Monoclonal antibody light chain with prothrombinase activity

Area of Science:

• Biochemistry
• Immunology
• Hematology

Background:

• Autoantibodies to prothrombin are linked to thromboembolism.
• The precise mechanisms by which these antibodies influence coagulation remain unclear.

Purpose of the Study:

• To investigate prothrombinase activity in monoclonal antibody light chains from multiple myeloma patients.
• To elucidate the catalytic mechanism and substrate specificity of identified prothrombin-activating light chains.

Main Methods:

• Screening of 34 monoclonal antibody light chains for prothrombinase activity using electrophoresis.
• Characterization of active light chains via gel filtration, peptide substrate cleavage assays, and N-terminal sequencing.
• Analysis of prothrombin fragment activity on thrombin substrates and fibrinogen conversion.

Main Results:

• Two light chains exhibited prothrombinase activity, with one characterized further.
• The active light chain demonstrated Michaelis-Menten-Henri kinetics in cleaving a prothrombin-derived peptide.
• Four specific cleavage sites on prothrombin were identified, and the light chain showed specificity for prothrombin over other proteins.
• Generated prothrombin fragments activated a thrombin substrate and accelerated fibrinogen to fibrin conversion.

Conclusions:

• Certain monoclonal antibody light chains possess intrinsic prothrombinase activity.
• This activity involves specific cleavage of prothrombin, leading to its activation.
• Antibodies may induce a procoagulant state through direct prothrombin activation, offering a novel mechanism for antibody-mediated thrombosis.