Essential role of monocyte chemoattractant protein-1 in gram-negative bacterial pneumonia

Acute gram-negative bacterial infections are a leading cause of mortality among the nosocomial infections. Increasing numbers of immunosuppressed individuals and growing numbers of antibiotic resistant strains make antibiotic treatment difficult. Neutrophils are the first cells recruited to the site of infection and are critical players in the host defense against gram-negative bacterial pneumonia. Therefore, identification of targets that boost neutrophil-associated host defense in the lung is essential in designing better therapies to control pulmonary infections. Production of chemokines is an important step for neutrophil recruitment. Monocyte chemoattractant protein-1 (MCP-1) is a chemokine that is important for monocyte and T-lymphocyte influx. It is important for the host defense during Listeria monocytogenes and Streptococcus pneumoniae infection. However, the role of MCP-1 during pulmonary gram-negative infections is not known. We hypothesized that MCP-1 is essential for the host defense during a gram-negative infection. To test the hypothesis, we infected MCP-1 gene-deficient (MCP-1-/-) mice and controls intratracheally (i.t.) with E. coli (106 CFUs/mouse) and Klebsiella pneumoniae (103 CFUs/mouse). We found that MCP-1 is critical for host defense against gram-negative infections, mainly by recruiting neutrophils to the site of infection. MCP-1 utilizes it’s receptor, CCR2, to recruit neutrophils directly and indirectly by regulating the expression of cytokines (IL-6, TNF-a) and chemokines (KC, MIP-2) through activation of NF-kB and MAPKs. We also observed that MCP-1 can regulate expression of G-CSF and thereby neutrophil numbers in circulation during Kp infection. In addition, exogenous administration of rG-CSF can restore the defects in host defense in MCP-1-/- mice following gram-negative Kp infection. This study demonstrates an unrecognized role of MCP-1 in host defense during gram negative bacterial pneumonia. These findings bolster pleiotropic effects of MCP-1 in the host defense and demonstrate a potential role as a therapeutic agent to augment host defense during acute bacterial pneumonia

Review: Toll-like receptors and NOD-like receptors in pulmonary antibacterial immunity

Lung diseases caused by bacteria are a leading cause of death in both immunocompromised and immunocompetent individuals as well as in children. Although neutrophil recruitment is critical to augment the host defence, excessive neutrophil accumulation results in life-threatening diseases, such as acute lung injury (ALI) and acute respiratory distress syndrome (ARDS). Therefore, it is important to modulate excessive neutrophil influx in ALI/ARDS to mitigate lung damage and mortality. A better understanding of the basic mechanisms underlying neutrophil influx is crucial to designing novel and innovative treatment strategies for ALI/ARDS. Recognition of bacteria in the lung is the critical first step leading to neutrophil influx. Pattern recognition receptors, such as Toll-like receptors and NOD-like receptors, play an important role in the recognition of bacterial pathogens. Understanding the molecular and cellular mechanisms associated with the recognition of bacterial pathogens by the host is critical for the development of effective therapeutic strategies to control parenchymal damage via modulating neutrophil accumulation in the lung

Intrapulmonary administration of leukotriene B(4) augments neutrophil accumulation and responses in the lung to Klebsiella infection in CXCL1 knockout mice

In prior studies, we demonstrated that 1) CXCL1/KC is essential for NF-κB and MAPK activation and expression of CXCL2/MIP-2 and CXCL5/LPS-induced CXC chemokine in Klebsiella-infected lungs, and 2) CXCL1 derived from hematopoietic and resident cells contributes to host immunity against Klebsiella. However, the role of CXCL1 in mediating neutrophil leukotriene B(4) (LTB(4)), reactive oxygen species (ROS), and reactive nitrogen species (RNS) production is unclear, as is the contribution of these factors to host immunity. In this study, we investigated 1) the role of CXCL1 in LTB(4), NADPH oxidase, and inducible NO synthase (iNOS) expression in lungs and neutrophils, and 2) whether LTB(4) postinfection reverses innate immune defects in CXCL1(-/-) mice via regulation of NADPH oxidase and iNOS. Our results demonstrate reduced neutrophil influx, attenuated LTB(4) levels, and decreased ROS and iNOS production in the lungs of CXCL1(-/-) mice after Klebsiella pneumoniae infection. Using neutrophil depletion and repletion, we found that neutrophils are the predominant source of pulmonary LTB(4) after infection. To treat immune defects in CXCL1(-/-) mice, we intrapulmonarily administered LTB(4). Postinfection, LTB(4) treatment reversed immune defects in CXCL1(-/-) mice and improved survival, neutrophil recruitment, cytokine/chemokine expression, NF-κB/MAPK activation, and ROS/RNS production. LTB(4) also enhanced myeloperoxidase, H(2)O(2,) RNS production, and bacterial killing in K. pneumoniae-infected CXCL1(-/-) neutrophils. These novel results uncover important roles for CXCL1 in generating ROS and RNS in neutrophils and in regulating host immunity against K. pneumoniae infection. Our findings suggest that LTB(4) could be used to correct defects in neutrophil recruitment and function in individuals lacking or expressing malfunctional CXCL1

Intrapulmonary G-CSF rescues neutrophil recruitment to the lung and neutrophil release to blood in Gram-negative bacterial infection in MCP-1-/- mice

We previously demonstrated that MCP-1 is important for E. coli-induced neutrophil migration to the lungs. However, E. coli neither disseminates nor induces death in mice. Furthermore, the cell types and the host defense mechanisms that contribute to MCP-1-dependent neutrophil trafficking have not been defined. In this study, we sought to explore the cell types and the mechanisms associated with Klebsiella pneumoniae-mediated MCP-1-dependent neutrophil influx. MCP-1(-/-) mice are more susceptible to pulmonary K. pneumoniae infection and show higher bacterial burden in the lungs and dissemination. MCP-1(-/-) mice also display attenuated neutrophil influx, cytokine/chemokine production, and activation of NF-κB and MAPKs following intratracheal K. pneumoniae infection. rMCP-1 treatment in MCP-1(-/-) mice following K. pneumoniae infection rescued impairment in survival, bacterial clearance, and neutrophil accumulation in the lung. Neutrophil numbers in the blood of MCP-1(-/-) mice were associated with G-CSF concentrations in bronchoalveolar lavage fluid and blood. Bone marrow or resident cell-derived MCP-1 contributed to bacterial clearance, neutrophil accumulation, and cytokine/chemokine production in the lungs following infection. Furthermore, exogenous MCP-1 dose dependently increased neutrophil counts and G-CSF concentrations in the blood. Intriguingly, administration of intratracheal rG-CSF to MCP-1(-/-) mice after K. pneumoniae infection rescued survival, bacterial clearance and dissemination, and neutrophil influx in MCP-1(-/-) mice. Collectively, these novel findings unveil an unrecognized role of MCP-1 in neutrophil-mediated host immunity during K. pneumoniae pneumonia and illustrate that G-CSF could be used to rescue impairment in host immunity in individuals with absent or malfunctional MCP-1

Mechanisms of Neutrophil Accumulation in the Lungs Against Bacteria

Bacterial lung diseases are a major cause of morbidity and mortality both in immunocompromised and in immunocompetent individuals. Neutrophil accumulation, a pathological hallmark of bacterial diseases, is critical to host defense, but may also cause acute lung injury/acute respiratory distress syndrome. Toll-like receptors, nucleotide-binding oligomerization domain (NOD)-like receptors, transcription factors, cytokines, and chemokines play essential roles in neutrophil sequestration in the lungs. This review highlights our current understanding of the role of these molecules in the lungs during bacterial infection and their therapeutic potential. We also discuss emerging data on cholesterol and ethanol as environmentally modifiable factors that may impact neutrophil-mediated pulmonary innate host defense. Understanding the precise molecular mechanisms leading to neutrophil influx in the lungs during bacterial infection is critical for the development of more effective therapeutic and prophylactic strategies to control the excessive host response to infection

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