Granulocyte Kinetics in Health and Disease

Abstract

The innate immune system is indispensable for a successful defence against pathogens. Yet, little is known about the life cycle of innate immune cells in the body. For example, there is little consensus regarding the circulatory lifespans of neutrophils, eosinophils and basophils in healthy individuals. For the lifespans and behaviour of these cells in tissues and in acute or chronic inflammation, no information is available at all. Without this knowledge, it is very difficult to interpret and design clinical trials targeting these cells. Therefore, we first estimated the circulatory lifespans of neutrophils, eosinophils and basophils in healthy controls by applying in vivo deuterium labelling with 2H-glucose. This method allows labelling of inflammatory cells without toxic side effects and without ex vivo manipulation. We found that these cells remain longer in the bloodstream than previously anticipated. In addition to estimating circulatory lifespans, deuterium labelling can also be used to gain information about the developmental relationships between phenotypes of immune cells. For example, three different subsets of monocytes are recognised in the circulation, but it is uncertain whether these three subsets represent different maturation stages of the same cell or whether they represent different cell lines developing separately in the bone marrow. By combining deuterium labelling results with mathematical modelling, we show that the three monocyte subsets can indeed be different maturation stages of the same cell, if part of this differentiation process takes place outside the bloodstream and if only few cells reach the third and last maturation stage. Also, neutrophils occur in three different subsets, although two of these subsets are only seen in the peripheral blood during acute inflammation. Using the same methodology combined with proteomics, we show that one of the three neutrophil subsets develops separately from the others. This immunomodulatory, hypersegmented neutrophil subset differs from the other two subsets based on its proteome, whilst having similar deuterium labelling kinetics as the normally segmented neutrophil population, suggesting that these two cell populations develop in parallel. Lastly, the behaviour of neutrophils and eosinophils were assessed in the blood and lungs of patients with asthma or cystic fibrosis (CF). We show that these cells in the lungs of asthma patients are fully activated, regardless of the inflammatory asthma phenotype. Regarding the life-cycle of these cells in the blood of CF and asthma patients, we found that in CF patients, the circulatory behaviour of neutrophils is severely affected, while in asthma patients, it is the circulatory behaviour of eosinophils that is affected. We found that neutrophils are very short-lived in the lungs, regardless of the type of disease, while our data suggest that eosinophils in the lungs are relatively long-lived. Taken together, this thesis provides critical insight into the behaviour of innate immune cells under both healthy and diseased conditions, which will help interpreting and predicting how these cells behave in response to intervention studies.