Elsevier

The Lancet

Volume 353, Issue 9160, 10 April 1999, Pages 1232-1237
The Lancet

Early Report
Damage to surfactant-specific protein in acute respiratory distress syndrome

https://doi.org/10.1016/S0140-6736(98)09449-5Get rights and content

Summary

Background

Acute respiratory distress syndrome (ARDS) develops in association with many serious medical disorders. Mortality is at least 40%, and there is no specific therapy. A massive influx of activated neutrophils, which damage pulmonary vascular endothelium and alveolar epithelium, leads to alveolar oedema and pulmonary surfactant dysfunction. In-vitro studies show that neutrophil elastase can cleave surfactant-specific proteins and impair surfactant function. If this happens in vivo in ARDS, the response to surfactant therapy will be limited.

Methods

Samples of pulmonary surfactant were obtained from the lungs of 18 patients with ARDS and six healthy controls by bronchoalveolar lavage. We separated proteins in these samples according to molecular weight by sodium dodecyl sulphate polyacrylamide gel electrophoresis (SDS-PAGE). We then used western blotting with monoclonal antibody E8 to detect the major surfactant-specific protein A (SP-A).

Findings

By contrast with controls, 14 of 18 patients had evidence of in-vivo damage to SP-A that resembled damage caused to SP-A when it is cleaved by neutrophil elastase. Controls showed a single band of normal dimers at 66 kDa, whereas 14 of 18 patients showed multiple bands at 66 kDa, 55 kDA, and 30–36 kDa, and six showed additional bands at 36–40 kDa.

Interpretation

Direct damage to surfactant-specific proteins occurs in lungs of patients with ARDS, probably by proteolysis. Trials of protein-containing therapeutic surfactant are in progress in ARDS, and our results indicate that the frequent failure to maintain response may result from continuing damage to surfactant by products of activated neutrophils. A combination of surfactant and antiprotease therapy may improve therapeutic prospects.

Introduction

Pulmonary surfactant is a mixture of phopholipids and surfactant-specific proteins, which reduces alveolar surface tension during respiration.1, 2 Specific proteins (SP) A, B, and C contribute to the biophysical functions of surfactant,3 and SP-A and SP-D also have antimicrobial properties.4, 5 The catastrophic consequences of major disturbance to surfactant are shown in the infant respiratory distress syndrome, which results from deficiency of surfactant at birth.1 Surfactant dysfunction also contributes substantially to pathogenesis in acute respiratory distress syndrome (ARDS).1 This severe form of adult lung injury develops in association with many serious conditions including sepsis, pneumonia, traumatic injury, and major surgery.2 Mortality exceeds 40%, and there is no effective therapy apart from mechanical ventilation and other supportive measures. Therapeutic surfactant is beneficial in infant respiratory distress syndrome1 and has potential for ARDS,2, 6 in which clinical trials have shown that artificial surfactant containing phospholipid without surfactant proteins is of no benefit7 but that the more effective preparations containing protein and phospholipid can improve gas exchange and reduce mortality.6 One outstanding question is whether antiinflammatory drugs are also needed to suppress products of activated neutrophils, which might damage surfactant.

The injury to the lungs in ARDS is caused by damage to the pulmonary vessels and alveoli, mediated in part by activated neutrophils,8 resulting in massive pulmonary oedema, neutrophilia, and surfactant dysfunction. The capacity of surfactant to lower surface tension is impaired in vivo in ARDS,2 owing to protein in oedema fluid inhibiting surfactant function,9 and to deficient production of surfactant.2 We sought evidence that direct damage to surfactant-specific proteins also contributes to surfactant dysfunction in ARDS because of their susceptibility to cleavage by proteases from activated neutrophils. This hypothesis is supported by several in-vitro observations. First, activated human neutrophils impair the function of surfactant in vitro by a mechanism that degrades SP-A, generating various bands of abnormal molecular weight.10 These abnormalities are inhibited by α1-antiprotease but not superoxide dismutase; thus they result from proteolysis and not from oxidant injury.10 Second, incubation of surfactant with neutrophil elastase in vitro also impairs the surface-tension-lowering capacity, and reduces the rate of surface absorption of surfactant; these changes are associated not only with proteolytic cleavage of SP-A, but also with damage to SP-B and SP-C.11 Third, all three of these surfactant-specific proteins can be cleaved by neutrophil elastase in vitro.12

In normal lungs, the inhibitor α1-antiprotease inactivates free elastase. However, bronchoalveolar lavage samples from patients with ARDS contain raised amounts of active neutrophil elastase13, 14, 15 and decreased antiprotease activity,16 indicating that proteolytic damage may occur as a result of imbalance between protease and inhibitor. Elastase inhibitor can attenuate lipopolysaccharide-induced acute lung injury.17 The availability of recombinant α1-antiprotease and other protease inhibitors for clinical trials in human beings now makes antiprotease therapy feasible.

We studied surfactant from patients with ARDS and healthy controls, looking for damage to SP-A.

Section snippets

Methods

We selected for investigation 18 patients with ARDS (nine male, nine female; median age 57 [range 16–74] years; seven never-smokers, five current smokers, six ex-smokers; six survivors, 12 non-survivors) from whom sufficient bronchoalveolar lavage fluid was recovered. All met the diagnostic criteria of the American-European Consensus,18 and all had severe lung injury according to the Murray lung-injury score.19 The study protocol was approved by the ethics committee of the Royal Brompton

Results

The six surviving and 12 non-surviving patients were of similar age (table) and had had a similar duration of mechanical ventilation at the time of bronchoalveolar lavage (median 8 days for both). All 18 patients had higher than normal numbers of neutrophils in their bronchoalveolar lavage samples (median percentage count 87·4%; number 20·5×104/mL). Under reducing conditions, the purified SP-A from the patient with alveolar lipoproteinosis separated into two major protein bands (Figure 1, A): a

Discussion

The lungs of patients with ARDS contain higher than normal numbers of activated neutrophils8 and amounts of free elastase released from the neutrophils.13, 14, 15 The incomplete inactivation of elastase seems to be the result of damage to its main inhibitor, α1- antiprotease.14, 16 On the basis of these observations, others have proposed that the specific proteins of the pulmonary surfactant system might be at risk of cleavage by the active elastase in the lungs of patients with ARDS. This

References (35)

  • TJ Gregory et al.

    Bovine surfactant therapy for patients with acute respiratory distress syndrome

    Am J Respir Crit Care Med

    (1997)
  • A Anzueto et al.

    Aerosolised surfactant in adults with sepsis-induced acute respiratory distress syndrome: Exosurf Acute Respiratory Distress Syndrome Sepsis Study Group

    N Engl J Med

    (1996)
  • JE Weiland et al.

    Lung neutrophils in the adult respiratory distress syndrome: clinical and pathological significance

    Am Rev Respir Dis

    (1986)
  • W Seeger et al.

    Surfactant inhibition by plasma proteins: differential sensitivity of various surfactant preparations

    Eur Respir J

    (1993)
  • SF Ryan et al.

    Effects of activated polymorphonuclear leukocytes upon pulmonary surfactant in vitro

    Am J Respir Cell Mol Biol

    (1991)
  • CT Lee et al.

    Elastolytic activity in pulmonary lavage fluid from patients with adult respiratory-distress syndrome

    N Engl J Med

    (1981)
  • WW McGuire et al.

    Studies on the pathogenesis of the adult respiratory distress syndrome

    J Clin Invest

    (1982)
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