Therapeutic strategies in FcγIIA receptor-dependent thrombosis and thromboinflammation as seen in heparin-induced thrombocytopenia (HIT) and vaccine-induced immune thrombocytopenia and thrombosis (VITT)

Figures & data

Figure 1. Signaling cascade events during FcγRIIA-mediated platelet activation.

(1) Immunocomplexes of PF4, heparin and anti-PF4 antibodies bind to FcγRIIA activating Src family kinases. (2) This results in the phosphorylation of Syk and (3) activation of PLCγ and PI3 kinase. (4) PLCγ catalyzes the conversion of PIP2 to IP3 and DAG. IP3 induces calcium mobilization in the cells which ultimately phosphorylates light chain of non-muscle myosin IIA through calmodulin-dependent mechanism. Simultaneously, DAG activates PKC, which in turn directly catalyzes the phosphorylation events on light chain of non-muscle myosin IIA. (5) These events prompt cytoskeletal reorganization and (6) shape changes in the cell, inducing conformational changes in integrin, resulting in the association with fibrinogen. (7) Concomitantly, activated PI3K converts PIP2 to PIP3 enabling the recruitment of (8) BTK which might induce granule secretion. (9) The exocytosis of granules, harboring PF4, calcium, and other procoagulatory agents is accomplished by the concerted efforts of calcium-dependent SNARE proteins in the plasma membrane. (10) On the activated cell membrane prothrombinase complex forms along with factor Xa and factor Va, which directly participates in the coagulation pathway generating thrombin. Fibrin is formed from fibrinogen upon the action of thrombin, eventually leading thrombus formation. (11) Furthermore, anti-PF4 antibodies are able to bind to the newly released PF4 forming immune complexes that are able to activate FcγRIIA, starting the signaling cascade following FcγRIIA again.

Figure 2. Different therapeutic interventions in HIT.

(a) Anticoagulants: either directly inhibit factor Xa or thrombin or indirectly by activating antithrombin. (b) (1) In addition to its anticoagulatory effect, danaparoid exerts a stoichiometric effect by direct interaction with PF4, thus limiting the binding of pathogenic anti-PF4/heparin antibodies to PF4. (2) Partially cleaved anti-PF4 monoclonal antibodies (e.g. deglycosylated monoclonal antibodies KKO or 5B9) directly interact with PF4, constraining the binding of pathogenic anti-PF4 antibodies to PF4. FcγRIIA-mediated activation of platelets is absent as these therapeutic antibodies do not have a functional Fc-part. (c) Therapeutic plasma exchange removes pathogenic anti-PF4/heparin antibodies from patient plasma. (d) (1) IVIG and also monoclonal antibodies directly bind to FcRn, inhibiting downstream signaling which accelerates degradation of pathogenic antibodies. However, the main effect of IVIG and DARPins is mediated by competitively inhibiting binding of the Fc-part of pathogenic immune-complexes to FcγRIIA. (2) BTKs, which are part of the FcγRIIA downstream signaling cascade can also be inhibited by pharmaceutical agents (BTK-inhibitors, e.g. ibrutinib), which in turn inhibits platelet activation.

Table 1. Treatment options for anticoagulation in HIT and VITT.

	Medication	Therapeutic mechanism	Regular monitoring	Preferred in	Not recommended	Applicable in HIT	Applicable in VITT	To be considered
Oral	Direct oral anticoagulants (DOACs): − rivaroxaban − apixaban − (edoxaban) − dabigatran	Direct FXa- inhibition direct thrombin inhibition	Not required	Stable patients	Severe renal failure	+ +	+ +	Especially suggested in patients with subacute HIT [19] and VITT [22]; Off-label-use, but widely used in clinical practice
	Vitamin K antagonists	Inhibition of vitamin K-dependent carboxylation of coagulation factors II, VII, IX, and X	Required (International normalized ratio)	Stable patients	Not to be used during the acute phase until platelet count has normalized because of Protein C depletion with subsequent risk for microvascular thrombosis and limb loss	(-)	(-)	Might be considered for long-term anticoagulation after normalization of platelet count
Subcutaneous	Fondaparinux	Synthetic pentasaccharide with antithrombin-mediated FXa inhibition	Not required but can be helpful during acute HIT to avoid underdosing	Stable patients	Severe renal failure	+ +	+ +	Long half-life ; rare possibility to cause fondaparinux-associated HIT [23–25]; Off-label-use, but widely used in clinical practice
Intravenous	Argatroban	Synthetic anticoagulant with direct thrombin inhibition	Required (activated partial thromboplastin time (aPTT), anti-IIa-activity)	Critically ill patients	Severe liver failure	+ +	+ +	Short elimination half-life: recommended in patients with high-bleeding risk and planned interventions; no renal elimination: preferred in patients with renal failure; monitoring very difficult in patients with prothrombin deficiency: risk of underdosing due to false long aPTT
	Bivalirudin	Synthetic anticoagulant with direct thrombin inhibition	Not required	Critically ill patients	Severe renal failure	++	++	Short elimination half-life
Intravenous or subcutaneous	Danaparoid	Heparansulfate, dermatan sulfate and chondroitin sulfate with anti-FXa activity, low anti-thrombin activity	Recommended until a steady state is reached (anti-Xa-activity) requires specific calibrator	All patients	High risk of bleeding or possible surgery in near future	++	++	Long elimination half-life; subcutaneous application of therapeutic dose possible (especially in non-acute HIT)
	Heparin: − unfractionated heparin (UFH) − low molecular weight heparin (LMWH)	Inhibition of FXa (UFH and LMWH) and thrombin (UFH)	Recommended for UFH (aPTT)	Both critically ill and stable patients	LMWH: severe renal failure	− Discontinuation of heparin is one of the mainstays of treatment in HIT	+ heparin inhibits binding of anti-PF4-antibodies to PF4	WHO recommends heparin in VITT when non-heparin anticoagulants are not available [26]

Cuker A, Arepally GM, Chong BH, et al. American Society of Hematology 2018 guidelines for management of venous thromboembolism: heparin-induced thrombocytopenia. Blood Adv. 2018;2(22):3360–3392. doi: 10.1182/bloodadvances.2018024489

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Carré J, Jourdi G, Gendron N, et al. Recent advances in anticoagulant treatment of immune thrombosis: a focus on direct oral anticoagulants in heparin-induced thrombocytopenia and anti-phospholipid syndrome. Int J Mol Sci. 2021;23(1):93. doi: 10.3390/ijms23010093

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WHO. Guidance for clinical case management of thrombosis with thrombocytopenia syndrome (TTS) following vaccination to prevent coronavirus disease (COVID-19). Geneva: World Health Organization; 2023. https://www.who.int/publications/i/item/9789240061989

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