Heparin-induced thrombocytopenia/thrombosis in a transgenic mouse model demonstrates the requirement for human platelet factor 4 and platelet activation through fcyriia
Heparin-induced thrombocytopenia/thrombosis (HIT/HITT) is a severe, lifethreatening complication in 3-5% of patients exposed to heparin. The factors responsible for development of the disease are heparin, platelet factor 4 (PF4), antibodies to the heparin/PF4 complex, and the platelet Fc receptor for IgG, FcyRIIA. Our goal has been to create a mouse model that recapitulates the disease process in humans in order to understand the factors that predispose some patients to development of thrombosis and to investigate therapeutic approaches. We generated transgenic mice which express human FcyRIIA on their platelets and macrophages at levels equivalent to human cells and showed that FcvRIIA expression is required to observe anti-platelet antibody-induced thrombocytopenia and thrombosis. We developed transgenic mice that express human PF4 (hPF4) in their platelets. We crossed the two lines to produce mice with both hPF4 and FcyRIIA. The FcvRIIA/hPF4 mice were injected intraperitoneally with 400 (ig of KKO, a mouse monoclonal IgG2bK antibody specific for hPF4/heparin complexes, on Day 0. Control mice were transgenic for FcyRIIA only and for hPF4 only. All mice received 20 U unfractionated heparin subcutaneously daily on Days 0-4. Platelet counts were obtained prior to antibody injection then daily while the mice received heparin. Nadir platelet counts (n=9; 0.21xlCC±0.13/1) for the FcyRIIA/hPF4 mice treated with KKO/heparin were 20% of baseline (0.99x106±0.38/iil) and were significantly different (p<0.001) from those of similarly treated mice transgenic for FcyRIIA only (n=6; nadir:0.73x 1010.19/n.l) or for hPF4 only (n=6; nadir:0.76xlO ±0.08/ll). There was no significant difference in nadir platelet counts for any of the mice treated with isotype control antibody/heparin. One of three mice that received 50 U heparin/day developed a shock-like phenotype and died. Histopathologic analysis showed fibrin precipitation in multiple organs, with thrombosis in the lung vasculature. This is the first mouse HIT model to show the severe disease found in humans and has allowed us to begin to define the determinants that contribute to thrombocytopenia and development of thrombosis in patients with HIT.
Reilly, MP; Taylor, SM; Hartman, NK; Arepally, GM; Cines, DB; Poncz, M; McKenzie, SE
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