Interferon-gamma is key in limiting Mycobacterium tuberculosis infection. Here we show that vaccination triggered an accelerated interferon-gamma response by CD4(+) T cells in the lung during subsequent M. tuberculosis infection. Interleukin 23 (IL-23) was essential for the accelerated response, for early cessation of bacterial growth and for establishment of an IL-17-producing CD4(+) T cell population in the lung. The recall response of the IL-17-producing CD4(+) T cell population occurred concurrently with expression of the chemokines CXCL9, CXCL10 and CXCL11. Depletion of IL-17 during challenge reduced the chemokine expression and accumulation of CD4(+) T cells producing interferon-gamma in the lung. We propose that vaccination induces IL-17-producing CD4(+) T cells that populate the lung and, after challenge, trigger the production of chemokines that recruit CD4(+) T cells producing interferon-gamma, which ultimately restrict bacterial growth.

Tuberculosis is primarily a disease of the lung and dissemination is dependent upon productive infection of this critical organ. Upon aerosol infection with Mycobacterium tuberculosis (Mtb), the acquired cellular immune response is slow to be induced and to be expressed within the lung. This slowness allows infection to become established and forces the acquired response to be expressed in a context of an inflammatory site that has been initiated and modulated by the bacterium. Mtb has a variety of surface molecules that interact with the innate response and this interaction along with the auto-regulation of the immune response by several mechanisms results in less than optimal control of bacterial growth. To improve current vaccine strategies we must understand the factors that mediate induction, expression and regulation of the immune response in the lung. We must also determine how to induce both known and novel immunoprotective responses without inducing immunopathologic consequences.

The cytokine interleukin-12 (IL-12) was thought to have a central role in T cell-mediated responses in inflammation for more than a decade after it was first identified. Discovery of the cytokine IL-23, which shares a common p40 subunit with IL-12, prompted efforts to clarify the relative contribution of these two cytokines in immune regulation. Ustekinumab, a therapeutic agent targeting both cytokines, was recently approved to treat psoriasis and psoriatic arthritis, and related agents are in clinical testing for a variety of inflammatory disorders. Here we discuss the therapeutic rationale for targeting these cytokines, the unintended consequences for host defense and tumor surveillance and potential ways in which these therapies can be applied to treat additional immune disorders.

Immunity to Mycobacterium tuberculosis infection is associated with the emergence of protective CD4 T cells that secrete cytokines, resulting in activation of macrophages and the recruitment of monocytes to initiate granuloma formation. The cytokine-mediating macrophage activation is interferon-γ (IFN-γ), which is largely dependent on interleukin-12 (IL-12) for its induction. To address the role of IL-12 in immunity to tuberculosis, IL-12 p40−/− mice were infected with M. tuberculosis and their capacity to control bacterial growth and other characteristics of their immune response were determined. The IL-12 p40−/− mice were unable to control bacterial growth and this appeared to be linked to the absence of both innate and acquired sources of IFN-γ. T cell activation as measured by delayed type hypersensitivity and lymphocyte accumulation at the site of infection were both markedly reduced in the IL-12 p40−/− mice. Therefore, IL-12 is essential to the generation of a protective immune response to M. tuberculosis, with its main functions being the induction of the expression of IFN-γ and the activation of antigen-specific lymphocytes capable of creating a protective granuloma.