Mechanisms of neutrophil accumulation in the lungs against bacteria

Effect on extrapulmonary sepsis-induced acute lung injury by hemoperfusion with neutral microporous resin column

The aim of this study was to investigate the effect of neutral microporous resin hemoperfusion on oxygenation improvement, removal of inflammatory cytokines in plasma and bronchoalveolar lavage, and mortality in acute lung injury induced by extrapulmonary sepsis. Forty-six patients with acute lung injury induced by extrapulmonary sepsis were randomized to HA type hemoperfusion treatment (N=25) or standard therapy (N=21). Those undergoing hemoperfusion treatment received HA330 hemoperfusion. We measured the plasma and bronchoalveolar lavage concentrations of TNF-α and IL-1, and the following parameters were compared between the control group and the hemoperfusion group on days 0, 3 and 7: lung injury measurements (arterial oxygen tension/fractional inspired oxygen ratio, lung injury score, chest X-ray score); interstitial edema of lung (extravascular lung water). Duration of mechanical ventilation, hospital, 28-day, and intensive care unit mortality were also observed. Patients treated with HA hemoperfusion showed a significant removal of plasma and bronchoalveolar lavage TNF-α and IL-1 over time while in the study. Patients in the HA group also demonstrated not only significant improvement of PaO2 /FiO2 , but also decreased Lung Injury Score and chest X-ray score at days 3 and 7. Furthermore, the measurements of the arterial oxygen tension/fractional inspired oxygen ratio, lung injury score and extravascular lung water (EVLWI) significantly correlated with and the concentration of cytokines in the plasma (all P<0.05). The HA hemoperfusion treatment group had a significant reduction in duration of mechanical ventilation, length of intensive care unit stay, and intensive care unit mortality. Significant removal of inflammatory cytokines from circulation and lung by hemoperfusion treatment using the HA type cartridge may contribute to the improvement of lung injury and intensive care unit outcome in extrapulmonary septic patients.

Delayed resolution of lung inflammation in Il-1rn-/- mice reflects elevated IL-17A/granulocyte colony-stimulating factor expression

IL-1 has been associated with acute lung injury (ALI) in both humans and animal models, but further investigation of the precise mechanisms involved is needed, and may identify novel therapeutic targets. To discover the IL-1 mediators essential to the initiation and resolution phases of acute lung inflammation, knockout mice (with targeted deletions for either the IL-1 receptor-1, i.e., Il-1r1(-/-), or the IL-1 receptor antagonist, i.e., Il-1rn(-/-)) were exposed to aerosolized LPS, and indices of lung and systemic inflammation were examined over the subsequent 48 hours. The resultant cell counts, histology, protein, and RNA expression of key cytokines were measured. Il-1r1(-/-) mice exhibited decreased neutrophil influx, particularly at 4 and 48 hours after exposure to LPS, as well as reduced bronchoalveolar lavage (BAL) expression of chemokines and granulocyte colony-stimulating factor (G-CSF). On the contrary, Il-1rn(-/-) mice demonstrated increased BAL neutrophil counts, increased BAL total protein, and greater evidence of histologic injury, all most notably 2 days after LPS exposure. Il-1rn(-/-) mice also exhibited higher peripheral neutrophil counts and greater numbers of granulocyte receptor-1 cells in their bone marrow, potentially reflecting their elevated plasma G-CSF concentrations. Furthermore, IL-17A expression was increased in the BAL and lungs of Il-1rn(-/-) mice after exposure to LPS, likely because of increased numbers of γδ T cells in the Il-1rn(-/-) lungs. Blockade with IL-17A monoclonal antibody before LPS exposure decreased the resultant BAL neutrophil counts and lung G-CSF expression in Il-1rn(-/-) mice, 48 hours after exposure to LPS. In conclusion, Il-1rn(-/-) mice exhibit delayed resolution in acute lung inflammation after exposure to LPS, a process that appears to be mediated via the G-CSF/IL-17A axis.

NOD2 signaling contributes to host defense in the lungs against Escherichia coli infection

Bacterial pneumonia remains a significant cause of mortality in the United States. The innate immune response is the first line of defense against invading bacteria. Neutrophil recruitment to the lungs is the first step in a multistep sequence leading to bacterial clearance. Ligand interaction with pattern-recognizing receptors (PRRs) leads to chemokine production, which drives neutrophils to the site of infection. Although we demonstrated that RIP2 is important for host defense in the lungs against Escherichia coli, the individual roles of NOD1 and NOD2 in pulmonary defense have not been addressed. Here, we explored the role of NOD2 in neutrophil-mediated host defense against an extracellular pathogen, E. coli. We found enhanced bacterial burden and reduced neutrophil and cytokine/chemokine levels in the lungs of NOD2⁻/⁻ mice following E. coli infection. Furthermore, we observed reduced activation of NF-κB and mitogen-activated protein kinases (MAPKs) in the lungs of NOD2⁻/⁻ mice upon E. coli challenge. Moreover, NOD2⁻/⁻ neutrophils show impaired intracellular bacterial killing. Using NOD2/RIP2⁻/⁻ mice, we observed bacterial burden and neutrophil accumulation in the lungs similar to those seen with NOD2⁻/⁻ mice. In addition, bone marrow-derived macrophages obtained from NOD2/RIP2⁻/⁻ mice demonstrate a reduction in activation of NF-κB and MAPKs similar to that seen with NOD2⁻/⁻ mice in response to E. coli. These findings unveil a previously unrecognized role of the NOD2-RIP2 axis for host defense against extracellular Gram-negative bacteria. This pathway may represent a novel target for the treatment of lung infection/inflammation.

NLRC4 inflammasome-mediated production of IL-1β modulates mucosal immunity in the lung against gram-negative bacterial infection

Bacterial flagellin is critical to mediate NLRC4 inflammasome-dependent caspase-1 activation. However, Shigella flexneri, a nonflagellated bacterium, and a flagellin (fliC) knockout strain of Pseudomonas aeruginosa are known to activate NLRC4 in bone marrow-derived macrophages. Furthermore, the flagellin-deficient fliC strain of P. aeruginosa was used in a mouse model of peritonitis to show the requirement of NLRC4. In a model of pulmonary P. aeruginosa infection, flagellin was shown to be essential for the induction of NLRC4-dependent caspase-1 activation. Moreover, in all P. aeruginosa studies, IL-1β production was attenuated in NLRC4(-/-) mice; however, the role of IL-1β in NLRC4-mediated innate immunity in the lungs against a nonflagellated bacterium was not explored. In this article, we report that NLRC4 is important for host survival and bacterial clearance, as well as neutrophil-mediated inflammation in the lungs following Klebsiella pneumoniae infection. NLRC4 is essential for K. pneumoniae-induced production of IL-1β, IL-17A, and neutrophil chemoattractants (keratinocyte cell-derived chemokines, MIP-2, and LPS-induced CXC chemokines) in the lungs. NLRC4 signaling in hematopoietic cells contributes to K. pneumoniae-induced lung inflammation. Furthermore, exogenous IL-1β, but not IL-18 or IL-17A, partially rescued survival, neutrophil accumulation, and cytokine/chemokine expression in the lungs of NLRC4(-/-) mice following infectious challenge. Furthermore, IL-1R1(-/-) mice displayed a decrease in neutrophilic inflammation in the lungs postinfection. Taken together, these findings provide novel insights into the role of NLRC4 in host defense against K. pneumoniae infection.

Host immune transcriptional profiles reflect the variability in clinical disease manifestations in patients with Staphylococcus aureus infections

Staphylococcus aureus infections are associated with diverse clinical manifestations leading to significant morbidity and mortality. To define the role of the host response in the clinical manifestations of the disease, we characterized whole blood transcriptional profiles of children hospitalized with community-acquired S. aureus infection and phenotyped the bacterial strains isolated. The overall transcriptional response to S. aureus infection was characterized by over-expression of innate immunity and hematopoiesis related genes and under-expression of genes related to adaptive immunity. We assessed individual profiles using modular fingerprints combined with the molecular distance to health (MDTH), a numerical score of transcriptional perturbation as compared to healthy controls. We observed significant heterogeneity in the host signatures and MDTH, as they were influenced by the type of clinical presentation, the extent of bacterial dissemination, and time of blood sampling in the course of the infection, but not by the bacterial isolate. System analysis approaches provide a new understanding of disease pathogenesis and the relation/interaction between host response and clinical disease manifestations.

Syndecan-4 regulates early neutrophil migration and pulmonary inflammation in response to lipopolysaccharide

Proteoglycans (PGs) and their associated glycosaminoglycan side chains are effectors of inflammation, but little is known about changes to the composition of PGs in response to lung infection or injury. The goals of this study were to identify changes to heparan sulfate PGs in a mouse model of gram-negative pneumonia, to identify the Toll-like receptor adaptor molecules responsible for these changes, and to determine the role of the heparan sulfate PG in the innate immune response in the lungs. We treated mice with intratracheal LPS, a component of the cell wall of gram-negative bacteria, to model gram-negative pneumonia. Mice treated with intratracheal LPS had a rapid and selective increase in syndecan-4 mRNA that was regulated through MyD88-dependent mechanisms, whereas expression of several other PGs was not affected. To determine the role of syndecan-4 in the inflammatory response, we exposed mice deficient in syndecan-4 to LPS and found a significant increase in neutrophil numbers and amounts of CXC-chemokines and total protein in bronchoalveolar lavage fluid. In studies performed in vitro, macrophages and epithelial cells treated with LPS had increased expression of syndecan-4. Studies performed using BEAS-2B cells showed that pretreatment with heparin and syndecan-4 decreased the expression of CXCL8 mRNA in response to LPS and TNF-α. These findings indicate that the early inflammatory response to LPS involves marked up-regulation of syndecan-4, which functions to limit the extent of pulmonary inflammation and lung injury.

Role of CXCL5 in leukocyte recruitment to the lungs during secondhand smoke exposure

Chronic obstructive pulmonary disease (COPD) is the third leading cause of mortality in the United States. The major cause of COPD is cigarette smoking. Extensive leukocyte influx into the lungs, mediated by chemokines, is a critical event leading to COPD. Although both resident and myeloid cells secrete chemokines in response to inflammatory stimuli, little is known about the role of epithelial-derived chemokines, such as CXC chemokine ligand (CXCL)5, in the pathogenesis of cigarette smoke-induced inflammation. To explore the role of CXCL5, we generated CXCL5 gene-deficient mice and exposed them to secondhand smoke (SHS) for 5 hours/day for 5 days/week up to 3 weeks (subacute exposure). We observed a reduced recruitment of leukocytes to the lungs of CXCL5(-/-) mice compared with their wild-type (WT) counterparts, and noted that macrophages comprised the predominant leukocytes recruited to the lungs. Irradiation experiments performed on CXCL5(-/-) or WT mice transplanted with WT or CXCL5(-/-) bone marrow revealed that resident but not hematopoietic cell-driven CXCL5 is important for mediating SHS-induced lung inflammation. Interestingly, we observed a significant reduction of monocyte chemotactic protein-1 (MCP-1/CC chemokine ligand 2) concentrations in the lungs of CXCL5(-/-) mice. The instillation of recombinant MCP-1 in CXCL5(-/-) mice reversed macrophage recruitment. Our results also show the reduced activation of NF-κB/p65 in the lungs, as well as the attenuated activation of C-Jun N-terminal kinase, p42/44, and p38 mitogen-activated protein kinases and the expression of intercellular adhesion molecule-1 in the lungs of SHS-exposed CXCL5(-/-) mice. Our findings suggest an important role for CXCL5 in augmenting leukocyte recruitment in SHS-induced lung inflammation, and provide novel insights into CXCL5-driven pathogenesis.

On, around, and through: neutrophil-endothelial interactions in innate immunity

This manuscript will review our current understanding of neutrophilic polymorphonuclear leukocyte (neutrophil) interactions with the endothelium during immune and inflammatory responses, focusing on the molecular mechanisms regulating neutrophil adhesion to and migration through the endothelium in response to infection or tissue injury. This is a complex and dynamic area of research and one that has been the topic of several recent comprehensive reviews to which the interested reader is referred (64, 118, 131). By design, this review will begin with a brief review of some basic aspects of neutrophil biology and endothelial adhesion to provide a foundation. The remainder of the review will focus on selected areas of this complex field, specifically the role of the endothelial glycocalyx in regulating neutrophil adhesion and the mechanisms and consequences of migration of neutrophils between (paracellular) and through (transcellular) endothelial cells during egress from the vasculature.

Induction of TNF-alfa and CXCL-2 mRNAs in different organs of mice infected with pathogenic Leptospira

The role of innate immune response in protection against leptospirosis is poorly understood. We examined the expression of the chemokine CXCL2/MIP-2 and the cytokine TNF-α in experimental resistant and susceptible mice models, C3H/HeJ, C3H/HePas and BALB/c strains, using a virulent strain of Leptospira interrogans serovar Copenhageni. Animals were infected intraperitoneally with 10(7) cells and the development of the disease was followed. Mortality of C3H/HeJ mice was observed whereas C3H/HePas presented jaundice and BALB/c mice remained asymptomatic. The infection was confirmed by the presence of leptospiral DNA in the organs of the animals, demonstrated by PCR. Sections of the organs were analyzed, after H&E stain. The relative expression of mRNA of chemokine CXCL2/MIP-2 and cytokine TNF-α was measured in lung, kidney and liver of the mice by qPCR. The concentrations of these proteins were measured in extracts of tissues and in serum of the animals, by ELISA. Increasing levels of transcripts and protein CXCL2/MIP-2 were detected since the first day of infection. The highest expression was observed at third day of infection in kidney, liver and lung of BALB/c mice. In C3H/HeJ the expression of CXCL2/MIP-2 was delayed, showing highest protein concentration in lung and kidney at the 5th day. Increasing in TNF-α transcripts were detected after infection, in kidney and liver of animals from the three mice strains. The expression of TNF-α protein in C3H/HeJ was also delayed, being detected in kidney and lung. Our data demonstrated that Leptospira infection stimulates early expression of CXCL2/MIP-2 and TNF-α in the resistant strain of mice. Histological analysis suggests that the expression of those molecules may be related to the influx of distinct immune cells and plays a role in the naturally acquired protective immunity.

The bactericidal/permeability-increasing protein (BPI) in the innate defence of the lower airways

The human BPI (bactericidal/permeability-increasing protein), stored in primary azurophilic granula of neutrophil granulocytes and produced by mucosal epithelia, has been known for decades to bind LPS (lipopolysaccharide) with very high affinity and to efficiently kill Gram-negative bacteria. Thus BPI potentially represents a central component of the innate immune system to directly combat microbes and modulate subsequent adaptive immune responses. Especially in the lungs, which are frequently exposed to a variety of inhaled pathogens, antimicrobial innate defence molecules such as BPI, are of exceptional relevance. In the present review, we highlight possible functions of BPI during acute pneumonia and CF (cystic fibrosis)-associated chronic infections in the lung.

Nuclear factor-κB: a key regulator in health and disease of lungs

Rel/NF-κB transcription factors play a key role in modulating the response of immunoregulatory genes including cytokines and chemokines, cell adhesion molecules, acute phase proteins, and anti-microbial peptides. Furthermore, an array of genes important for angiogenesis, tumor invasion and metastasis is also regulated by nuclear factor-κB (NF-κB). Close association of NF-κB with inflammation and tumorigenesis makes it an attractive target for basic research as well as for pharmaceutical industries. Studies involving various animal and cellular models have revealed the importance of NF-κB in pathobiology of lung diseases. This review (a) describes structures, activities, and regulation of NF-κB family members; (b) provides information which implicates NF-κB in pathogenesis of pulmonary inflammation and cancer; and (c) discusses information about available synthetic and natural compounds which target NF-κB or specific components of NF-κB signal transduction pathway and which may provide the foundation for development of effective therapy for lung inflammation and bronchogenic carcinomas.

Vaccine immunity to coccidioidomycosis occurs by early activation of three signal pathways of T helper cell response (Th1, Th2, and Th17)

We have previously reported that C57BL/6 mice vaccinated with a live, attenuated mutant of Coccidioides posadasii, referred to as the ΔT vaccine, are fully protected against pulmonary coccidioidomycosis. This model was used here to explore the nature of vaccine immunity during the initial 2-week period after intranasal challenge. Elevated neutrophil and eosinophil infiltration into the lungs of nonvaccinated mice contrasted with markedly reduced recruitment of these cells in vaccinated animals. The numbers of lung-infiltrated macrophages and dendritic cells showed a progressive increase in vaccinated mice and corresponded with reduction of the lung infection. Concentrations of selected inflammatory cytokines and chemokines were initially higher in lung homogenates of vaccinated mice but then generally decreased at 14 days postchallenge in correlation with containment of the organism and apparent dampening of the inflammation of host tissue. Profiles of cytokines detected in lung homogenates of ΔT-vaccinated mice were indicative of a mixed T helper 1 (Th1)-, Th2-, and Th17-type immune response, a conclusion which was supported by detection of lung infiltration of activated T cells with the respective CD4(+) gamma interferon (IFN-γ)(+), CD4(+) interleukin-5 (IL-5)(+), and CD4(+) IL-17A(+) phenotypes. While Th1 and Th2 immunity was separately dispensed of by genetic manipulation without loss of ΔT vaccine-mediated protection, loss of functional Th17 cells resulted in increased susceptibility to infection in immunized mice. Characterization of the early events of protective immunity to Coccidioides infection in vaccinated mice contributes to the identification of surrogates of immune defense and provides potential insights into the design of immunotherapeutic protocols for treatment of coccidioidomycosis.

Intratracheal transplantation of human umbilical cord blood-derived mesenchymal stem cells attenuates Escherichia coli-induced acute lung injury in mice

BACKGROUND

Human umbilical cord blood (UCB)-derived mesenchymal stem cells (MSCs) attenuate hyperoxic neonatal lung injury primarily through anti-inflammatory effects. We hypothesized that intratracheal transplantation of human UCB-derived MSCs could attenuate Escherichia coli (E. coli)-induced acute lung injury (ALI) in mice by suppressing the inflammatory response.

METHODS

Eight-week-old male ICR mice were randomized to control or ALI groups. ALI was induced by intratracheal E. coli instillation. Three-hours after E. coli instillation, MSCs, fibroblasts or phosphate-buffered saline were intratracheally administered randomly and survival was analyzed for 7 days post-injury. Lung histology including injury scores, myeloperoxidase (MPO) activity, and protein levels of interleukin (IL)-1α, IL-1β, IL-6, tumor necrosis factor (TNF)-α, and macrophage inflammatory protein (MIP)-2 as well as the wet-dry lung ratio and bacterial counts from blood and bronchoalveolar lavage (BAL) were evaluated at 1, 3, and 7 days post-injury. Levels of inflammatory cytokines in the lung were also profiled using protein macroarrays at day 3 post-injury which showed peak inflammation.

RESULTS

MSC transplantation increased survival and attenuated lung injuries in ALI mice, as evidenced by decreased injury scores on day 3 post-injury and reduced lung inflammation including increased MPO activity and protein levels of IL-1α, IL-1β, IL-6, TNF-α, and MIP-2 on day 3 and 7 post-injury. Inflammatory cytokine profiles in the lungs at day 3 post-injury were attenuated by MSC transplantation. MSCs also reduced the elevated lung water content at day 3 post-injury and bacterial counts in blood and BAL on day 7 post-injury.

CONCLUSIONS

Intratracheal transplantation of UCB-derived MSCs attenuates E. coli-induced ALI primarily by down-modulating the inflammatory process and enhancing bacterial clearance.

IL-17A and TNF-α exert synergistic effects on expression of CXCL5 by alveolar type II cells in vivo and in vitro

CXCL5, a member of the CXC family of chemokines, contributes to neutrophil recruitment during lung inflammation, but its regulation is poorly understood. Because the T cell-derived cytokine IL-17A enhances host defense by triggering production of chemokines, particularly in combination with TNF-α, we hypothesized that IL-17A would enhance TNF-α-induced expression of CXCL5. Intratracheal coadministration of IL-17A and TNF-α in mice induced production of CXCL1, CXCL2, and CXCL5, which was associated with increased neutrophil influx in the lung at 8 and 24 h. The synergistic effects of TNF-α and IL17A were greatly attenuated in Cxcl5(-/-) mice at 24 h, but not 8 h, after exposure, a time when CXCL5 expression was at its peak in wild-type mice. Bone marrow chimeras produced using Cxcl5(-/-) donors and recipients demonstrated that lung-resident cells were the source of CXCL5. Using differentiated alveolar epithelial type II (ATII) cells derived from human fetal lung, we found that IL-17A enhanced TNF-α-induced CXCL5 transcription and stabilized TNF-α-induced CXCL5 transcripts. Whereas expression of CXCL5 required activation of NF-κB, IL-17A did not increase TNF-α-induced NF-κB activation. Apical costimulation of IL-17A and TNF-α provoked apical secretion of CXCL5 by human ATII cells in a transwell system, whereas basolateral costimulation led to both apical and basolateral secretion of CXCL5. The observation that human ATII cells secrete CXCL5 in a polarized fashion may represent a mechanism to recruit neutrophils in host defense in a fashion that discriminates the site of initial injury.

Soluble endothelial selectin in acute lung injury complicated by severe pneumonia

BACKGROUND

Pneumonia is still one of the most frequent causes of death in the elderly. Complication of acute lung injury (ALI)/acute respiratory distress syndrome (ARDS) by pneumonia makes patients very ill due to severe respiratory failure. Biomarkers that can discriminate the presence of complicating ALI/ARDS are required for early detection. The aim of this research was to investigate whether soluble endothelial selectin (sES) could be a biomarker for ALI.

METHODS

Serum sES levels were measured in 27 pneumonia patients, who were enrolled between April 2006 and September 2007. Among these patients, six had ALI or a condition that was clinically comparable to ALI (cALI). All patients who were enrolled were successfully treated and survived.

RESULTS

Circulating sES levels were elevated in pneumonia patients with ALI/cALI, and sES levels decreased following treatment of their pneumonia. Univariate and multivariate logistic regression analyses showed that sES was the only significant factor for identifying complicating ALI/cALI, independently of C-reactive protein (CRP) and lactate dehydrogenase (LDH). By receiver operating characteristic (ROC) curve analysis, the cut-off value for sES was 40.1 ng/mL, with a sensitivity of 0.8 and a specificity of 0.8.

CONCLUSION

sES may be a useful biomarker for discriminating complicating ALI/cALI in patients with severe pneumonia.

Lung deposition of inhaled alpha-1-proteinase inhibitor (alpha 1-PI) - problems and experience of alpha1-PI inhalation therapy in patients with hereditary alpha1-PI deficiency and cystic fibrosis

Alpha-1-proteinase inhibitor (α1-PI) is the most relevant protease inhibitor in the lung. Patients with hereditary deficiency of α1-PI suffer from an impaired hepatic synthesis of α1-PI in the liver and in consequence an insufficient concentration of the protease inhibitor in the lung followed by development of lung emphysema due to an impaired protease antiprotease balance and a local relative excess of neutrophil elastase (NE). In contrast, patients with cystic fibrosis (CF) are characterised by a normal synthesis of α1-PI and a severe pulmonary inflammation with a strong excess of NE in the lung followed by progressive loss of lung function. In principle, both patient groups may benefit from an augmentation of α1-PI. Intravenous augmentation, which is established in patients with α1-PI deficiency only, is very expensive, subject to controversial discussions and only about 2% of the administered protein reaches lung interstitium. Inhalation of α1-PI may serve as an alternative to administer high α1-PI doses into the lungs of both patient groups to restore the impaired protease antiprotease balance and to diminish the detrimental effects of NE. However, prerequisites of this therapy are the reproducible administration of sufficient doses of active α1-PI into the lung without adverse effects. In our review we describe the results of studies investigating the inhalation of α1-PI in patients with α1-PI deficiency and CF. The data demonstrate the feasibility of α1-PI inhalation for restoration of the impaired protease antiprotease balance, attenuation of the inflammation and neutralisation of the excess activity of NE. Likely, inhalation of α1-PI serves as cheaper and more convenient therapy than intravenous augmentation. However, inhalation will be further optimised by use of novel nebulisers and optimised breathing techniques.

CXCL1 regulates pulmonary host defense to Klebsiella Infection via CXCL2, CXCL5, NF-kappaB, and MAPKs

Pulmonary bacterial infections are a leading cause of death. Since the introduction of antibiotics, multidrug-resistant Klebsiella pneumoniae became an escalating threat. Therefore, development of methods to augment antibacterial defense is warranted. Neutrophil recruitment is critical to clear bacteria, and neutrophil migration in the lung requires the production of ELR(+) CXC chemokines. Although lung-specific CXCL1/keratinocyte cell-derived chemokine (KC) transgene expression causes neutrophil-mediated clearance of K. pneumoniae, the mechanisms underlying KC-mediated host defense against K. pneumoniae have not been explored. In this study, we delineated the host defense functions of KC during pulmonary K. pneumoniae infection using KC(-/-) mice. Our findings demonstrate that KC is important for expression of CXCL2/MIP-2 and CXCL5/LPS-induced CXC chemokine, and activation of NF-κB and MAPKs in the lung. Furthermore, KC derived from both hematopoietic and resident cells contributes to host defense against K. pneumoniae. Neutrophil depletion in mice before K. pneumoniae infection reveals no differences in the production of MIP-2 and LPS-induced CXC chemokine or activation of NF-κB and MAPKs in the lung. Using murine bone marrow-derived and alveolar macrophages, we confirmed KC-mediated upregulation of MIP-2 and activation of NF-κB and MAPKs on K. pneumoniae infection. Moreover, neutralizing KC in bone marrow-derived macrophages before K. pneumoniae challenge decreases bacteria-induced production of KC and MIP-2, and activation of NF-κB and MAPKs. These findings reveal the importance of KC produced by hematopoietic and resident cells in regulating pulmonary host defense against a bacterial pathogen via the activation of transcription factors and MAPKs, as well as the expression of cell adhesion molecules and other neutrophil chemoattractants.