Heparin-Induced Thrombocytopenia and Thrombosis: Therapeutic Strategy Using a Single-Chain Variable Fragment (scFv) Antibody

Heparin-Induced Thrombocytopenia and Thrombosis (HIT) is a life threatening disorder that affects 1–5% of patients receiving heparin therapy. A low platelet count is usually recorded (<150,000 per cubic millimetre) with a decrease of 50% or more from the baseline. The occurrence of HIT is due to the presence of an IgG antibody that recognizes the immune complex formed between Platelet Factor 4 (PF4) and heparin. The antibody/PF4/Heparin complex binds to the FcγRIIa receptor on platelets, leading to platelet activation and thrombotic complications in patients receiving heparin. IV.3 is a murine monoclonal antibody that was raised against the FcγRIIa receptor and has been used as an inhibitor in specificity assays to confirm HIT in patients. We have developed a humanized single-chain variable fragment (scFv) antibody based on the IV.3 monoclonal antibody that binds to the FcγRIIa receptor on platelets and prevents platelet aggregation induced by HIT antibodies.

The variable heavy chain (VH) and light chain (VL) of the IV.3 antigen binding fragment (Fab) moiety were amplified using polymerase chain reaction (PCR). These two fragments were then coupled with a linker (Glycine₄ and Serine)₆. This was followed by introduction of several components including fusion tags (FLAG and c-Myc) at both termini for cloning, detection and purification purposes. The construct was transformed into E. coli (strain-BL21) for protein expression of the scFv. The presence of the protein was detected via immunostaining using anti-FLAG and anti-c-Myc antibodies. The scFv was purified by affinity chromatography and the binding activity was detected using flow cytometry and confocal microscopy. The functional activity was determined using Platelet Aggregation Assay. The scFv was then humanized to minimize potential immunogenicity. Humanization was achieved by introducing specific mutations that rendered the molecule human-like but did not affect binding specificity. The humanized scFv was also expressed in E. coli, purified and tested as before.

The scFv protein (32kDa) was expressed, purified and confirmed via immunostaining. The created humanized scFv exhibits binding activity against the FcγRIIa on human platelets as determined by flow cytometry and confocal microscopy. In addition, the protein successfully inhibits platelet aggregation at micro molar concentrations in aggregation assays conducted in vitro in the presence of HIT antibodies.

The humanized scFv was successful in recapitulating the properties of the IV.3 murine monoclonal antibody. It demonstrated binding activity against the FcgRIIa on human platelets and exhibited functional activity by inhibiting platelet activation and aggregation in vitro. This implies that our scFv is able to stop binding of the antibody/PF4/Heparin immune complex to platelets, thus hindering one of the critical initial steps in HIT. The scFv described here may be able to ameliorate the unwanted side effects of heparin therapy and could serve as a potential therapeutic drug for HIT patients.

No relevant conflicts of interest to declare.