Lung Microenvironments Influence on Mast Cell Plasticity and Spatial Distribution of Immune cells

Inflammatory airway diseases and lower respiratory tract infections are leading causes of severe illness, disability and death worldwide. Much of today ́s hypotheses about the underlying inflammatory processes and pathogenic contribution by immune cells come from in vitro experiments or animal models. As a consequence, our knowledge of the actual immunopathological events in real patient tissues is surprisingly limited. To grasp the true nature of the disease it is therefore important to assess the phenotypes and interplay of immune cells under disease-relevant in vivo conditions. The overarching aim of the present study is to investigate how lung microenvironments dictate immune cell phenotypes, plasticity and their spatial coordination in health and airway diseases. Focus is on mast cells, a key player in immune surveillance and tissue homeostasis, and cells associated with type 2 immunity. The methodological focus is on innovative computer integrated histology-based approaches that have been applied on human lung explant models, humanized mouse models (NSG-SGM3) and clinically relevant lung tissue samples from patients with COPD, CF, IPF, and COVID-19. Individuals with no history of chronic lung diseases served as control. Papers 1 and 2 profoundly alter the view on mast cell protease heterogeneity by showing that, rather than the prevailing division of mast cells into binary MCt and MCtc protease profiles, both the mRNA expression and protein granule storage of tryptase, chymase and carboxypeptidase A3 are under a fine-tuned regulation by the local tissue microenvironment. These studies also provide strong indications that individual proteases may be selectively released already at healthy base-line conditions and then further up-regulated in COPD, IPF, CF, and COVID-19. Paper 3 represents the first extensive exploration of eosinophils, basophils, and the type 2 immunity surrogate marker GATA3 across multiple anatomical sites in COPDaffected lungs. Computerized quantification and spatial analysis revealed a surprisingly patchy eosinophil distribution that co-localized with GATA3, basophil, and ILC2-containing type 2 microenvironments. Importantly, the occurrence of eosinophils, basophils and patchy type 2 microenvironments correlated with disease severity. In Paper 4 a cutting-edge humanized mouse model (NSG-SGM3) was used to show that sophisticated highly spatially organized human leukocyte tissue patterns spontaneously develop in mouse lung and lymphoid tissues. Although some of the patterns had a striking resemblance to human COPD, confounding species cross reactivity events must be taken into account before establishing humanized immune system mouse models for any human disease. In a nutshell, the present thesis reveals several new important aspects of mast cell protease plasticity, type 2 immunity, and leukocyte pattern formation and how these features are dynamically controlled by the local tissue microenvironment in health and disease. As such, the thesis also provides a powerful example of the need for “direct” explorations of patient tissues in order to understand the true complexity of inflammatory diseases and infections.