Atomic features of an autoantigen in heparin-induced thrombocytopenia (HIT)

Abstract

Autoantigen development is poorly understood at the atomic level. Heparin-induced thrombocytopenia (HIT) is an autoimmune thrombotic disorder caused by antibodies to an antigen composed of platelet factor 4 (PF4) and heparin or cellular glycosaminoglycans (GAGs). In solution, PF4 exists as an equilibrium among monomers, dimers and tetramers. Structural studies of these interacting components helped delineate a multi-step process involved in the pathogenesis of HIT. First, heparin binds to the 'closed' end of the PF4 tetramer and stabilizes its conformation; exposing the 'open' end. Second, PF4 arrays along heparin/GAG chains, which approximate tetramers, form large antigenic complexes that enhance antibody avidity. Third, pathogenic HIT antibodies bind to the 'open' end of stabilized PF4 tetramers to form an IgG/PF4/heparin ternary immune complex and also to propagate the formation of 'ultralarge immune complexes' (ULCs) that contain multiple IgG antibodies. Fourth, ULCs signal through FcγRIIA receptors, activating platelets and monocytes directly and generating thrombin, which transactivates hematopoietic and endothelial cells. A non-pathogenic anti-PF4 antibody prevents tetramer formation, binding of pathogenic antibody, platelet activation and thrombosis, providing a new approach to manage HIT. An improved understanding of the pathogenesis of HIT may lead to novel diagnostics and therapeutics for this autoimmune disease.

Keywords: Autoimmunity; FcγRIIA; Heparin-induced thrombocytopenia; Immune complex structure; Pathogenesis.

Figure 1

Model of step-wise pathogenesis of HIT. Step 0, PF4 (green circles) released from activated platelets exists as an equilibrium among tetramers, dimers and monomers in plasma or whole blood. Tetramers may be transient and may switch between ‘open’-‘closed’ and ‘closed’-‘open’ conformations. Step 1, Heparin or GAGs (orange circles) bind to the ‘closed’ end of PF4 and stabilize the PF4 tetramers. Based on steric contraints, it is not possible to for a similar switch in conformations to occur in heparin/GAG-bound tetramers. Step 2, Binding of the first tetramer enhances the binding and approximation of additional tetramers, which eventuates in the formation of large PF4/heparin antigenic complexes. Step 3, Binding of PF4 tetramers also enhances binding of pathogenic antibodies (black) to the ‘open’ end of tetramer, forming ultralarge IgG/PF4/heparin immune complexes. Step 4, immune complexes activate platelets and monocytes though FcγRIIA’s and through expression of tissue factor, which leads to thrombin-dependent transactivation of platelets and endothelial cells.