Dissecting intensive treatment as risk factor for inhibitor development in haemophilia

Thrombocytopenia and bleeding in myelosuppressed transfusion-dependent patients: a simulation study exploring underlying mechanisms

Background

Hematology–oncology patients often become severely thrombocytopenic and receive prophylactic platelet transfusions when their platelet count drops below 10×10⁹ platelets/L. This so-called “platelet count trigger” of 10×10⁹ platelets/L is recommended because currently available evidence suggests this is the critical concentration at which bleeding risk starts to increase. Yet, exposure time and lag time may have biased the results of studies on the association between platelet counts and bleeding risks.

Methods

We performed simulation studies to examine possible effects of exposure time and lag time on the findings of both randomized trials and observational data.

Results

Exposure time and lag time reduced or even reversed the association between the risk of clinically relevant bleeding and platelet counts. The frequency of platelet count measurements influenced the observed bleeding risk at a given platelet count trigger. A transfusion trigger of 10×10⁹ platelets/L resulted in a severely distorted association, which closely resembled the association reported in the literature. At triggers of 0, 5, 10, and 20×10⁹ platelets/L the observed percentages of patients experiencing bleeding were 18, 19, 19, and 18%. A trigger of 30×10⁹ platelets/L showed an observed bleeding risk of 16% and triggers of 40 and 50×10⁹ platelets/L both resulted in observed bleeding risks of 13%.

Conclusion

The results from our simulation study show how minimal exposure times and lag times may have influenced the results from previous studies on platelet counts, transfusion strategies, and bleeding risk and caution against the generally recommended universal trigger of 10×10⁹ platelets/L.

To serve and protect: The modulatory role of von Willebrand factor on factor VIII immunogenicity

Hemophilia A is a bleeding disorder characterized by the absence or dysfunction of blood coagulation factor VIII (FVIII). Patients are treated with regular infusions of FVIII concentrate. In response to treatment, approximately 30% of patients with severe hemophilia A develop inhibitory antibodies targeting FVIII. Both patient and treatment related risk factors for inhibitor development have been described. Multiple studies comparing the immunogenicity of recombinant and plasma-derived FVIII have yielded conflicting results. The randomized controlled SIPPET (Survey of Inhibitors in Plasma-Product Exposed Toddlers) trial demonstrated an increased risk of inhibitor development of recombinant FVIII when compared to von Willebrand factor (VWF)-containing plasma-derived FVIII. Presently, it is unclear which mechanism underlies the reduced immunogenicity of plasma-derived FVIII. In this review we address the potential role of VWF on FVIII immunogenicity and we discuss how VWF affects the immune recognition, processing and presentation of FVIII. We also briefly discuss the potential impact of glycan-composition on FVIII immunogenicity. It is well established that VWF shields the uptake of FVIII by antigen presenting cells. We have recently shown that VWF binds to the surface of dendritic cells. Here, we present a novel model in which surface bound FVIII-VWF complexes regulate the internalization of FVIII. Binding of FVIII to VWF is critically dependent on sulfation of Tyr1699 (HVGS numbering) in the light chain of FVIII. Incomplete sulfation of Tyr1699 has been suggested to occur in several recombinant FVIII products resulting in a loss of VWF binding. We hypothesize that this results in alternative pathways of FVIII internalization by antigen presenting cells which are not regulated by VWF. This hypothetical mechanism may explain the reduced immunogenicity of VWF containing plasma-derived FVIII concentrates as found in the SIPPET study.