Molecular mechanisms in acute lung injury

Reciprocal backcross mice confirm major loci linked to hyperoxic acute lung injury survival time

Morbidity and mortality associated with acute lung injury (ALI) and acute respiratory distress syndrome remain substantial. Although many candidate genes have been tested, a clear understanding of the pathogenesis is lacking, as is our ability to predict individual outcome. Because ALI is a complex disease, single gene approaches cannot easily identify effectors that must be treated concurrently. We employed a strategy to help identify critical genes and gene combinations involved in ALI mortality. Using hyperoxia to induce ALI, a mouse model for genetic analyses of ALI survival time was identified: C57BL/6J (B) mice are sensitive (i.e., die early), whereas 129X1/SvJ (S) mice are significantly more resistant, but with low penetrance. Segregation analysis of reciprocal F(2) mice generated from B and S strains revealed significant sex, cross, and parent of origin effects. Quantitative trait locus (QTL) analysis identified five chromosomal regions significantly linked to hyperoxic ALI survival time (named Shali1-Shali5). Further analyses demonstrated that both parental strains contribute resistance alleles to their offspring and that the phenotype demonstrated parent of origin effects. To validate earlier findings, we generated and tested mice from all eight possible B-S-derived backcrosses. Results from segregation and QTL analyses of 935 backcrosses, alone and combined with the previous 840 B-S-derived F(2) population, further supported the highly significant QTLs on chromosomes 1 (Shali1) and 4 (Shali2) and confirmed that the sex, cross, and parent of origin all contribute to survival time with hyperoxic ALI.

Role of IL-18 in acute lung inflammation

We have examined the role of IL-18 after acute lung inflammation in rats caused by intrapulmonary deposition of IgG immune complexes. Constitutive IL-18 mRNA and protein expression (precursor form, 26 kDa) were found in normal rat lung, whereas in inflamed lungs, IL-18 mRNA was up-regulated; in bronchoalveolar (BAL) fluids, the 26-kDa protein form of IL-18 was increased at 2-4 h in inflamed lungs and remained elevated at 24 h, and the "mature" protein form of IL-18 (18 kDa) appeared in BAL fluids 1-8 h after onset of inflammation. ELISA studies confirmed induction of IL-18 in inflamed lungs (in lung homogenates and in BAL fluids). Prominent immunostaining for IL-18 was found in alveolar macrophages from inflamed lungs. When rat lung macrophages, fibroblasts, type II cells, and endothelial cells were cultured in vitro with LPS, only the first two produced IL-18. Intratracheal administration of rat recombinant IL-18 in the lung model caused significant increases in lung vascular permeability and in BAL content of neutrophils and in BAL content of TNF-alpha, IL-1beta, and cytokine-induced neutrophil chemoattractant, whereas intratracheal instillation of anti-IL-18 greatly reduced these changes and prevented increases in BAL content of IFN-gamma. Intratracheal administration of the natural antagonist of IL-18, IL-18 binding protein, resulted in suppressed lung vascular permeability and decreased BAL content of neutrophils, cytokines, and chemokines. These findings suggest that endogenous IL-18 functions as a proinflammatory cytokine in this model of acute lung inflammation, serving as an autocrine activator to bring about expression of other inflammatory mediators.

Leukocyte-endothelial interactions in antineutrophil cytoplasmic antibody-associated systemic vasculitis

The etiology of ANCA-associated vasculitis is unknown. Currently, it is believed that disease may be triggered by infection with the release of proinflammatory cytokines in genetically susceptible individuals. Priming of PMNs and endothelial cells by these cytokines allows ANCAs to activate PMNs, with damage localized to the endothelium, resulting in early lesions. Damage and activation of endothelial cells produces proinflammatory chemokines and cytokines with influxes of monocytes and T cells that intensify endothelial damage. In the kidney, these changes eventually lead to crescent formation. Antigen-specific memory T cells persist after disease remission with the potential of reactivation and disease relapse. Although our understanding of the pathophysiologic mechanisms of ANCA-associated vasculitis is far greater now than when ANCAs were first identified in 1982, more experimental work in combination with clinical observations is required to further elucidate these mechanisms.

Pathogenesis of ANCA-associated systemic vasculitis

The aetiology of primary systemic vasculitides remains unknown. Recent advances have been made in the understanding of relevant mechanisms of inflammation, particularly the role of the endothelium and interactions with inflammatory mediators and immune effector cells. In Wegener's granulomatosis and microscopic polyangiitis the evidence suggests an autoimmune inflammatory process, characterized by an early lesion involving neutrophils and endothelial cells as both targets and active participants; priming of neutrophils and endothelial cells allows ANCA to activate neutrophils with damage localized to the endothelium. In the absence of immune complex deposition, the role of the ANCA is particularly intriguing. Endothelial cell damage and activation produces pro-inflammatory mediators with influx monocytes and T cells intensifying damage. Increased understanding of the pathogenesis of systemic vasculitis is likely to provide the basis for the use of more selective immunomodulatory therapies in the future.

Preservation of complement-induced lung injury in mice with deficiency of NADPH oxidase

Mice with chronic granulomatous disease (X-CGD mice) generated by mutating the X-linked gene for a subunit of NADPH oxidase have been analyzed for their ability to respond to intravenous injection of purified cobra venom factor (CVF). This agent in wild-type mice produces a neutrophil-dependent and catalase-sensitive form of lung injury. Lung injury was evaluated by measuring the accumulation of extravascular albumin. Quite unexpectedly, the lungs of X-CGD mice showed no difference in the increased accumulation of extravascular albumin after injection of CVF when compared to wild-type mice. In both X-CGD and wild-type mice, full development of injury required neutrophils. While catalase was highly protective in wild-type mice, its protective effects were completely lost in the X-CGD mice. Furthermore, a competitive antagonist of L-arginine, N(G)-methyl-L-arginine, was protective in X-CGD mice but not in wild-type mice. Allopurinol was protective in both types of mice. Both the basal and the CVF-inducible lung mRNA for inducible nitric oxide synthase and IL-1beta was similar in X-CGD and wild-type mice. These data indicate that oxygen radical production and lung injury in response to injection of CVF occurs through alternative pathways in mice with genetic deletion of NADPH oxidase.

Protective effect of platelet activating factor antagonists on cultured endothelial cell lysis induced by elastase or activated neutrophils

1. The mechanism(s) responsible for injury of endothelial cells induced by human leukocyte elastase (HLE) was investigated in an immortalized venous human endothelial cell line (IVEC). 2. First, the proteinase concentrations and incubation delays necessary to trigger a significant IVEC cytotoxicity were determined by chromium assays. Thus, exposure of IVEC for 6 h to 10 micrograms ml-1 HLE resulted in 22 +/- 2.8% lysis and 36.4 +/- 5.4% detachment (mean +/- s.e. mean; n = 4; P < 0.05). 3. WEB 2086, a specific platelet-activating factor (PAF) receptor antagonist, induced a significant concentration-dependent decrease of such a lysis (39.6 +/- 7.7% protection at 100 microM; n = 4). This potential role for PAF was confirmed with two other antagonists of this lipid mediator, i.e., BN 52021 and RP 48740. 4. Finally, we demonstrated that pretreatment of IVEC with WEB 2086 protected significantly against cell lysis induced by stimulated human neutrophils, an experimental model in which HLE participates.

Streptococcal cysteine protease augments lung injury induced by products of group A streptococci

Streptococcus pyogenes infections in humans may be associated with severe clinical manifestations, including adult respiratory distress syndrome and a toxic shock-like syndrome. These observations have led to the investigation of products of group A streptococci that may contribute to increased virulence. Streptococcal pyrogenic exotoxin B is a highly conserved precursor of an extracellular cysteine protease that is secreted by S. pyogenes. We investigated the ability of this streptococcal cysteine protease (SCP) to act synergistically with either streptococcal cell wall antigen (SCW) or streptolysin-O (SLO) to augment lung injury in rats. Intratracheal administration of either SCW or SLO alone caused lung injury, as measured by pulmonary vascular leak. Bronchoalveolar lavage (BAL) fluid analysis showed that SCW induced neutrophil accumulation and appearance of interleukin-1beta and tumor necrosis factor alpha. In contrast, SLO induced neither neutrophil influx nor significant cytokine elevations in BAL fluids. Intratracheal administration of SCP with either SCW or SLO resulted in synergistic augmentation of lung vascular permeability and accumulation of BAL neutrophils. The synergy was reduced when SCP was either heat inactivated or coinstilled with a peptide inhibitor of the protease. SCP in the presence of SCW resulted in a significant increase in BAL fluid tumor necrosis factor alpha content but not in immunoreactive interleukin-1beta. Moreover, the copresence of SAP with SAW resulted in increased BAL fluid nitrite-nitrate levels, indicative of nitric oxide production. These data demonstrate that SCP acts synergistically with other S. pyogenes products (SCW or SLO) to increase tissue injury and provide additional evidence that SCP may function as an important virulence factor in group A streptococcal infections.

Regulation of superoxide anion generation in bovine alveolar macrophages by bacterial lipopolysaccharide, serum proteins, and modulators of signal transduction

The respiratory burst of phagocytes in an important leukocyte function which results in generation of oxygen species that are both microbicidal and potentially damaging to host tissues. We investigated regulation of the respiratory burst of alveolar macrophages in response to lipopolysaccharide (LPS) derived from gram-negative bacteria, serum proteins, and several modulators of signal transduction. When employed as a single stimulus, LPS (E. coli 055:B5, 10 ng/ml-1 microgram/ml) was a weak stimulus for generation of superoxide anion (O2-) as compared to the potent effect of the protein kinase C activator, phorbol 12-myristate 13-acetate (PMA; 500 ng/ml). However, when LPS was combined with fetal bovine serum (FBS; 0.4-1.0% vol/vol, equivalent to 128-320 micrograms protein/ml), O2- generation was enhanced approximately two-fold over LPS alone. A chromatographically-derived bovine serum fraction which contained bovine lipopolysaccharide-binding protein (bLBP; 0.25-1.0 microgram/ml) was an effective substitute for FBS at a much lower protein concentration than whole FBS, and a similar synergistic effect with LPS on O2- generation was observed. Stimulation of macrophages for generation of O2- either with LPS alone or with LPS plus serum/serum fraction was suppressed by the protein tyrosine kinase inhibitor heribimycin A (0.2 ng/ml), and the calcium chelator BAPTA (12 microM), but not by modulators of G-proteins, including pertussis toxin (10 ng/ml) and cholera toxin (5 micrograms/ml protein). Essentially complete inhibition of O2- synthesis by herbimycin A and BAPTA occurred in the presence of LPS and the bLBP-containing serum fraction (1 microgram/ml protein), but only partial inhibition (46.7% and 64.1%, respectively) was observed in the presence of LPS plus FBS (256 micrograms/ml protein). These results indicate that when LPS is used as a sole stimulus it induces modest respiratory burst activity. However, when LPS is combined with appropriate serum components, it stimulates alveolar macrophages to generate larger amounts of O2-. Cellular signaling pathways important in stimulation of macrophages by LPS and serum components are protein tyrosine kinase- and Ca(++)-dependent, but do not relay on G-protein-mediated signaling.

Interleukin-6 secretion by bacterial lipopolysaccharide-stimulated bovine alveolar macrophages in vitro

Interleukin-6 (IL-6) is a pluripotent cytokine that may play a role in pulmonary defense against bacterial pathogens. We have quantitated the response of bovine alveolar macrophages (bAM) to bacterial lipopolysaccharide (LPS; E. coli 055: B5) in vitro using the IL-6 sensitive 7TD1 cell line. Bacteria LPS in the absence of serum induced IL-6 secretion from bAM (1 x 10(6) ml-1) over a range of LPS concentrations from 10 ng ml-1 to 10 micrograms ml-1. This resulted in IL-6 levels ranging from approximately 5 to over 200 U ml-1.IL-6 secretion by from approximately 5 to over 200 U ml-1.IL-6 secretion by LPS-stimulated bAM was increased by 24 h poststimulation, and continued to increase up to 72 h after stimulation. Fetal bovine serum (FBS, 1% vol/vol; 320 micrograms ml-1) enhanced IL-6 secretion from macrophages in the presence of LPS by approximately 10-fold compared with LPS alone. A bovine serum fraction (1 microgram ml-1 protein) prepared using ion-exchange chromatography also markedly enhanced IL-6 secretion versus LPS alone. The stimulatory effect of IL-6-like activity in the bAM supernatants was neutralized by an anti-human IL-6 polyclonal antibody. Northern blot analysis revealed increased IL-6 mRNA at 2 h poststimulation with LPS + FBS, peak levels at 4 h, and levels were decreased by 6 h poststimulation. Results suggest that IL-6 is secreted by bovine alveolar macrophages, and that bacterial LPS and serum components synergize to produce this response.

Cytokines, inflammation, and autoimmune diseases

The understanding of cytokines is in its infancy, but it appears that overproduction or deficiency of these intracellular mediators may contribute to inflammatory and autoimmune diseases such as insulin-dependent diabetes mellitus and rheumatoid arthritis. As the complexities of cytokine actions and interactions are unraveled, therapeutic blockade or upregulation may be possible.

CD14 and tissue factor expression by bacterial lipopolysaccharide-stimulated bovine alveolar macrophages in vitro

The membrane-associated CD14 receptor (mCD14) is a monocyte/macrophage differentiation antigen, and it has been demonstrated to serve as a receptor for bacterial lipopolysaccharide (LPS; endotoxin). Binding of LPS to mCD14 has been shown to be associated with LPS-induced macrophage, monocyte, and neutrophil activation in humans. In this report, we describe the presence and function of an mCD14-like receptor on bovine alveolar macrophages (bAM). An immunofluorescence technique and flow cytometric analysis indicated binding of anti-human CD14 monoclonal antibodies (MAb) My4, 3C10, and 60bd to bAM. Binding of anti-CD14 MAb (3C10 and MY4) was reduced over 20% by pretreatment of bAM with phosphatidylinositol-specific phospholipase C (0.5 to 1.0 U/ml), indicating that bovine mCD14 is a glycosyl phosphatidylinositol-anchored protein. In addition, pretreatment of bAM with anti-CD14 MAb decreased binding of 125I-labeled LPS to macrophages, suggesting that bovine mCD14 serves as a receptor for LPS. A cDNA probe based on the human sequence for CD14 was used in Northern (RNA) blot analysis, and hybridization to human monocyte CD14 yielded the expected 1.5-kb band. Hybridization to bovine mRNA yielded a 1.5-kb band plus an unexpected 3.1-kb band. Constitutive expression of bovine CD14 mRNA was observed, and the expression level was modestly elevated in bAM stimulated for 24 h with LPS (1 ng/ml) in the presence of bovine serum. The function and activation of bAM were assessed by quantitation of tissue factor (TF) expression on the cells using an activated factor X-related chromogenic assay and S-2222 substrate. LPS (1 ng/ml)-mediated upregulation of TF expression on bAM was dependent on the presence of bovine serum components, and TF expression was inhibited by anti-CD14 MAb. In addition, TF mRNA levels in LPS-stimulated bAM were decreased by pretreatment of cells with anti-CD14 MAb (MAb 60bd, 10 micrograms/ml).