COPD has become one of the major causes of ill health throughout the world. COPD has major economic impact being the second commonest causes of hospital admission and days lost to work in the UK where an estimated 10% of the population over 40 are affected. The disease causing mechanisms are not well defined but a central feature is development of a chronic inflammatory process. Inflammation in COPD is progressive and persists after inciting factors, such as cigarette smoke, are removed. The stimuli to persistent inflammation and the failure of normal cues to resolve this inflammation are poorly characterised. Lung inflammation in COPD is largely resistant to the antiinflammatory actions of corticosteroids, currently used as treatment, which fail to modify disease progression or mortality. Existing therapeutic approaches for COPD are therefore flawed and do not alter the central chronic inflammatory process.
Better understanding of COPD pathogenesis is essential if new therapeutic strategies are to be developed that will alter the course of the disease. A sub-set of COPD patients have more frequent exacerbations, which are associated with more rapid decline in pulmonary function tests and increased mortality. Bacterial infection is a frequent cause of these exacerbations. In addition some of the high-risk patients with COPD are more susceptible to bacterial pneumonia. In COPD, the lower respiratory tract is colonised with a higher density of bacteria (including Haemophilus influenzae or Streptococcus pneumoniae), whereas in non-smoking subjects and smokers with normal lung function the density of bacteria is much lower. This suggests there may be a defect in immune responses to bacteria in patients with COPD, in particular involving the part of the immune response which is termed 'innate', which provides the most rapid and generic response to bacteria.
Lung host defence against bacteria requires the co-ordinated action of both immune cells and factors released into the airway termed humoral factors. Alveolar macrophage (AM) competence is essential to maintain sterility in the lower airway and bacterial phagocytosis and killing is complemented by a delayed programme of cell death termed apoptosis, which provides a significant increment to bacterial killing and shuts off the inflammatory response. When this component of host defence is over-whelmed recruited cells that ingest bacteria termed phagocytes (i.e. neutrophils and macrophages) become key effectors of the host response. Research suggests that monocyte-derived macrophages (MDM) isolated from patients with COPD have impaired capacity for phagocytosis of bacteria as compared to healthy donors or smokers without COPD. That this defect is apparent in cells isolated from the blood suggests it may reflect a systemic defect in bacterial clearance. In addition there is a marked defect in AM isolated from the lung, suggesting additional defects imposed by the lung environment, that impacts uptake of bacteria coated with antibody. The investigators have also shown that macrophages are impaired in their ability to kill the bacteria they ingest. The molecular basis of the decreased clearance of bacteria is unclear but research by the investigators has found that macrophages fail to produce an increment in a factor that kills bacteria that is produced by mitochondria, termed mitochondrial reactive oxygen species (mROS) and this may contribute to both impaired ingestion and killing of bacteria.
Mitochondria are structures in cell that generate energy but how they burn fuels in the form of molecules adapts to aid immune function. The investigators believe that in COPD mitochondria in macrophages fail to adapt during bacterial challenge and this underpins the immune defect observed for bacteria. Similarly, the investigators have shown that peripheral blood neutrophils have impaired killing of bacteria in COPD and that this is associated with impaired gluconeogenesis, a process that stores glucose in the form of glycogen. This glycogen is normally used by neutrophils as a source of glucose to help prime responses to bacteria and this defect in COPD neutrophils impairs their capacity to kill bacteria This has not been previously addressed and the investigators will focus on four main aspects: i) the molecules myeloid cells burn as fuel sources during metabolism in COPD in at rest and in response to bacteria and other inflammatory stimuli, ii) alterations in mitochondrial function in myeloid cells in COPD, iii) whether alterations are confined to myeloid cells in the lung or expressed at sites removed from the lung and the mechanisms of these effects and iv) methods to recalibrate altered responses determined by the investigators in myeloid cells in COPD. The cells and materials that will be generated will allow additional research and will be used to help test other hypotheses and identify additional mechanism underlying immune dysfunction and promotion of inflammation in COPD that emerge as work by the investigators and the field progresses during the lifetime of this research.
