Further characterization of antibody and antigen in heparin-induced thrombocytopenia

Patients with immune heparin-induced thrombocytopenia (HIT) possess antibod ies that bind to a complex of platelet factor 4 (PF4) and heparin. We obser ved that HIT antibodies will also bind to PP4 alone adsorbed on polystyrene ELISA wells but not to soluble PF4 in the absence of heparin, Having devel oped a technique to affinity-purify anti-PF4-heparin HIT IgG, we are able t o provide the first estimates of the avidity of HIT IgG, HIT IgG displayed relatively high functional affinity for both PF4-heparin (K-d = 7-30 nM) an d polystyrene adsorbed PF4 alone (K-d = 20-70 nM). Furthermore, agarose bea ds coated with PF4 alone were almost as effective as beads coated with PF4 plus heparin in depleting HIT plasmas of anti-PF4-heparin antibodies. We co nclude that the HIT antibodies which bind to polystyrene adsorbed PF4 witho ut heparin are largely the same IgG molecules that bind PF4-heparin and the refore 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 t o heparin (optimal) or polystyrene/agarose (suboptimal) promotes recognitio n of this epitope.