Experimental Models of Anti-PF4 Immunothrombosis: Mechanistic Insights, Translational Value, and Unmet Needs.

Antibody-mediated immune responses against platelet factor 4 (PF4) underlie a spectrum of rare but severe thromboinflammatory disorders, including heparin-induced thrombocytopenia (HIT), autoimmune HIT variants, vaccine-induced immune thrombocytopenia and thrombosis (VITT), and VITT-like syndromes. Despite differences in triggering factors and antibody-binding requirements, these conditions share a common pathogenic mechanism in which anti-PF4-IgG immune complexes engage platelet FcγRIIa, thereby triggering platelet activation, procoagulant transformation, and amplification of immunothrombosis. Experimental models have been instrumental in defining the molecular and cellular basis of anti-PF4 immunothrombosis. Platelet-centric in vitro assays and whole-blood systems established FcγRIIa-dependent platelet activation as a central effector mechanism and revealed functional heterogeneity among anti-PF4 antibodies. Flow-based vascular models further incorporate shear stress and endothelial interfaces, enabling analysis of thrombus formation under near-physiological conditions. In vivo, passive antibody transfer models in humanized mice expressing human PF4 and FcγRIIa reproduce key clinical features, including thrombocytopenia and thrombosis, and have enabled mechanistic and therapeutic studies. Despite these advances, important conceptual gaps remain. Most existing models focus on the effector phase of disease. Conversely, active immunization models can induce anti-PF4 antibodies but rarely reproduce the downstream thrombotic phenotype. As a result, the mechanisms linking antigen formation, innate immune activation, B cell recruitment, and acquisition of pathogenic antibody function remain incompletely understood. Developing integrated experimental systems that connect immune initiation with FcγRIIa-dependent effector pathways will be essential to advance mechanistic insight, improve therapeutic strategies, and establish predictive preclinical models to assess the risk of anti-PF4 antibody induction during vaccine development.