Charting mast cell development in health and systemic mastocytosis

Hematopoietic stem and progenitor cells give rise to various blood and immune cells, including mast cells. In adults, bone marrow hematopoietic stem cells give rise to mast cell progenitors. These mast cell progenitors circulate in the peripheral blood and mature in peripheral tissues. Mast cells are activated by the cross-linking of immunoglobulin E (IgE) immune complexes bound to the high-affinity receptors, FcεRI. This event causes the release of a series of bioactive substances involved in various physiological and pathological processes.

Mast cells play a key role in immunological homeostasis as essential immune cells. By contrast, mast cell dysfunction causes mast cell-related diseases. Systemic mastocytosis is a rare systemic disorder caused by abnormal accumulation of aberrant mast cells. However, in-depth studies of mast cell development in healthy individuals and in patients with mastocytosis are scarce. In the past decade, single-cell RNA sequencing technology has revolutionized the hematopoietic models, making it possible to analyze differentiation trajectories at single cell resolution. Leveraging single-cell RNA-sequencing and cell culture assays, this thesis aims to systematically explore mast cell development in healthy individuals and patients with systemic mastocytosis.

In study I, we examined the cell-forming capacity of FcεRI+ progenitors in the bone marrow. In this study we found that CD203c distinguishes the erythroid and mast cell/basophil differentiation trajectories within the FcεRI+ progenitors.

In study II, we analyzed the hematopoietic landscape in peripheral blood using single-cell RNA sequencing (scRNA-seq). Transcriptome analysis indicated that the genes encoding FcεRI were expressed at the hematopoietic progenitor cell stage and increased in cells that showed a mast cell gene expression signature. In vitro culture revealed the ability of the FcεRI+ progenitor population to rapidly differentiate into mature mast cells. Together, the transcriptome and cell culture assays demonstrated that CD34+ c-Kit+ FcεRI+ progenitors constitute mast cell progenitors (MCPs) in peripheral blood. Subsequent screening of cell surface receptors on the gene expression level and in vitro culture experiments identified novel regulators of MCPs. IL-3 and IL-5 promoted MCP survival. In addition, IL-3 showed a proproliferative effect on MCPs, whereas IL-5 did not. Interestingly, IL-33 significantly downregulated the expression of FcεRI on MCP, suggesting that FcεRI expression can be influenced by the extracellular environment.

In study III, we aimed to explore the hematopoietic landscape in the bone marrow of patients with systemic mastocytosis (SM). In this study, we utilized single-cell RNA-sequencing to analyze the hematopoietic landscape of c-Kit+ hematopoietic progenitors isolated from bone marrow, with a focus on mast cells. Our results provide a comprehensive and in-depth analytical resource for research related to hematopoiesis in mastocytosis. Furthermore, using integrated single-cell transcriptome and immunophenotype data, we identified two distinct mast cell subpopulations with distinct expression of CD25, which is as diagnostic marker. Further comparison of CD25+ aberrant mast cells with CD25− mast cells identified a panel of disease-associated markers and revealed new potential pathogenic factors.

Altogether, this thesis provides a comprehensive map of mast cell development in healthy individuals and systemic mastocytosis, laying the foundation for further studies on mast cell development in physiological and pathological states.

List of scientific papers

I. Grootens J, Ungerstedt JS, Wu Chenyan, Hamberg Levedahl K, Nilsson G, Dahlin JS. CD203c distinguishes the erythroid and mast cell-basophil differentiation trajectories among human FcεRI+ bone marrow progenitors. Allergy. 2020 Jan;75(1):211-214.
https://doi.org/10.1111/all.13981

II. Wu Chenyan*, Boey D*, Bril O, Grootens J, Vijayabaskar MS, Sorini C, Ekoff M, Wilson NK, Ungerstedt JS, Nilsson G, Dahlin JS. Single-cell transcriptomics reveals the identity and regulators of human mast cell progenitors. Blood Advances. 2022 Aug 9;6(15):4439-4449. *These authors contributed equally to this study.
https://doi.org/10.1182/bloodadvances.2022006969

III. Boey D*, Wu Chenyan*, Mo J, Papavasileiou S, Ungerstedt JS, Nilsson G, Dahlin JS. Single-cell transcriptional analyses uncovers the hematopoietic landscape in systemic mastocytosis. *These authors contributed equally to this study. [Manuscript]