Interleukin-6 Induces Early Gamma Interferon Production in the Infected Lung but Is Not Required for Generation of Specific Immunity toMycobacterium tuberculosisInfection

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Abstract
Immunity toMycobacterium tuberculosisis dependent upon the generation of a protective gamma interferon (IFN-γ)-producing T-cell response. Recent studies have suggested that interleukin-6 (IL-6) is required for the induction of a protective T-cell response and that IL-4 may suppress the induction of IFN-γ. To evaluate the role of the cytokines IL-6 and IL-4 in the generation of pulmonary immunity toM. tuberculosis, IL-6 and IL-4 knockout mice were infected withM. tuberculosisvia aerosol. The absence of IL-6 led to an early increase in bacterial load with a concurrent delay in the induction of IFN-γ. However, mice were able to contain and control bacterial growth and developed a protective memory response to secondary infection. This demonstrates that while IL-6 is involved in stimulating early IFN-γ production, it is not essential for the development of protective immunity againstM. tuberculosis. In contrast, while the absence of IL-4 resulted in increased IFN-γ production, this had no significant effect upon bacterial growth.