abnormality could explain discordant previous reports about the prevalence of the activated protein C resistance phe notype in patients with primary or secondary forms of the an tiphospholipid syndrome.

Lupus anticoagulant interference in activated protein C resistance testing: in vitro phenomenon or in vivo pathophysiologic effect?

Second-generation activated protein C resistance (APC-R) assay was developed to avert interferences from lupus anticoagulant (LA) and warfarin therapy by prediluting the patient sample with factor V (FV)-depleted plasma. We investigated the effect of LA on the second generation APC-R assay in 121 LA-positive patients. Twenty-five APC-R-positive patients were tested for the mutation in FV (Leiden, Hong Kong, and Cambridge). Eleven had FV Leiden and twelve were negative for any mutation (2 were not tested). Of 12, 8 had APC-R suggestive of heterozygous and 4 had APC-R suggestive of homozygous defects. These patients had strong LA activity, compared to those with concurrent FVL. This was associated with a trend toward increased thrombosis risk compared to those with normal APC-R. These findings suggest that LA causes acquired APC-R, reflecting an in vivo pathophysiologic effect of LA rather than merely an in vitro phenomenon even with the second generation APC-R assay.

Mechanisms of antiphospholipid-induced thrombosis: effects on the protein C system

An acquired resistance to activated protein C (APC) has been demonstrated in patients with antiphospholipid antibodies (aPL). Recent studies report interactions between beta2 glycoprotein I (beta2GPI) and prothrombin-binding antibodies and the protein C system. Some aPL in patients recognize one or more conformational epitopes shared by beta2GPI and catalytic domains of APC. Both beta2GPI and anti-prothrombin antibodies are associated with APC resistance. Several clinical studies have focused on an association between aPL and APC resistance, determined by classic activated partial thromboplastin time-based tests. It has been shown in most studies that APC resistance was associated with lupus anticoagulants. APC resistance is also associated with thrombosis, especially venous thromboembolism. Several recent studies have reported a novel integrated approach of coagulation using calibrated automated thrombography. This technique allows an approach of APC sensitivity without interference with lupus anticoagulants. Clinical associations between APC resistance and thromboembolic events have been demonstrated.

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.

Evaluation of a highly specific functional test for the detection of factor V Leiden

In the present study, a new functional test for the detection of increased resistance of coagulation factor V to degradation by activated protein C (factor V Leiden mutation) was evaluated. The STA-STACLOT APC-R Test (Diagnostica Stago, Asnieres, France) is based on the specific activation of factor X by Crotalus viridis helleri snake venom. The results are given as clotting time in seconds of the patient's plasma in the presence of venom and activated protein C. The intra-assay coefficient of variation was 2.17% (n=20) for samples within the normal range, and 1.70% and 1.42% (n=20) for the plasma of a heterozygous or a homozygous carrier of the factor V Leiden mutation, respectively. The inter-assay coefficient of variation (n=10) was 7.75% for the plasma of a healthy donor, 5.05% for the plasma of a heterozygous carrier and 3.38% for the plasma of a homozygous individual. The normal range (5th-95th percentile) of 136.4 s-174.7 s was derived from the clotting time of the plasma of 38 healthy controls. Values below 136 s were found in every sample from patients carrying the factor V Leiden mutation (n=52), whereas no patient with protein C (n=11) or protein S deficiency (n=10) had reduced clotting times. Homozygous carriers of the factor V Leiden mutation had clotting times shorter than 66.0 s and heterozygous carriers had clotting times longer than 80.0 s. Thus, based upon the individual clotting time, patients homozygous for factor V Leiden mutation could easily be distinguished from normals or heterozygous individuals. The influence of coagulation factor X, V, or II deficiency on the STACLOT APC-R Test was evaluated and revealed prolonged clotting times at factor V activities below 50%. In the presence of lupus anticoagulant the specificity of the STA-STACLOT APC-R Test was clearly decreased. In the present study, we clearly show that the STA-STACLOT APC-R Test is able to discriminate carriers of the factor V Leiden mutation from healthy controls or patients with protein C or protein S deficiency.

Acquired activated protein C resistance is associated with lupus anticoagulants and thrombotic events in pediatric patients with systemic lupus erythematosus

Acquired activated protein C resistance (APCR) has been hypothesized as a possible mechanism by which antiphospholipid antibodies (APLAs) cause thrombotic events (TEs). However, available evidence for an association of acquired APCR with APLAs is limited. More importantly, an association of acquired APCR with TEs has not been demonstrated. The objective of the study was to determine, in pediatric patients with systemic lupus erythematosus (SLE), whether (1) acquired APCR is associated with the presence of APLAs, (2) APCR is associated with TEs, and (3) there is an interaction between APCR and APLAs in association with TEs. A cross-sectional cohort study of 59 consecutive, nonselected children with SLE was conducted. Primary clinical outcomes were symptomatic TEs, confirmed by objective radiographic tests. Laboratory testing included lupus anticoagulants (LAs), anticardiolipin antibodies (ACLAs), APC ratio, protein S, protein C, and factor V Leiden. The results revealed that TEs occurred in 10 (17%) of 59 patients. Acquired APCR was present in 18 (31%) of 58 patients. Acquired APCR was significantly associated with the presence of LAs but not ACLAs. Acquired APCR was also significantly associated with TEs. There was significant interaction between APCR and LAs in the association with TEs. Presence of both APCR and LAs was associated with the highest risk of a TE. Protein S and protein C concentrations were not associated with the presence of APLAs, APCR, or TEs. Presence of acquired APCR is a marker identifying LA-positive patients at high risk of TEs. Acquired APCR may reflect interference of LAs with the protein C pathway that may represent a mechanism of LA-associated TEs. (Blood. 2001;97:844-849)

The Leiden mutation of coagulation factor V in Hungarian SLE patients

We studied the prevalence and the effect of coagulation factor V Leiden mutation on the occurrence of thrombotic episodes in 120 Hungarian patients having systemic lupus erythematosus (SLE) with or without antiphospholipid antibody. The frequency of the factor V Leiden mutation in Hungarian SLE patients was 13%, which is comparable with those found previously in a healthy Caucasian population. The incidence of venous thrombosis among factor V Leiden carriers has been found to be higher (odds ratio [OR] 1.7) than it is in patients without Leiden mutation (38% vs 29%). In addition, the frequency of venous thrombosis in the heterozygous SLE patients (OR 8.4 [confidence interval (CI) 0.8-83.9] P = 0.06) is dependent on the coexistence of other risk factors, such as antiphospholipid antibody. Moreover, among heterozygous factor V SLE patients, the Leiden mutation could explain the tendency to have significantly higher prevalence of fetal losses (OR 3.9 [CI 1.2-12.0] P = 0.02) and higher prevalence of cerebrovascular lesions, cardiac valvular abnormalities, and Raynaud's syndrome than that found in individuals without factor V Leiden mutation of those having antiphospholipid antibody. Systemic lupus erythematosus patients with combined defects suffer more severely from thrombosis than those with a single risk factor do, suggesting that thrombophilia is a multifactorial disorder in SLE, also. Although, the factor V Leiden mutation does not seem to be a significant risk factor for venous thrombosis in SLE, these data demonstrate that Leiden mutation can be regarded as an additive thrombogenic factor providing higher predisposition to several vasoocclusive disorders in SLE.

Mechanisms of autoantibody-mediated thrombosis

It is widely hypothesized that autoantibodies directly contribute to the prothrombotic state in the antiphospholipid syndrome (APS). The discovery that antiphospholipid autoantibodies are specific for phospholipid-binding plasma proteins (beta2-glycoprotein I, prothrombin, etc.) has allowed a much more precise investigation of the interactions of autoantibodies and antigens, and the effects of these interaction on hemostasis. Recent studies suggest that two types of interactions may be important in the pathophysiology of APS: (1) antibody cross-linking of membrane bound antigens may alter the kinetics of phospholipid-dependent reactions; and (2) antibody cross-linking of antigens bound to cell surface receptors may trigger signal transduction and cellular activation. In light of these findings, previous reports implicating various mechanisms of autoantibody-mediated thrombosis are being re-evaluated.

Activated protein C resistance phenotype in patients with antiphospholipid antibodies

The effect of antiphospholipid antibodies (aPL) on the action of activated protein C (APC) was examined in 32 patients: 19 with lupus anticoagulant (LA), 6 with anticardiolipin antibodies (aCL), and 7 with LA and aCL. Eighteen patients had a ratio of activated partial thromboplastin time (APTT) with APC to APTT without APC (APTT ratio) <2.06 (cut-off level) and no factor V Leiden mutation; these patients showed APC-resistance (APC-R) phenotype. The mean prolongation of APTT after addition of APC in a control group was 45.3 seconds, with a lower limit of 31.4 seconds. Only 3 of the 18 patients with low APTT ratio had a prolongation of <31.4 seconds; they were classified as true APC-R phenotype, whereas the other 15 patients were classified as spurious APC-R. Of the 3 patients with true APC-R, 2 had deep venous thrombosis, 1 with pulmonary embolism, and the third had recurrent abortion. Of the other 15 patients, 2 had had ischemic stroke, 1 had recurrent abortion, and 12 were asymptomatic. Circulating APC level was measured in 14 of the 18 aPL patients with a low APTT ratio; it was lower than the normal lower limit in 4 patients and within the lower limit in 2. Three of the 4 patients with reduced APC levels had a history of thrombosis. We conclude that patients with aPL who show APC-R phenotype due to a low APTT ratio without the factor V Leiden mutation can be classified into two groups: true and spurious APC-R phenotype. Since those with true APC-R phenotype could have greater thrombotic risk, adequate classification of these patients is important. Moreover, aPL can sometimes interfere with the activation of protein C, thus reducing the circulating levels of APC, and this could constitute another thrombotic risk factor.

A simple procedure that increases the specificity of the activated protein C resistance test in samples containing antiphospholipid antibodies

It is well known that plasmas with lupus anticoagulants (LA) may give false low activated protein C (APC) ratios, and these false positive tests are not necessarily corrected by mixing with factor V deficient plasma (FVdef). In the present study, we show that repeating the test after mixing 1 + 1 with pooled normal plasma (NP) instead of mixing with FVdef confirm the presence of the Leiden mutation (FV-Leiden) in patients with antiphospholipid antibodies (APA). Samples from sixteen patients with a low APC-ratio were examined. Eight samples contained APAs, including five samples with LA.

RESULTS

The APC-ratios of eleven samples, including four with APAs, became normal when retested after mixing the plasma 1 + 1 with NP. All of them were heterozygous for FV-Leiden. One additional patient, who had partial correction of the APC-ratio, proved to be homozygous for the Leiden mutation. The remaining four samples, all of which had high positive tests for APAs, remained unchanged. One of these four was heterozygous for FV-Leiden, while the other three had normal FV.

CONCLUSION

Normalisation of a low APC-ratio after dilution 1 + 1 with NP confirms the diagnosis of FV-Leiden. PCR analysis could be reserved for cases not affected by this procedure.

Evaluation of a new chronometric assay for factor V Leiden-dependent APC resistance

Since the first description by Dahlbäck et al (1) of a hereditary defect in the plasma of three unrelated thrombophilic patients characterized by a poor anticoagulant response to activated protein C (APC), many reports have shown a high prevalence of the APC resistance in both the general and thrombophilic populations (2-6). In almost all cases APC resistance is due to a single point mutation in the factor V gene, (a G to A substitution at nucleotide position 1691), which predicts the synthesis of a factor V molecule (FVQ506 or FV Leiden) in which the Arg 506 in one of the APC cleavage sites is replaced by Gln (7), rendering factor Va resistant to inactivation by APC. The FVQ506 mutation is the most frequent hereditary risk factor for venous thrombosis (2, 5, 6, 8-10) making an assay for APC resistance with a high predictive value for the mutation highly desirable, especially when molecular diagnosis at the gene level is not possible. The aim of this study was to evaluate the performance of a new factor Xa-based assay for the FV Leiden-dependent APC resistance.

Clinical significance of activated protein C resistance as a potential marker for hypercoagulable state

The activated protein C (APC)-resistance test is a simple and reliable method for detecting reduced sensitivity to the anticoagulant action of this protein. We investigated the sensitivity to APC in 180 Japanese controls and in 96 Japanese patients with venous and arterial thrombosis (28 with deep vein thrombosis; 13 with pulmonary thromboembolism; 41 with cerebral infarction; and 14 with coronary artery disease). All of the patient groups showed significantly reduced sensitivity to APC, reflected by the lower normalized APC-sensitivity ratio (n-APC-SR), as compared with healthy control. The APC-sensitivity ratio was negatively correlated with plasma activated factor VII levels. These results suggest that the low n-APC-SR is related to venous or arterial thrombotic disease. The APC resistance may serve as a potential marker for assessing the hypercoagulable state.

Anticardiolipin antibodies do not seem to be associated with APC resistance in vivo or in vitro

Anticardiolipin antibodies (aCL) or lupus anticoagulants (LA) have been found to exert an inhibitory action upon the activation and function of protein C, a natural coagulation inhibitor. Recently an in vitro phenomenon called resistance to activated protein C (APC resistance) has been described as the most frequent cause of hereditary thrombophilia. In order to see whether a positive association of APC resistance with aCL exists we examined plasma of 162 consecutive outpatients referred for thrombophilia screening. Further, the IgG fraction was isolated from plasma of two aCL-positive and LA-negative patients and of two aCL-negative healthy subjects by means of protein A affinity chromatography. Each of these isolates was mixed with normal plasma, and the APC resistance was assayed; 25/162 (15.4%) patients had confirmed abnormal APC resistance. Only 1/25 (4.0%) APC resistance-positive patients and 11/137 (8.0%) APC resistance-negative patients had positive IgG- and/or IgM-aCL (p = 0.5, nonsignificant). In the in vitro test system the APC resistance ratio remained unaffected after addition of normal IgG or aCL-IgG fraction in the tested normal plasma and did not deviate from the range of buffer controls. These data do not suggest any association of aCL with abnormal APC resistance. aCL-IgG fractions from aCL-positive and LA-negative plasmas do not interfere with the APC resistance test system in vitro in low concentration.

Resistance to activated protein C activity of an anti-beta 2-glycoprotein I antibody in the presence of beta 2-glycoprotein I

Some researchers claim that lupus anticoagulant-positive plasma may cause a false-positive reaction in the test for activated protein C (APC) resistance, a hereditary thrombophilic state characterized by abnormal factor V, which frequently causes venous thrombosis. We investigated whether anti-beta 2-glycoprotein I antibody (aGPI), which has recently come to be regarded as an anti-cardiolipin antibody (aCL) itself, might have an effect on the APC resistance test.