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 γδ 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 γδ T cells, may represent a central innate protective response to pulmonary infection.

Local production of IL-17 is a significant factor in effective host defense against Gram-negative bacteria. However, the proximal events mediating IL-17 elaboration by T cells remain unclear. In this study, we show in vivo that intact Toll-like receptor 4 signaling in the lung is required for induction of both the p19 transcript of IL-23 and IL-17 protein elaboration in response to Klebsiella pneumoniae. Although IL-17 is widely considered a CD4+ T cell product, we also demonstrate significant in vitro IL-17 production by CD8+ T cells after culture in medium from dendritic cells exposed to these bacteria. The dominant portion of this IL-17-inducing activity for both CD4+ and CD8+ T cells is IL-23. These data demonstrate the critical signaling pathway for IL-17 induction in the host response to Gram-negative pulmonary infection and suggest a direct role for IL-23 in CD8+ T cell IL-17 production.

Interleukin-23 (IL-23The resurgence of Mycobacterium tuberculosis infection linked to the human immunodeficiency virus epidemic highlights the importance of cellular immunity in controlling the growth of this pathogen. Effective host defense against pulmonary infection with M. tuberculosis requires the coordinated actions of both the innate and adaptive immune systems (10). Interleukin-12 (IL-12) is well-established as a cytokine released by antigen-presenting cells early in M. tuberculosis infection, and this cytokine is critical for the generation of a Th1 polarized adaptive immune response and subsequent host defenses. Indeed, the exogenous administration of IL-12 augments M. tuberculosis clearance (6,11,25). IL-23 has recently been identified as a member of the IL-12 cytokine family and may also play a role in host defense against this pathogen. IL-23 is a heterodimer that shares an identical p40 subunit with IL-12 but contains a unique p19 chain that closely resembles IL-12 p35 (26). Secreted by dendritic cells and other antigenpresenting cells, IL-23 stimulates the production of gamma interferon (IFN-␥) by activated/memory CD45RO ϩ T cells but not naïve CD45RA ϩ cells. In contrast, IL-12 elicits IFN-␥ from both subsets. IL-23 also induces the proliferation of activated/ memory T cells but not naïve T cells. IL-12 and IL-23 share binding affinity for the IL-12R␤1 subunit; however, IL-23 binds to a distinct IL-23 receptor (IL-23R), whereas IL-12 utilizes IL-12R␤2 as its coreceptor (27).The comparative roles of IL-12 and IL-23 in host defense against a variety of infections are actively under investigation. Through Toll-like receptor 4 signaling, IL-23, but not IL-12, has been shown to play a critical role in stimulating T-cell release of IL-17 following infection with Klebsiella pneumoniae (15), while specific roles for IL-23 in immune defense against the intracellular bacteria Salmonella enteritidis and Francisella tularensis have also been found (9, 21). In regard to M. tuberculosis infections, recent studies have demonstrated a greater sensitivity in IL-12/23 p40 Ϫ/Ϫ mice than in IL-12 p35 Ϫ/Ϫ animals (5, 17). Moreover, macrophages rapidly express IL-23 when exposed to mycobacterial antigens, suggesting an immune-stimulatory role for this cytokine during infection (4, 33).The evidence of a role for IL-23 in the development of immunity against several intracellular pathogens, together with increasing interest in the use of biologic response modifiers to treat disease, led us to study the effects of localized, transient expression of IL-23 during early pulmonary M. tuberculosis infection. In this report, we show that local IL-23 gene delivery using a replication-defective adenovirus vector (AdIL-23) resulted in markedly decreased mycobacterial burden in the lungs up to 28 days after infection. AdIL-23 treatment significantly decreased lung inflammation in infected animals despite increasing numbers of activated CD4 ϩ T cells in lungs and draining lymph nodes. Cultures of T cells from draining

Summary Bacillus Calmette Guerin (BCG) has been reported to show variable efficacy as a vaccine against tuberculosis. We demonstrated that the secretion of biologically active IL-2 (rBCG/IL-2), but not IL-18 (rBCG/ IL-18), by BCG improves its ability to induce and maintain a strong type 1 immune response in BALB/c mice. rBCG/IL-2 induced significantly higher Ag-specific proliferative responses, high IFN-γ production and serum titres of IgG2a 16 weeks after vaccination. This immune profile was correlated to an increased rate of clearance of nonpathogenic mycobacteria (live BCG delivered intranasally). Surprisingly, however, this strong type 1 immune profile induced no greater protective immunity against aerosol challenge with virulent Mycobacterium bovis than that induced by normal BCG (nBCG). By comparison, vaccination with rBCG/IL-18 was found to induce significantly less IFN-γ production in splenic lymphocytes than nBCG. This impaired induction of IFN-γ was correlated to a significantly lower protective efficacy against M. bovis challenge, as compared to nBCG. The data suggest that manipulation of the immune response to tuberculosis and tuberculosis vaccines will require a more complete understanding of the factors that are important in generating a protective immune response.

The host immune response is generally sufficient to contain Mycobacterium tuberculosis infection. It does not, however, efficiently prevent subsequent infection with M. tuberculosis or provide sterilizing immunity. While the understanding of the immune response generated against this pathogen is incomplete, improvements have been achieved due to advances in immunological tools. In this study, we analyzed the multifunctional nature of primary and memory CD8 T-cell responses generated during murine M. tuberculosis infection. We generated a recombinant M. tuberculosis strain expressing ovalbumin (OVA) epitopes in order to expand the peptides for the detection of CD8 T cells during M. tuberculosis infection and enable us to use OVA-specific reagents. Our results indicate that the majority of M. tuberculosis-specific CD8 T cells are limited to either cytotoxicity or the secretion of gamma interferon (IFN-␥), with cytotoxicity being far more prevalent than IFN-␥ secretion. Memory CD8 T cells responded earlier and reached higher levels in the lungs than naïve CD8 T cells, as was expected. They were, however, less cytotoxic and secreted less IFN-␥ than newly primed CD8 T cells, suggesting that one factor contributing to bacterial persistence and lack of sterilizing immunity may be the low quality of memory cells that are generated.