Complement activation as a biomarker for platelet activating antibodies in heparin-induced thrombocytopenia (HIT)

Abstract

Background: IgG antibodies (Abs) to platelet factor 4 complexed to heparin (PF4/H) commonly occur after heparin exposure but cause life-threatening complications of heparin-induced thrombocytopenia (HIT) in only a few patients. Presently, only platelet activation assays reliably distinguish anti-PF4/H Abs that cause disease (HIT Abs) from those likely to be asymptomatic (AAbs).

Objectives: Recent studies indicate that complement activation is an important serologic property of HIT Abs and is essential for FcγRIIA-mediated cellular activation. As platelet activation by HIT Abs also rely on FcγRIIA activation, we correlated the complement- and platelet activating properties of anti-PF4/H Abs in a clinically annotated patient cohort.

Patients/methods: Clinical and laboratory features of patients with HIT (n=8) and asymptomatic anti-PF4/H Abs (AAb+, n=14) were correlated with properties of complement, platelet and monocyte/neutrophil activation.

Results: Expected clinical and laboratory differences were seen between HIT and AAb+ patients, with HIT patients having lower mean platelet counts, greater % drop in platelet counts, higher 4T and HEP scores, higher anti-PF4 polyclonal and IgG Ab levels, and SRA positivity. Ex vivo assays revealed significant differences in complement activation by HIT versus AAb+ patients, with the extent of complement activation closely correlated with percent serotonin release by anti-PF4/H Abs, and MMP9 and IL-8 release in whole blood.

Conclusions: These findings suggest that complement activation strongly correlates with cellular activation endpoints, including platelet and monocyte/neutrophil activation and if confirmed in a larger prospective study, may serve as a "functional" biomarker for pathogenic HIT Abs.

Keywords: antibodies; complement; heparin-induced thrombocytopenia (HIT); platelet factor 4 (PF4); platelets; serotonin release assay (SRA).