Extracellular matrix antigen of human muscle defined by a monoclonal antibody.

A monoclonal antibody (7.1B3) has been produced in mice previously immunised with a glycoprotein fraction prepared from human myotube cells grown in tissue culture. The antibody precipitates an antigen complex of four glycoprotein components from detergent extracted human muscle myotube cultures. These are two chains of about 205K MW and two chains of about 130-140K MW. The antigen complex is trypsin-sensitive collagenase insensitive and bands as a complex of 10S on sucrose gradients in NP40. Indirect immunofluorescence studies on cell cultures of myoblasts and myotubes showed that the antigen existed in a number of locations: as a granular stain on the cell surface of myotubes, and as short streaks of immunoreactivity in the extracellular matrix. Clonal populations of myoblasts and myotubes gave a similar result showing that 7.1B3 is specifically synthesized by cells of the myogenic lineage. Double indirect immunofluorescence staining with 7.1B3 antibody and fibronectin antibody showed no overlap of immunoreactivity indicating that these two antigens are different. 7.1B3 antigen was found during human fetal muscle development in vivo at all time points studied from 15 weeks onwards. Immunoreactivity was localised to the loose extracellular matrix and in the endomysium. Perymysium staining was very prominent at 25 weeks and in adult muscle. Double staining of 7.1B3 antibody and alpha-bungarotoxin was carried out to determine whether 7.1B3 was present in synaptic junctions. Although 7.1B3 was found in the endomysium it is excluded from synaptic areas. We conclude that 7.1B3 antibody defines a new and novel antigen in the extracellular matrix of human muscle in vivo and in vitro.