Prevention and amelioration of rodent endotoxin-induced lung injury with administration of a novel therapeutic tripeptide feG

Abstract

Background

The synthetic tripeptide feG is a novel pharmacological agent that decreases neutrophil recruitment, infiltration, and activation in various animal models of inflammatory disease. In human and rat cell culture models, feG requires pre-stimulation in order to decrease in vitro neutrophil chemotaxis. We aimed to investigate the effect of feG on neutrophil chemotaxis in a lipopolysaccharide-induced acute lung injury model without pre-stimulation.

Methods

The efficacy of feG as both a preventative treatment, when administered before lung injury (prophylactic), or as a therapeutic treatment, administered following lung injury (therapeutic), was investigated.

Results

Prophylactic or therapeutic feG administration significantly reduced leukocyte infiltration, ameliorated the severity of inflammatory damage, and restored lung function. feG was demonstrated to significantly decrease bronchoalveolar lavage cell infiltration, lung myeloperoxidase activity, lung oedema, histological tissue injury scores, and improve arterial blood oxygenation and respiratory mechanics.

Conclusions

feG reduced leukocyte infiltration, ameliorated the severity of inflammatory damage, and restored lung function when administered prophylactically or therapeutically in a rodent model of lipopolysaccharide-induced acute lung injury, without the need for pre-stimulation, suggesting a direct rather than indirect mechanism of action in the lung.

Introduction

Acute lung injury (ALI) is a common problem in the intensive care unit setting with the incidence of ALI estimated at between 34 and 78.9 cases per 100,000 persons/year in Australia and the USA, respectively [1], [2]. The reported in-hospital mortality rates from ALI are 32% in Australia [1] and 38.5% in the USA [2]. Common causes of ALI include pneumonia, aspiration of gastric contents, transfusion, trauma, and sepsis. It is clear that ALI outcomes can be modified by ventilation strategies. In the landmark ARDS Network Study [3], ALI mortality was reduced from 39.8% to 31% by using low tidal volume ventilation (6 vs 12 ml/kg predicted body weight). However, greater reduction in ALI morbidity and mortality may still be achieved through the development of a clinically relevant pharmacological agent.

The synthetic tripeptide feG ((d-Phe)-(d-Glu)-Gly) is a novel therapeutic pharmacological agent that in animal models, reduces the severity of acute pancreatitis and acute pancreatitis-associated lung injury, spinal cord injury, intestinal anaphylactic reactions, pulmonary inflammation, asthma and ventilation-induced lung injury [4], [5], [6], [7]. In vitro experiments previously suggested that feG requires pre-stimulation (such as with platelet-activating factor-stimulation, ovalbumin or carrageenan) in order to decrease neutrophil chemotaxis in both humans and rat cell culture models [8]. In vivo we have similarly shown that inflammation and tissue damage in injured lung and pancreas tissue is significantly reduced, and respiratory function is improved, in mouse and rat models of acute pancreatitis-associated lung injury where feG is administered prior to lung injury (prophylactically) or after the initiation of lung injury (therapeutically), but following the potential pre-stimulation of pancreatic injury in both incidences [4], [6]. However, the ability of feG to similarly prevent or ameliorate inflammatory damage in the lung without pre-stimulation has not been previously investigated.

Endotoxin-induced lung injury is a highly reproducible model of ALI that induces clinically relevant responses including a neutrophilic inflammatory activation and infiltration, with an increase in intrapulmonary cytokines [9]. The endotoxin lipopolysaccharide (LPS) is found in the outer membrane of Gram-negative bacteria, and is capable of eliciting all of the clinical symptoms of septic shock [10]. Activated neutrophils are attracted to the lung, by pro-inflammatory and chemotactic cytokines, and stimulate further leukocyte chemotaxis and a respiratory burst together with resident macrophages, resulting in pulmonary epithelial cell apoptosis and necrosis [11]. Pulmonary epithelial barrier permeability is subsequently increased, leading to an influx of protein-rich oedematous fluid [11], [12], [13], which may cause surfactant inactivation, resulting in decreased lung compliance and impaired gas exchange [13].

Direct exposure in the lung via intratracheal LPS causes a pronounced inflammatory response initiated primarily through the activation of alveolar macrophages, and ultimately resulting in the progressive influx of leukocytes into the alveolar space [9]. In this study, we use a rodent model to investigate the effect of feG as a preventative treatment when administered before (prophylactic) or after (therapeutic) the initiation of LPS-induced ALI.