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.

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.

Toll-like receptor 4 (TLR4) has been identified as a receptor for lipopolysaccharide. However, the precise role of TLR4 in regulating gene expression in response to an infection caused by gram-negative bacteria has not been fully elucidated. The role of TLR4 signaling in coordinating gene expression was assessed by gene expression profiling in lung tissue in a mouse model of experimental pneumonia with a low-dose infection of Klebsiella pneumoniae. We analyzed four mouse strains: C57BL/6 mice, which are resistant to bacterial dissemination; 129/SvJ mice, which are susceptible; C3H/HeJ mice, which are susceptible and have defective TLR4 signaling; and their respective control strain, C3H/HeN (intermediate resistance). At 4 h after infection, C57BL/6 and C3H/HeN mice demonstrated the greatest number of genes, with 67 shared induced genes which were TLR4 dependent and highly associated with the resistance phenotype. These genes included cytokine and chemokine genes required for neutrophil activation or recruitment, growth factor receptors, MyD88 (a critical adaptor protein for TLR signaling), and adhesion molecules. TLR4 signaling accounted for over 74% of the gene expression in the C3H background. These data suggest that early TLR4 signaling controls the vast majority of gene expression in the lung in response to an infection caused by gram-negative bacteria and that this subsequent gene expression determines survival of the host.

Host defenses are profoundly compromised in HIV-infected hosts due to progressive depletion of CD4+ T lymphocytes. A hallmark of HIV infection is Pneumocystis carinii (PC) pneumonia. Recently, CD8+ T cells, which are recruited to the lung in large numbers in response to PC infection, have been associated with some level of host defense as well as contributing to lung injury in BALB/c mice. In this study, we show that CD8+ T cells that have a T cytotoxic-1 response to PC in BALB/c mice, as determined by secretion of IFN-γ, have in vitro killing activity against PC and effect clearance of the organism in adoptive transfer studies. Moreover, non-T cytotoxic-1 CD8+ T cells lacked in vitro effector activity and contributed to lung injury upon adoptive transfer. This dichotomous response in CD8+ T cell response may in part explain the clinical heterogeneity in the severity of PC pneumonia.

Although a clear relationship between ␣␤ T-cell receptor-positive (␣␤-TCR؉ ) CD4 ؉ T cells and susceptibility to Pneumocystis carinii infection exists, the role of other T-cell subsets is less clearly defined. Previous studies have shown that ␥␦-TCR ؉ T cells infiltrate into the lung during P. carinii pneumonia. Therefore, the present study examined the role of ␥␦-TCR ؉ T cells in host defense against P. carinii pneumonia. C57BL/6 (control) and B6.129P2-Tcrd tm1Mom (␥␦-TCR ؉ T-cell-deficient) mice were inoculated intratracheally with P. carinii. At specific time points, mice were sacrificed and analyzed for P. carinii burden, T-cell subsets, and cytokine levels in lung tissue. Analysis of P. carinii burden showed a more rapid and complete resolution of infection in ␥␦-TCR ؉ T-cell-deficient mice than in C57BL/6 controls. This augmented resolution was associated with elevated gamma interferon (IFN-␥) levels in bronchoalveolar lavage fluid predominantly produced by CD8؉ T cells, as well as an increased recruitment of CD8 ؉ T cells in general. In separate experiments, neutralization of IFN-␥ or depletion of CD8 ؉ T cells early during infection abolished the augmented resolution previously observed in ␥␦-TCR ؉ T-cell-deficient mice. These results show that the presence of ␥␦-TCR ؉ T cells modulates host susceptibility to P. carinii pneumonia through interactions with pulmonary CD8