M. van Eijk

Intratracheal Aerosolization of Endotoxin (LPS) in the Rat: A Comprehensive Animal Model to Study Adult (Acute) Respiratory Distress Syndrome

The aim of the study was to extend existing evidence that intratracheal aerosolization of LPS may serve as a very relevant model to study ARDS. The authors investigated the sequence of pathogenic events reflected by changes in levels of tumor necrosis factor alpha (TNF alpha), surfactant-associated protein A (SP-A) in BAL fluid, in addition to cell count, edema formation, and respiratory function. Within 24 h following intratracheal aerosolization of LPS in the rat, ARDS could be diagnosed according to the lung injury score for patients. This score includes the extent of the inflammatory density on chest X-rays, the severity of hypoxemia, the decline in lung compliance, and the level of PEEP (positive end expiratory pressure). In addition, other typical features of human ARDS appeared to be present in this model: (1) increased microvascular permeability reflected by edema, elevated levels of protein and of LDH, and increased numbers of PMNs in BAL fluid; (2) high levels of TNF alpha in BAL fluid preceding the appearance of PMNs; (3) changes in breathing pattern and a gradual development of respiratory failure with decreased compliance. SP-A levels in BAL fluid doubled within one hour after LPS administration, suggesting that this collectin may play a role in the immediate inflammatory response. Taken together, the findings presented here suggest that intratracheal LPS administration mimics the clinical development of ARDS very closely.

Aerosolized endotoxin is immediately bound by pulmonary surfactant protein D in vivo

Collectins are carbohydrate binding proteins that are implicated in innate host defense. The lung collectins, surfactant proteins A and D (SP-A and SP-D), bind a variety of pathogens in vitro and influence phagocytosis by alveolar macrophages. In this report we show that SP-D binds endotoxin (lipopolysaccharide, LPS) in vivo in a rat model of acute respiratory distress syndrome (ARDS). Intratracheal aerosolization of LPS in rats resulted in the typical features of human ARDS. Total amounts of SP-D, as well as the carbohydrate binding properties of SP-D were measured in lung lavage as a function of time. The amount of SP-D did not change during 24 h. Interestingly, SP-D in lung lavage isolated from rats during the first 2 h after LPS treatment, was not able to bind to carbohydrate. Further analysis revealed that the carbohydrate binding sites of SP-D were occupied by LPS, suggesting that SP-D is an LPS scavenging molecule in vivo. Electron microscopic analysis indicated that, 1 h after LPS aerosolization, aggregates of SP-D with LPS were found in lysosomal structures in alveolar macrophages. We conclude that the lung collectin SP-D binds inhaled endotoxin in vivo, which may help to protect the lung from endotoxin-induced disease.

Very low surfactant protein C contents in newborn Belgian White and Blue calves with respiratory distress syndrome

We have studied a respiratory distress syndrome (RDS) occurring in newborn calves of the Belgian White and Blue (BWB) breed that represents the large majority of beef cattle in Belgium. Pulmonary surfactant isolated from 14 BWB newborn calves that died from RDS and from 7 healthy controls was analysed for composition and surface activity. An extremely low content or, in some instances, an absence of surfactant protein C (SP-C) was detected in the RDS samples by Western blotting and differential amino acid analysis [0.03+/-0.01% (w/w) relative to total phospholipids, compared with 0.39+/-0.06% for healthy controls (means+/-S.E.M., P < 0.001)]. The contents of surfactant protein B (SP-B) were similar in RDS and control samples. The crude surfactant samples isolated from RDS calves had higher ratios of total protein to total phospholipid, altered phospholipid profiles and lower SP-A contents. Both crude and organic extracts of RDS surfactant samples showed increased dynamic surface tension compared with healthy controls when evaluated with a pulsating-bubble surfactometer. The addition of purified SP-C to organic extracts of RDS surfactant samples lowered surface tension. Strongly decreased levels of mature SP-C associated with fatal RDS and altered surface activity in vitro have, to the best of our knowledge, not been previously reported. The mechanisms underlying RDS and the decrease in SP-C in BWB calves remain to be established.

Pulmonary Surfactant Protein D in First-Line Innate Defence against Influenza A Virus Infections

Influenza A viruses (IAV) cause respiratory tract infections annually associated with excess mortality and morbidity. Nonspecific, innate immune mechanisms play a key role in protection against viral invasion at early stages of infection. A soluble protein present in mucosal secretions of the lung, surfactant protein D (SP-D), is an important component of this initial barrier that helps to prevent and limit IAV infections of the respiratory epithelium. This collagenous C-type lectin binds IAVs and thereby inhibits attachment and entry of the virus but also contributes to enhanced clearance of SP-D-opsonized virus via interactions with phagocytic cells. In addition, SP-D modulates the inflammatory response and helps to maintain a balance between effective neutralization/killing of IAV, and protection against alveolar damage resulting from IAV-induced excessive inflammatory responses. The mechanisms of interaction between SP-D and IAV not only depend on the structure and binding properties of SP-D but also on strain-specific features of IAV, and both issues will be discussed. SP-D from pigs exhibits distinct anti-IAV properties and is discussed in more detail. Finally, the potential of SP-D as a prophylactic and/or therapeutic antiviral agent to protect humans against infections by IAV is discussed.