MICA: Mitochondrial dysfunction in macrophages and impaired bacterial clearance in chronic obstructive pulmonary disease (COPD)

Chronic obstructive pulmonary disease (COPD) is a progressive lung disease caused by inflammation and narrowing of the small airways, leading to breathlessness. COPD is triggered by cigarette smoke, but inflammation persists after stopping smoking and causes disease progression. Identifying what drives inflammation is vital since no treatment can stop progression. Frequent bacterial chest infections are associated with worsening COPD symptoms. We believe that ineffective clearance of bacteria from the airways causes COPD progression. We and others have found that in COPD, immune cells in the air sacs in the lung display faulty responses to bacteria that commonly cause chest infections resulting in bacteria persisting in the airway. These cells, termed alveolar macrophages (AM), are less able to eat and kill bacteria in people with COPD. We will examine why this happens.

The process by which cells, such as AM, produce energy (metabolism) changes dynamically based on the cells function (e.g. killing bacteria). Our preliminary results suggest that COPD AM are less able to adjust their metabolism as needed, and this prevents bacterial clearance. Key parts of the cell involved in metabolism are mitochondria. Normally, after macrophages have eaten bacteria, the function of their mitochondria changes away from metabolism and towards producing substances to kill bacteria (mitochondrial reactive oxygen species; "mROS"). This requires the mitochondria to break up into smaller units ("mitochondrial fission"). Our work suggests that COPD AM normally produce too much mROS so cannot increase production to kill bacteria. We believe that in COPD, AM mitochondria are less able to adapt their function when trying to kill bacteria, leading to susceptibility to infection. However, the precise details of how these processes normally function, or go wrong in COPD, are not fully understood. A better understanding is needed to identify new treatments to enhance these processes in COPD. We will determine the key changes in metabolism, production of mROS and mitochondrial fission in macrophages required to kill bacteria effectively in healthy people and determine how COPD alters this response.

To do this, we will study AM from the blood or lungs of healthy non-smokers, healthy current smokers, and people with COPD. We will isolate AM from the lungs by bronchoscopy, where a fibre-optic tube is passed into the airways and a segment of the lung is flushed with fluid to obtain the cells. We will also use mouse models of infection and airway disease. First, we will characterise in detail the metabolic response of AM to infection by isolating AM and labelling them with chemicals to track metabolism ("mass spectrometry"). We will measure patterns of genes and proteins involved in responding to bacteria, to identify metabolic pathways engaged during infection. We will confirm these metabolic responses in healthy AM and then determine how they are altered in COPD. Next, we will measure mROS production and investigate how it is produced following infection and in COPD AM. We will also examine the timing and mechanism of mitochondrial fission in these conditions. Our current findings suggest several potential mechanisms for mROS production and fission, and our analyses of metabolism and gene expression here will help determine which theories to test. Key findings from human cells will be validated in mouse models. We will also validate findings using macrophages derived from cells from people with genetic defects impacting mitochondrial function. We will use chemical and gene editing techniques in cells to modify pathways we have identified as altered in COPD that impact bacterial responses. Finally, to develop potential treatments we will screen libraries of drugs to identify ways of improving key responses. These will be tested in mouse models and patient samples to help prioritise approaches for future trials in COPD.

Chronic obstructive pulmonary disease (COPD) results from small airway obstruction. No treatment halts progression. Adaption in metabolism and mitochondrial function is essential for macrophage responses to bacteria. Macrophages show impaired responses to bacteria in COPD, combined with reduced flexibility in adaption of metabolism and generation of mitochondrial reactive oxygen species (mROS). We hypothesise macrophage defects in mitochondrial antibacterial function underly COPD susceptibility.

In COPD there is a systemic macrophage defect compounded by the lung environment. Therefore, we will study both monocyte-derived macrophages and alveolar macrophages (AM) in healthy volunteers (non-smokers and current smokers) and in patients with COPD. To examine findings in vivo we will use mouse models of infection and airway disease. We will identify metabolic adaptions to bacteria in mouse models of infection and healthy AM, using high-performance liquid chromatography-mass spectrometry (HLPC-MS) tracing, RNA-Sequencing and proteomics. We will repeat this approach to examine which pathways are altered in COPD macrophages and in an LPS/elastase mouse model of emphysema. Next, we will explore the kinetics and mechanisms of mROS production and mitochondrial fission following bacterial challenge and how COPD modulates these. Mechanisms will be validated using induced pluripotent stem cell derived macrophages (iPSCDM) and CRISPR/Cas9 gene editing combined with genetically modified mice or zebrafish. We will use iPSCDM from patients with genetic defects in mitochondrial functions we identify to confirm key findings. To recalibrate mitochondrial antibacterial responses in COPD we will perform a high content drug screen complemented by candidate approaches (Nrf2 agonists and BH3 mimetics). The ultimate goal will be to identify therapeutic candidates for future clinical trials to recalibrate antimicrobial responses in AM and prevent COPD progression.