Reaction of human smooth muscle autoantibody with skeletal muscle, cardiac muscle, and thymic myoid cells

Fourteen sera containing smooth muscle antibody (SMA) obtained from patients with active chronic hepatitis (ACH) reacted, in indirect immunofluorescence tests, with smooth muscle, striated muscle including thymic myoid cells, hepatocytes, glomeruli, the brush borders and peritubular fibrils of renal tubules, and thymic medulla. Muscle tissue staining was most intense in smooth muscle, moderate in skeletal muscle and thymic myoid cells, and weak in cardiac muscle; skeletal muscle staining was best seen with stretched preparations. Reactivity with smooth muscle, striated muscle, and non-muscle tissues was inhibited by serum absorption with actin derived from smooth or skeletal muscle but not by skeletal muscle myosin, tropomyosin, or troponin. The amount of skeletal muscle F-actin required to inhibit SMA staining of smooth muscle was greater than that required to neutralize SMA staining of striated muscle or nonmuscle tissues; G-actin was more effective in neutralizing SMA activity than F-actin. These results suggest that the intensity of SMA binding to muscle and nonmuscle tissues may depend on the accessibility of actin, its concentration, and its state of polymerization.