The Possible Role of Interleukin (IL)-12 and Interferon-γ-Inducing Factor/IL-18 in Protection against ExperimentalMycobacterium lepraeInfection in Mice

Cell-mediated immunity participates in host defense against mycobacterial infection. Both interleukin 12 (IL-12) and interferon-γ-inducing factor (IGIF/IL-18), produced mainly by macrophages, play a critical role in expression of cell-mediated immunity. To investigate the role of IL-12 and IGIF/IL-18in vivo,we examined cytokine profile, bacterial growth, and the potential benefit of cytokine therapy in susceptible and resistant mice infected withMycobacterium leprae.The early expression of IL-12 p40 and IGIF/IL-18 at the site of inoculation was found in resistant mice 3–72 h after the infection, but not in susceptible mice. Both strains of mice did not show expression of IFN-γ and IL-4. IL-12 administration resulted in a significant reduction of bacterial counts in mice with establishedM. lepraeinfection. The results imply that susceptible mice exhibit decreased expression of type 1 helper T (Th1) response without reciprocal increased Th2 response and show responsiveness to exogenous IL-12. IL-12 therapy may be a possible rationale for treatment ofM. lepraeinfection.