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.

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.

SummaryExposure 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. Two factors might influence clinical outcome: (i) the amount and type of pre-existing anti-SK Abs; (ii) the known interindividual variability of the platelet response to binding and activation by IgG involving the CD32 molecule.