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Clinical & Experimental Allergy

Impaired macrophage phagocytosis in non-eosinophilic asthma

Authors

  • J. L. Simpson,

    1. Centre for Asthma and Respiratory Disease, The University of Newcastle, Newcastle, Australia
    2. Department of Respiratory and Sleep Medicine, Hunter Medical Research Institute, Newcastle, Australia
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  • P. G. Gibson,

    1. Centre for Asthma and Respiratory Disease, The University of Newcastle, Newcastle, Australia
    2. Department of Respiratory and Sleep Medicine, Hunter Medical Research Institute, Newcastle, Australia
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  • I. A. Yang,

    1. School of Medicine, The University of Queensland, St Lucia, Australia
    2. The Prince Charles Hospital, Chermside, Australia
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  • J. Upham,

    1. School of Medicine, The University of Queensland, St Lucia, Australia
    2. Princess Alexandra Hospital, Brisbane, Australia
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  • A. James,

    1. Sir Charles Gairdner Hospital, Perth, Australia
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  • P. N. Reynolds,

    1. Department of Thoracic Medicine, Royal Adelaide Hospital, Adelaide, Australia
    2. Lung Research Laboratory, Hanson Institute, Adelaide, Australia
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  • S. Hodge,

    Corresponding author
    1. Department of Thoracic Medicine, Royal Adelaide Hospital, Adelaide, Australia
    2. Lung Research Laboratory, Hanson Institute, Adelaide, Australia
    • Centre for Asthma and Respiratory Disease, The University of Newcastle, Newcastle, Australia
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  • AMAZES Study Research Group


Correspondence:

S. Hodge, Lung Research, Hanson Institute, Frome Rd, Adelaide 5001, South Australia.

E-mail: sandra.hodge@health.sa.gov.au

Summary

Background

Many patients with non-eosinophilic asthma have increased numbers of neutrophils in the airways. The explanation for this chronic inflammation remains unclear, but may result from an impaired ability of alveolar macrophages to phagocytose apoptotic cells (a process termed ‘efferocytosis’), as we have shown in chronic obstructive pulmonary disease (COPD).

Objectives

To examine induced sputum as a non-invasive technique to characterize efferocytosis in chronic lung diseases and to compare efferocytosis in patients with non-eosinophilic asthma, eosinophilic asthma and COPD.

Methods

Participants with stable asthma (20 with eosinophilic and 30 with non-eosinophilic) and COPD (= 11) underwent clinical assessment including allergy skin tests, saline challenge and sputum induction. Sputum cells were dispersed using dithiothreitol and resuspended in culture medium. Efferocytosis of apoptotic bronchial epithelial cells by sputum-derived macrophages was determined using flow cytometry.

Results

There were no significant differences in efferocytosis between paired sputum and bronchoalveolar lavage macrophages from three subjects. Efferocytosis was significantly impaired in patients with non-eosinophilic asthma [mean (SD) 0.95 (0.24)] compared with eosinophilic asthma [1.17 (0.19)] and to a similar degree as patients with COPD [1.04 (0.16)]. Sputum neutrophils were significantly higher in patients with COPD and non-eosinophilic asthma compared with eosinophilic asthma.

Conclusion and Clinical Relevance

Induced sputum provides a reliable and non-invasive method for studying macrophage efferocytosis in chronic lung disease. Macrophage efferocytosis is impaired in non-eosinophilic asthma to a similar degree as that in COPD and may explain the persistent airway neutrophilia and chronic inflammation that characterizes this asthma subtype.

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