Mesenchymal heterogeneity in the adult human lung

Sammanfattning: The human lung is a complex tissue consisting of a heterogeneous mix of different structures and cells, each which a specific role that together contribute to the primary function of the organ: the gas exchange. Different mesenchymal cells populate the interstitial areas around airways and blood vessels and form the connective tissue together with the extracellular matrix. Fibroblasts constitute one group of mesenchymal cells which are potent producers of extracellular matrix, an important part of the tissue niche which is heavily altered during remodeling in fibrotic lung diseases. In addition to providing structure and support, the extracellular matrix is also a part of the dynamic microenvironment that interacts and communicates with the cells. Yet, the different functions of fibroblasts and other mesenchymal cells in the lung under homeostatic and pathophysiological conditions remain elusive. Furthermore, the extensive heterogeneity displayed by fibroblasts and other mesenchymal cells in terms of phenotype and function, which in part can be related to their spatial localization, is not fully understood. Deciphering this heterogeneity is important to better understand specific functions of specialized mesenchymal cells.The aim of this thesis was to investigate the heterogeneity of fibroblastic mesenchymal cells that reside in the human lungs and to explore how different cellular phenotypes can contribute to mesenchymal-associated functions such as tissue remodeling. The studies included in the thesis are based on experimental work on primary cells derived from human lung tissue. Mass-spectrometry and RNA-sequencing technologies were used to perform comprehensive analysis of the proteome and transcriptome of targeted cell populations. To study mesenchymal cells in heterogeneous tissue material, we also utilized techniques with single-cell resolution, including flow cytometry, single-cell RNA-sequencing and histology, together with distinct anatomical tissue sampling (proximal vs distal airways).Based on these approaches we could confirm the existence of heterogeneity within the mesenchymal cell compartment, with a specific phenotypical pattern associated with fibroblasts in the peribronchovascular compartment emerging. In paper I, we characterize the proteome of fibroblast from proximal and distal airways and confirm specific protein expression in the bronchvascular regions. In paper II, we explore the functional attribute of colony formation (reflecting proliferative capacity) that has been connected to a mesenchymal progenitor population, and describe a link between this property and an adventitial localization and an adventitial fibroblasts phenotype. In paper III, we describe the existence of a CD26+ adventitial fibroblast phenotype in human lung and show that it is not associated to an actived profibrotic phenotype in idiopathic pulmonary fibrosis, contrary to what other studies have suggested. Finally, in paper IV, we applied our bioinformatical approach to characterize non-fibroblastic cells, alveolar epithelial cell, with a focus on their contribution to ECM remodeling, and revealed an overlooked complexity in their matrix producing capacity that include interstitial matrix proteins commonly associated with mesenchymal cells.Taken together, the results presented in the thesis describe a phenotypic heterogeneity among mesenchymal cells that appear to be intricately connected to their local environment, highlighting the importance of the extracellular context for cell function. The generated information could contribute to reduce the inconsistency and confusion regarding different terminology used and provide information regarding potential functions of these elusive cells in lung tissue.