Mechanisms of increased pneumonia risk with
inhaled corticosteroid use in COPD
Host immunity in COPD exacerbations
Corticosteroids have immunosuppressive eff ects and
increased susceptibility to infections is recognised as an
adverse eff ect of long-term use of oral corticosteroids.
The aetiology of COPD exacerbations and pneumonia
are remarkably similar, with bacteria such as Streptococcus
pneumoniae and Haemophilus infl uenzae and viruses such
as rhinoviruses commonly detected in both conditions.60,61
Therefore, the completely opposite eff ect of inhaled
corticosteroids on two syndromes with similar aetiology
seems paradoxical. The key diff erence between
pneumonia and COPD exacerbations is the anatomical
site, with exacerbations caused by infection mainly
within the larger airways,62 whereas pneumonia includes
the lung parenchyma and small airways.63 The
distribution of immune cells diff ers within the lung and
between these airway compartments. Induced sputum
shows the cellular composition of the large airways and
60–80% of infl ammatory cells in sputum in people with
COPD are neutrophils, whereas only 15–25% are
macrophages.64 Conversely, bronchoalveolar lavage
samples derive from smaller airways and the alveolar
spaces, and greater than 90% of cells are macrophages
and only 2% are neutrophils.65 Therefore, if inhaled
corticosteroids have diff erent eff ects on neutrophils and
macrophages, their eff ects on host immunity could vary
in diff erent airway compartments and this might result
in diverse eff ects on exacerbations and pneumonia.