A human monoclonal antibody obtained from EBV-transformed B cells with specificity for myosin

We describe the preparation of a stable human lymphoblastoid cell line obtained during ex vivo studies in which peripheral blood lymphocytes of a Glanzmann's thrombasthenia patient were transformed with Epstein-Barr virus. Somatic hybrids secreted an IgM monoclonal antibody (B7) that reacted with the myosin heavy chain of human platelets by immunoblotting. Flow cytometry showed that B7 barely recognized unstimulated intact platelets, but bound abundantly after permeabilization of fixed cells with Triton X-100. The reactivity of the antibody on thin sections of human myocardium and aorta was studied by immunohistochemistry. B7 specifically stained myosin of myocytes, but there was no labelling of aortic smooth muscle cells. The epitope was conserved in cardiac or skeletal myosin prepared from pig or rabbit. Measurement of the dissociation constant in a competitive ELISA showed that B7 bound with high affinity (10(-8) M). Purified Fab fragments retained their ability to bind to myosin, suggesting that B7 may be useful in the imaging of myocardial necrosis after myocardial infarction, myocarditis, cardiac drug toxicosis or graft rejection. This work also shows that EBV transformation of B cells may uncover naturally occurring autoantibodies which under normal circumstances are inhibited by the immune surveillance system.