Strain-dependent protective effect of adult thymectomy on murine infection by Mycobacterium lepraemurium.

C57BL/6, DBA/2, BALB/c and CBA mice were thymectomized as adults, or sham-thymectomized, and infected subcutaneously with 10(6) MLM. The number of MLM in the spleen and in the inoculated footpad was measured after 1 year of infection as well as the DTH reactions and the IgM and IgG antibody levels to MLM. Non-thymectomized mice exhibited a broad spectrum of resistance to MLM infection and of T cell mediated immunity grading from the highly resistant C57BL/6 strain to the highly susceptible CBA strain. In between, DBA/2 was found more resistant than BALB/c mice. Adult thymectomy reduced by 100 times the MLM number in the spleen of infected DBA/2 mice, without affecting that measured in the inoculated footpad, and significantly decreased DTH reaction in the same strain. No effect of adult thymectomy was observed in any other strain, except for an increase of anti MLM antibodies in BALB/c mice. These results may suggest that the medium-resistant DBA/2 strain develops after MLM infection suppressor T cells which favour MLM dissemination and are sensitive to adult.