New insights into the immunopathology of tuberculosis

Tuberculosis is characterised by fever, weight loss and necrosis in both lesions and tuberculin skin test sites (Koch phenomenon), although the antigens of Mycobacterium tuberculosis are not directly toxic to the tissues. The tissue damage appears to be due to several interacting factors. First, M. tuberculosis induces an immunoregulatory disorder of which a raised percentage of agalactosyl IgG is a marker. This is seen also in rheumatoid arthritis and Crohn's disease and is associated with tissue-damaging inflammation. Subsequently, several properties of M. tuberculosis exacerbate this disorder by triggering cytokine release, and rendering tissues sensitive to the toxicity of tumor necrosis factor (TNF). Moreover, M. tuberculosis, but not bacillus Calmette-Guérin or several Mycobacterium avium strains, produces a factor which increases the toxicity of TNF for individual cells. Thus, M. tuberculosis may distort the normal protective role of TNF so that this cytokine becomes toxic to the host. The immunoregulatory disorder associated with agalactosyl IgG appears to be susceptible to immunotherapy, so novel types of treatment for the immunopathological component of tuberculosis are being explored.