Neutrophil-derived TRAIL induces apoptosis of alveolar macrophages, limiting the spread of S. pneumoniae infection.

Neutrophil serine proteases cathepsin G (CG), neutrophil elastase (NE), and proteinase 3 (PR3) have recently been shown to contribute to killing of Streptococcus pneumoniae in vitro. However, their relevance in lung-protective immunity against different serotypes of S. pneumoniae in vivo has not been determined so far. Here, we examined the effect of CG and CG/NE deficiency on the lung host defense against S. pneumoniae in mice. Despite similar neutrophil recruitment, both CG knockout (KO) mice and CG/NE double-KO mice infected with focal pneumoniainducing serotype 19 S. pneumoniae demonstrated a severely impaired bacterial clearance, which was accompanied by lack of CG and NE but not PR3 proteolytic activity in recruited neutrophils, as determined using fluorescence resonance energy transfer (FRET) substrates. Moreover, both CG and CG/NE KO mice but not wild-type mice responded with increased lung permeability to infection with S. pneumoniae, resulting in severe respiratory distress and progressive mortality. Both neutrophil depletion and ablation of hematopoietic CG/NE in bone marrow chimeras abolished intra-alveolar CG and NE immunoreactivity and led to bacterial outgrowth in the lungs of mice, thereby identifying recruited neutrophils as the primary cellular source of intra-alveolar CG and NE. This is the first study showing a contribution of neutrophil-derived neutral serine proteases CG and NE to lung-protective immunity against focal pneumonia-inducing serotype 19 S. pneumoniae in mice. These data may be important for the development of novel intervention strategies to improve lung-protective immune mechanisms in critically ill patients suffering from severe pneumococcal pneumonia.

Our data unravel a novel mechanism by which infection with Spn through Ply release induces progression of established lung fibrosis, which can be attenuated by protein-based vaccination of mice.

The growth factor granulocyte/macrophage-colony stimulating factor (GM-CSF) has an important role in pulmonary surfactant metabolism and the regulation of antibacterial activities of lung sentinel cells. However, the potential of intra-alveolar GM-CSF to augment lung protective immunity against inhaled bacterial pathogens has not been defined in preclinical infection models. We hypothesized that transient overexpression of GM-CSF in the lungs of mice by adenoviral gene transfer (Ad-GM-CSF) would protect mice from subsequent lethal pneumococcal pneumonia. Our data show that intra-alveolar delivery of Ad-GM-CSF led to sustained increased pSTAT5 expression and PU.1 protein expression in alveolar macrophages during a 28 day observation period. Pulmonary Ad-GM-CSF delivery two or four weeks prior to infection of mice with S. pneumoniae significantly reduced mortality rates relative to control vector treated mice. This increased survival was accompanied by increased iNOS expression, antibacterial activity and a significant reduction in caspase 3 dependent apoptosis and secondary necrosis of lung sentinel cells. Importantly, therapeutic treatment of mice with recombinant GM-CSF improved lung protective immunity and accelerated bacterial clearance after pneumococcal challenge. We conclude that prophylactic delivery of GM-CSF triggers long-lasting immunostimulatory effects in the lung in vivo and rescues mice from lethal pneumococcal pneumonia by improving antibacterial immunity. These data support use of novel antibiotic-independent immunostimulatory therapies to protect patients against bacterial pneumonias.

Daptomycin is a novel lipopeptide antibiotic with excellent activity against Gram-positive bacterial pathogens, but its therapeutic value for the treatment of invasive pneumococcal disease compared to that for the treatment of pneumococcal pneumonia is incompletely defined. We investigated the efficacy of daptomycin in two models of Streptococcus pneumoniae-induced lung infection, i.e., pneumococcal pneumonia and septic pneumococcal disease. Mice were infected with a bioluminescent, invasive serotype 2 S. pneumoniae strain or a less virulent serotype 19 S. pneumoniae strain and were then given semitherapeutic or therapeutic daptomycin or ceftriaxone. Readouts included survival; bacterial loads; and septic disease progression, as determined by biophotonic imaging. Semitherapeutic daptomycin treatment fully protected the mice against the progression of septic disease induced by serotype 2 S. pneumoniae, while therapeutic treatment of the mice with daptomycin or ceftriaxone led to ϳ70% or ϳ60% survival, respectively. In contrast, mice infected with serotype 19 S. pneumoniae developed severe pneumonia and lung leakage even in the presence of increased intra-alveolar daptomycin levels, resulting in only 40% survival, whereas the ceftriaxone-treated mice had 100% survival. Together, although daptomycin demonstrates little efficacy in the treatment of pneumococcal pneumonia, daptomycin is highly effective in preventing S. pneumoniae-induced septic death, thus possibly offering a therapeutic option for patients with life-threatening septic pneumococcal disease.