IL-23 and IL-17 in tuberculosis

Tuberculosis is a chronic disease requiring the constant expression of cellular immunity to limit bacterial growth. The constant expression of immunity also results in chronic inflammation, which requires regulation. While IFN-gamma-producing CD4+ T helper cells (Th1) are required for control of bacterial growth they also initiate and maintain a mononuclear inflammatory response. Other T cell subsets are induced by Mycobacterium tuberculosis (Mtb) infection including those able to produce IL-17 (Th17). IL-17 is a potent inflammatory cytokinecapable of inducing chemokine expression and recruitment of cells to parenchymal tissue. Both the IL-17 and the Th17 response to Mtb are largely dependent upon IL-23. Although both Th17 and Thl cells are induced following primary infection with Mtb, the protective response is significantly altered in the absence of Thl cells but not in the absence of Th17. In contrast, in vaccinated animals the absence of memory Th17 cells results in loss of both the accelerated memory Thl response and protection. Thl and Th17 responses cross-regulate each other during mycobacterial infection and this may be important for immunopathologic consequences not only in tuberculosis but also other mycobacterial infections. (C) 2007 Elsevier Ltd. All rights reserved.