Mechanism of Platelet Activation and Hypercoagulability by Antithymocyte Globulins (ATG)

Abstract

T cell depletion with antithymocyte globulins (ATG) can be complicated by thrombopenia and hypercoagulability. The underlying mechanism is still unclear. We found that binding of ATG to platelets caused platelet aggregation, a-granule release, membrane phosphatidylserine exposure and the rapid release of procoagulant platelet microvesicles (MV). Platelet activation and MV release were complement-dependent and required membrane insertion of C5b-8 but not stable lytic pore formation by C5b-9. ATG also activated platelets via binding to the low-affinity Fc gamma receptor FcgRII. However, only complement inhibition but not blockade of FcgRII prevented MV release and subsequent thrombin activation in plasma. In 19 hematopoietic stem cell and kidney transplant patients, ATG treatment resulted in thrombopenia and increased plasma levels of d-dimer and thrombin-antithrombin complexes. Flow cytometric analysis of complement fragments on platelet MV in patient plasma confirmed dose-dependent complement activation by ATG. However, the rapid rise in MV numbers observed in vitro was not seen during ATG treatment. In vitro experiments suggested that this was due to adherence of C3b-tagged MV to red blood cells via complement receptor CR1. These data suggest a clinically relevant link between complement activation and thrombin generation and offer a potential mechanism underlying ATG-induced hypercoagulability.

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“…Furthermore, C3 levels have been associated with sclerotic cutaneous GVHD patients (106) and patients with sclerotic GVHD have shown abnormalities in complement factor H and APC functional assays (107). Complement activation has been also linked with thrombin generation in patients treated with antithymocyte globulins (ATG) (108).…”

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“…demonstrated ATG also binds and depletes platelets in a complement dependent manner. 48 Ayuk et al. suggested that ATG also causes caspase-dependent cell death, but that this was complement-independent.…”

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“…Anti‐thymocyte globulin (ATG) is used in the management of patients with aplastic anaemia (AA) and hypoplastic myelodysplastic syndrome (H‐MDS). Administration of ATG causes significant thrombocytopenia due to non‐specific binding of platelets resulting in activation and degranulation (Cumpelik et al , ). In the recently published AA guidelines, the British Committee for Standards in Haematology (BCSH) provides guidance on ATG therapy and platelet count thresholds (Killick et al , ).…”

“…Bleeding may have been mitigated in our patients with prophylactic tranexamic acid. ATG is known to be associated with an increase in tissue factor and thrombin‐antithrombin complexes, however it has not correlated with bleeding or thromboembolic manifestations (Weber et al , ; Inbal et al , ; Cumpelik et al , ).…”