Structural Deficiencies in Granuloma Formation in TNF Gene-Targeted Mice Underlie the Heightened Susceptibility to Aerosol Mycobacterium tuberculosis Infection, Which Is Not Compensated for by Lymphotoxin

Abstract

TNF and lymphotoxin-α (LTα) may act at various stages of the host response to Mycobacterium tuberculosis. To dissect the effects of TNF independent of LTα, we have used C57BL/6 mice with a disruption of the TNF gene alone (TNF^−/−). Twenty-one days following aerosol M. tuberculosis infection there was a marked increase in the number of organisms in the lungs of TNF^−/− mice, and by 28–35 days all animals had succumbed, with widespread dissemination of M. tuberculosis. In comparison with the localized granulomas containing activated macrophages and T cells in lungs and livers of C57BL/6 wild-type (wt) mice, cellular infiltrates in TNF^−/− mice were poorly formed, with extensive regions of necrosis and neutrophilic infiltration of the alveoli. Phenotypic analysis of lung homogenates demonstrated similar numbers of CD4⁺ and CD8⁺ T cells in TNF^−/− and wt mice, but in TNF-deficient mice the lymphocytes were restricted to perivascular and peribronchial areas rather than colocated with macrophages in granulomas. T cells from TNF^−/− mice retained proliferative and cytokine responses to purified protein derivative, and delayed-type hypersensitivity to purified protein derivative was demonstrable. Macrophages within the lungs of TNF^−/− and wt mice showed similar levels of MHC class II and inducible nitric oxide synthase expression, and levels of serum nitrite were comparable. Thus, the enhanced susceptibility of TNF^−/− is not compensated for by the presence of LTα, and the critical role of TNF is not in the activation of T cells and macrophages but in the local organization of granulomas.