Immune heparin-induced thrombocytopenia (HIT) is a potentially serious complication of heparin therapy and is associated with antibodies directed against a complex of platelet factor 4 (PF4) and heparin. Early diagnosis of HIT is important to reduce morbidity and mortality. I developed an enzyme immunoassay that detects the binding of HIT IgG to PF4-heparin in the fluid phase. This required techniques to purify and biotinylate PF4. The fluid phase assay produces consistently low background and can detect low levels of anti-PF4-heparin. It is suited to testing alternative anticoagulants because, unlike in an ELISA, a clearly defined amount of antigen is available for antibody binding. I was able to detect anti-PF4-heparin IgG in 93% of HIT patients. I also investigated cross-reactivity of anti-PF4-heparin antibodies with PF4 complexed to alternative heparin-like anticoagulants. Low molecular weight heparins cross-reacted with 88% of the sera from HIT patients while half of the HIT sera weakly cross-reacted with PF4-danaparoid (Orgaran). The thrombocytopenia and thrombosis of most of these patients resolved during danaparoid therapy, indicating that detection of low affinity antibodies to PF4-danaparoid by immunoassay may not be an absolute contraindication for danaparoid administration. While HIT patients possess antibodies to PF4-heparin, I observed that HIT antibodies will also bind to PF4 alone adsorbed on polystyrene ELISA wells but not to soluble PF4 in the absence of heparin. Having developed a technique to affinity-purify anti-PF4-heparin HIT IgG, I provide the first estimates of the avidity of HIT IgG. HIT IgG displayed relatively high functional affinity for both PF4-heparin (Kd=7-30nM) and polystyrene adsorbed PF4 alone (Kd=20-70nM). Furthermore, agarose beads coated with PF4 alone were almost as effective as beads coated with PF4 plus heparin in depleting HIT plasmas of anti-PF4-heparin antibodies. I conclude that the HIT antibodies which bind to polystyrene adsorbed PF4 without heparin are largely the same IgG molecules that bind PF4-heparin and thus most HIT antibodies bind epitope(s) on PF4 and not epitope(s) formed by part of a PF4 molecule and part of a heparin molecule. Binding of PF4 to heparin (optimal) or polystyrene/agarose (sub-optimal) promotes recognition of this epitope. Under conditions that are more physiological and sensitive than previous studies, I observed that affinity-purified HIT IgG will cause platelet aggregation upon the addition of heparin. Platelets activated with HIT IgG increased their release and surface expression of PF4. I quantitated the binding of affinity-purified HIT 125I-IgG to platelets as they activate in a plasma milieu. Binding of the HIT IgG was dependent upon heparin and some degree of platelet activation. Blocking the platelet Fc??? receptor-II with the monoclonal antibody IV.3 did not prevent HIT IgG binding to activated platelets. I conclude that anti-PF4-heparin IgG is the only component specific to HIT plasma that is required to induce platelet aggregation. The Fab region of HIT IgG binds to PF4-heparin that is on the surface of activated platelets. I propose that only then does the Fc portion of the bound IgG activate other platelets via the Fc receptor. My data support a dynamic model of platelet activation where released PF4 enhances further antibody binding and more release.

Immune heparin-induced thrombocytopenia (HIT) is a potentially serious complication of heparin therapy and is associated with antibodies directed against a complex of platelet factor 4 (PF4) and heparin. Early diagnosis of HIT is important to reduce morbidity and mortality. I developed an enzyme immunoassay that detects the binding of HIT IgG to PF4-heparin in the fluid phase. This required techniques to purify and biotinylate PF4. The fluid phase assay produces consistently low background and can detect low levels of anti-PF4-heparin. It is suited to testing alternative anticoagulants because, unlike in an ELISA, a clearly defined amount of antigen is available for antibody binding. I was able to detect anti-PF4-heparin IgG in 93% of HIT patients. I also investigated cross-reactivity of anti-PF4-heparin antibodies with PF4 complexed to alternative heparin-like anticoagulants. Low molecular weight heparins cross-reacted with 88% of the sera from HIT patients while half of the HIT sera weakly cross-reacted with PF4-danaparoid (Orgaran). The thrombocytopenia and thrombosis of most of these patients resolved during danaparoid therapy, indicating that detection of low affinity antibodies to PF4-danaparoid by immunoassay may not be an absolute contraindication for danaparoid administration. While HIT patients possess antibodies to PF4-heparin, I observed that HIT antibodies will also bind to PF4 alone adsorbed on polystyrene ELISA wells but not to soluble PF4 in the absence of heparin. Having developed a technique to affinity-purify anti-PF4-heparin HIT IgG, I provide the first estimates of the avidity of HIT IgG. HIT IgG displayed relatively high functional affinity for both PF4-heparin (Kd=7-30nM) and polystyrene adsorbed PF4 alone (Kd=20-70nM). Furthermore, agarose beads coated with PF4 alone were almost as effective as beads coated with PF4 plus heparin in depleting HIT plasmas of anti-PF4-heparin antibodies. I conclude that the HIT antibodies which bind to polystyrene adsorbed PF4 without heparin are largely the same IgG molecules that bind PF4-heparin and thus most HIT antibodies bind epitope(s) on PF4 and not epitope(s) formed by part of a PF4 molecule and part of a heparin molecule. Binding of PF4 to heparin (optimal) or polystyrene/agarose (sub-optimal) promotes recognition of this epitope. Under conditions that are more physiological and sensitive than previous studies, I observed that affinity-purified HIT IgG will cause platelet aggregation upon the addition of heparin. Platelets activated with HIT IgG increased their release and surface expression of PF4. I quantitated the binding of affinity-purified HIT 125I-IgG to platelets as they activate in a plasma milieu. Binding of the HIT IgG was dependent upon heparin and some degree of platelet activation. Blocking the platelet Fc??? receptor-II with the monoclonal antibody IV.3 did not prevent HIT IgG binding to activated platelets. I conclude that anti-PF4-heparin IgG is the only component specific to HIT plasma that is required to induce platelet aggregation. The Fab region of HIT IgG binds to PF4-heparin that is on the surface of activated platelets. I propose that only then does the Fc portion of the bound IgG activate other platelets via the Fc receptor. My data support a dynamic model of platelet activation where released PF4 enhances further antibody binding and more release.