Studies on the specificity of smooth-muscle antibodies

Purified contractile proteins from smooth and striated muscles have been used to test the specificity of human smooth muscle antibodies (SMA) from patients with chronic liver disease (IgG-SMA) and acute hepatitis (IgM-SMA). The reactions, as detected by indirect immunofluorescence, of IgG-SMA with renal vessel walls, renal glomeruli, peritubular fibrils and the luminal part of the tubular cells could be completely abolished by absorption with either smooth muscle or skeletal muscle F-actin, while absorption with myosin and tropomyosin had no effect. The specificity of IgG-SMA for actin was confirmed by their staining of the actin-rich I-bands of skeletal muscle myofibrils and by the blocking of this reaction by pretreatment of myofibrils and isolated smooth muscle cells with smooth muscle myosin subfragment 1 (S-1). IgM-SMA from patients with acute hepatitis-stained renal vessel walls and some sera also stained renal glomeruli. The IgM-SMA titres decreased after absorption both with myosin and F-actin but not with tropomyosin. The reactivity of some IgM-SMA could be blocked by S-1 while others could not. Thus the specificity of IgM-SMA seemed to be variable, and apparently differed from IgG-SMA in some cases.