Signal transducer and activator of transcription 4 (STAT4), but not IL-12 contributes to Pseudomonas aeruginosa-induced lung inflammation in mice

STAT4 is expressed in neutrophils and promotes antimicrobial immunity

Signal transducer and activator of transcription 4 (STAT4) is expressed in hematopoietic cells and plays a key role in the differentiation of T helper 1 cells. Although STAT4 is required for immunity to intracellular pathogens, the T cell-independent protective mechanisms of STAT4 are not clearly defined. In this report, we demonstrate that STAT4-deficient mice were acutely sensitive to methicillin-resistant Staphylococcus aureus (MRSA) infection. We show that STAT4 was expressed in neutrophils and activated by IL-12 via a JAK2-dependent pathway. We demonstrate that STAT4 was required for multiple neutrophil functions, including IL-12-induced ROS production, chemotaxis, and production of the neutrophil extracellular traps. Importantly, myeloid-specific and neutrophil-specific deletion of STAT4 resulted in enhanced susceptibility to MRSA, demonstrating the key role of STAT4 in the in vivo function of these cells. Thus, these studies identify STAT4 as an essential regulator of neutrophil functions and a component of innate immune responses in vivo.

PTP1B negatively regulates STAT1-independent Pseudomonas aeruginosa killing by macrophages

Pseudomonas aeruginosa is the main conditional pathogen of immunodeficiency individuals. The mechanisms governing immune response to P. aeruginosa infection by macrophages remain incompletely defined. Herein, we demonstrate that protein tyrosine phosphatase-1B (PTP1B) is a critical negative regulator of P. aeruginosa infection response by macrophages. PTP1B-deficient macrophages display greatly enhanced bacterial phagocytosis and killing, accompanied by increased lysosome formation during P. aeruginosa infection. We also found that PTP1B repressed nitric oxide (NO) production and nitric oxide synthase (iNOS) induction following P. aeruginosa infection. PTP1B deficiency tended to upregulate the production of TRIF-interferon (IFN) pathway cytokines and chemokines, including IFN-β and interferon γ-inducible protein 10 (CXCL10, IP-10). Unexpectedly, the phosphorylation level of STAT1 was not regulated by PTP1B. In vivo experiments also confirmed that the regulatory function of PTP1B was not dependent on STAT1. These findings demonstrate that STAT1 is dispensable for negative regulation of P. aeruginosa clearance by macrophages.

PTP1B negatively regulates nitric oxide-mediated Pseudomonas aeruginosa killing by neutrophils

Neutrophils play a critical role in host defense against Pseudomonas aeruginosa infection. Mechanisms underlying the negative regulation of neutrophil function in bacterial clearance remain incompletely defined. Here, we demonstrate that protein tyrosine phosphatase-1B (PTP1B) is a negative regulator of P. aeruginosa clearance by neutrophils. PTP1B-deficient neutrophils display greatly enhanced bacterial phagocytosis and killing, which are accompanied by increased Toll-like receptor 4 (TLR4) signaling activation and nitric oxide (NO) production following P. aeruginosa infection. Interestingly, PTP1B deficiency mainly upregulates the production of IL-6 and IFN-β, leads to enhanced TLR4-dependent STAT1 activation and iNOS expression by neutrophils following P. aeruginosa infection. Further studies reveal that PTP1B and STAT1 are physically associated. These findings demonstrate a negative regulatory mechanism in neutrophil underlying the elimination of P. aeruginosa infection though a PTP1B-STAT1 interaction.

CXCL4 contributes to host defense against acute Pseudomonas aeruginosa lung infection

Platelets have been implicated in pulmonary inflammation following exposure to bacterial stimuli. The mechanisms involved in the platelet-mediated host response to respiratory bacterial infection remain incompletely understood. In this study, we demonstrate that platelet-derived chemokine (C-X-C motif) ligand 4 (CXCL4) plays critical roles in a mouse model of acute bacterial pneumonia using Pseudomonas aeruginosa. Platelets are activated during P. aeruginosa infection, and mice depleted of platelets display markedly increased mortality and impaired bacterial clearance. CXCL4 deficiency impairs bacterial clearance and lung epithelial permeability, which correlate with decreased neutrophil recruitment to BALF. Interestingly, CXCL4 deficiency selectively regulates chemokine production, suggesting that CXCL4 has an impact on other chemokine expression. In addition, CXCL4 deficiency reduces platelet-neutrophil interactions in blood following P. aeruginosa infection. Further studies revealed that platelet-derived CXCL4 contributes to the P. aeruginosa-killing of neutrophils. Altogether, these findings demonstrate that CXCL4 is a vital chemokine that plays critical roles in bacterial clearance during P. aeruginosa infection through recruiting neutrophils to the lungs and intracellular bacterial killing.

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.

Role of excessive inflammatory response to Stenotrophomonas maltophilia lung infection in DBA/2 mice and implications for cystic fibrosis

Stenotrophomonas maltophilia is a pathogen that causes infections mainly in immunocompromised patients. Despite increased S. maltophilia isolation from respiratory specimens of patients with cystic fibrosis (CF), the real contribution of the microorganism to CF pathogenesis still needs to be clarified. The aim of the present study was to evaluate the pathogenic role of S. maltophilia in CF patients by using a model of acute respiratory infection in DBA/2 mice following a single exposure to aerosolized bacteria. The pulmonary bacterial load was stable until day 3 and then decreased significantly from day 3 through day 14, when the bacterial load became undetectable in all infected mice. Infection disseminated in most mice, although at a very low level. Severe effects (swollen lungs, large atelectasis, pleural adhesion, and hemorrhages) of lung pathology were observed on days 3, 7, and 14. The clearance of S. maltophilia observed in DBA/2 mouse lungs was clearly associated with an early and intense bronchial and alveolar inflammatory response, which is mediated primarily by neutrophils. Significantly higher levels of interleukin-1beta (IL-1beta), IL-6, IL-12, gamma interferon (IFN-gamma), tumor necrosis factor alpha (TNF-alpha), GROalpha/KC, MCP-1/JE, MCP-5, macrophage inflammatory protein 1alpha (MIP-1alpha), MIP-2, and TARC were observed in infected mice on day 1 with respect to controls. Excessive pulmonary infection and inflammation caused systemic effects, manifested by weight loss, and finally caused a high mortality rate. Taken together, our results show that S. maltophilia is not just a bystander in CF patients but has the potential to contribute to the inflammatory process that compromises respiratory function.

Infection in systemic lupus erythematosus: friend or foe?

Infectious agents have long been implicated in the pathogenesis of systemic lupus erythematosus. Common viruses, such as the Epstein-Barr virus, transfusion transmitted virus, parvovirus and cytomegalovirus, have an increased prevalence in patients with systemic lupus erythematosus. They may contribute to disease pathogenesis through triggering autoimmunity via structural or functional molecular mimicry, encoding proteins that induce cross-reactive immune responses to self antigens or modulate antigen processing, activation, or apoptosis of B and T cells, macrophages or dendritic cells. Alternatively, some infectious agents, such as malaria, Toxoplasma gondii and Helicobacter pylori, may have a protective effect. Vaccinations may play dual roles by protecting against friend and foe alike.

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.