The triggering receptor expressed on myeloid cells 2 inhibits complement component 1q effector mechanisms and exerts detrimental effects during pneumococcal pneumonia.

Phagocytosis and inflammation within the lungs is crucial for host defense during bacterial pneumonia. Triggering receptor expressed on myeloid cells (TREM)-2 was proposed to negatively regulate TLR-mediated responses and enhance phagocytosis by macrophages, but the role of TREM-2 in respiratory tract infections is unknown. Here, we established the presence of TREM-2 on alveolar macrophages (AM) and explored the function of TREM-2 in the innate immune response to pneumococcal infection in vivo. Unexpectedly, we found Trem-2(-/-) AM to display augmented bacterial phagocytosis in vitro and in vivo compared to WT AM. Mechanistically, we detected that in the absence of TREM-2, pulmonary macrophages selectively produced elevated complement component 1q (C1q) levels. We found that these increased C1q levels depended on peroxisome proliferator-activated receptor-δ (PPAR-δ) activity and were responsible for the enhanced phagocytosis of bacteria. Upon infection with S. pneumoniae, Trem-2(-/-) mice exhibited an augmented bacterial clearance from lungs, decreased bacteremia and improved survival compared to their WT counterparts. This work is the first to disclose a role for TREM-2 in clinically relevant respiratory tract infections and demonstrates a previously unknown link between TREM-2 and opsonin production within the lungs.

The scavenger receptor CD36 downmodulates the early inflammatory response while enhancing bacterial phagocytosis during pneumococcal pneumonia.

CD36 is a scavenger receptor that exhibits pleiotropic functions, including adhesion to thrombospondin, inhibition of angiogenesis, transport of long-chain fatty acids, and clearance of apoptotic cells. In addition, it has been implicated in the host immune response because it acts as a coreceptor for TLR2 and plays a role in Staphylococcus aureus infection. However, its role in other Gram-positive bacterial infections is unclear. In this study, using mice deficient in CD36, we sought to examine the role of CD36 in pneumococcal pneumonia, a major cause of morbidity and mortality worldwide. We show that CD36 is expressed on both alveolar macrophages and respiratory epithelial cells. Early in infection, CD36(-/-) mice have an exaggerated inflammatory response compared with wild-type littermate controls. In vitro studies using CD36(-/-) primary cells confirm the enhanced early inflammation in response to S. pneumoniae and its lipoteichoic acid, demonstrate that S. pneumoniae binds to cells via its phosphocholine residues, and suggest a role for CD36 in reducing inflammation induced by the phosphocholine residues of pneumococcal lipoteichoic acid. Later in infection, although CD36(-/-) mice exhibit impaired bacterial clearance, owing to a decreased capacity of CD36(-/-) macrophages to phagocytose S. pneumoniae, minor effects on mortality occur, in comparison with those in wild-type littermate control mice. These data show that CD36 contributes to the pulmonary host response during S. pneumoniae infection by virtue of its ability to act as a phagocytic receptor and as a modulator of the early innate immune response.

Apolipoprotein B100 is a suppressor of Staphylococcus aureus-induced innate immune responses in humans and mice.

Plasma lipoproteins such as LDL (low-density lipoprotein) are important therapeutic targets as they play a crucial role in macrophage biology and metabolic disorders. The impact of lipoprotein profiles on host defense pathways against Gram-positive bacteria is poorly understood. In this report, we discovered that human serum lipoproteins bind to lipoteichoic acid (LTA) from Staphylococcus aureus and thereby alter the immune response to these bacteria. Size-exclusion chromatography and solid-phase-binding analysis of serum revealed the direct interaction of LTA with apolipoproteins (Apo) B100, ApoA1, and ApoA2. Only ApoB100 and the corresponding LDL exerted biological effects as this binding significantly inhibited LTA-induced cytokine releases from human and murine immune cells. Serum from hypercholesterolemic mice or humans significantly diminished cytokine induction in response to S. aureus or its LTA. Sera taken from the patients with familial hypercholesterolemia before and after ApoB100-directed immuno-apheresis confirmed that ApoB100 inhibited LTA-induced inflammation in humans. In addition, mice in which LDL secretion was pharmacologically inhibited, displayed significantly increased serum cytokine levels upon infection with S. aureus in vivo. The present study identifies ApoB100 as an important suppressor of innate immune activation in response to S. aureus and its LTA.

TLR 2 and CD14 mediate innate immunity and lung inflammation to staphylococcal Panton-Valentine leukocidin in vivo.

The pore-forming toxin Panton-Valentine leukocidin (PVL) is carried by community-acquired methicillin-resistant Staphylococcus aureus and associated with necrotizing pneumonia together with poor prognosis of infected patients. Although the cell-death-inducing properties of PVL have previously been examined, the pulmonary immune response to PVL is largely unknown. Using an unbiased transcriptional profiling approach, we show that PVL induces only 29 genes in mouse alveolar macrophages, which are associated with TLR signaling. Further studies indicate that PVL directly binds to TLR2 and induces immune responses via NF-κB in a TLR2, CD14, MyD88, IL-1R-associated kinase 1, and TNFR-associated factor 6-dependent manner. PVL-mediated inflammation is independent of pore formation but strongly depends on the LukS subunit and is suppressed in CD14/TLR2(-/-) cells. In vivo PVL or LukS induced a robust inflammatory response in lungs, which was diminished in CD14/TLR2(-/-) mice. These results highlight the proinflammatory properties of PVL and identify CD14/TLR2 as an essential receptor complex for PVL-induced lung inflammation.

Myeloid PTEN promotes inflammation but impairs bactericidal activities during murine pneumococcal pneumonia.

Phosphatidylinositol 3-kinase has been described as an essential signaling component involved in the chemotactic cell influx that is required to eliminate pathogens. At the same time, PI3K was reported to modulate the immune response, thus limiting the magnitude of acute inflammation. The precise role of the PI3K pathway and its endogenous antagonist phosphatase and tensin homolog deleted on chromosome 10 (PTEN) during clinically relevant bacterial infections is still poorly understood. Utilizing mice lacking myeloid cell-specific PTEN, we studied the impact of PTEN on the immune response to Streptococcus pneumoniae. Survival analysis disclosed that PTEN-deficient mice displayed less severe signs of disease and prolonged survival. The inflammatory response to S. pneumoniae was greatly reduced in macrophages in vitro and in vivo. Unexpectedly, neutrophil influx to the lungs was significantly impaired in animals lacking myeloid-cell PTEN, whereas the additional observation of improved phagocytosis by alveolar macrophages lacking PTEN ultimately resulted in unaltered lung CFUs following bacterial infection. Together, the absence of myeloid cell-associated PTEN and consecutively enhanced PI3K activity dampened pulmonary inflammation, reduced neutrophil influx, and augmented phagocytic properties of macrophages, which ultimately resulted in decreased tissue injury and improved survival during murine pneumococcal pneumonia.

TREM-1 activation alters the dynamics of pulmonary IRAK-M expression in vivo and improves host defense during pneumococcal pneumonia.

Triggering receptor expressed on myeloid cells-1 (TREM-1) is an amplifier of TLR-mediated inflammation during bacterial infections. Thus far, TREM-1 is primarily associated with unwanted signs of overwhelming inflammation, rendering it an attractive target for conditions such as sepsis. Respiratory tract infections are the leading cause of sepsis, but the biological role of TREM-1 therein is poorly understood. To determine the function of TREM-1 in pneumococcal pneumonia, we first established TREM-1 up-regulation in infected lungs and human plasma together with augmented alveolar macrophage responsiveness toward Streptococcus pneumoniae. Mice treated with an agonistic TREM-1 Ab and infected with S. pneumoniae exhibited an enhanced early induction of the inflammatory response that was indirectly associated with lower levels of negative regulators of TLR signaling in lung tissue in vivo. Later in infection, TREM-1 engagement altered S. pneumoniae-induced IRAK-M (IL-1R-associated kinase-M) kinetics so as to promote the resolution of pneumonia and remarkably led to an accelerated elimination of bacteria and consequently improved survival. These data show that TREM-1 exerts a protective role in the innate immune response to a common bacterial infection and suggest that caution should be exerted in modulating TREM-1 activity during certain clinically relevant bacterial infections.

Bbeta(15-42) protects against acid-induced acute lung injury and secondary pseudomonas pneumonia in vivo.

RATIONALE: Acute lung injury (ALI) is a serious condition in critically ill patients that predisposes to secondary bacterial pneumonia. Vascular leak is a hallmark in the pathogenesis of ALI. The fibrin-derived peptide Bbeta(15-42) was shown to preserve endothelial barriers, thereby reducing vascular leak. The potential therapeutic role of Bbeta(15-42) in ALI has not been addressed so far. OBJECTIVES: To investigate the therapeutic potential of Bbeta(15-42) in ALI and secondary pneumonia induced by Pseudomonas aeruginosa. METHODS: The effect of the fibrin-derived peptide Bbeta(15-42) was studied in models of ALI, induced either by pulmonary administration of LPS or hydrochloric acid. Lung inflammation was analyzed by quantifying cell influx, cytokine levels, and oxidized lipids. Vascular leak was determined by Evans Blue extravasations and alveolar protein content. In subsequent two-hit studies, mice were infected with P. aeruginosa 24 hours after induction of aspiration pneumonitis and effects of Bbeta(15-42) on inflammation, bacterial clearance, and survival were evaluated. MEASUREMENTS AND MAIN RESULTS: After LPS or acid inhalation, proinflammatory cytokine levels, neutrophil influx, and vascular leak were found diminished in mice treated with Bbeta(15-42). Acid aspiration impaired macrophage functions and rendered mice more susceptible to subsequent P. aeruginosa infection, whereas mice that received Bbeta(15-42) during acid aspiration and were subsequently challenged with bacteria displayed reduced inflammation, enhanced bacterial clearance, and ultimately improved survival. CONCLUSIONS: The fibrin-derived peptide Bbeta(15-42) exerted protective effects during ALI, resulting in diminished lung injury and preserved antibacterial properties of macrophages, which improved outcome during subsequent P. aeruginosa pneumonia.

WAVE1 mediates suppression of phagocytosis by phospholipid-derived DAMPs.

Clearance of invading pathogens is essential to preventing overwhelming inflammation and sepsis that are symptomatic of bacterial peritonitis. Macrophages participate in this innate immune response by engulfing and digesting pathogens, a process called phagocytosis. Oxidized phospholipids (OxPL) are danger-associated molecular patterns (DAMPs) generated in response to infection that can prevent the phagocytic clearance of bacteria. We investigated the mechanism underlying OxPL action in macrophages. Exposure to OxPL induced alterations in actin polymerization, resulting in spreading of peritoneal macrophages and diminished uptake of E. coli. Pharmacological and cell-based studies showed that an anchored pool of PKA mediates the effects of OxPL. Gene silencing approaches identified the A-kinase anchoring protein (AKAP) WAVE1 as an effector of OxPL action in vitro. Chimeric Wave1(-/-) mice survived significantly longer after infection with E. coli and OxPL treatment in vivo. Moreover, we found that endogenously generated OxPL in human peritoneal dialysis fluid from end-stage renal failure patients inhibited phagocytosis via WAVE1. Collectively, these data uncover an unanticipated role for WAVE1 as a critical modulator of the innate immune response to severe bacterial infections.

Lipocalin 2 deactivates macrophages and worsens pneumococcal pneumonia outcomes.

Macrophages play a key role in responding to pathogens and initiate an inflammatory response to combat microbe multiplication. Deactivation of macrophages facilitates resolution of the inflammatory response. Deactivated macrophages are characterized by an immunosuppressive phenotype, but the lack of unique markers that can reliably identify these cells explains the poorly defined biological role of this macrophage subset. We identified lipocalin 2 (LCN2) as both a marker of deactivated macrophages and a macrophage deactivator. We show that LCN2 attenuated the early inflammatory response and impaired bacterial clearance, leading to impaired survival of mice suffering from pneumococcal pneumonia. LCN2 induced IL-10 formation by macrophages, skewing macrophage polarization in a STAT3-dependent manner. Pulmonary LCN2 levels were tremendously elevated during bacterial pneumonia in humans, and high LCN2 levels were indicative of a detrimental outcome from pneumonia with Gram-positive bacteria. Our data emphasize the importance of macrophage deactivation for the outcome of pneumococcal infections and highlight the role of LCN2 and IL-10 as determinants of macrophage performance in the respiratory tract.