Secretion of cytokines by human macrophages upon infection by pathogenic and non-pathogenic mycobacteria

The interaction of various pathogenic (Mycobacterium tuberculosis, M. avium, M. kansasii, M. xenopi), and non-pathogenic mycobacteria (M. smegmatis, M. phlei) with human macrophages at the level of macrophage cytokine expression (TNFalpha, IL1, IL6 and GM-CSF) was investigated. Both for TNFalpha and GM-CSF, the lowest levels were obtained with pathogenic mycobacterial species, whereas about 2-8 times higher levels were observed for non-pathogenic species. Contrary to the above, the differences for IL6 and IL1 were not marked, although IL6 appeared to be more elevated for non-pathogenic species. Heat-killed bacteria induced a lower level of the cytokines for all the three cytokines assayed (TNFalpha, IL6 and IL1), except for M. tuberculosis for whom a significantly higher proportion of TNFalpha was induced by killed bacilli. The RT-PCR experiments performed on M. avium (as a low inducer of the cytokines) and M. smegmatis (as a high inducer of the cytokines) showed that the differences observed among pathogenic vs non-pathogenic strains were also reflected at the transcriptional level for TNFalpha and to a lesser extent for IL6, but not for IL1. This investigation underlined important differences existing between the pathogenic and non-pathogenic species, particularly as regards TNFalpha and GM-CSF.