Introduction

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Pneumonia is the sixth leading cause of death overall and is the leading infectious cause of death in patients 65 yr and older in the United States (1, 2). Estimated costs of treatment, including direct patient-care costs and lost wages, exceed $20 billion yearly (2). Although much progress has been made in the treatment of pneumonia, the emergence of new pathogens such as Legionella and Chlamydia, antibiotic resistant strains of Streptococcus pneumoniae, and increasing numbers of immunosuppressed patients represent great therapeutic challenges.

One strategy to improve therapeutic outcomes is to use immunotherapy with cytokines or chemokines as an adjunct to antibiotic therapy (3, 4). Interleukin (IL)-17 is a novel cytokine secreted by activated CD4+ T cells (5, 6). In contrast to the relatively restricted expression of IL-17, IL-17 receptors are found on virtually all cells and tissues (6, 7). One of the first biologic effects described for IL-17 was its ability to support granulopoiesis in vitro by stimulating hematopoietic growth factors from a bone-marrow feeder cell monolayer (8). Our laboratory has also reported that IL-17 can stimulate granulopoiesis in vivo via proliferation and expansion of granulocyte precursors in the spleen and bone marrow (9). This expansion of precursors is due in part to upregulation of the transmembrane form of stem-cell factor and IL-17-mediated stimulation of granulocyte colony-stimulating factor (G-CSF) (10). IL-17 has also been shown to induce secretion of IL-1 and tumor necrosis factor (TNF)- by human monocyte- derived macrophages (11).

Studies investigating intratracheal administration of recombinant IL-17 have shown that administration of IL-17 to the lungs of normal mice can stimulate the local release of macrophage inflammatory protein (MIP)-2, and IL-1, which resulted in recruitment of polymorphonuclear leukocytes (PMNs) into the lung (12). On the basis of these data, we hypothesized that upregulation of IL-17 in the lung would augment local MIP-2, IL-1, and G-CSF, key factors of innate host defense, and thus improve antibacterial host defenses in a model of bacterial pneumonia.

	Materials and Methods

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Construction, Propagation, and Purification of AdIL-17

To achieve local production of IL-17 in the lung, we constructed a recombinant adenovirus encoding murine IL-17 (AdIL-17) (9). Briefly, the murine IL-17 complementary DNA encoding secreted IL-17 preceded by the IL-7 signal peptide (provided by Immunex Corp., Seattle, WA) was amplified by polymerase chain reaction (PCR) using KlenTaq (Clontech, Palo Alto, CA) and specific primers. The 5' primer was modified to contain a Kozak consensus sequence immediately before the start codon and contained a KpnI site at the 5' end. The 3' primer contained an XbaI site in the 3' end. A single 498-base pair PCR product was obtained and cloned into pACCMV.PLA (13). The sequence of this construct was verified by dideoxynucleotide thermal cycling sequencing. This vector was cotransfected into 911 cells (14) with XbaI-restricted AdCMVLacZ DNA using calcium-phosphate precipitation, and plaques were screened by blue-white screen as described by Schaack and colleagues (15). AdIL-17 clones were further screened by PCR, protein production was confirmed by an IL-17 enzyme-linked immunosorbent assay (ELISA), and bioassay was tested on the ability of secretion of IL-6 by NIT3T3 cells after incubation with the supernatant of AdIL-17 (16). One high-producing clone was chosen for all subsequent in vivo studies.

Viruses were propagated on 911 cells using endotoxin-free conditions and purified by CsCl. Virus preparations were screened for replication-competent adenovirus by propagation on A549 cells as previously described (17). This assay has a sensitivity of 1 contaminant per 10⁸ plaque-forming units (PFU). All lots of virus had a PFU/particle ratio < 100:1 and contained less than 1 endotoxin unit/mL as measured by the Limulus amebocyte lysate assay (BioWhittaker, Walkersville, MD).

Animals and Animal Procedures

Specific pathogen-free C57BL/6 mice (males, 6 to 8 wk old, average weight 25 g; NCI, Frederick, MD) were used in all experiments. All mice were housed in a specific pathogen-free room within the animal care facility of the Louisiana State University Health Sciences Center vivarium under an IACUC-approved protocol, provided with water and food ad libitum, and housed under 12-h light/dark cycles until the date of the experiment.

To overexpress IL-17 in the lung, AdIL-17 was administered through intratracheal (i.t.) injection. Pilot experiments were performed with 10⁸, 5 × 10⁸, and 10⁹ PFU of AdIL-17 given intratracheally to mice. Mice were killed 72 h later and IL-17 and neutrophil recruitment were measured in lavage fluid. The quantity of 10⁹ PFU of AdIL-17 resulted in the greatest amount of IL-7 in the bronchoalveolar lavage fluid (BALF) as well as neutrophil recruitment (data not shown), and thus this dose was chosen for subsequent studies. Mice were anesthetized using ether, the trachea was exposed, and 10⁹ PFU of either AdIL-17 or AdLuc, a control adenovirus expressing firefly luciferase (obtained from Dr. Robert Gerard, University of Texas Southwestern Medical Center, Dallas, TX) diluted with phosphate-buffered saline (PBS) to a final volume of 100 µL was injected through a 28-gauge needle. A separate group of mice received 100 µL of PBS as a control. For pharmacokinetic studies of IL-17, mice were anesthetized at designated time points with inhaled ether and killed by transection of the abdominal aorta, and the lungs were removed. Serum was frozen at 80°C. The lungs were lavaged with 11 ml of PBS plus 0.5 mM ethylenediaminetetraacetic acid, and the first milliliter of BALF was centrifuged at 500 × g for cell recovery. The supernatant was frozen at 80°C for later cytokine analysis. The cell pellet was combined with the remaining BALF (9.5 to 10 ml in all cases) and was used for total cell count and cytospins for cell differential.

Experimental Klebsiella pneumoniae Infection

Klebsiella pneumoniae ATCC strain 43816, serotype 2 (ATCC, Rockville, MD) was grown in 100 mL of tryptic soy broth (Difco, Detroit, MI) for 18 h at 37°C. The quantity of 1 mL of the culture was added to 100 mL of fresh tryptic soy broth, grown for 2 h, allowing the culture to reach early log phase. The concentration of K. pneumoniae was determined by measuring the absorbance at 600 nm. A standard curve of absorbance units based on known colony-forming units (CFU) was used to calculate inoculum concentration. Bacteria were pelleted by centrifugation at 5,000 rpm for 15 min, washed twice in PBS, and resuspended at the desired concentration.

For induction of experimental pneumonia, mice were anesthetized with ether. The trachea was exposed, and 100 µL of the K. pneumoniae inoculum was administered via a 28-gauge needle. The skin incision was then closed with surgical staples. To investigate the effect of IL-17 pretreatment on early bacterial clearance and survival, mice were randomized to receive 10⁹ PFU of AdIL-17, AdLuc, or PBS intratracheally 72 h before i.t. challenge with 10⁴ CFU/mouse K. pneumoniae. To determine lung bacterial clearance, lungs were removed aseptically at 0 or 6 h after bacterial inoculation and placed in sterile PBS. The tissues were homogenized in a tissue homogenizer, and 1:10 serial dilutions were made, plated on soy-based blood agar plates (Difco), and incubated for 18 h at 37°C; the colonies were then counted and expressed as CFU/mouse. A subgroup of mice (n = 10 each group) were monitored every 12 h for survival up to 10 d. To investigate whether AdIL-17 had a beneficial effect on survival in a simultaneous treatment model, mice were randomized to receive 10⁹ PFU of AdIL-17, AdLuc, or PBS at the same time as the i.t. with 10⁴ CFU/mouse K. pneumoniae. As described earlier, these mice (n = 10-14 each group) were also monitored every 12 h for survival up to 10 d.

ELISA

ELISA kits for mouse IL-17, MIP-2, IL-1, and TNF- were purchased from R&D Systems (Minneapolis, MN) and assays were run following the manufacturer's protocol. G-CSF protein concentrations were determined using a specific enzyme-linked immunoassay developed in our laboratory using described procedures previously reported in detail (10). Briefly, the ELISA was performed using native and biotinylated forms of the previously described anti-G-CSF rabbit polyclonal antibody (Ab) as the capture and detection Abs, respectively. G-CSF concentrations were calculated from a standard curve of recombinant G-CSF (Amgen, Thousand Oaks, CA) using log-log linear regression. The assay had an interassay coefficient of variation of 4.4%, and the assay failed to detect 1200 pg/ml homologous mouse proteins (growth hormone, prolactin) or murine granulocyte macrophage- CSF, IL-3, or IL-6, or Escherichia coli lipopolysaccharide (LPS).

CD11b/c or CD18 Expression

To measure CD11b/c or CD18 expression on neutrophils recruited to the lung, lavage fluid from five mice was pooled and neutrophils were isolated using a discontinuous Ficoll-Hypaque density gradient (Sigma 1119-1; Sigma Chemical, St. Louis, MO). Viability of the purified PMNs were > 95% as determined by trypan blue exclusion. For the measurement of CD11b/c or CD18 expression, 1 × 10⁶ of PMNs were suspended in 100 µL Dulbecco's modified Eagle's medium (DMEM) containing 5% fetal bovine serum (FBS) and 1 µg of fluorescein isothiocyanate-conjugated anti-CD11b/c or anti-CD18 monoclonal Ab or the related isotype Ab (B-D Pharmingen, San Diego, CA). The mixtures were incubated at 4°C in the dark for 30 min. At the end of incubation, the cells were washed twice with cold PBS and finally fixed with 0.5 mL of PBS-buffered 1% paraformaldehyde. Samples were analyzed by flow cytometry as described later.

Phagocytosis

The quantity of 10⁶ PMNs was suspended in 100 µL DMEM containing 1.25 × 10⁸ latex microspheres and 5% FBS in a metabolic shaker at 37°C for 60 min. To measure the nonspecific adhesion of latex microspheres on the surface of cells, an aliquot of the cell/bead mixture was incubated at 4°C simultaneously. At the end of incubation, the cells were washed twice with cold PBS containing 5 mM glucose and 0.1% gelatin. Cells were resuspended in 0.5 mL of the same solution. Samples were analyzed by flow cytometry as described later.

Flow Cytometry

CD11b/c, CD18, and phagocytic activity were measured by flow cytometry (Becton Dickinson FACScalibur) using a 488-nm excitation line from an argon laser. Green fluorescence was monitored at 525-nm light scatter gates (using 90-degree and forward angle light scatter) to identify the PMNs and alveolar macrophages (AMs). A total of 5,000 cells was analyzed within the gated region in each sample. Results are expressed as percentage of cells that engaged in phagocytosis (percentage phagocytosis) and mean channel fluorescence intensity (MCF) for phagocytosis and CD11b/c and CD18 expression.

Statistical Analysis

Statistical comparisons were made between experimental and control groups. Where differences were noted among treatment responses, multiple comparison tests were conducted using analysis of variance (ANOVA) with Fisher's PLSD follow-up testing at the 0.05 significance level. Statistical analyses of survival curves were performed by the log-rank test using the Prism software program (GraphPad Software).

	Results

Top Abstract Introduction Materials and Methods Results Discussion References

IL-17 Is Produced in a Compartmentalized Fashion in Response to Intrapulmonary K. pneumoniae Infection

Male C57BL/6 mice, 6 to 8 wk old, were challenged with 10⁴ CFU of K. pneumoniae or PBS as a control and killed at serial intervals for determinations of IL-17 in serum and BALF, and lung burden of K. pneumoniae. IL-17 was detectable in BALF 12 h after bacterial challenge and was maintained at 451 ± 28 pg/ml 48 h after bacterial challenge (Figure 1A). The IL-17 level correlated with the bacterial burden (Figure 1B) in this animal model. Despite significant levels of IL-17 in BALF, no IL-17 was detectable in serum at any time studied after challenge. Moreover, no IL-17 was detected in BALF or serum in mice inoculated with PBS (data not shown). We did not report IL-17 data after 48 h because mortality in this model would bias the results to surviving animals.

View larger version (15K): [in this window] [in a new window]   Figure 1.   (A) Local production of IL-17 in lung homogenate in response to K. pneumoniae. C57BL/6 mice (n = 6) were challenged with 104 CFU of K. pneumoniae and IL-17 was measured at serial time points in lung homogenate by ELISA. No IL-17 was detected in mice challenged with PBS (n = 4-6 per time point, data not shown; *P < 0.05 compared with 0- or 6-h data by ANOVA). (B) Growth curve of K. pneumoniae in C57BL/6 mouse lung homogenates (n = 6 each time point).

AdIL-17 Pharmacokinetics

To investigate the local release of IL-17 in the lung after adenoviral-mediated gene transfer of the cytokine gene, mice were given intratracheal injections of 10⁹ PFU/mouse of AdIL-17 or AdLuc or an equal volume of PBS as a control. Mice were killed at 24, 48, 72, and 96 h and at 7 and 14 d, and IL-17 was measured in BALF and serum. IL-17 levels peaked at 72 h after AdIL-17 administration with a concentration of 1,625 ± 184 pg/ml in BALF (Figure 2). No IL-17 was detectable in the serum of AdIL-17-treated animals or in the serum or BALF of AdLuc or PBS control animals. BALF IL-17 levels fell to undetectable levels by Day 14, similar to other cytokine genes delivered intratracheally by E1-deleted adenoviral vectors (18).

View larger version (11K): [in this window] [in a new window]   Figure 2.   Pharmacokinetics of AdIL-17. C57BL/6 mice received 109 PFU/mouse of AdIL-17 or AdLuc, or an equal volume of PBS intratracheally (n = 6-8; only AdIL-17 data shown). Mice were killed at specific time points and IL-17 was measured in BALF by ELISA. No IL-17 was detectable in AdLuc- or PBS-treated mice (*P < 0.05 compared with Day 0 or AdLuc-treated mice).

IL-17 Induces the Local Release of MIP-2, TNF-, IL-1, and G-CSF

On the basis of prior studies demonstrating that IL-17 can stimulate CXC chemokine and G-CSF production, we investigated whether AdIL-17 stimulated the local expression of MIP-2, TNF-, G-CSF, and IL-1 in lung BALF. These chemokines/cytokines were measured by ELISA in BALF at serial intervals after i.t. administration of 10⁹ PFU/mouse of AdIL-17, AdLuc, or PBS. All four growth factors were detectable in BALF as early as 12 h after AdIL-17 administration (Figure 3). Maximal TNF-, MIP-2, and IL-1 levels were measured at 12 h and the levels of these cytokines slowly declined up to 72 h. Interestingly, G-CSF, a growth factor, previously shown by our group not to be compartmentalized in the lung (19), showed a maximal response at 24 h. This G-CSF response in BALF is similar to the maximal systemic G-CSF response, which occurs 24 h, after systemic administration of AdIL-17 (9). The levels of MIP-2, TNF-, G-CSF, and IL-1 were all < 50 pg/ml at all time points in AdLuc or PBS control BALF (data not shown).

View larger version (16K): [in this window] [in a new window]   Figure 3.   Release of proinflammatory cytokines by AdIL-17 in the lung. C57BL/6 mice received 109 PFU/mouse of AdIL-17 or AdLuc, or an equal volume of PBS intratracheally (n = 6-8 each group; only AdIL-17 data shown). Mice were killed at specific time points and MIP-2, TNF, IL-1, and G-CSF were measured in BALF by ELISA. MIP-2, TNF, IL-1, and G-CSF levels in BALF from AdLuc- and PBS-treated mice were all < 50 pg/ml at each time point. All points were statistically significant compared with control mice.

AdIL-17 Induces Lung Neutrophil Recruitment

We investigated the cellular profile in the BALF after the i.t. injection of 10⁹ PFU/mouse of AdIL-17, AdLuc, or PBS. At serial intervals after i.t. injection, total and differential cell counts were determined in BALF. AdIL-17 resulted in a significant increase in the BALF absolute neutrophil count (ANC) compared with AdLuc or PBS control groups (Figure 4). In a separate experiment, mice were divided into three groups (n = 8) and received 10⁹ PFU of AdIL-17, or AdLuc, or 100 µL of i.t. PBS. At 72 h later, lung PMNs were isolated for CD11b/c and CD18 phenotyping and PMN phagocytosis by flow cytometry. These data show that over 99% of both the AdIL-17-recruited and AdLuc-recruited PMNs into the lung were CD11b/c- and CD18-positive and over 97 ± 3.2% had phagocytic ability (defined as the ability to phagocytose at least one bead in the assay; data not shown). Moreover, the numbers of beads phagocytized by PMNs in the assay, as measured by the MCF, were similar between AdIL-17 and AdLuc, with values of 573.2 ± 48.6 and 546.8 ± 32.9, respectively (n = 3-4; P = not significant). The BALF in the PBS group contained over 97% macrophages, and thus there were too few PMNs to analyze in the PBS control group.

View larger version (12K): [in this window] [in a new window]   Figure 4.   IL-17 induced lung neutrophil recruitment. C57BL/6 mice received 109 PFU/mouse of AdIL-17 or AdLuc, or an equal volume of PBS intratracheally (n = 6-8). Mice were killed at specific time points and ANC was determined in the total recovered BALF as outlined in MATERIALS AND METHODS. *P < 0.05 compared with respective controls.

AdIL-17 Improves Lung Host Defense in a Pretreatment Model

C57BL/6 mice were pretreated with 10⁹ PFU/mouse of AdIL-17 or AdLuc or an equal volume of PBS. Mice were challenged intranasally 72 h later with 10⁴ CFU/mouse of K. pneumoniae, and killed immediately or 6 h later to assess bacterial clearance. A separate group of mice were monitored for survival. AdIL-17 pretreatment resulted in a significant enhancement in lung bacterial clearance (Figure 5A). This increase in lung bacterial clearance was associated with a significant increase in pre- and post-challenge ANC in AdIL-17-treated animals (Figure 5B) and improved survival (Figure 5C). Moreover, this increase in lung neutrophil recruitment, as measured by ANC in BALF, was also associated with a significantly elevated pre-bacterial challenge level of the CXC chemokine MIP-2, IL-1, and G-CSF (Figure 6). AdIL-17 pretreatment also significantly augmented the local release of MIP-2, IL-1, and G-CSF in BALF after the challenge with K. pneumoniae, compared with AdLuc- or PBS-treated control animals (Figure 6; note logarithmic scale). At 6 h after K. pneumoniae challenge, AdIL-17-treated animals had 973 ± 49 pg/ml of MIP-2 compared with 583 ± 98 and 316 ± 46 in AdLuc and PBS controls, respectively. Moreover, AdIL-17-treated mice showed higher G-CSF levels after bacterial challenge with 138 ± 9.6 pg/ml compared with 56.3 ± 8.2 and 27.5 ± 2.8 pg/ml in AdLuc and PBS controls, respectively. Simultaneous administration of AdIL-17 with K. pneumoniae, however, had no survival benefit (data not shown).

View larger version (20K): [in this window] [in a new window]   Figure 5.   (A) AdIL-17 augmented lung clearance of K. pneumoniae. C57BL/6 mice received 109 PFU/mouse of AdIL-17 or AdLuc, or an equal volume of PBS intratracheally (n = 10-12; *P < 0.05 compared with AdLuc or PBS controls by ANOVA). Mice were then challenged with 104 CFU/mouse K. pneumoniae intranasally and killed immediately or 6 h later. Data are expressed as the percentage of bacteria remaining in lung homogenates at 6 h. (B) AdIL-17 augmented basal and bacterial-induced increases in lung neutrophils. C57BL/6 mice received 109 PFU/mouse of AdIL-17 or AdLuc, or an equal volume of PBS intratracheally (n = 10-12; *P < 0.05 compared with AdLuc and PBS controls as well as pre-AdIL-17 levels by ANOVA). Mice were killed at 72 h or challenged with 104 CFU/mouse K. pneumoniae intranasally and killed 6 h later. ANC was determined in BALF and plotted as pre- or post-bacterial challenge. (C) AdIL-17 pretreatment increased early survival after K. pneumoniae challenge. C57BL/6 mice received 109 PFU/mouse of AdIL-17 or AdLuc, or an equal volume of PBS intratracheally (data represent the average values of two separate experiments; n = 10-12 each group; *P < 0.05 compared with respective controls by log-rank test). Mice were then challenged with 104 CFU/mouse K. pneumoniae intranasally and monitored every 12 h for survival.

View larger version (19K): [in this window] [in a new window]   Figure 6.   AdIL-17 increased basal bacterial-induced MIP-2, IL-1, and G-CSF production in the lung. C57BL/6 mice received 109 PFU/mouse of AdIL-17 (A) or AdLuc (B), or an equal volume of PBS (C) intratracheally (n = 10-12). Mice were killed at 72 h or challenged with 104 CFU/mouse K. pneumoniae intranasally and killed 6 h later. MIP-2, TNF-, IL-1, and G-CSF were determined in BALF by ELISA and plotted as pre- or post-bacterial challenge. *P < 0.05 compared with both AdIL-17 pre-challenge, and AdLuc and PBS post-challenge controls.

	Discussion

Top Abstract Introduction Materials and Methods Results Discussion References

Bacterial pneumonia is a leading cause of morbidity and mortality in both developed and developing countries. Pneumonia is also a very common disease in patients with human immunodeficiency virus (HIV). Although tremendous advances have been made in the treatment of pneumonia using broad-spectrum antibiotic regimens, these approaches have also resulted in the recent emergence of antibiotic-resistant bacteria. A better understanding of the innate immune response to bacterial pathogens may offer improved therapy.

In this study we used a well described mouse model of K. pneumoniae lung infection (20, 21) to study IL-17 in the host response to this bacterial infection. Our results indicate that IL-17 is produced as part of the normal immune response to this organism and that the release of IL-17 parallels lung neutrophil recruitment and the lung bacterial burden of K. pneumoniae. Although the cellular source is not defined, it is likely a product of resident CD4 or CD8+ T cells in the lung, inasmuch as these cells are detectable by flow cytometry in BALF as early as 12 h after K. pneumoniae challenge (our unpublished observations). This is the first in vivo report that bacterial pathogens elicit IL-17. Our data are consistent with the data of Infante-Duarte and colleagues, who demonstrated that microbial lipopeptides from Borrellia burgdorferi can elicit IL-17 from CD4+ T cells in vitro (22). Moreover, it has recently been reported that passive neutralization of IL-17 with an anti-IL-17 antibody inhibits LPS-induced lung neutrophil recruitment by over 90% (23, 24). Taken together, these data suggest that IL-17 is a critical cytokine involved in the innate immune response to K. pneumoniae and perhaps other pathogens.

Overexpression of IL-17 in the lung through the administration of AdIL-17 resulted in the enhanced production of cytokines critical to innate lung immunity, including MIP-2, G-CSF, TNF-, and IL-1. We believe that increased release of these cytokines explains in part the capacity of IL-17 gene transfer to improve bacterial clearance and survival. Our data are consistent with those of Laan and colleagues, who demonstrated local of release of MIP-2 in the lung by administration of recombinant IL-17 (12); and data from Jovanovic and associates demonstrating that recombinant IL-17 can induce IL-1 and TNF in human macrophages in vitro (11). The cellular target of IL-17 in lung tissue remains unclear at present. Our laboratory has not been able to detect IL-17 receptor expression on murine alveolar macrophages, or augmented TNF production in AMs stimulated with IL-17 ex vivo (unpublished observation). Thus, IL-17 may act on other cells in the lung, such as airway epithelial cells, a significant source of CXC chemokines (25, 26), or may augment macrophage production of cytokines via an indirect mechanism. Enhanced elaboration of MIP-2 and other growth factors by IL-17 was associated with significant lung neutrophil recruitment and augmented bacterial clearance. Based on CD11/CD18 and PMN phagocytosis studies, our results indicate that the PMNs recruited into lung by IL-17 are mature and functional. In addition to increasing basal levels of MIP-2, IL-1, and G-CSF, AdIL-17 primed lung tissue for enhanced release of these growth factors after bacterial challenge. Thus, these are the first in vivo data to suggest that IL-17 is capable of amplifying or priming the proinflammatory response in the lung.

Human IL-17 is a novel cytokine secreted by activated T cells, which in humans can induce IL-6 and IL-8 from fibroblasts and stimulate granulopoiesis in bone marrow partially through G-CSF (10). IL-17 may increase G-CSF by enhancing G-CSF messenger RNA stability (27). In our study, G-CSF was locally released in response to AdIL-17 in the lung. We have previously shown that G-CSF is detectable in the circulation after bacterial pulmonary challenge, whereas MIP-2 and TNF are compartmentalized in the BALF (28, 29). Thus, local IL-17 production may be a mechanism to enhance local G-CSF production, which serves to stimulate bone-marrow granulopoiesis in response to lung infections. Moreover, as a product of CD4+ T cells, a lack of IL-17 production may in part explain the pulmonary host defense defect observed in patients with deficient numbers of CD4+ T cells, such as those with HIV infection.

In summary, our studies demonstrate that IL-17 is produced locally in lung tissue as part of the normal host response to a bacterial challenge with K. pneumoniae. Further, overexpression of IL-17 in lung through adenovirus IL-17 gene transfer can augment lung host defense and early survival after bacterial infection, and reduce bacterial growth. These effects are likely due to induction of MIP-2 and G-CSF in lung, which then recruit PMNs into the lungs. Further work will be needed to understand how IL-17 contributes to lung host defense and the potential role of IL-17 in immunotherapy.