IL-23 and IL-17 in tuberculosis

Abstract

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 cytokine capable 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 Th1 cells are induced following primary infection with Mtb, the protective response is significantly altered in the absence of Th1 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 Th1 response and protection. Th1 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.

Figures

Figure 1. Tuberculosis represents a chronic interaction between host and pathogen with IL-23 and IL-17 playing roles throughout

Bacteria are deposited at low numbers within the lung and γδ T cells (green nuclei) make IL-17 (green lined cells) to recruit neutrophils (purple nuclei, purple lines). If mice have been vaccinated, memory Th17 cells (purple nuclei, green lines) make chemokines and accelerate the accumulation of memory Th1 cells (purple nuclei, red lines). In either the memory or the primary response it is the arrival of IFN-γ-producing cells (purple nuclei, red lines) at sufficient numbers that correlates with cessation of bacterial growth. Initially a mononuclear granuloma develops in the lung parenchyma (pink), the extent and content of which depends upon IL-17. As infection progresses, lymphocyte numbers decrease while polymorphonuclear cells increase. We propose here that the balance between the IL-12/IFN-γ and IL-23/IL-17 pathways define the late consequences of mycobacterial infection with disruption of the lung parenchyma (dark pink) and bacterial recrudescence being the consequences of imbalance.