SummaryDuring storage, erythrocytes undergo changes that alter their clearance and function after transfusion and there is increasing evidence that these changes contribute to the complications observed in transfused patients. Stored erythrocytes were incubated overnight at 37°C to mimic the temperature after transfusion. After incubation, several markers for erythrocyte damage were analysed. After overnight incubation, stored erythrocytes showed increased potassium leakage, haemolysis, PS exposure and vesicle formation, and all these effects increased with increasing storage time. Furthermore, we demonstrated that long-term stored erythrocytes develop decreased flippase activity and increased scrambling activity after overnight incubation, leading to PS exposure and the release of vesicles. Reduced intracellular potassium was identified as the cause of the decreased flippase activity. Lastly, we provide evidence that erythrocytes can return to a PSnegative state by shedding parts of their membrane as PS-containing vesicles and that these vesicles can serve as a platform for the coagulation cascade. These findings reveal that potassium leakage, a well-known phenomenon of prolonged erythrocyte storage, primes erythrocytes for PS exposure. PS exposure will lead to vesicle formation and might have an important impact on the post-transfusion function and side effects of stored erythrocytes.

Background: Antibody KM33 blocks factor VIII (FVIII) endocytosis and phospholipid binding. Results: Hydrogen-deuterium exchange mass spectrometry reveals that KM33 binds C1 domain spikes 2092-2093 and 2158 -2159. Glycosylated FVIII-R2159N shows reduced endocytosis and decreased binding to phospholipid membranes with low phosphatidylserine content. Conclusion: Spikes 2092-2093 and 2158 -2159 modulate FVIII endocytosis and phospholipid binding. Significance: Novel insight is obtained about the role of the C1 domain for FVIII biology. The C1 domain of factor VIII (FVIII) has been implicated in binding to multiple constituents, including phospholipids, vonWillebrand factor, and low-density lipoprotein receptor-related protein (LRP). We have previously described a human monoclonal antibody called KM33 that blocks these interactions as well as cellular uptake by LRP-expressing cells. To unambiguously identify the apparent "hot spot" on FVIII to which this antibody binds, we have employed hydrogen-deuterium exchange mass spectrometry. The results showed that KM33 protects FVIII regions 2091-2104 and 2157-2162 from hydrogen-deuterium exchange. These comprise the two C1 domain spikes 2092-2093 and 2158 -2159. Spike 2092-2093 has been demonstrated recently to contribute to assembly with lipid membranes with low phosphatidylserine (PS) content. Therefore, spike 2158 -2159 might serve a similar role. This was assessed by replacement of Arg-2159 for Asn, which introduces a motif for N-linked glycosylation. Binding studies revealed that the purified, glycosylated R2159N variant had lost its interaction with antibody KM33 but retained substantial binding to von Willebrand factor and LRP. Cellular uptake of the R2159N variant was reduced both by LRP-expressing U87-MG cells and by human monocytederived dendritic cells. FVIII activity was virtually normal on membranes containing 15% PS but reduced at low PS content. These findings suggest that the C1 domain spikes 2092-2093 and 2158 -2159 together modulate FVIII membrane assembly by a subtle, PS-dependent mechanism. These findings contribute evidence in favor of an increasingly important role of the C1 domain in FVIII biology.Activated coagulation factor VIII (FVIIIa) is a cofactor that assembles with activated factor IX (FIXa) 3 on lipid membranes that expose phosphatidylserine (PS) in the outer leaflet (1). This complex effectively generates activated factor X, ultimately leading to blood clot formation at sites of vascular injury. Current treatment of hemophilia A patients, who lack functional factor VIII (FVIII), involves intravenous infusion with either recombinant or plasma-derived FVIII (2). This treatment is, however, limited by the particularly effective clearance of FVIII from the circulation. Moreover, about 20% of patients develop antibodies against FVIII (3). The molecular determinants that drive the assembly of FVIII with membranes, including those of cells involved in clearance and immune response, remain incompletely understood.FVIII is a multidomain protein that ...

It has been proposed that von Willebrand factor might affect factor VIII immunogenicity by reducing factor VIII uptake by antigen presenting cells. Here we investigate the interaction of recombinant von Willebrand factor with immature monocyte-derived dendritic cells using flow cytometry and confocal microscopy. Surprisingly, von Willebrand factor was not internalized by immature dendritic cells, but remained bound to the cell surface. As von Willebrand factor reduces the uptake of factor VIII, we investigated the repertoire of factor VIII presented peptides when in complex with von Willebrand factor. Interestingly, factor VIII-derived peptides were still abundantly presented on major histocompatibility complex class II molecules, even though a reduction of factor VIII uptake by immature dendritic cells was observed. Inspection of peptide profiles from 5 different donors showed that different core factor VIII peptide sequences were presented upon incubation with factor VIII/von Willebrand factor complex when compared to factor VIII alone. No von Willebrand factor peptides were detected when immature dendritic cells were pulsed with different concentrations of von Willebrand factor, confirming lack of von Willebrand factor endocytosis. Several von Willebrand factor derived peptides were recovered when cells were pulsed with von Willebrand factor/factor VIII complex, suggesting that factor VIII promotes endocytosis of small amounts of von Willebrand factor by immature dendritic cells. Taken together, our results establish that von Willebrand factor is poorly internalized by immature dendritic cells. We also show that von Willebrand factor modulates the internalization and presentation of factor VIII-derived peptides on major histocompatibility complex class II.

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Background: Mim8 is a novel, next-generation factor VIIIa mimetic in development for subcutaneous prophylactic treatment of patients with hemophilia A with and without inhibitors. In vitro and in vivo models indicate that Mim8 has a distinct hemostatic potential.Objectives: To test the nonclinical safety and pharmacodynamics of Mim8. Methods:The Mim8 nonclinical safety program in cynomolgus monkeys consisted of three studies of 4-26 weeks in duration with Mim8 doses ranging from 0.3-60 mg/ kg/week intravenously or subcutaneously. After sacrifice, macroscopic and microscopic pathological examinations were performed.Results: Mim8 was well tolerated with no noteworthy clinical observations. No signs of excessive coagulation or pathological macroscopic or microscopic findings were observed at doses 0.3-3 mg/kg/week subcutaneous. Thrombosis-related findings were detected during histopathological examination in a small proportion of animals (16%) receiving doses ranging 6-20 mg/kg/week. Dose-dependent increases in factor X (FX) and factor IX (FIX) concentrations were observed. Shortening of activated partial thromboplastin time (APTT) and increased thrombin generation under ex vivo hemophilia A-like conditions were observed at all Mim8 dose levels.Conclusions: Thrombosis-related findings observed at doses above 6 mg/kg/week Mim8 may have been exaggerated pharmacological reactions to a procoagulant compound in normocoagulant animals. Increases in FX and FIX concentrations could be because of a half-life prolongation due to binding to Mim8, but were limited at clinically relevant exposure levels. Subcutaneous administration of up to 3 mg/kg/week (several fold greater than expected clinical exposure) for 26 weeks resulted in relevant

Background: von Willebrand factor (VWF) protects factor VIII (FVIII) from rapid clearance and degradation. Results: Mass spectrometric footprinting revealed that FVIII protects Lys-773 and the N-terminal Ser-764 of VWF from chemical modification. VWF(S764A) showed increased and VWF(K773A) showed decreased FVIII binding. Conclusion:The N terminus of VWF is critical for FVIII binding. Significance: This study sheds new light on the mechanism of FVIII-VWF complex assembly.

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Factor VIII C-domains are believed to have specific functions in cofactor activity and in interactions with von Willebrand factor. We have previously shown that factor VIII is co-targeted with von Willebrand factor to the Weibel-Palade bodies in blood outgrowth endothelial cells, even when factor VIII carries mutations in the light chain that are associated with defective von Willebrand factor binding. In this study, we addressed the contribution of individual factor VIII C-domains in intracellular targeting, von Willebrand factor binding and cofactor activity by factor VIII/V C-domain swapping. Blood outgrowth endothelial cells were transduced with lentivirus encoding factor V, factor VIII or YFP-tagged C-domain chimeras, and examined by confocal microscopy. The same chimeras were produced in HEK293-cells for in vitro characterization and chemical foot-printing by mass spectrometry. In contrast to factor VIII, factor V did not target to Weibel-Palade bodies. The chimeras showed reduced Weibel-Palade body targeting, suggesting that this requires the factor VIII C1–C2 region. The factor VIII/V-C1 chimera did not bind von Willebrand factor and had reduced affinity for activated factor IX, whereas the factor VIII/V-C2 chimera showed a minor reduction in von Willebrand factor binding and normal interaction with activated factor IX. This suggests that mainly the C1-domain carries factor VIII-specific features in assembly with von Willebrand factor and activated factor IX. Foot-printing analysis of the chimeras revealed increased exposure of lysine residues in the A1/C2- and C1/C2-domain interface, suggesting increased C2-domain mobility and disruption of the natural C-domain tandem pair orientation. Apparently, this affects intracellular trafficking, but not extracellular function.