Bacterial pneumonia is an increasing complication of HIV infection and inversely correlates with the CD4+ lymphocyte count. Interleukin (IL)-17 is a cytokine produced principally by CD4+ T cells, which induces granulopoiesis via granulocyte colony-stimulating factor (G-CSF) production and induces CXC chemokines. We hypothesized that IL-17 receptor (IL-17R) signaling is critical for G-CSF and CXC chemokine production and lung host defenses. To test this, we used a model of Klebsiella pneumoniae lung infection in mice genetically deficient in IL-17R or in mice overexpressing a soluble IL-17R. IL-17R–deficient mice were exquisitely sensitive to intranasal K. pneumoniae with 100% mortality after 48 h compared with only 40% mortality in controls. IL-17R knockout (KO) mice displayed a significant delay in neutrophil recruitment into the alveolar space, and had greater dissemination of K. pneumoniae compared with control mice. This defect was associated with a significant reduction in steady-state levels of G-CSF and macrophage inflammatory protein (MIP)-2 mRNA and protein in the lung in response to the K. pneumoniae challenge in IL-17R KO mice. Thus, IL-17R signaling is critical for optimal production of G-CSF and MIP-2 and local control of pulmonary K. pneumoniae infection. These data support impaired IL-17R signaling as a potential mechanism by which deficiency of CD4 lymphocytes predisposes to bacterial pneumonia.

Interleukin (IL)-23 is a heterodimeric cytokine that shares the identical p40 subunit as IL-12 but exhibits a unique p19 subunit similar to IL-12 p35. IL-12/23 p40, interferon γ (IFN-γ), and IL-17 are critical for host defense against Klebsiella pneumoniae. In vitro, K. pneumoniae–pulsed dendritic cell culture supernatants elicit T cell IL-17 production in a IL-23–dependent manner. However, the importance of IL-23 during in vivo pulmonary challenge is unknown. We show that IL-12/23 p40–deficient mice are exquisitely sensitive to intrapulmonary K. pneumoniae inoculation and that IL-23 p19−/−, IL-17R−/−, and IL-12 p35−/− mice also show increased susceptibility to infection. p40−/− mice fail to generate pulmonary IFN-γ, IL-17, or IL-17F responses to infection, whereas p35−/− mice show normal IL-17 and IL-17F induction but reduced IFN-γ. Lung IL-17 and IL-17F production in p19−/− mice was dramatically reduced, and this strain showed substantial mortality from a sublethal dose of bacteria (103 CFU), despite normal IFN-γ induction. Administration of IL-17 restored bacterial control in p19−/− mice and to a lesser degree in p40−/− mice, suggesting an additional host defense requirement for IFN-γ in this strain. Together, these data demonstrate independent requirements for IL-12 and IL-23 in pulmonary host defense against K. pneumoniae, the former of which is required for IFN-γ expression and the latter of which is required for IL-17 production.

Bacterial pneumonia remains an important cause of morbidity and mortality worldwide, especially in immune-compromised patients. Cytokines and chemokines are critical molecules expressed in response to invading pathogens and are necessary for normal lung bacterial host defenses. Here we show that interleukin (IL)-17, a novel cytokine produced largely by CD4+ T cells, is produced in a compartmentalized fashion in the lung after challenge with Klebsiella pneumoniae. Moreover, overexpression of IL-17 in the pulmonary compartment using a recombinant adenovirus encoding murine IL-17 (AdIL-17) resulted in the local induction of tumor necrosis factor-alpha, IL-1beta, macrophage inflammatory protein-2, and granulocyte colony-stimulating factor (G-CSF); augmented polymorphonuclear leukocyte recruitment; and enhanced bacterial clearance and survival after challenge with K. pneumoniae. However, simultaneous treatment with AdIL-17 provided no survival benefit after intranasal K. pneumoniae challenge. These data show that IL-17 may have a role in priming for enhanced chemokine and G-CSF production in the context of lung infection and that optimally timed gene therapy with IL-17 may augment host defense against bacterial pneumonia.

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.

Every year in the United States approximately 200,000 people die from pulmonary infections, such as influenza and pneumonia, or from lung disease that is exacerbated by pulmonary infection. In addition, respiratory diseases such as, asthma, affect 300 million people worldwide. Therefore, understanding the mechanistic basis for host defense against infection and regulation of immune processes involved in asthma are crucial for the development of novel therapeutic strategies. The identification, characterization, and manipulation of immune regulatory networks in the lung represents one of the biggest challenges in treatment of lung associated disease. Recent evidence suggests that the gastrointestinal (GI) microbiota plays a key role in immune adaptation and initiation in the GI tract as well as at other distal mucosal sites, such as the lung. This review explores the current research describing the role of the GI microbiota in the regulation of pulmonary immune responses. Specific focus is given to understanding how intestinal “dysbiosis” affects lung health.

Innate immune mechanisms against Pneumocystis carinii, a frequent cause of pneumonia in immunocompromised individuals, are not well understood. Using both real time polymerase chain reaction as a measure of organism viability and fluorescent deconvolution microscopy, we show that nonopsonic phagocytosis of P. carinii by alveolar macrophages is mediated by the Dectin-1 β-glucan receptor and that the subsequent generation of hydrogen peroxide is involved in alveolar macrophage–mediated killing of P. carinii. The macrophage Dectin-1 β-glucan receptor colocalized with the P. carinii cyst wall. However, blockage of Dectin-1 with high concentrations of anti–Dectin-1 antibody inhibited binding and concomitant killing of P. carinii by alveolar macrophages. Furthermore, RAW 264.7 macrophages overexpressing Dectin-1 bound P. carinii at a higher level than control RAW cells. In the presence of Dectin-1 blockage, killing of opsonized P. carinii could be restored through FcγRII/III receptors. Opsonized P. carinii could also be efficiently killed in the presence of FcγRII/III receptor blockage through Dectin-1–mediated phagocytosis. We further show that Dectin-1 is required for P. carinii–induced macrophage inflammatory protein 2 production by alveolar macrophages. Taken together, these results show that nonopsonic phagocytosis and subsequent killing of P. carinii by alveolar macrophages is dependent upon recognition by the Dectin-1 β-glucan receptor.

Pulmonary infections with Pneumocystis carinfi are an important cause of morbidity and mortality in patients with AIDS. P. carinii infections are seen in patients with decreased numbers of helper T lymphocytes, suggesting that these cells are important in preventing infection. To test this hypothesis, we sought to establish experimental infection with P. carinfi in mice selectively depleted of helper T lymphocytes. Weekly injections of a monoclonal anti-CD4 antibody produced sustained depletion of helper T lymphocytes from blood and lymphoid organs. To establish pulmonary infection, lymphocytedepleted mice were then given intratracheal inoculations of P. carinji organisms derived from the lungs of chronically infected athymic mice. Pulmonary infection with P. carinli was demonstrable in the antibody-treated mice and was centered around the conducting airways. Infection was persistent for up to 3 mo with continued antibody treatments, and yet could be cleared from the lungs if antibody treatments were discontinued. This experimental model of P. carinji infection permits

Host defense mechanisms against Pneumocystis carinii are not fully understood. Previous work in the murine model has shown that host defense against infection is critically dependent upon host CD4 ؉ T cells. The recently described Th17 immune response is predominantly a function of effector CD4 ؉ T cells stimulated by interleukin-23 (IL-23), but whether these cells are required for defense against P. carinii infection is unknown. We tested the hypothesis that P. carinii stimulates the early release of IL-23, leading to increases in IL-17 production and lung effector CD4 ؉ T-cell population that mediate clearance of infection. In vitro, stimulation of alveolar macrophages with P. carinii induced IL-23, and IL-23p19 mRNA was expressed in lungs of mice infected with this pathogen. To address the role of IL-23 in resistance to P. carinii, IL-23p19؊/؊ and wild-type control C57BL/6 mice were infected and their fungal burdens and cytokine/chemokine responses were compared. IL-23p19؊/؊ mice displayed transient but impaired clearance of infection, which was most apparent 2 weeks after inoculation. In confirmatory studies, the administration of either anti-IL-23p19 or anti-IL-17 neutralizing antibody to wild-type mice infected with P. carinii also caused increases in fungal burdens. IL-17 and the lymphocyte chemokines IP-10, MIG, MIP-1␣, MIP-1␤, and RANTES were decreased in the lungs of infected IL-23p19؊/؊ mice in comparison to their levels in the lungs of wild-type mice. In IL-23p19؊/؊ mice infected with P. carinii, there were fewer effector CD4 ؉ T cells in the lung tissue. Collectively, these studies indicate that the IL-23-IL-17 axis participates in host defense against P. carinii.