The role of platelet factor 4 in platelet aggregation induced by the antibodies implicated in heparin-induced thrombocytopenia.

Heparin-induced thrombocytopenia (HIT) is a severe side effect of heparin treatment. Recent studies using immunological methods demonstrated that antibodies contained in plasma, or in purified total immunoglobulin (Ig)G from patients suffering HIT, recognize as target antigen the complex heparin/platelet factor (PF4). In the present study, the role of PF4 in in-vitro platelet aggregation induced by purified total IgG or platelet-poor plasma from patients suffering HIT was investigated. In order to demonstrate the functional role of PF4, an anti-PF4 antibody that specifically blocked PF4 was used. In an experimental system composed of washed platelet suspension, incubation of F(ab')2 fragments (0.125 microg/ml) of the polyclonal anti-PF4 antibody resulted in complete inhibition of platelet aggregation triggered by purified total IgG from patients suffering HIT and heparin. In platelet-rich plasma, a significantly higher concentration (4.25 microg/ml) of the anti-PF4 F(ab')2 was required to inhibit platelet aggregation induced by HIT-PPP and heparin. Intermediate concentrations of the anti-PF4 antibody partially inhibited platelet aggregation. In plasma milieu, the concentration of PF4 was about five-fold higher in comparison with that measured in the purified system. The intensity of platelet aggregation depended on the concentration of HIT-IgG. Platelet aggregation was abolished in the presence of high concentrations of heparin (superior or equal to 10 IU/ml). The present study shows that PF4 is essential for platelet aggregation triggered by the antibodies related to HIT in the presence of heparin. The concentration of PF4 that is available to bind with heparin or with the HIT-related antibodies is critical for platelet aggregation induced by HIT antibodies.

[Staphylokinase]. / La staphylokinase.

Myocardial infarction is due to thrombotic occlusion of a coronary artery. Current thrombolytic agents have demonstrated their major value by inducing a significant reduction of mortality, but they nevertheless present certain limits: 1) excellent arterial patency is obtained in only about 50% of cases; 2) reocclusions persist in 5 to 10% of cases; 3) very severe complications, such as cerebral haemorrhages, have not disappeared (about 0.5% of cases). There is therefore a large field of action for development of the optimal thrombolytic agent. Research concerning staphylokinase, classified among third generation thrombolytic agents, is therefore situated in this context. Staphylokinase is a protein known to possess profibrinolytic properties for more than four decades, but has been the subject of renewed interest over recent years. This article reviews the characteristics, mechanism of action and fibrinolytic properties of staphylokinase. It recalls the main results obtained in animal models and summarizes the main clinical trials in man.

[Staphylokinase and its mutants: a new generation of thrombolytic agents]. / La staphylokinase et ses mutants: agents thrombolytiques de nouvelle génération.

Staphylokinase which is extracted from Staphylococci has been known for more than 40 years as a profibrinolytic compound. It has been obtained more recently by genetic engineering. Staphylokinase activates plasminogen into plasmin. It is a promising new thrombolytic agent for the treatment of acute myocardial infarction and thromboembolic disease in general. It is a fibrin specific thrombolytic agent which does not decrease plasma fibrinogen level in treated patients. However it is antigenic and the group of Louvain has made much effort to reduce its antigenicity without decreasing its thrombolytic activity. Mutants with a reduced antigenic activity have been recently obtained. These new thrombolytic agents could be superior to currently used drugs. Staphylokinase mutants could have a better benefit/cost ratio than other thrombolytic agents.

Human anti-streptokinase antibodies induce platelet aggregation in an Fc receptor (CD32) dependent manner.

Exposure to streptokinase (SK) elicits anti-SK antibodies (Abs), which inhibit fibrinolysis and induce platelet aggregation. The mechanism of the latter is not fully understood, although it seems to involve platelet binding by a plasminogen streptokinase and anti-SK ternary complex. Anti-SK Abs were purified by affinity chromatography from serum of patients having received SK for acute myocardial infarction (AMI), and were shown to be of the IgG type. Their effects were studied with (i) human platelets in citrated plasma in the presence of SK or acetylated plasminogen-SK activator complex (APSAC), and (ii) in washed platelets, resuspended in Tyrode buffer after lowering the ionic strength, in the presence of APSAC (which provides both SK and plasminogen). An antibody concentration-response curve was obtained, showing a plateau in the presence of 0.1 mg/ml IgG. By increasing the concentration of APSAC, we obtained a unimodal response curve, the optimal concentration of APSAC being 0.05 U/ml. Aggregation was suppressed by chelating calcium with EDTA, blocking fibrinogen binding by the synthetic peptide Arg-Gly-Asp-Ser (RGDS), and raising intraplatelet cAMP with Iloprost (a prostacyclin analogue). Aggregation required the interaction of the anti-SK Ab Fc domain with the platelet Fc-gamma receptor type II, also known as CD32, since: (i) it was blocked by the monoclonal antibody IV-3 directed against CD32, (ii) it did not occur with F(ab)'2 fragments, which block the response to the intact IgG. The clinical relevance of these platelet-activating anti-SK antibodies remains to be determined.(ABSTRACT TRUNCATED AT 250 WORDS)

Original Article: Streptokinase Modifies in Vitro Platelet Aggregation by Two Mechanisms: Reduced Aggregation due to Fibrinogenolysis and Enhanced Aggregation via an Immunological Reaction.

Both inhibition and enhancement of platelet aggregation have been observed after exposure to streptokinase (SK) in vitro. Recently we have shown that inhibition of aggregation appears to be related to the fraction containing the fibrinogen degradation product, fragment E. In addition, SK may initiate platelet aggregation by a mechanism involving specific anti-SK antibodies and plasminogen. Two monoclonal antibodies (MoAbs) (PL2-49 and LeoA1) were used to assess the immunological activation of platelets in SK-induced platelet aggregation and in SK-enhanced ADP-induced platelet aggregation. The anti-SK titers in healthy volunteers' and patients' (previously treated with SK for acute myocardial infarction) plasma, were measured using a one-site non-competitive ELISA. Serum from patients was used for the purification of IgG anti-SK by affinity chromatography. We confirmed that the degree of fibrinogen degradation is a major determinant of the aggregation inhibition induced by SK. SK-induced platelet aggregation and SK-enhanced ADP-induced platelet aggregation require the interaction of the Fc domain of the anti-SK antibodies with the FcyRII located on the platelet membrane, since they are blocked by the MoAb IV-3 directed against FcyRII. Classification of the subjects according to their responses to specific MoAbs (PL2-49 and LeoA1) supports the essential role played by immunological activation of platelets in SK-induced platelet aggregation and in SK-enhanced ADP-induced platelet aggregation. The ability of anti-SK antibodies to promote SK-induced platelet aggregation and SK-enhanced ADP-induced platelet aggregation, seems to result from the interaction between two separate mechanisms: the fist mechanism is based on immunological activation of platelets and the second is related to the intervention of a defined subset of anti-SK antibodies.

In vitro effect of plasmin on human platelet function in plasma. Inhibition of aggregation caused by fibrinogenolysis.

BACKGROUND: Plasmin has been reported both to activate platelets and to inhibit platelet functions. The latter effect was thought to be caused by proteolysis of the main membrane glycoproteins. METHODS AND RESULTS: We found that incubation of citrated human platelet-rich plasma with streptokinase (SK) (300 IU/ml) does not produce any detectable activation but leads to a time-dependent inhibition of ADP-induced aggregation accompanied by substantial fibrinogenolysis. These effects were abrogated by previous addition of a plasmin inhibitor, aprotinin. Crossover experiments (SK-treated or control platelets mixed with SK-treated or control plasma) demonstrated that the platelets remained functional and that the aggregation defect was caused by fibrinogenolysis. Further experiments (addition of purified fibrinogen to fibrinogen-depleted plasma with either SK or thrombin) suggested that in addition to the low residual level of fibrinogen, fibrinogen degradation products had an inhibitory effect. Under the same conditions, tissue-type plasminogen activator (t-PA) (3,000 ng/ml) had no effect on platelet aggregation, and plasma fibrinogen was not significantly lowered. The effects on glycoproteins IIb-IIIa of incubation with SK, t-PA, or plasmin were assessed with immunoblots with murine monoclonal antibodies directed against either part of the complex, which is the receptor for fibrinogen. Proteolysis was detected only in the presence of EDTA, a potent chelator of divalent cations. CONCLUSIONS: The incubation of human platelets in citrated plasma with SK concentrations obtained during therapy leads to an aggregation defect that is related to the decrease in fibrinogen, the adhesive protein involved in this function, and to the impeding effect of fibrinogen degradation products on its binding onto platelets but not to an alteration of the corresponding platelet receptor, the heterodimer glycoproteins IIb-IIIa.