IL-17 Production Is Dominated by  T Cells rather than CD4 T Cells during Mycobacterium tuberculosis Infection 1 Euan Lockhart, Angela M. Green, and JoAnne L. Flynn 2 IL-17 is a cytokine produced by T cells in response to IL-23. Recent data support a new subset of CD4 Th cells distinct from Th1 or Th2 cells that produce IL-17 and may contribute to inflammation. In this study, we demonstrate that, in naive mice, as well as during Mycobacterium tuberculosis infection, IL-17 production is primarily from  T cells and other non-CD4  CD8  cells, rather than CD4 T cells. The production of IL-17 by these cells is stimulated by IL-23 alone, and strongly induced by the cytokines, including IL-23, produced by M. tuberculosis-infected dendritic cells. IL-23 is present in the lungs early in infection and the IL-17-producing cells, such as  T cells, may represent a central innate protective response to pulmonary infection. The Journal of Immunology, 2006, 177: 4662– 4669. I nterleukin-23 is a member of the IL-12 family of cytokines and shares the IL-12p40 subunit with IL-12. IL-23 appears to be a driving force in chronic inflammatory disease. This cy- tokine promotes T cell production of IL-17A, IL-17F, TNF, and IL-6, which drive inflammation in experimental autoimmune en- cephalomyelitis (1). IL-23 also promotes IL-17-mediated chronic inflammation in collagen-induced arthritis (2). The profile of T cell genes induced by IL-23 is mostly distinct from that induced by IL-12, and IL-23 does not significantly prime for high IFN- pro- duction in mice (1, 3). A new subset of CD4 helper cells, distinct from that of Th1 and Th2 cells, termed Th17, has been described (4, 5). The development of this IL-17-producing subset is inhibited by IFN- or IL-4, and may be involved in autoimmune diseases. However, mature Th17 cells are resistant to the effects of IL-4 and IFN-, and are able to secrete IL-17 in Th1 or Th2 environments (4). Overexpression of IL-17 in lung epithelium results in lung pathology including epithelial hypertrophy and the presence of multinucleated macrophages in the parenchyma (5). IL-23 induction of IL-17 in the lung leads to downstream events that mobilize cell infiltration. IL-17 induces chemokines, growth factors, and adhesion molecules, and augments neutrophil accu- mulation (6, 7). It has been demonstrated to play a role in control of pulmonary infection due to Klebsiella, an extracellular bacterial pathogen, in mouse models. There are several lines of evidence that suggest  cells con- tribute to the immune response to Mycobacterium tuberculosis. In the macaque model of infection with Mycobacterium bovis bacillus Calmette-Gue ´rin (BCG), 3 the V2V2  T cell subset expand rapidly during primary exposure and also to secondary challenge with BCG or virulent M. tuberculosis (8). Similarly, intranasal infection of mice expanded resident V2 T cells and caused an influx of other  subsets into the lung. These cells were capable of producing IFN- and were cytotoxic toward infected macrophages in vitro (9). In humans,  cells from BCG-vaccinated individuals expand upon restimulation with mycobacterial Ag, thereby displaying a memory-like phenotype (10). Human alveolar macrophages infected with M. tubercu- losis release chemoattractants such as CXCL10 that cause che- motaxis of  cells and cytokine secretion (11). These chemoat- tractants are found in the lungs and axillary lymph nodes of patients with active disease. In the mouse model of M. tuber- culosis infection,  knockout (KO) mice have a more pyogenic granulomatous response, implying a regulatory role in granu- loma formation (12). IL-12 has a central role in priming T cells to produce IFN- in response to M. tuberculosis (13, 14). IL-23 also contributes to resistance as suggested by the increased susceptibility of IL- 12p40 / compared with IL-12p35 / mice during experimental infection (15). Somewhat surprisingly in light of that study, mice deficient in the p19 subunit of IL-23 did not display obviously increased susceptibility to M. tuberculosis (as measured by bacte- rial numbers). The relatively less susceptible phenotype of IL- 12p35 / mice may be due to the fact that IL-23 can also make a minor contribution to IFN- production in the absence of IL-12 (16). It was also demonstrated that IL-23 is an absolute require- ment in CD4  T cell production of IL-17 during M. tuberculosis infection (16). The role of the IL-17-producing T cells in control of M. tuberculosis infection is not known. In this study, we have used murine M. tuberculosis infection as a model to investigate the interaction between IL-23 and IL-17, with particular emphasis on the cells that respond to IL-23. By using a slow growing bacterium against which immune responses develop slowly, we have dissected the early IL-17 responses in the lungs, and the chronicity of cytokine production. Recently, a homeostatic loop involving IL-23-induced IL-17 production by unconventional  T cells and  cells has been described (17). Whereas IL-23 and IL-17 promote granulopoiesis and influx of neutrophils into the lung, phagocytosis of apoptotic lung neutrophils reduces macrophage IL-23 secretion, and induc- tion of IL-17, thereby limiting influx. The data presented here sim- ilarly demonstrate that, in naive mice, IL-17 production can be induced in  T cells. However, our study also demonstrates that Department of Molecular Genetics and Biochemistry, University of Pittsburgh School of Medicine, Pittsburgh, PA 15261 Received for publication April 26, 2006. Accepted for publication July 14, 2006. The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked advertisement in accordance with 18 U.S.C. Section 1734 solely to indicate this fact. 1 This work was supported by National Institutes of Health Grants RO1 AI50732 and AI37859 (to J.L.F.), T32 CA82084-07 (to E.L.), and AI060525 (to A.M.G.) and C Advisors Grant. 2 Address correspondence and reprint requests to Dr. JoAnne L. Flynn, W1157 Bio- medical Science Tower, Pittsburgh, PA 15261. E-mail address: joanne@pitt.edu 3 Abbreviations used in this paper: BCG, bacillus Calmette-Gue ´rin; KO, knockout; DC, dendritic cell. The Journal of Immunology Copyright © 2006 by The American Association of Immunologists, Inc. 0022-1767/06/$02.00