Summary
Platelet agonists and RGD-containing peptides can convert platelet membrane glycoprotein
(GP) Ilb-IIIa from its resting state to an activated state competent to bind soluble
fibrinogen. We examined the effects of two anti-GPIIb-IIIa monoclonal antibodies,
PMA1 and PMA5, on fibrinogen binding to agonist- and RGD-activated GPIIb-IIIa. PMA1
abolished aggregation of both agonist- and RGDS peptide-activated fixed platelets,
and inhibited the binding of 125I-fibrinogen to these platelets almost completely.
PMA5 had the same effects on agonist-activated platelets, but had little effect on
the aggregation of RGDS-activated fixed platelets, and inhibited fibrinogen binding
to RGDS-activated fixed platelets by only 44%. PMA5 bound to agonist- and RGDS-activated
platelets equally. Immunoblot analysis showed that PMA5 bound to intact GPIIIa, but
not to a 66 kDa fragment of GPIIIa digested by chymotrypsin. Although PMA5 inhibited
platelet adhesion to immobilized fibrinogen by 94%, 44% of the remaining adherent
platelets were spread. In contrast, no platelet spreading was observed in the presence
of PMA1. These findings indicate that PMA5 is a novel anti-GPIIIa monoclonal antibody
with the ability to inhibit fibrinogen binding to agonist- and RGD-activated states
of GPIIb-IIIa differentially, and suggest that binding of immobilized fibrinogen to
RGD-activated GPIIb-IIIa is necessary for platelet spreading.
