Prevalence, isotype, and functionality of antiheparin-platelet factor 4 antibodies in patients treated with heparin and clinically suspected for heparin-induced thrombocytopenia. The pathogenic role of IgG.

Antibodies to heparin-platelet factor 4 (PF4) complexes have been observed in patient with heparin-induced thrombocytopenia (HIT) syndrome. These antibodies may be of various isotypes and differ with respect to their ability to activate platelets/endothelial cells. This study determined the isotypes and functionality of antiheparin-platelet factor 4 (AHPF4) antibodies in 111 patients treated with heparin and clinically suspected for HIT. In this patient population, 50% had detectable AHPF4 cumulative IgA, IgG, and IgM (determined by enzyme-linked immunosorbent assay, ELISA), but only 35% was positive when tested with the (14)C-serotonin release assay (SRA). Using antihuman Ig specific for different isotypes, we found that 50% of the 111 samples was positive for IgG, 45% for IgM, and 37% for IgA. In 50 normal human serum (NHS) samples, only two were positive for IgG, but 33 were positive for IgM, indicating a potential humoral response to the heparin-PF4 complex prior to heparin administration. Patients that were ELISA(+) for AHPF4 antibody titer were subdivided into SRA-positive (+) and SRA-negative (-) groups. The SRA(+) group had a mean ELISA optical density (OD) for AHPF4 IgA/IgG/IgM of 2.1, while the SRA(-) group had a mean OD of 0.8 (P<.001). The SRA(+) group had greater mean OD values for all three individual isotypes. Using flow cytometry, we determined the ability of different patient samples to activate platelets. Samples that contained IgG and were SRA(+) activated platelets (as measured by microparticle generation and P-selectin expression) in the presence of therapeutic concentrations of heparin. NHS and samples containing IgA and/or IgM that were SRA(-) were not able to produce microparticles nor were they able to increase expression of P-selectin. Together, these data indicate that IgG is the principal mediator of platelet activation in patients with HIT, with IgA and IgM playing a less significant role in the pathophysiology of this syndrome.