Biological activity in sensitized guinea pigs of MPB 70, a protein specific for some strains of Mycobacterium bovis BCG.

MPB 70 is a protein found in large quantities in the culture filtrate (CF) of the Tokyo and some other strains of Mycobacterium bovis BCG, and it has a remarkable degree of specificity for these strains. We estimated the molecular weight of MPB 70 to 22,000 by sodium dodecyl sulphate-polyacrylamide gel electrophoresis (SDS-PAGE). SDS-PAGE and immunoblotting showed that MPB 70 was present in high quantities in CF from BCG Tokyo, that it could be also demonstrated in BCG Copenhagen, and that it was absent in CF from M. tuberculosis H37Rv. When the purified MPB 70 preparation used in the present study was run in SDS-PAGE, blotted and stained with a polyclonal rabbit or a monoclonal mouse anti-MPB 70 antibody, several bands in addition to the main 22 kDa band were seen, indicating a tendency of the MPB 70 molecules and/or fragments thereof to form very stable aggregates with themselves. The biological activity of MPB 70 was studied in groups of guinea pigs sensitized with live BCG of the Tokyo and Copenhagen strains. Guinea pigs from both groups developed reactivity to tuberculin PPD as assessed by skin tests and lymphocyte stimulation tests with peripheral blood or lymph node lymphocytes. In addition, a strong and persistent reactivity to MPB 70 was demonstrated in the BCG Tokyo group with both methods. Guinea pigs sensitized with the Copenhagen strain were only weakly reactive to MPB 70. Skin reactions in guinea pigs that had been repeatedly tested with MPB 70 and tuberculin were compared with reactions in animals tested only once. Reactions to MPB 70 in BCG Tokyo sensitized guinea pigs were suppressed by repeated tests, whereas tuberculin reactions were boosted by the interim tests. The levels of specific anti-MPB 70 antibodies were higher in BCG Tokyo- than in BCG Copenhagen-sensitized guinea pigs. MPB 70 has a high degree of specificity and is a strongly immunogenic protein, which may prove useful in studies of mycobacterial immunology.