Mycobacterium bovis BCG but not Mycobacterium leprae induces TNF-alpha secretion in human monocytic THP-1 cells

In this study, we compared the level of TNF-a secretion induced in monocyti c THP-1 cells after phagocytosis of Mycobacterium leprae, the causative age nt of leprosy, and M. bovis BCG, an attenuated strain used as a vaccine aga inst leprosy and tuberculosis. The presence of M. leprae and BCG was observ ed in more than 80% of the cells after 24 h of exposure. However, BCG but n ot M. leprae was able to induce TNF-alpha secretion in these cells. Moreove r, THP-1 cells treated simultaneously with BCG and M. leprae secreted lower levels of TNF-alpha compared to cells incubated with BCG alone. M. leprae was able, however, to induce TNF-alpha secretion both in blood-derived mono cytes as well as in THP-1 cells pretreated with phorbol myristate acetate. The inclusion of streptomycin in our cultures, together with the fact that the use of both gamma-irradiated M. leprae and heat-killed BCG gave similar results, indicate that the differences observed were not due to difference s in viability but in intrinsic properties between M. leprae and BCG. These data suggest that the capacity of M. leprae to induce TNF-alpha is depende nt on the stage of cell maturation and emphasize the potential of this mode l to explore differences in the effects triggered by vaccine strain versus pathogenic species of mycobacteria on the host cell physiology and metaboli sm.