Immunopathologic Effects of Tumor Necrosis Factor Alpha in Murine Mycobacterial Infection Are Dose Dependent

Abstract

ABSTRACT In experimental mycobacterial infection, tumor necrosis factor alpha (TNF-α) is required for control of bacillary growth and the protective granulomatous response, but may cause immunopathology. To directly examine the positive and detrimental effects of this cytokine, a murine model was used in which different amounts of TNF-α were delivered to the site of infection. Mice with a disruption in the TNF-α gene (TNF-KO) or wild-type mice were infected with low or high doses of recombinant Mycobacterium bovis BCG that secreted murine TNF-α (BCG-TNF). Infection of TNF-KO mice with BCG containing the vector (BCG-vector) at a low dose led to increased bacillary load in all organs and an extensive granulomatous response in the lungs and spleen. The mice succumbed to the infection by ∼40 days. However, when TNF-KO mice were infected with low doses of BCG-TNF, bacillary growth was controlled, granulomas were small and well differentiated, the spleen was not enlarged, and the mice survived. Infection with high inocula of BCG-TNF resulted in bacterial clearance, but was accompanied by severe inflammation in the lungs and spleen and earlier death compared to the results from the mice infected with high inocula of BCG-vector. Wild-type mice controlled infection with either recombinant strain, but showed decreased survival following high-dose BCG-TNF infection. The effects of TNF-α required signaling through an intact receptor, since the differential effects were not observed when TNF-α receptor-deficient mice were infected. The results suggest that the relative amount of TNF-α at the site of infection determines whether the cytokine is protective or destructive.