Anti-PF4 disorders: Pathogenesis, diagnosis and treatment

Abstract

Platelet factor 4 (PF4) is a cationic protein, able to form complexes with negatively charged molecules upon its self-assembly into PF4 tetramers. The targeting of these PF4 complexes by immunoglobulin G (IgG) antibodies underlies anti-PF4 disorders such as heparin-induced thrombocytopenia (HIT) and Vaccine-Induced Immune Thrombocytopenia and Thrombosis (VITT)/VITT-like disorders. The formation of IgG/PF4 immune complexes facilitates uncontrolled activation of platelets, neutrophils and monocytes, via IgG-mediated Fcγ receptor binding. This promotes the thrombocytopenia and thrombosis characteristic of anti-PF4 disorders. HIT is predominantly triggered by heparin exposure. VITT is a recently recognised anti-PF4 disorder, which developed following specific SARS-CoV-2 vaccinations. It is thought that hexon proteins, components of adenoviral vectors, may form complexes with PF4 to trigger anti-PF4 antibody production in VITT. A novel anti-PF4 disorder has been recognised causing platelet activation without the administration of heparin or SARS-CoV-2 vaccination and referred to as 'VITT-like disorder.' Clinical evaluation of HIT and VITT/VITT-like disorders is based on thrombotic events, platelet counts and D-dimer levels. Laboratory assays such as heparin/PF4-induced platelet activation assays can be used to distinguish between HIT and VITT. Treatment plans for HIT and VITT may differ across patient groups. In this review, we discuss the pathogenesis, diagnosis and management of anti-PF4 disorders.

Keywords: heparin; heparin‐induced thrombocytopenia; immunoglobulin G; platelet factor 4; platelet‐activating antibodies; thrombosis; vaccine‐induced immune thrombocytopenia and thrombosis.

FIGURE 1

Flow diagram classifying HIT, VITT and VITT‐like syndromes by anti‐PF4 antibodies and heparin dependence. cHIT occurs with heparin exposure and involves the production of heparin‐dependent (anti‐PF4/H) antibodies. In aHIT, which can occur with or without proximate exposure, both heparin‐dependent and heparin‐independent antibodies may be produced. aHIT can be classified into four subtypes: delayed‐onset HIT, persistent HIT, HF HIT and fondaparinux‐associated HIT. Spontaneous HIT exclusively involves the production of heparin‐independent antibodies, occurring in the absence of heparin. VITT, involving heparin‐independent antibodies, has been shown to arise post‐vaccination (predominantly associated with the SARS‐CoV‐2 vaccination programmes). Heparin‐independent antibody production, despite no heparin exposure or vaccination, but could be following adenovirus/other viral infection is termed as a VITT‐like syndrome. Abs, antibodies; aHIT, autoimmune heparin‐induced thrombocytopenia; cHIT, classic heparin‐induced thrombocytopenia; HF, heparin ‘flush’; HIT, heparin‐induced thrombocytopenia; VITT, vaccine‐induced thrombocytopenia and thrombosis.

FIGURE 2

Formation of PF4/IgG immune complexes in HIT and PF4's charge approximation model. Panel (A) PF4 tetramer structure and charge approximation process. PF4 tetramers lose their intermolecular distance as negatively charged polyanions bind to the positively charged PF4 tetramer, neutralising its charge and inducing conformational changes. This exposes neoepitopes, leading to the formation of anti‐PF4/polyanion antibodies, a key step in the pathogenesis of heparin‐induced thrombocytopenia (HIT). This enables the formation of PF4/heparin/IgG complexes leading to platelet activation and thrombosis. Panel (B) PF4 immune complex forms with heparin‐dependent and/or heparin‐independent IgGs, depending on HIT subtype. Heparin‐dependent IgG binds to antigen binding sites on the exterior of the tetramer. HIT‐independent IgG binds to N‐terminal lysines in the heparin binding antigen site. Adapted from [6]. aHIT, autoimmune heparin‐induced thrombocytopenia; cHIT, classic heparin‐induced thrombocytopenia; IgG, immunoglobulin G; PF4, platelet factor 4; VITT, vaccine‐induced thrombocytopenia and thrombosis.

FIGURE 3

IgG‐FcγRIIa receptor binding is essential for the pathogenesis of thrombosis in HIT and VITT anti‐PF4 disorders. Panel (A) Platelet factor 4 (PF4), released from alpha granules of activated platelets, binds to heparin, forming PF4–heparin complexes. IgG antibodies recognise and bind these complexes, forming PF4–heparin–IgG multimolecular immune complexes. These immune complexes engage platelet FcγRIIa receptors, triggering platelet activation, further PF4 release and amplification of the prothrombotic cascade. This process leads to the release of procoagulant microparticles, platelet aggregation and potential thrombosis. Panel (B) PF4 binds to the adenoviral vector, forming PF4–viral complexes. IgG antibodies recognise these complexes, leading to platelet activation through FcγRIIa receptor engagement. This activation promotes the release of procoagulant microparticles, platelet aggregation and creates a prothrombotic state., , , FcγRIIa receptor, Fc gamma receptor IIa; IgG, immunoglobulin G; PF4, platelet factor 4.

FIGURE 4

A flow chart illustrating the proposed management and treatment of PF4 disorders HIT and VITT. HIT management involves discontinuation of all forms of heparin and administration of an alternative anticoagulant, with transition to DOACs upon platelet count recovery. VITT management varies depending on the platelet count status and can be treated with heparin or non‐heparin anticoagulant., , , , , , , , DOAC, direct oral anticoagulant; HIT, heparin‐induced thrombocytopenia; IVIG, intravenous immunoglobulin; PEx, plasma exchange; PF4, platelet factor 4; VITT, vaccine‐induced thrombocytopenia and thrombosis.