Dipeptidyl peptidase I controls survival from Klebsiella pneumoniae lung infection by processing surfactant protein D.

Prior work established that a deficiency in the cysteine protease dipeptidyl peptidase I (DPPI) improves survival following polymicrobial septic peritonitis. To test whether DPPI regulates survival from severe lung infections, DPPI(-/-) mice were studied in a Klebsiella pneumoniae lung infection model, finding that survival in DPPI(-/-) mice is significantly better than in DPPI(+/+) mice 8d after infection. DPPI(-/-) mice have significantly fewer bacteria in the lung than infected DPPI(+/+) mice, but no difference in lung histopathology, lung injury, or cytokine levels. To explore mechanisms of enhanced bacterial clearance in DPPI(-/-) mice, we examined the status of pulmonary collectins, finding that levels of surfactant protein D, but not of surfactant protein A, are higher in DPPI(-/-) than in DPPI(+/+) BAL fluid, and that DPPI(-/-) BAL fluid aggregate bacteria more effectively than control BAL fluid. Sequencing of the amino terminus of surfactant protein D revealed two or eight additional amino acids in surfactant protein D isolated from DPPI(-/-) mice, suggesting processing by DPPI. These results establish that DPPI is a major determinant of survival following Klebsiella pneumoniae lung infection and suggest that the survival disadvantage in DPPI(+/+) mice is in part due to processing of surfactant protein D by DPPI.

Alveolar epithelial cells express mesenchymal proteins in patients with idiopathic pulmonary fibrosis.

Prior work has shown that transforming growth factor-ß (TGF-ß) can mediate transition of alveolar type II cells into mesenchymal cells in mice. Evidence this occurs in humans is limited to immunohistochemical studies colocalizing epithelial and mesenchymal proteins in sections of fibrotic lungs. To acquire further evidence that epithelial-to-mesenchymal transition occurs in the lungs of patients with idiopathic pulmonary fibrosis (IPF), we studied alveolar type II cells isolated from fibrotic and normal human lung. Unlike normal type II cells, type II cells isolated from the lungs of patients with IPF express higher levels of mRNA for the mesenchymal proteins type I collagen, α-smooth muscle actin (α-SMA), and calponin. When cultured on Matrigel/collagen, human alveolar type II cells maintain a cellular morphology consistent with epithelial cells and expression of surfactant protein C (SPC) and E-cadherin. In contrast, when cultured on fibronectin, the human type II cells flatten, spread, lose expression of pro- SPC, and increase expression of vimentin, N-cadherin, and α-SMA; markers of mesenchymal cells. Addition of a TGF-ß receptor kinase inhibitor (SB431542) to cells cultured on fibronectin inhibited vimentin expression and maintained pro-SPC expression, indicating persistence of an epithelial phenotype. These data suggest that alveolar type II cells can acquire features of mesenchymal cells in IPF lungs and that TGF-ß can mediate this process.

Neutrophil-derived IL-6 limits alveolar barrier disruption in experimental ventilator-induced lung injury.

IL-6 is a biological marker of ventilator-associated lung injury that may contribute to alveolar barrier dysfunction in acute respiratory distress syndrome. To determine whether IL-6 affects alveolar barrier disruption in a model of ventilator-induced lung injury, we examined alveolar barrier albumin flux in wild-type (WT) mice given an IL-6-blocking Ab (IL6AB) and mice deficient in IL-6 (IL6KO). Albumin flux was significantly higher in mice given IL6AB compared with mice given a control Ab. Unexpectedly, albumin flux was similar in WT and IL6KO mice. To examine the mechanisms for these findings, lung neutrophil accumulation (myeloperoxidase activity) was compared, revealing a correlation between lung neutrophil accumulation and albumin flux. IL6AB mice had significantly more lung neutrophils than WT and IL6KO mice, which were similar. Therefore, to determine whether the cellular source of IL-6 influences neutrophil accumulation and alveolar barrier function, chimeric mice were compared. WT/KO chimeras (WT mice with IL6KO hematopoietic cells) showed significantly greater albumin flux and neutrophil accumulation with mechanical ventilation than WT/WT mice. Neutrophil depletion decreased albumin flux in WT and WT/KO mice. IL6KO neutrophils were more adherent in an in vitro assay compared with WT neutrophils. IL-6 from a hematopoietic cell source limits alveolar barrier disruption potentially by reducing neutrophil contact with the endothelium. Modulation of IL-6 signaling in a cell type-specific fashion may be a therapeutic target for patients with acute lung injury.

Mast cell IL-6 improves survival from Klebsiella pneumonia and sepsis by enhancing neutrophil killing.

The pleiotropic cytokine IL-6 has favorable and harmful effects on survival from bacterial infections. Although many innate immune cells produce IL-6, little is known about relevant sources in vivo and the nature of its contributions to host responses to severe bacterial infections. To examine these roles, we subjected mast cell-specific IL-6-deficient mice to the cecal ligation and puncture model of septic peritonitis, finding that survival in these mice is markedly worse than in controls. Following intranasal or i.p. inoculation with Klebsiella pneumoniae, IL-6 (-/-) mice are less likely to survive than wild-type controls and at the time of death have higher numbers of bacteria but not inflammatory cells in lungs and peritoneum. Similarly, mast cell-specific IL-6-deficient mice have diminished survival and higher numbers of K. pneumoniae following i.p. infection. Neutrophils lacking IL-6 have greater numbers of live intracellular K. pneumonia, suggesting impaired intracellular killing contributes to reduced clearance in IL-6(-/-) mice. These results establish that mast cell IL-6 is a critical mediator of survival following K. pneumoniae infection and sepsis and suggest that IL-6 protects from death by augmenting neutrophil killing of bacteria.

Systemic mast cell degranulation increases mortality during polymicrobial septic peritonitis in mice.

MCs are required for an effective host response during septic peritonitis. Local MC degranulation facilitates neutrophil recruitment, activation, and bacterial killing. However, the role of MCs located distant from the site of infection is unknown. We studied the temporal and spacial degranulation of MCs following CLP-induced septic peritonitis. The functional importance of systemic MC degranulation during infection was evaluated by compartment-specific MC reconstitution. Serum histamine, reflecting MC degranulation, was elevated 4 h after onset of septic peritonitis. Histologic examination revealed progressive MC degranulation in select tissues during the first 24 h of infection. MC-deficient Wsh mice, reconstituted only in the peritoneal compartment, had improved survival after CLP compared with controls. However, reconstitution in peritoneal plus systemic compartments worsened survival after CLP. IL-6 contributed to the detrimental effects of systemic MCs on survival, as mice systemically reconstituted with IL-6(-/-) MCs were more likely to survive than control mice. These results indicate that in contrast to the benefits of local MC activation during infection, systemic MC activation worsens survival during CLP-induced sepsis.

Parasitic infection improves survival from septic peritonitis by enhancing mast cell responses to bacteria in mice.

Mammals are serially infected with a variety of microorganisms, including bacteria and parasites. Each infection reprograms the immune system's responses to re-exposure and potentially alters responses to first-time infection by different microorganisms. To examine whether infection with a metazoan parasite modulates host responses to subsequent bacterial infection, mice were infected with the hookworm-like intestinal nematode Nippostrongylus brasiliensis, followed in 2-4 weeks by peritoneal injection of the pathogenic bacterium Klebsiella pneumoniae. Survival from Klebsiella peritonitis two weeks after parasite infection was better in Nippostrongylus-infected animals than in unparasitized mice, with Nippostrongylus-infected mice having fewer peritoneal bacteria, more neutrophils, and higher levels of protective interleukin 6. The improved survival of Nippostrongylus-infected mice depends on IL-4 because the survival benefit is lost in mice lacking IL-4. Because mast cells protect mice from Klebsiella peritonitis, we examined responses in mast cell-deficient Kit(W-sh)/Kit(W-sh) mice, in which parasitosis failed to improve survival from Klebsiella peritonitis. However, adoptive transfer of cultured mast cells to Kit(W-sh)/Kit(W-sh) mice restored survival benefits of parasitosis. These results show that recent infection with Nippostrongylus brasiliensis protects mice from Klebsiella peritonitis by modulating mast cell contributions to host defense, and suggest more generally that parasitosis can yield survival advantages to a bacterially infected host.

Cathepsins L and S are not required for activation of dipeptidyl peptidase I (cathepsin C) in mice.

The cysteine protease dipeptidyl peptidase I (DPPI) activates granule-associated immune-cell serine proteases. The in vivo activator of DPPI itself is unknown; however, cathepsins L and S are candidates because they activate pro-DPPI in vitro. In this study, we tested whether cathepsins L and S activate pro-DPPI in vivo by characterizing DPPI activity and processing in cells lacking cathepsins L and S. DPPI activity, and the relative size and amounts of DPPI heavy and light chains, were identical in mast cells from wild-type and cathepsin L/S double-null mice. Furthermore, the activity of DPPI-dependent chymase was preserved in tissues of cathepsin L/S double-null mice. These results show that neither cathepsin L nor S is required for activation of DPPI and suggest that one or more additional proteases is responsible.