An Inhibitory Single-Domain Antibody against Protein Z-Dependent Protease Inhibitor Promotes Thrombin Generation in Severe Hemophilia A and FXI Deficiency.

BACKGROUND: Protein Z-dependent protease inhibitor (ZPI) is an anticoagulant serpin that targets factor Xa (FXa) in the presence of protein Z (PZ), and factor XIa (FXIa). In factor-VIII-deficient mice, PZ or ZPI gene knock-out mitigates the bleeding phenotype, and pharmacological inhibition of PZ enhances thrombin generation in plasma from patients with hemophilia. AIMS: To develop a single-domain antibody (sdAb) directed against ZPI to inhibit its anticoagulant activity. METHODS: We screened for anti-ZPI sdAbs in a llama-derived phage display immune library of sdAbs. The sdAbs that bound ZPI were produced and purified for characterization. The binding of sdAbs to ZPI or other serpins was evaluated using ELISAs, and ZPI inhibition was measured in an anti-FXa or anti-FXIa chromogenic assay. The sdAbs's procoagulant activity was assessed in a thrombin generation assay in normal plasma, factor VIII- and FXI-deficient plasma. RESULTS: Of the four sdAbs found to bind to ZPI, one (referred to as ZPI-sdAb2) dose-dependently inhibited ZPI's anti-FXa and anti-FXIa activities with a mean half-maximal inhibitory concentration of 1.8 and 1.3 µM, respectively. ZPI-sdAb2 did not cross-react with other plasma serpins, such as antithrombin and α1-antitrypsin. ZPI-sdAb2 induced a significant increase in thrombin generation in plasma samples from healthy donors, patients with severe hemophilia A, and patients with FXI deficiency. CONCLUSION: ZPI-sdAb2 is the first specific, direct ZPI inhibitor found to exhibit procoagulant activity in plasma. This sdAb might have potential as a treatment for hemophilia or other bleeding disorders.

Antithrombotic potential of a single-domain antibody enhancing the activated protein C-cofactor activity of protein S.

BACKGROUND: Protein S (PS) is a natural anticoagulant acting as a cofactor for activated protein C (APC) in the proteolytic inactivation of activated factors V (FVa) and VIII (FVIIIa), but also for tissue factor pathway inhibitor α (TFPIα) in the inhibition of activated factor X (FXa). OBJECTIVE: For therapeutic purposes, we aimed at generating single-domain antibodies (sdAbs) that could specifically modulate the APC-cofactor activity of PS in vivo. METHODS: A llama-derived immune library of sdAbs was generated and screened on recombinant human PS by phage display. PS binders were tested in a global activated partial thromboplastin time (APTT)-based APC-cofactor activity assay. RESULTS: A PS-specific sdAb (PS003) was found to enhance the APC-cofactor activity of PS in our APTT-based assay, and this enhancing effect was greater for a bivalent form of PS003 (PS003biv). Further characterization of PS003biv demonstrated that PS003biv also enhanced the APC-cofactor activity of PS in a tissue factor (TF)-induced thrombin generation assay and stimulated APC in the inactivation of FVa, but not FVIIIa, in plasma-based assays. Furthermore, PS003biv was directed against the sex hormone-binding globulin (SHBG)-like domain but did not inhibit the binding of PS to C4b-binding protein (C4BP) and did not interfere with the TFPIα-cofactor activity of PS. In mice, PS003biv exerted an antithrombotic effect in a FeCl3 -induced thrombosis model, while not affecting physiological hemostasis in a tail-clip bleeding model. DISCUSSION: Altogether, these results showed that pharmacological enhancement of the APC-cofactor activity of PS through an original anti-PS sdAb might constitute a promising and safe antithrombotic strategy.

Measuring beta-galactose exposure on platelets: Standardization and healthy reference values.

BACKGROUND: Correct diagnosis of the cause of thrombocytopenia is crucial for the appropriate management of patients. Hyposialylation/desialylation (characterized by abnormally high ß-galactose exposure) accelerates platelet clearance and can lead to thrombocytopenia. However, the reference range for ß-galactose exposure in healthy individuals has not been defined previously. OBJECTIVE: The objective of the present study was to develop a standardized assay of platelet ß-galactose exposure for implementation in a clinical laboratory. METHODS: ß-Galactose exposure was measured in platelet-rich plasma by using flow cytometry and Ricinus communis agglutinin (RCA). A population of 120 healthy adults was recruited to study variability. RESULTS: We determined an optimal RCA concentration of 12.5 µg/mL. The measure was stable for up to 4 hours (mean fluorescence intensity [MFI]-RCA: 1233 ± 329 at 0 hour and 1480 ± 410 at 4 hours). The platelet count did not induce a variation of RCA and the measure of RCA was stable when tested up to 24 hours after blood collection (MFI-RCA: 1252 ± 434 at day 0 and 1140 ± 297 24 hours after blood sampling). To take into account the platelet size, results should be expressed as RCA/forward scatter ratio. We used the assay to study variability in 120 healthy adults, and we found that the ratio is independent of sex and blood group. CONCLUSION: We defined a normal range in a healthy population and several preanalytical and analytical variables were evaluated, together with positive and negative controls. This assay may assist in the diagnosis of thrombocytopenic diseases linked to changes in ß-galactose exposure.