Human Platelet Activating Antibodies

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Abstract
Platelet antibodies identified in the plasma of three multiply transfused patients and a woman who had delivered a baby with neonatal alloimmune thrombocytopenia were investigated for their platelet activating properties. Three patients possessed multispecific HLA antibodies reactive with 90 to 100% of the cells on a lymphocytotoxic panel. These antibodies were also detected using the MAIPA assay and MAb w6/32, which recognizes an epitope common to all HLA class I molecules. In addition to HLA antibodies, three of the patients possessed platelet-specific antibodies that were identified by the MAIPA assay as anti-HPA-1a and anti-HPA-3a (one patient) and anti-HPA-1b (two patients). Each of the HLA antibodies when reacted with platelets expressing the corresponding HLA antigens, potently induced aggregation and release of ATP from dense granules. In contrast, the HPA-1b antibodies induced platelet agglutination, but failed to trigger ATP release. However, platelets coated with these latter antibodies were now refractory to subsequent stimulation by ADP. Similarly, when HLA antibodies were reacted with platelets to produce suboptimal activation, the platelets could now be stimulated only poorly or not at all by either epinephrine or thrombin. This was also true for anti-HPA-1b, which, although not inducing aggregation or ATP release by itself, was capable of almost completely blocking thrombin-induced platelet activation. The thrombin-inhibiting activity of these antibodies could partially be reversed by pretreating antibody-coated platelets with epinephrine immediately followed by stimulation with thrombin. These findings suggest that transfused platelets may either be activated or inhibited by reaction with various platelet antibodies. Therefore it is conceivable that the presence of platelet reactive antibodies in multiply transfused recipients may contribute to the increased thrombotic and hemorrhagic symptoms often observed among these patients.

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