S Lévy-Toledano

The monoclonal antibody ALB6 directed against the leukocyte differentiation antigen CD9 (p24) increases the calcium incorporation into isolated platelet membrane vesicles enriched in internal membranes. The similarities of the effects of both the monoclonal antibody and the catalytic subunit of the cAMP-dependent protein kinase (C, subunit), which phosphorylates a protein of an apparent molecular mass of 23 kDa, led us to investigate the relationship between CD9 (p24) and the 23-kDa phosphoprotein (p23). ALB6IgG does not inhibit the C.subunit-induced phosphorylation of p23 and the immunoadsorption by ALB6IgG of p24 associated to membrane vesicles does not alter the phosphorylation pattern. Thus, proteins of similar molecular mass appear to be involved in calcium fluxes: one is recognized by the ALB6 antibody while the other can be phosphorylated by the C-subunit.

A large number of similarities have previously been noted between the blood and milk clotting phenomena [Jollès, P. (1975) Mol. Cell. Biochem. 7, 73-85; Jollès, P. & Henschen, A. (1982) Trends Biochem. Sci. 7, 325-328]: some analogous features have also been found between fibrinogen and kappa-casein. In this connection, the effect of a natural and a synthetic peptide derived from kappa-casein on platelet function was studied: the undecapeptide Met-Ala-Ile-Pro-Pro-Lys-Lys-Asn-Gln-Asp-Lys (residues 106----116 of cow kappa-casein) inhibited both aggregation of ADP-treated platelets and binding of 125I-fibrinogen to ADP-treated platelets: its behaviour was similar to that of the structurally related C-terminal dodecapeptide of human fibrinogen gamma-chain.

A decreased platelet adhesion to rabbit aorta subendothelium (Baumgartner technique) is confirmed in the Bernard Soulier (giant platelet) syndrome. Electron microscope techniques using a purified antibody against Factor VIII/von Willebrand protein, revealed an apparently normal presence of the Factor VIII/von Willebrand protein on the Bernard Soulier platelets. Electrophoretic characterization of the major protein and glycoprotein components of the Bernard Soulier platelets following sodium dodecyl sulfate solubilization indicated a relatively normal protein content but suggested a reduced content of the 155,000 molecular weight major platelet glycoprotein. This was confirmed by a reduced release of high molecular weight acidic glycopeptides following incubation of washed Bernard Soulier platelets with trypsin. It is proposed that this abnormality may be related to the previously reported reduced sialic acid content and the reduced electrophoretic mobility of the Bernard Soulier platelets and that a glycoprotein reduced or abnormal in the Bernard Soulier platelets is necessary for the normal adhesion of platelets to subendothelium.

In a group of 6 patients with lupus anticoagulant (LA) and antiphospholipid (aPL) antibodies detected by ELISA overnight urine and blood were simultaneously collected. A significantly increased urinary excretion of the platelet-derived thromboxane (TX) metabolite 11-dehydro-TXB2 was found in this group, as compared to 12 healthy individuals. In contrast, a small but significant reduction of the vascular prostacyclin (PGI2) metabolite 2,3-dinor-6-keto-prostaglandin F1 alpha was observed. To further elucidate the effect of these antibodies on platelet activation we isolated the F(ab')2 fragments from IgG of the 6 patients and 5 controls, and we evaluated the effect of these fragments on the responses of isolated normal platelets to thrombin. Patients' F(ab')2 increased platelet aggregation and serotonin release of platelets stimulated by low dose thrombin (0.01 U/ml). At threshold thrombin concentration (0.05 U/ml) an enhanced TXB2 production was also observed. In summary, our results show, in addition to the altered TXA2/PGI2 balance observed in vivo, a direct stimulatory effect of aPL antibodies on platelet activation in vitro. This effect is related to recognition of phospholipid epitopes on platelets as shown by its neutralization upon preincubation with phospholipids. This phenomenon may be relevant for the thrombotic tendency of these patients.