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The relationship of the dose of vaccine to the immune response was determined in CF-1 mice vaccinated intraperitoneally with viable cells of the attenuated H37Ra strain ofMycobacterium tuberculosis and in mice vaccinated with cells of the same strain killed by autoclaving at 121 C for 15 min. The results showed, in terms of increased resistance to tuberculous infection, that the immune response with both living and killed cells was dependent upon the dose of vaccine, whereas only the living cells were dependent upon the time of challenge after vaccination. The dose response curves show dramatically that viable cells, which do not multiply in vivo, are several hundred times more effective immunizing agents against tuberculous infection than are autoclaved cells. Viable 2-week-old H37Ra cells were far more immunogenic than viable 4-week-old cells. Autoclaved 2-week-old cells, however, were no more immunogenic than autoclaved 4-week-old cells. H37Ra cells killed by boiling (98 C), exposure to 65 C for 30 min, treating with 2% phenol, or by being dried with acetone also lost most of their capacity to immunize mice. The effect of adjuvant on the immune response of mice to tuberculous infection was tested by incorporating both viable and autoclaved cells in Freund's incomplete adjuvant. We found that this vehicle had little or no effect on the immunizing capacity of either viable or heat-killed mycobacterial cells. The relationship of all the findings to the specificity of the immune response to tuberculosis is discussed.

Immunizing Capacity of Viable and Killed Attenuated Mycobacterial Cells Against Experimental Tuberculous Infection