Origin and ontogeny of human lung macrophages

Abstract

The lung is a vital organ occupied by a large diversity of immune cells due to its close proximity with the external environment. This thesis focuses on a particular type of immune cell: the macrophage. The lungs are composed of three separate anatomical compartments: the airways that start from the trachea and end at the alveoli, the blood capillaries and the connective tissue supporting the lung structure. Correspondingly, three macrophage subsets have been described in the lungs: alveolar, intravascular and interstitial macrophages.

Contrary to the reigning dogma that macrophages were generated from circulating monocytes, studies in mice demonstrated that fetal monocytes seeding the lungs during embryonic stages gave rise to alveolar macrophages after birth. The origin of lung macrophages in humans remains unknown as invasive experiment to track cellular origin in vivo are ethically impossible to conduct. To face this challenge, we used the MISTRG humanized mouse model. MISTRG mice support the development of human myeloid cells (especially human monocytes and macrophages). MISTRG mice were transplanted intrahepatically, intranasally and intravenously with different adult and embryonic precursors to assess their macrophage progenitor capacity as well as migration potential. In addition, we performed intravascular cell labelling, bead-based fate-mapping, microarray gene expression analysis, bulk and single-cell RNA sequencing to investigate the precise localization of macrophage progenitors and the heterogeneity of the human lung macrophage compartment.

We demonstrated in Paper I that two distinct developmental pathways from circulating blood monocytes exist: CD14+ CD16- classical monocytes give rise to interstitial and alveolar macrophages while their CD14low CD16+ non-classical counterparts generate pulmonary intravascular macrophages. In Paper II, we defined the CD116+ CD64- human fetal progenitor of alveolar macrophages, resembling Myb-expressing erythro-myeloid progenitors (EMPs) and CD64- myeloid progenitors derived from EMPs in mice. These fetal progenitors had a highly proliferative gene expression profile and were consequently better at occupying the alveolar niche than the adult monocytes. Gene expression analysis of alveolar macrophages of adult and embryonic origin revealed that the environment had a strong impact on macrophage function as very few genes were differentially expressed between alveolar macrophages of different origin. However, adult precursors preferentially generated interferon-responsive macrophages, similar to a subset expanding in inflammatory disease in humans such as COVID-19.

This thesis provides new insights into the origin and development of human lung macrophages, thereby contributing to the development of macrophage-based therapies for lung diseases in humans.