Anti-factor V auto-antibody in the plasma and platelets of a patient with repeated gastrointestinal bleeding

Abstract number: P0618

Ajzner^* E., Balogh† I., Haramura† G., Boda† Z., Kalmar† K., Pfliegler† G. Y., Dahlback‡ B., Muszbek† L.

†Hungary; ‡Sweden ^*University of Lund, Sweden;

A 78-year-old female with repeated gastrointestinal bleeding presented antifactor V (FV) antibody titers of 1.25–2.0 Bethesda Unit in her plasma. The aim of our study was to characterize the FV inhibitor responsible for the clinical symptoms and establish the pathomechanisms of an acquired FV deficiency.

Methods  

FV antigen and FV-containing immune complexes were measured by ELISA. Structural abnormalities of the FV molecule, which might lead to allo-FV-antibody formation using blood replacement therapy, were analyzed by Western blotting and by sequencing the f5 gene. The binding of the patient's IgG to FV and activated FV (FVa) was demonstrated in an ELISA system and its effect on the procoagulant activity of FVa was tested in clotting tests and in a chromogenic prothrombinase assay. Localization of the epitope for the antibody was performed by blocking ELISA.

Results  

FV antigen levels were normal both in the patient's plasma and platelet lysate by ELISA using polyclonal anti-FV antibody, but depressed when HV1 monoclonal antibody against the C2 domain in the FV light-chain was used as capture antibody. The FV molecule was found intact. In the patient's plasma and IgG fraction, an antibody reacting with both FV and FVa was detected. The binding of the patient's IgG to FV could be inhibited by HV1 antibody. The isolated IgG fraction of the patient induced PT and APTT prolongation, when added to normal plasma, and inhibited FV cofactor function in a prothrombinase assay. FV-containing immune-complexes were detected in the patient's plasma and platelet lysate.

Conclusions  

An anti-FV IgG was present in the patient's plasma and platelets. The autoantibody reacted with an epitope in the C2 domain of FV light chain and neutralized the procoagulant function of FVa.