Epithelial Myeloid-Differentiation Factor 88 Is Dispensable during Klebsiella Pneumonia

Host defense against the infection of Klebsiella pneumoniae: New strategy to kill the bacterium in the era of antibiotics?

Klebsiella pneumoniae (K. pneumoniae) is a typical gram-negative iatrogenic bacterium that often causes bacteremia, pneumonia and urinary tract infection particularly among those with low immunity. Although antibiotics is the cornerstone of anti-infections, the clinical efficacy of β-lactamase and carbapenems drugs has been weakened due to the emergence of drug-resistant K. pneumoniae. Recent studies have demonstrated that host defense plays a critical role in killing K. pneumoniae. Here, we summarize our current understanding of host immunity mechanisms against K. pneumoniae, including mechanical barrier, innate immune cells, cellular immunity and humoral immunity, providing a theoretical basis and the new strategy for the clinical treatment of K. pneumoniae through improving host immunity.

Alveolar epithelial TET2 is not involved in the development of bleomycin-induced pulmonary fibrosis

Idiopathic pulmonary fibrosis (IPF) is a chronic lung disease of unknown etiology with minimal treatment options. Repetitive alveolar epithelial injury has been suggested as one of the causative mechanisms of this disease. Type 2 alveolar epithelial cells (AEC2) play a crucial role during fibrosis by functioning as stem cells able to repair epithelial damage. The DNA demethylase Tet methylcytosine dioxygenase 2 (TET2) regulates the stemness of multiple types of stem cells, but whether it also affects the stemness of AEC2 during fibrosis remains elusive. To study the role of TET2 in AEC2 during fibrosis, we first determined TET2 protein levels in the lungs of IPF patients and compared TET2 expression in AEC2 of IPF patients and controls using publicly available data sets. Subsequently, pulmonary fibrosis was induced by the intranasal administration of bleomycin to wild-type and AEC2-specific TET2 knockout mice to determine the role of TET2 in vivo. Fibrosis was assessed by hydroxyproline analysis and fibrotic gene expression. Additionally, macrophage recruitment and activation, and epithelial injury were analyzed. TET2 protein levels and gene expression were downregulated in IPF lungs and AEC2, respectively. Bleomycin inoculation induced a robust fibrotic response as indicated by increased hydroxyproline levels and increased expression of pro-fibrotic genes. Additionally, increased macrophage recruitment and both M1 and M2 activation were observed. None of these parameters were, however, affected by AEC2-specific TET2 deficiency. TET2 expression is reduced in IPF, but the absence of TET2 in AEC2 cells does not affect the development of bleomycin-induced pulmonary fibrosis.

de Vos A, Sirard JC, J. T. H. Roelofs J, P. Scicluna B, van der Poll T. Bronchial epithelial DNA methyltransferase 3b dampens pulmonary immune responses during Pseudomonas aeruginosa infection

DNA methyltransferase (Dnmt)3b mediates de novo DNA methylation and modulation of Dnmt3b in respiratory epithelial cells has been shown to affect the expression of multiple genes. Respiratory epithelial cells provide a first line of defense against pulmonary pathogens and play a crucial role in the immune response during pneumonia caused by Pseudomonas (P.) aeruginosa, a gram-negative bacterium that expresses flagellin as an important virulence factor. We here sought to determine the role of Dntm3b in respiratory epithelial cells in immune responses elicited by P. aeruginosa. DNMT3B expression was reduced in human bronchial epithelial (BEAS-2B) cells as well as in primary human and mouse bronchial epithelial cells grown in air liquid interface upon exposure to P. aeruginosa (PAK). Dnmt3b deficient human bronchial epithelial (BEAS-2B) cells produced more CXCL1, CXCL8 and CCL20 than control cells when stimulated with PAK, flagellin-deficient PAK (PAKflic) or flagellin. Dnmt3b deficiency reduced DNA methylation at exon 1 of CXCL1 and enhanced NF-ĸB p65 binding to the CXCL1 promoter. Mice with bronchial epithelial Dntm3b deficiency showed increased Cxcl1 mRNA expression in bronchial epithelium and CXCL1 protein release in the airways during pneumonia caused by PAK, which was associated with enhanced neutrophil recruitment and accelerated bacterial clearance; bronchial epithelial Dnmt3b deficiency did not modify responses during pneumonia caused by PAKflic or Klebsiella pneumoniae (an un-flagellated gram-negative bacterium). Dnmt3b deficiency in type II alveolar epithelial cells did not affect mouse pulmonary defense against PAK infection. These results suggest that bronchial epithelial Dnmt3b impairs host defense during Pseudomonas induced pneumonia, at least in part, by dampening mucosal responses to flagellin.

Bronchial Epithelial Tet2 Maintains Epithelial Integrity during Acute Pseudomonas aeruginosa Pneumonia

Respiratory epithelial cells are important for pulmonary innate immune responses during Pseudomonas aeruginosa infection. Tet methylcytosine dioxygenase 2 (Tet2) has been implicated in the regulation of host defense by myeloid and lymphoid cells, but whether Tet2 also contributes to epithelial responses during pneumonia is unknown. The aim of this study was to investigate the role of bronchial epithelial Tet2 in acute pneumonia caused by P. aeruginosa To this end, we crossed mice with Tet2 flanked by two Lox-P sites (Tet2^fl/fl mice) with mice expressing Cre recombinase under the bronchial epithelial cell-specific Cc10 promoter (Cc10^Cre mice) to generate bronchial epithelial cell-specific Tet2-deficient (Tet2^fl/fl Cc10^Cre ) mice. Six hours after infection with P. aeruginosa, Tet2^fl/fl Cc10^Cre and wild-type mice had similar bacterial loads in bronchoalveolar lavage fluid (BALF). At this time point, Tet2^fl/fl Cc10^Cre mice displayed reduced mRNA levels of the chemokines Cxcl1, Cxcl2, and Ccl20 in bronchial brushes. However, Cxcl1, Cxcl2, and Ccl20 protein levels and leukocyte recruitment in BALF were not different between groups. Tet2^fl/fl Cc10^Cre mice had increased protein levels in BALF after infection, indicating a disturbed epithelial barrier function, which was corroborated by reduced mRNA expression of tight junction protein 1 and occludin in bronchial brushes. Differences detected between Tet2^fl/fl Cc10^Cre and wild-type mice were no longer present at 24 h after infection. These results suggest that bronchial epithelial Tet2 contributes to maintaining epithelial integrity by enhancing intracellular connections between epithelial cells during the early phase of P. aeruginosa pneumonia.

Caspase-11 contributes to pulmonary host defense against Klebsiella pneumoniae and local activation of coagulation

Klebsiella (K.) pneumoniae is a common cause of gram-negative pneumonia and sepsis. Caspase-11 is an intracellular receptor for lipopolysaccharide and regulates pyroptosis, a specific form of inflammatory cell death, which aids in host defense against intracellular gram-negative bacteria. Recently, caspase-11 has also been implicated in blood coagulation. Previously, we found that local fibrin formation contributes to protective immunity against Klebsiella infection of the lung. The aim of the present study was to determine the role of caspase-11 in host defense during K. pneumoniae-evoked pneumonia and sepsis. Therefore, we infected wild-type and caspase-11-deficient (Casp11^-/-) mice with a low-dose K. pneumoniae via the airways to induce a gradually evolving pneumosepsis. Casp11^-/- mice displayed increased bacterial numbers in the lung 12 h and 48 h after inoculation. Analysis of pulmonary IL-1α, IL-1β, and TNF levels showed reduced IL-1α levels in bronchoalveolar lavage fluid and increased TNF levels in the lung of Casp11^-/- mice at 48 h after inoculation. Lung γH2AX staining (marker for cell death), lung pathology and neutrophil influx in the lung, as well as bacterial dissemination and organ damage, however, were not altered in Casp11^-/- mice after Klebsiella infection. Strikingly, analysis of cross-linked fibrin and D-dimer (markers for coagulation) revealed significantly less fibrin formation in the lungs of Casp11^-/- mice at either time point after Klebsiella infection. These data reveal that caspase-11 contributes to protective immunity against K. pneumoniae possibly by activation of blood coagulation in the lung.

Klebsiella pneumoniae infection biology: living to counteract host defences

Klebsiella species cause a wide range of diseases including pneumonia, urinary tract infections (UTIs), bloodstream infections and sepsis. These infections are particularly a problem among neonates, elderly and immunocompromised individuals. Klebsiella is also responsible for a significant number of community-acquired infections. A defining feature of these infections is their morbidity and mortality, and the Klebsiella strains associated with them are considered hypervirulent. The increasing isolation of multidrug-resistant strains has significantly narrowed, or in some settings completely removed, the therapeutic options for the treatment of Klebsiella infections. Not surprisingly, this pathogen has then been singled out as an 'urgent threat to human health' by several organisations. This review summarises the tremendous progress that has been made to uncover the sophisticated immune evasion strategies of K. pneumoniae. The co-evolution of Klebsiella in response to the challenge of an activated immune has made Klebsiella a formidable pathogen exploiting stealth strategies and actively suppressing innate immune defences to overcome host responses to survive in the tissues. A better understanding of Klebsiella immune evasion strategies in the context of the host-pathogen interactions is pivotal to develop new therapeutics, which can be based on antagonising the anti-immune strategies of this pathogen.