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Survival of Mycobacterium lepraemurium in C57BL mice after acquired protective immunity.

The protective immune response against Mycobacterium lepraemurium (MLM) in C57BL mice has been shown to stop the increase in bacillary numbers and the dissemination of bacilli, but the acid fast bacilli are not cleared from the tissues. Persistence of viable bacilli was indicated by a significant increase in the number of acid fast bacilli in the footpad of C57BL mice that were treated with cortisone acetate several weeks after the onset of the immune response. Bacilli harvested 9 and 16 days after inoculation into immune C57BL mice showed only a marginally detectable loss of viability as determined by bacillary multiplication after transfer into susceptible C3H mice. Twenty-six weeks after being inoculated into immune C57BL mice a small proportion of the bacilli was found still to be alive. A similar finding was done 15 weeks after primary inoculation of MLM into mice that developed an apparently effective protective immune response 4 weeks after being inoculated. Sixty-seven weeks after inoculation of immunized C57BL mice with MLM, bacillary numbers in the footpad were as with patent immunity, but the bacilli were found to be fully viable, suggesting incipient reactivation of the infection. When bacillary numbers were followed over a period of 52 weeks in the organs of normal C57BL mice inoculated with a low dose of bacilli it was found that after a plateau phase bacillary numbers started to increase again. Thus, in all experiments part of the bacillary population had survived the protective immune response against MLM in C57BL mice.