Immunolocalization of complement component C9 on necrotic and non-necrotic muscle fibres in myositis using monoclonal antibodies: a primary role of complement in autoimmune cell damage.

Monoclonal antibodies to human complement component C9 were used to localise the membrane attack complex (MAC) of human complement on muscle fibres from patients with autoimmune myositis. A pigeon erythrocyte 'model' system was used to screen the available monoclonal antibodies for their ability to bind C9 after its insertion into the MAC. Three of the five antibodies tested were demonstrated, using radioiodine- or rhodamine-labelled second antibody, to bind. The antibody giving maximum binding in this system was used to demonstrate that MACs were present on histologically normal as well as on necrotic fibres in myositis. No binding was seen in muscle from control subjects. These results suggest a primary role of complement in the pathogenesis of muscle fibre necrosis in myositis.