Toll-like receptors (TLR) and the downstream adaptor protein MyD88 are considered crucial for protective immunity during bacterial infections. Streptococcus (S.) pneumoniae is a human respiratory pathogen and a large majority of clinical pneumococcal isolates expresses an external polysaccharide capsule. We here sought to determine the role of pneumococcal capsule in MyD88-mediated antibacterial defense during S. pneumonia pneumonia. Wild type (WT) and Myd88(-/-) mice were inoculated intranasally with serotype 2 S. pneumoniae D39 or with an isogenic capsule locus deletion mutant (D39∆cps), and analysed for bacterial outgrowth and inflammatory responses in the lung. As compared to WT mice, Myd88(-/-) mice infected with D39 demonstrated a modestly impaired bacterial clearance accompanied by decreased inflammatory responses in the lung. Strikingly, while WT mice rapidly cleared D39∆cps, Myd88(-/-) mice showed 105-fold higher bacterial burdens in their lungs and dissemination to blood 24 hours after infection. These data suggest that the pneumococcal capsule impairs recognition of TLR ligands expressed by S. pneumoniae and thereby partially impedes MyD88-mediated antibacterial defense.

Klebsiella (K.) pneumoniae is a common cause of pneumonia-derived sepsis. Myeloid related protein 8 (MRP8, S100A8) and MRP14 (S100A9) are the most abundant cytoplasmic proteins in neutrophils. They can form MRP8/14 heterodimers that are released upon cell stress stimuli. MRP8/14 reportedly exerts antimicrobial activity, but in acute fulminant sepsis models MRP8/14 has been found to contribute to organ damage and death. We here determined the role of MRP8/14 in K. pneumoniae sepsis originating from the lungs, using an established model characterized by gradual growth of bacteria with subsequent dissemination. Infection resulted in gradually increasing MRP8/14 levels in lungs and plasma. Mrp14 deficient (mrp14(-/-)) mice, unable to form MRP8/14 heterodimers, showed enhanced bacterial dissemination accompanied by increased organ damage and a reduced survival. Mrp14(-/-) macrophages were reduced in their capacity to phagocytose Klebsiella. In addition, recombinant MRP8/14 heterodimers, but not MRP8 or MRP14 alone, prevented growth of Klebsiella in vitro through chelation of divalent cations. Neutrophil extracellular traps (NETs) prepared from wildtype but not from mrp14(-/-) neutrophils inhibited Klebsiella growth; in accordance, the capacity of human NETs to kill Klebsiella was strongly impaired by an anti-MRP14 antibody or the addition of zinc. These results identify MRP8/14 as key player in protective innate immunity during Klebsiella pneumonia.

Streptococcus pneumoniae is a major causative agent in community-acquired pneumonia and sepsis. Overwhelming lung inflammation during pneumococcal pneumonia may hamper lung function. Ibrutinib is an irreversible inhibitor of Bruton's tyrosine kinase (Btk), a key signaling protein controlling the activation of various immune cells, including macrophages and neutrophils. The aim of this study was to determine whether ibrutinib treatment ameliorates acute lung inflammation during pneumococcal pneumonia.

Streptococcus (S.) pneumoniae is the most common causative pathogen in community-acquired pneumonia. Nucleotide-binding oligomerization domain-containing (NOD) 2 is a pattern recognition receptor located in the cytosol of myeloid cells that is able to detect peptidoglycan fragments of S. pneumoniae. We here aimed to investigate the role of NOD2 in the host response during pneumococcal pneumonia. Phagocytosis of S. pneumoniae was studied in NOD2 deficient (Nod2-/-) and wild-type (Wt) alveolar macrophages and neutrophils in vitro. In subsequent in vivo experiments Nod2-/- and Wt mice were inoculated with serotype 2 S. pneumoniae (D39), an isogenic capsule locus deletion mutant (D39Δcps) or serotype 3 S. pneumoniae (6303) via the airways, and bacterial growth and dissemination and the lung inflammatory response were evaluated. Nod2-/- alveolar macrophages and blood neutrophils displayed a reduced capacity to internalize pneumococci in vitro. During pneumonia caused by S. pneumoniae D39 Nod2-/- mice were indistinguishable from Wt mice with regard to bacterial loads in lungs and distant organs, lung pathology and neutrophil recruitment. While Nod2-/- and Wt mice also had similar bacterial loads after infection with the more virulent S. pneumoniae 6303 strain, Nod2-/- mice displayed a reduced bacterial clearance of the normally avirulent unencapsulated D39Δcps strain. These results suggest that NOD2 does not contribute to host defense during pneumococcal pneumonia and that the pneumococcal capsule impairs recognition of S. pneumoniae by NOD2.

Klebsiella pneumoniae is a frequently isolated causative pathogen in respiratory tract infections. CD44 is a transmembrane adhesion molecule that has been implicated in several immunological processes. To determine the role of CD44 during Klebsiella pneumonia, we intranasally infected wild-type and CD44 knockout (KO) mice with 10(2) to 10(4) colony-forming units of K. pneumoniae or administered Klebsiella lipopolysaccharide. During lethal infection, CD44 deficiency was associated with reduced bacterial growth and dissemination accompanied by enhanced pulmonary inflammation. After infection with lower Klebsiella doses, CD44 KO mice but not wild-type mice demonstrated mortality. After infection with even lower bacterial doses, which were cleared by most mice of both strains, CD44 KO mice displayed enhanced lung inflammation 4 and 10 days postinfection, indicating that CD44 is important for the resolution of pulmonary inflammation after nonlethal pneumonia. In accordance, CD44 KO mice showed a diminished resolution of lung inflammation 4 days after intrapulmonary delivery of lipopolysaccharide. CD44 deficiency was associated with the accumulation of hyaluronan together with reduced gene expression levels of the negative regulators of Toll-like receptor signaling, interleukin-1R-associated kinase M, A20, and suppressor of cytokine signaling 3. In conclusion, the absence of CD44 affects various components and phases of the host response during Klebsiella pneumonia, reducing bacterial outgrowth and dissemination and enhancing pulmonary pathology during lethal infection, and diminishing the resolution of lung inflammation during sublethal infection.

Klebsiella species is the second most commonly isolated gram-negative organism in sepsis and a frequent causative pathogen in pneumonia. The receptor for advanced glycation end products (RAGE) is expressed on different cell types and plays a key role in diverse inflammatory responses. We here aimed to investigate the role of RAGE in the host response to Klebsiella (K.) pneumoniae pneumonia and intransally inoculated rage gene deficient (RAGE-/-) and normal wild-type (Wt) mice with K. pneumoniae. Klebsiella pneumonia resulted in an increased pulmonary expression of RAGE. Furthermore, the high-affinity RAGE ligand high mobility group box-1 was upregulated during K. pneumoniae pneumonia. RAGE deficiency impaired host defense as reflected by a worsened survival, increased bacterial outgrowth and dissemination in RAGE-/- mice. RAGE-/- neutrophils showed a diminished phagocytosing capacity of live K. pneumoniae in vitro. Relative to Wt mice, RAGE-/- mice demonstrated similar lung inflammation, and slightly elevated-if any-cytokine and chemokine levels and unchanged hepatocellular injury. In addition, RAGE-/- mice displayed an unaltered response to intranasally instilled Klebsiella lipopolysaccharide (LPS) with respect to pulmonary cell recruitment and local release of cytokines and chemokines. These data suggest that (endogenous) RAGE protects against K. pneumoniae pneumonia. Also, they demonstrate that RAGE contributes to an effective antibacterial defense during K. pneumoniae pneumonia, at least partly via its participation in the phagocytic properties of professional granulocytes. Additionally, our results indicate that RAGE is not essential for the induction of a local and systemic inflammatory response to either intact Klebsiella or Klebsiella LPS.

Klebsiella pneumoniae is an important cause of sepsis. The common Toll-like receptor adapter myeloid differentiation primary response gene (MyD)88 is crucial for host defense against Klebsiella. Here we investigated the role of MyD88 in myeloid and endothelial cells during Klebsiella pneumosepsis. Mice deficient for MyD88 in myeloid (LysM-Myd88(-/-)) and myeloid plus endothelial (Tie2-Myd88(-/-)) cells showed enhanced lethality and bacterial growth. Tie2-Myd88(-/-) mice reconstituted with control bone marrow, representing mice with a selective MyD88 deficiency in endothelial cells, showed an unremarkable antibacterial defense. Myeloid or endothelial cell MyD88 deficiency did not impact on lung pathology or distant organ injury during late stage sepsis, while LysM-Myd88(-/-) mice demonstrated a strongly attenuated inflammatory response in the airways early after infection. These data suggest that myeloid but not endothelial MyD88 is important for host defense during gram-negative pneumonia derived sepsis.

Bruton's tyrosine kinase (Btk) is a cytoplasmic kinase expressed in B cells and myeloid cells. It is essential for B cell development and natural antibody-mediated host defense against bacteria in humans and mice, but little is known about the role of Btk in innate host defense in vivo. Previous studies have indicated that lack of (natural) antibodies is paramount for impaired host defense against Streptococcus (S.) pneumoniae in patients and mice with a deficiency in functional Btk. In the present study, we re-examined the role of Btk in B cells and myeloid cells during pneumococcal pneumonia and sepsis in mice. The antibacterial defense of Btk-/- mice was severely impaired during pneumococcal pneumosepsis and restoration of natural antibody production in Btk-/- mice by transgenic expression of Btk specifically in B cells did not suffice to protect against infection. Btk-/- mice with reinforced Btk expression in MhcII+ cells, including B cells, dendritic cells and macrophages, showed improved antibacterial defense as compared to Btk-/- mice. Bacterial outgrowth in Lysmcre-Btkfl/Y mice was unaltered despite a reduced capacity of Btk-deficient alveolar macrophages to respond to pneumococci. Mrp8cre-Btkfl/Y mice with a neutrophil specific paucity in Btk expression, however, demonstrated impaired antibacterial defense. Neutrophils of Mrp8cre-Btkfl/Y mice displayed reduced release of granule content after pulmonary installation of lipoteichoic acid, a gram-positive bacterial cell wall component relevant for pneumococci. Moreover, Btk deficient neutrophils showed impaired degranulation and phagocytosis upon incubation with pneumococci ex vivo. Taken together, the results of our study indicate that besides regulating B cell-mediated immunity, Btk is critical for regulation of myeloid cell-mediated, and particularly neutrophil-mediated, innate host defense against S. pneumoniae in vivo.

The microbiome has emerged as an important player in the pathophysiology of a whole spectrum of diseases that affect the critically ill. We hypothesized that differences in microbiota composition across vendors can influence murine models of pulmonary lipopolysaccharide (LPS) inflammation and Gram-negative pneumonia.

Pneumonia represents a major health care burden and Gram-negative bacteria provide an increasing therapeutic challenge at least in part through the emergence of multidrug-resistant strains. IL-33 is a multifunctional cytokine belonging to the IL-1 family that can affect many different cell types. We sought here to determine the effect of recombinant IL-33 on the host response during murine pneumonia caused by the common Gram-negative pathogen Klebsiella pneumoniae. IL-33 pretreatment prolonged survival for more than 1 day during lethal airway infection and decreased bacterial loads at the primary site of infection and distant organs. Postponed treatment with IL-33 (3 h) also reduced bacterial growth and dissemination. IL-33-mediated protection was not observed in mice deficient for the IL-33 receptor component IL-1 receptor-like 1. IL-33 induced a brisk type 2 response, characterized by recruitment of type 2 innate lymphoid cells to the lungs and enhanced release of IL-5 and IL-13. However, neither absence of innate lymphoid cells or IL-13, nor blocking of IL-5 impacted on IL-33 effects in mice infected with Klebsiella. Likewise, IL-33 remained effective in reducing bacterial loads in mice lacking B, T, and natural killer T cells. Experiments using antibody-mediated cell depletion indicated that neutrophils and inflammatory monocytes were of importance for antibacterial defense. The capacity of IL-33 to restrict bacterial growth in the lungs was strongly reduced in mice depleted of both neutrophils and inflammatory monocytes, but not in mice selectively depleted of either one of these cell types. These results suggest that IL-33 boosts host defense during bacterial pneumonia by a combined effect on neutrophils and inflammatory monocytes. © 2020 The Authors. The Journal of Pathology published by John Wiley & Sons, Ltd. on behalf of The Pathological Society of Great Britain and Ireland.

Tet methylcytosine dioxygenase 2 (Tet2) is an important enzyme in the demethylation of DNA. Recent evidence has indicated a role for Tet2 in the regulation of macrophage activation by lipopolysaccharide (LPS) and mice with a myeloid cell Tet2 deficiency showed enhanced lung inflammation upon local LPS administration. However, mice with a global Tet2 deficiency showed reduced systemic inflammation during abdominal sepsis. Here, we sought to determine the role of myeloid cell Tet2 in the host response during gram-negative bacterial pneumonia. To this end we infected myeloid cell specific Tet2 deficient and control mice with two common gram-negative respiratory pathogens via the airways: Pseudomonas aeruginosa (PAK, causing acute infection that remains confined in the lungs) or Klebsiella pneumoniae (causing a gradually evolving pneumonia with subsequent dissemination and sepsis) and compared bacterial loads and host response parameters between mouse strains. Bone marrow derived macrophages from myeloid Tet2 deficient mice released more interleukin-6 than control macrophages upon stimulation with PAK or K. pneumoniae. However, bacterial loads did not differ between mouse strains upon infection with viable PAK or K. pneumoniae, and neither did cytokine levels or neutrophil recruitment. In addition, in the K. pneumoniae pneumosepsis model myeloid Tet2 deficiency did not affect systemic inflammation or organ injury. Together these data strongly argue against a role for myeloid cell Tet2 in the host response during gram-negative bacterial pneumonia and pneumosepsis.

Interleukin-1 receptor like 1 (ST2) is a negative regulator of Toll-like receptor (TLR) signaling. TLRs are important for host defense during respiratory tract infections by both influenza and Streptococcus (S.) pneumoniae. Enhanced susceptibility to pneumococcal pneumonia is an important complication following influenza virus infection. We here sought to determine the role of ST2 in primary influenza A infection and secondary pneumococcal pneumonia. ST2 knockout (st2(-/-)) and wild-type (WT) mice were intranasally infected with influenza A virus; in some experiments mice were infected 2 weeks later with S. pneumoniae. Both mouse strains cleared the virus similarly during the first 14 days of influenza infection and had recovered their weights equally at day 14. Overall st2(-/-) mice tended to have a stronger pulmonary inflammatory response upon infection with influenza; especially 14 days after infection modest but statistically significant elevations were seen in lung IL-6, IL-1β, KC, IL-10, and IL-33 concentrations and myeloperoxidase levels, indicative of enhanced neutrophil activity. Interestingly, bacterial lung loads were higher in st2(-/-) mice during the later stages of secondary pneumococcal pneumonia, which was associated with relatively increased lung IFN-γ levels. ST2 deficiency did not impact on gross lung pathology in either influenza or secondary S. pneumoniae pneumonia. These data show that ST2 plays a limited anti-inflammatory role during both primary influenza and postinfluenza pneumococcal pneumonia.

Tenascin C (TNC) is an extracellular matrix protein with immunomodulatory properties that plays a major role during tissue injury and repair. TNC levels are increased in patients with pneumonia and pneumosepsis, and they are associated with worse outcomes. Methicillin-resistant Staphylococcus aureus (MRSA) is a Gram-positive bacterium that is a major causative pathogen in nosocomial pneumonia and a rising cause of community-acquired pneumonia. To study the role of TNC during MRSA-induced pneumonia, TNC sufficient (TNC+/+) and TNC-deficient (TNC-/-) mice were infected with MRSA via the airways and euthanized after 6, 24, and 48 h for analysis. Pulmonary transcription of TNC peaked at 6 h, while immunohistochemistry revealed higher protein levels at later time points. Although TNC deficiency was not associated with changes in bacterial clearance, TNC-/- mice showed increased levels of TNF-α and IL-6 in bronchoalveolar lavage fluid during the acute phase of infection when compared with TNC+/+ mice. In addition, TNC-/- mice showed more severe pulmonary pathology at 6, but not at 24 or 48 h, after infection. Together, these data suggest that TNC plays a moderate protective role against tissue pathology during the acute inflammatory phase, but not during the bacterial clearance phase, of MRSA-induced pneumonia. These results argue against an important role of TNC on disease outcome during MRSA-induced pneumonia. IMPORTANCE Recently, the immunomodulatory properties of TNC have drawn substantial interest. However, to date most studies made use of sterile models of inflammation. In this study, we examine the pathobiology of MRSA-induced pneumonia in a model of TNC-sufficient and TNC-deficient mice. We have studied the immune response and tissue pathology both during the initial insult and also during the resolution phase. We demonstrate that MRSA-induced pneumonia upregulates pulmonary TNC expression at the mRNA and protein levels. However, the immunomodulatory role of TNC during bacterial pneumonia is distinct from models of sterile inflammation, indicating that the function of TNC is context dependent. Contrary to previous descriptions of TNC as a proinflammatory mediator, TNC-deficient mice seem to suffer from enhanced tissue pathology during the acute phase of infection. Nonetheless, besides its role during the acute phase response, TNC does not seem to play a major role in disease outcome during MRSA-induced pneumonia.

Staphylococcus (S.) aureus has emerged as an important cause of necrotizing pneumonia. Lung injury during S. aureus pneumonia may be enhanced by local release of damage associated molecular patterns such as high-mobility group box 1 (HMGB1). In the current study we sought to determine the functional role of HMGB1 and its receptors, toll-like receptor 4 (TLR4) and the receptor for advanced glycation end products (RAGE), in the injurious host response to S. aureus pneumonia.

Antibiotic resistance is a major problem, with methicillin-resistant Staphylococcus aureus (MRSA) being a prototypical example in surgical and community-acquired infections. S. aureus, like many pathogens, is immune evasive and able to multiply within host immune cells. Consequently, compounds that aid host immunity (e.g., by stimulating the host-mediated killing of pathogens) are appealing alternatives or adjuncts to classical antibiotics. Azithromycin is both an antibacterial and an immunomodulatory drug that accumulates in immune cells. We set out to improve the immunomodulatory properties of azithromycin by coupling the immune activators, nitric oxide and acetate, to its core structure. This new compound, designated CSY5669, enhanced the intracellular killing of MRSA by 45% ± 20% in monocyte-derived macrophages and by 55% ± 15% in peripheral blood leukocytes, compared with untreated controls. CSY5669-treated peripheral blood leukocytes produced fewer proinflammatory cytokines, while in both monocyte-derived macrophages and peripheral blood leukocytes, phagocytosis, ROS production, and degranulation were unaffected. In mice with MRSA pneumonia, CSY5669 treatment reduced inflammation, lung pathology and vascular leakage with doses as low as 0.01 μmol/kg p.o. CSY5669 had diminished direct in vitro antibacterial properties compared with azithromycin. Also, CSY5669 was immunomodulatory at concentrations well below 1% of the minimum inhibitory concentration, which would minimize selection for macrolide-resistant bacteria if it were to be used as a host-directed therapy. This study highlights the potential of CSY5669 as a possible adjunctive therapy in pneumonia caused by MRSA, as CSY5669 could enhance bacterial eradication while simultaneously limiting inflammation-associated pathology.

Liver kinase B1 (Lkb1, gene name Stk11) functions as a tumor suppressor in cancer. Myeloid cell Lkb1 potentiates lung inflammation induced by the Gram-negative bacterial cell wall component lipopolysaccharide and in host defense during Gram-negative pneumonia. Here, we sought to investigate the role of myeloid Lkb1 in lung inflammation elicited by the Gram-positive bacterial cell wall component lipoteichoic acid (LTA) and during pneumonia caused by the Gram-positive respiratory pathogen Streptococcus pneumoniae (Spneu).

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.

Hypoxia-inducible factor- (HIF-) 1α has been implicated in the ability of cells to adapt to alterations in oxygen levels. Bacterial stimuli can induce HIF1α in immune cells, including those of myeloid origin. We here determined the role of myeloid cell HIF1α in the host response during pneumonia and sepsis caused by the common human pathogen Klebsiella pneumoniae. To this end, we generated mice deficient for HIF1α in myeloid cells (LysM-cre × Hif1α fl/fl) or neutrophils (Mrp8-cre × Hif1α fl/fl) and infected these with Klebsiella pneumoniae via the airways. Myeloid, but not neutrophil, HIF1α-deficient mice had increased bacterial loads in the lungs and distant organs after infection as compared to control mice, pointing at a role for HIF1α in macrophages. Myeloid HIF1α-deficient mice did not show increased bacterial growth after intravenous infection, suggesting that their phenotype during pneumonia was mediated by lung macrophages. Alveolar and lung interstitial macrophages from LysM-cre × Hif1α fl/fl mice produced lower amounts of the immune enhancing cytokine tumor necrosis factor upon stimulation with Klebsiella, while their capacity to phagocytose or to produce reactive oxygen species was unaltered. Alveolar macrophages did not upregulate glycolysis in response to lipopolysaccharide, irrespective of HIF1α presence. These data suggest a role for HIF1α expressed in lung macrophages in protective innate immunity during pneumonia caused by a common bacterial pathogen.

Melioidosis, caused by the Gram-negative bacterium Burkholderia pseudomallei, is an emerging cause of pneumonia-derived sepsis in the tropics. The gut microbiota supports local mucosal immunity and is increasingly recognized as a protective mediator in host defenses against systemic infection. Here, we aimed to characterize the composition and function of the intestinal microbiota during experimental melioidosis.

Adult mesenchymal stem cells (MSCs) improve the host response during experimental sepsis in animals. MSCs from various sources express a procoagulant activity that has been linked to the expression of tissue factor. This study sought to determine the role of tissue factor associated with adipose-derived MSCs (ASCs) in their procoagulant and antibacterial effects during pneumonia-derived sepsis.