Susceptibility to Mycobacterium lepraemurium (MLM) infection markedly differed between two mouse strains, CBA/J and C57BL/6. CBA/J mice showed high susceptibility to MLM infection and developed either very weak or no delayedtype hypersensitivity (DTH) to MLM antigen after the injection of MLM. In contrast, C57BL/6 mice, which were resistant to MLM infection, showed significant DTH reaction to MLM antigen after the injection.Treatment of CBA/J mice with cyclophosphamide (Cy) conferred significant resistance to MLM infection on the CBA/J mice, and the treated mice developed a strong anti-MLM DTH response after the MLM injection. When spleen cells from MLM-infected CBAIJ mice were transferred to Cy-treated and MLM-infected syngeneic mice, the anti-MLM DTH reaction of the recipient mice was suppressed. Treatment of the spleen cells to be transferred with anti-Thy-1.2 antibody or anti-I-Jk antiserum plus complement abrogated the suppressive activity. Thus, it is suggested that the high susceptibility of CBAIJ mice to MLM infection is due to the generation of Cy-sensitive, I-Jk-positive suppressor T cells after infection with MLM.Mice of different strains exhibit markedly different levels of susceptibility to Mycobacterium lepraemurium (MLM) infection (2, 3, 6, 7-10, 14-16, 20, 21). Kawaguchi (14, 16) and Closs (7-10) demonstrated that C3H mice are susceptible to MLM infection, whereas C57BL/6 mice are resistant to the infection when they are given moderate doses of MLM subcutaneously. Histological findings suggested that the strain difference in susceptibility to MLM infection is closely related to the extent of cell-mediated immunity (CMI) generated during the infection. Though the strain differences in the susceptibility to MLM infection are affected somehow by the infecting dose and the route of the infection (17-19,21), we found a higher susceptibility of C3H than C57BL/6 mice to subcutaneous inoculation of moderate doses of MLM. Furthermore, we found that C3H mice infected with a moderate dose (5 X 10 6 ) ofMLM show a poor response to MLM antigen but a marked response to an unrelated immunizing antigen, i.e., sheep erythrocytes, human r-globulin, or PPD, in both in vivo DTH and in vitro lymphocyte transformation tests (13). C57BL/6 mice infected with the same dose of MLM showed a marked response to MLM antigen.
