Early Stages of Heparin-Induced Thrombocytopenia Revealed by X-ray Crystallography
In collaboration with scientists and equipment at NYSBC’s National Synchrotron Light Source beamline (X4), researchers from the University of Pennsylvania, the Children’s Hospital of Philadelphia and Duke University used X-ray crystallography to study the mechanisms leading to heparin-induced thrombocytopenia (HIT). Their findings, which provide a basis to develop new diagnostics and therapeutics for HIT, were published on September 22 in the journal Nature Communications (Cai et al. 2015).
Patients treated with the anticoagulant heparin often develop antibodies against a complex formed by this glycosaminoglycan and the protein Platelet Factor 4 (PF4). In some cases, this autoimmune reaction escalates further, leading to activation of platelets by the autoantibodies, which in turn promotes the formation of blood clots (thrombosis) and reduces the number of circulating platelets (thrombocytopenia). It is not clear why HIT develops in only some individuals and current diagnostic tests cannot distinguish between harmless autoantibodies and those that cause HIT, leading to over-diagnosing and over-treating of patients on heparin.
How an antigenic complex forms. Heparin (red) acts as a scaffold around which PF4 tetramers (green) aggregate. Aggregates are then more likely recognized as non-self, eliciting the binding of antibodies (blue) to the PF4 tetramers, further stabilizing the large structures.
To shed light on the molecular mechanisms of HIT, Cai et al. (2015) solved the crystal structure of human PF4, which exists as a monomer, dimer or tetramer, in complex with several molecules: fondaparinux (a short, synthetic heparin molecule amenable to crystallography), the Fab fragment of KKO (an anti-PF4 mouse monoclonal antibody that mimics HIT-inducing human antibodies), and the Fab fragment of RTO (an anti-PF4 mouse monoclonal antibody that does not elicit HIT). The first structure revealed that fondaparinux binds to and stabilizes PF4 tetramers and can tether two tetramers together. With longer heparin molecules, multiple PF4 tetramers can be bridged together, leading to large complexes. The second structure showed that KKO-Fab binds to PF4 tetramers but not dimers or monomers. A model based on the first and second structures suggests that heparin and antibodies collaborate to stabilize the tetramer and therefore also the ternary heparin/PF4/antibody complex. Finally, the third structure led to the unexpected finding that, despite considerable overlap of the RTO-Fab and the KKO-Fab epitopes on PF4, the former only binds PF4 monomers and moreover, prevents the formation of tetramers. Further experiments showed that RTO inhibits the activation and aggregation of platelets caused by heparin and HIT-inducing antibodies in vitro as well as thrombosis in vivo.
Despite the potential limitations due to the use of fondaparinux instead of heparin and of mouse rather than human antibodies, this study uncovers a stepwise process that can explain how a normal protein can become the target of autoantibodies and ultimately cause HIT (see image for details) The density and close proximity of antibodies in the macro-complex that forms likely triggers activation of Fc-gamma receptors on the surface of platelets, leading to their activation and thrombus formation. Anti-PF4 antibodies that do not trigger this mechanism probably recognize conformations of the protein that are not present in the large complexes. This was the case with RTO antibodies, which only bind PF4 monomers. Interestingly, this selectivity seems to confer therapeutic properties to RTO antibodies and further studies are underway to assess their effectiveness in treating HIT.
Research article: Cai et al. 2015, Nature Communications Sep 22;6:8277.