Summary
The aim of this study was to determine if there is a correlation between the activity
of a MoAb as an agonist and its ability to bind to the Fc platelet receptor, FcγRIIa.
A polymorphism at amino acid 131 [arginine (Arg) or histidine (His)] of FcγRIIa was
first shown to be determinant for MoAb-IgG1 binding on monocytes. To clarify the role of this polymorphism in platelet activation
by MoAb-IgG1 we (i) established the FCγRIIA polymorphism at the gene level by adapting the denaturating
gradient gel electrophoresis method, (ii) analyzed the binding affinity of the MoAbs
to FrγRIIa on platelets from homozygous Arg, homozygous His, and heterozygous Arg/His
donors, and (iii) characterized the different reactivities of platelets according
to the FCγRIIA polymorphism. Among 167 Caucasian donors we found 46% heterozygous
Arg/His, 36% homozygous His and 18% homozygous Arg. ALB6, an anti CD9, P256 an anti
GPIIb-IIIa, and AP3 an anti-GPIIIa were chosen according to their ability (ALB6, P256)
or not (AP3) to activate platelets. These 3 MoAbs-IgG1 bind to FcγRIIa with a stronger affinity for the Arg-form of FcγRIIa, a result which
was confirmed with the use of diverse MoAbs directed against various antigens. The
different abilities of MoAbs to bind to the two FcγRIIa forms were well correlated
to the different platelet responses induced by ALB6 and P256. However, low concentrations
of ALB6, which allow full activation of platelets from homozygous Arg donors, as did
P256, did not induce any activation of platelets from homozygous His donors, whereas
P256 is able to induce a low aggregation. The results further define the respective
roles of the antigen and the Fc receptor, depending on the MoAb, and the role of the
FcγRIIa polymorphism in platelet activation induced by MoAbs. In addition, the results
obtained with MoAbs unable to induce platelet activation provide evidence that the
binding of a MoAb on FcγRIIa does not predict its ability to activate platelets.
