Summary
To study the pathological functions of anti-phospholipid (anti-PL) antibodies, we
have analyzed their effect on platelet function. We identified an IgG anti-PL mAb,
designated PSG3, which cross-reacted specifically with glycoprotein (GP) IIIa in human
platelets and inhibited platelet aggregation. PSG3 bound also to certain polyanionic
substances, such as double-stranded DNA, heparan sulfate, dextran sulfate and acetylated-LDL,
but not to other polyanionic substances. The binding of PSG3 to GPIIIa was completely
inhibited by heparan sulfate and dextran sulfate, indicating that PSG3 recognizes
a particular array of negative charges expressed on both GPIIIa and the specified
polyanionic substances. Since neither neuraminidase- nor endoglycopeptidase F-treatment
of GPIIIa had any significant effect on the binding of PSG3, this array must be located
within the amino acid sequence of GPIIIa but not in the carbohydrate moiety. Reduction
of the disulfide bonds in GPIIIa greatly reduced its reactivity, suggesting that the
negative charges in the epitope are arranged in a particular conformation. PSG3 inhibited
platelet aggregation induced by either ADP or collagen, it also inhibited fibrinogen
binding to activated platelets in a dose-dependent fashion. PSG3, however, did not
inhibit the binding of GRGDSP peptide to activated platelets. These results suggest
that the PSG3 epitope on GPIIIa contains a particular array of negative charges, and
possibly affects the fibrinogen binding to GPIIb/IIIa complex necessary for platelet
aggregation.