False-positive immunoreactivity with muscle-specific actins in non-Hodgkin's lymphomas

Antibodies to forms of muscle actin have been developed that are highly specific for tissue of myogenic origin and for which false-positive reactions are seldom reported. We studied 23 cases of B-cell malignant lymphoma by immunohistochemistry using monoclonal antiactin antibodies of two different specificities. Eight of the 23 cases were positive with an antibody against smooth-muscle actin that had been produced as ascitic fluid. Reactivity was abolished by the use of antibody from the same clone (1A4) but produced as a tissue culture supernatant. Nine of 23 cases were positive with an antibody against muscle actin (clone HHF35) produced as ascitic fluid. An antibody from the same clone (HHF35), but produced as a tissue culture supernatant, gave weak staining of both lymphoid cells and connective tissue background. Lymphoma staining by both HHF35 antibodies (and background staining) was abolished by the addition of ethylenediaminetetraacetic acid (EDTA) to the antibody. (EDTA is reported to eliminate nonspecific protein binding.) Each ascitic fluid-derived antibody stained the lymphomas in an apparently specific manner. There was intense granular deposition of reaction product localized to specific portions of the cell (plasma membrane or diffuse cytoplasmic staining and, in two signet ring cell lymphomas, globular staining of intracytoplasmic inclusions), and there was substantial concordance of the staining of the two antibodies. This finding of false-positive reactivity by two different antiactin monoclonal antibodies, both prepared as ascitic fluid, supplements recent reports of a similar phenomenon with ascitic fluid preparations of the antimelanoma monoclonal antibody HMB45.(ABSTRACT TRUNCATED AT 250 WORDS)