Myosin autoantibodies detected by immunofluorescence

Autoantibodies to striated and smooth muscles myosins were detected by indirect immunofluorescence and confirmed by absorption with purified contractile proteins extracted from human, rabbit and chicken muscle. Myosin antibodies were rare: of fifty-five sera examined from patients with various skeletal and cardiac muscle disorders, only one serum, from a case of Coxsackie viral pericarditis, had anti-myosin activity. It reacted with cardiac muscle and type 1 fibres of skeletal muscle, staining the 'A' band of the sarcomere only. The antibody was absorbed by skeletal myosin and by skeletal heavy meromyosin fragments, but not by smooth muscle myosin. Two types of smooth muscle myosin autoantibodies are described. One is restricted to smooth muscle myosin and examples were found in polyclonal and monoclonal SMA sera. The second type of smooth muscle myosin antibody cross-reacted with skeletal and cardiac muscle and with cytoplasmic myosin in liver, kidney and thyroid cells. It was completely absorbed using either smooth or skeletal myosin and by heavy meromyosin fragments. The different types of myosin autoantibodies reflect the variety of myosins found in mammalian tissues. Cross-reacting myosin antibodies indicate epitopes on the heavy meromyosin fragment which are common to several different tissue myosins.