Heparin-independent, PF4-dependent binding of HIT antibodies to platelets: implications for HIT pathogenesis.

[heparin-induced thrombocytopenia]

Antibodies specific for platelet factor 4 (PF 4 )/ heparin complexes are the hallmark of heparin-induced thrombocytopenia /thrombosis (HIT ) but many antibody-positive patients have normal platelet counts . The basis for this is not fully understood , but it is believed that antibodies testing positive in the serotonin release assay (SRA ) are most likely to cause disease . We addressed this issue by characterizing PF 4 -dependent binding of HIT antibodies to intact platelets and found that most antibodies testing positive in the SRA , but none of those testing negative , bind to and activate platelets when PF 4 is present without any requirement for heparin (p <0 .0001 ). Binding of SRA-positive antibodies to platelets was inhibited by chondroitinase ABC digestion (p <0 .05 ) and by addition of chondroitin-4 -sulfate (CS ) or heparin in excess quantities . The findings suggest that , although all HIT antibodies recognize PF 4 in a complex with heparin , only a subset recognize more subtle epitopes induced in PF 4 when it binds to CS , the major platelet glycosaminoglycan . Antibodies having this property could explain "delayed HIT " seen in some individuals after discontinuation of heparin and the high risk of thrombosis that persists for weeks in patients recovered from HIT .