|Heparin is a glycosaminoglycan (GAG) consisting of linear polymers of repeating disaccharide subunits of variable length and grade of sulphation. Platelet factor 4 (PF4) is a 70-amino acid (7780 Da), platelet-specific member of the C-X-C subfamily of chemokines (chemotactic cytokines).
Four PF4 molecules associate to form a compact tetramer, globular in structure (31,000 Da). PF4, being rich in basic amino acids - lysine and arginine, forms a ‘ring of positive charge’, providing an interface between the PF4 tetramer and heparin. PF4 is normally found only in trace levels in human plasma and is stored in platelet α-granules. Following a heparin infusion, PF4 is displaced from endothelial cell surfaces and plasma levels are increased by about 30-fold.
Heparin binds to PF4, forming a highly reactive antigenic complex on the surface of platelets and on endothelial cell surfaces. The binding of heparin/PF4 complexes to platelets is via the negative charge of the highly sulphated heparin chains. Maximal binding of heparin/PF4 complexes, and thus optimal platelet activation, occurs when PF4 and heparin are present in an optimal stoichiometric concentration, ranging from equimolar to a PF4/heparin ratio of about 2:1. Platelet activation leads to PF4 release from α-granules.
Susceptible patients then develop an antibody (IgG) to the heparin/PF4 antigenic complex. Once produced, immunoglobulins, usually IgG, bind to the heparin/PF4 immune complex on the platelet surface via membrane Fc receptors (i.e. FcγRIIa). The Fc portion of the IgG then activates the platelets by binding to platelet Fc receptors. A consequence of platelet activation by HIT-IgG is the formation of procoagulant, platelet-derived microparticles. Studies have shown increased levels of thrombin/antithrombin (TAT) complexes in plasma, suggesting increased thrombin generation. Also, the antibody-coated platelets are cleared by the reticulo-endothelial system.
Endothelial and monocyte activation also occurs. PF4 binds to heparan sulphate, a GAG found on endothelial cell surfaces. IgG and IgM classes of HIT antibodies can recognise the PF4 bound to heparan sulphate and this could focus immunoinjury to the endothelium. HIT-IgG antibodies in the presence of PF4 can activate monocytes, and these activated monocytes express tissue factor and generate procoagulant activity. This is mediated by surface proteoglycans (e.g. chondroitin sulphate) and does not require heparin for PF4 binding to monocytes.