The roles of platelet membrane glycoprotein (GP) Ib in human platelet aggregation induced by non-hemolytic influenza virus (IV) were studied. The anti-GPIIIa monoclonal antibody (TM83) which had been shown to inhibit fibrinogen-binding to platelets activated by ADP and thrombin also inhibited the aggregation induced by high and low hemagglutinin (HA) titers of IV, suggesting that the aggregation is also mediated by GPIIb/IIIa complex. The anti-GPIb monoclonal antibody (TM60) inhibited only the aggregation induced by low titers (HA: 5, 120 and 7, 680) of IV, but it could not inhibit the aggregation by high titers (HA: 10, 240) of IV. Removal of N-terminal 45 kDa fragment from GPIb by treatment with elastase resulted in the complete loss of ristocetin-induced aggregation but did not cause any decrease in IV-induced aggregation. The aggregation of chymotrypsin-treated platelets induced by IV or ristocetin gradually dereased with increasing of chymotrypsin. However the platelets pretreated with 10U/ml of chymotrypsin showed complete loss of ristocetin-induced aggregation but still had IV-induced aggregation, indicating that not only GPIb but also other components exhibited binding sites for IV. Treatment of the platelets with sialidase dose-dependently inhibited both high and low titers of IV-induced aggregation but had no effect on ristocetin-induced aggregation. IV-induced aggregation of sialidase-pretreated platelets was further decreased by TM60, while that of chymotrypsin-pretreated platelets was not inhibited.
These results indicated that IV binds to not only sialic acid residue of GPIb with the highest affinity but also other sialic acid of the surface membrane with low affinity.