The specific cytotoxicity of nucleoprotein fractions from Brucella species was investigated using spleen explants from normal, Brucella-infected and tuberculous guinea pigs. The nucleoprotein preparations studied included Brucellergen and similar fractions isolated from smooth cultures of Brucella suis, Brucella melitensis and Brucella abortus . Cytotoxicity was determined by the inhibition of migration and growth of macrophages, leukocytes and fibroblasts and by the morphologic appearance of stained sections of cultures fixed following different periods of incubation. Brucellergen at a dilution of 1:10 was nontoxic for normal cells but had a significant effect on cells from Brucella-sensitive animals. Brucella antibodies did not influence the specific cytotoxic activity of Brucellergen. Splenic cell cultures from Brucella-infected guinea pigs were sensitive to Brucellergen but not to PPD-S, whereas similar cultures from tuberculous guinea pigs were sensitive to PPD-S but not to Brucellergen. The nucleoprotein fractions prepared from each of the three Brucella species elicited a delayed response on intracutaneous injection into a rabbit infected with Br. melitensis . In splenic cell culture, the toxicity of the fractions was very low for normal cells, but growth and migration was significantly inhibited in cultures from guinea pigs infected with each of the three species of bacteria. In all experiments, macrophages were more sensitive than leukocytes. The three nucleoprotein preparations varied in potency in the cytotoxic tests but they did not display species specificity when tested in a heterologous system. Splenic explants from guinea pigs infected with Br. suis displayed a higher degree of cellular sensitivity than tissues from guinea pigs infected with Br. melitensis or Br. abortus .
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