Detection of circulating mast cells in advanced systemic mastocytosis

Single-cell transcriptomics reveals the identity and regulators of human mast cell progenitors

Mast cell accumulation is a hallmark of a number of diseases, including allergic asthma and systemic mastocytosis. Immunoglobulin E-mediated crosslinking of the FcεRI receptors causes mast cell activation and contributes to disease pathogenesis. The mast cell lineage is one of the least studied among the hematopoietic cell lineages, and controversies remain about whether FcεRI expression appears during the mast cell progenitor stage or during terminal mast cell maturation. Here, we used single-cell transcriptomics analysis to reveal a temporal association between the appearance of FcεRI and the mast cell gene signature in CD34+ hematopoietic progenitors in adult peripheral blood. In agreement with these data, the FcεRI+ hematopoietic progenitors formed morphologically, phenotypically, and functionally mature mast cells in long-term culture assays. Single-cell transcriptomics analysis further revealed the expression patterns of prospective cytokine receptors regulating development of mast cell progenitors. Culture assays showed that interleukin-3 (IL-3) and IL-5 promoted disparate effects on progenitor cell proliferation and survival, respectively, whereas IL-33 caused robust FcεRI downregulation. Taken together, we showed that FcεRI expression appears at the progenitor stage of mast cell differentiation in peripheral blood. We also showed that external stimuli regulate FcεRI expression of mast cell progenitors, providing a possible explanation for the variable FcεRI expression levels during mast cell development.

Frequency and prognostic impact of blood-circulating tumor mast cells in mastocytosis

Circulating tumor mast cells (CTMC) have been identified in the blood of a small number of patients with advanced systemic mastocytosis (SM). However, limited data exists about their frequency and prognostic impact in patients with mast cell activation syndromes (MCAS), cutaneous and non-advanced SM. We investigated the presence of CTMC and mast cell-committed CD34+ precursors in blood of 214 patients with MCAS, cutaneous mastocytosis and SM using highly sensitive next-generation flow cytometry. CTMC were detected at progressively lower counts in almost all advanced SM (96%) and smoldering SM (100%), nearly half (45%) indolent SM cases and few (7%) bone marrow mastocytosis patients, but were systematically absent in cutaneous mastocytosis and MCAS (P<0.0001). In contrast to CTMC counts, the number of mast cell-committed CD34+ precursors progressively decreased from MCAS, cutaneous mastocytosis and bone marrow mastocytosis to indolent SM, smoldering SM and advanced SM (P<0.0001). Clinically, the presence (and number) of CTMC in blood of patients with SM in general and non-advanced SM (indolent SM and bone marrow mastocytosis) in particular was associated with more adverse features of the disease, poorer risk prognostic subgroups as defined by the International Prognostic Scoring System for advanced SM (P<0.0001) and the Global Prognostic Score for mastocytosis (P<0.0001) and a significantly shortened progression-free survival (P<0.0001) and overall survival (P=0.01). Based on our results, CTMC emerge as a novel candidate biomarker of disseminated disease in SM that is strongly associated with advanced SM and poorer prognosis in patients with indolent SM.

P2X Receptor-Dependent Modulation of Mast Cell and Glial Cell Activities in Neuroinflammation

Localisation of mast cells (MCs) at the abluminal side of blood vessels in the brain favours their interaction with glial cells, neurons, and endothelial cells, resulting in the activation of these cells and the release of pro-inflammatory mediators. In turn, stimulation of glial cells, such as microglia, astrocytes, and oligodendrocytes may result in the modulation of MC activities. MCs, microglia, astrocytes, and oligodendrocytes all express P2X receptors (P2XRs) family members that are selectively engaged by ATP. As increased concentrations of extracellular adenosine 5′-triphosphate (ATP) are present in the brain in neuropathological conditions, P2XR activation in MCs and glial cells contributes to the control of their communication and amplification of the inflammatory response. In this review we discuss P2XR-mediated MC activation, its bi-directional effect on microglia, astrocytes and oligodendrocytes and role in neuroinflammation.

A role for glia maturation factor dependent activation of mast cells and microglia in MPTP induced dopamine loss and behavioural deficits in mice

The molecular mechanism mediating degeneration of nigrostriatal dopaminergic neurons in Parkinson's disease (PD) is not yet fully understood. Previously, we have shown the contribution of glia maturation factor (GMF), a proinflammatory protein in dopaminergic neurodegeneration mediated by activation of mast cells (MCs). In this study, methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-induced nigrostriatal neurodegeneration and astro-glial activations were determined by western blot and immunofluorescence techniques in wild type (WT) mice, MC-deficient (MC-KO) mice and GMF-deficient (GMF-KO) mice, with or without MC reconstitution before MPTP administration. We show that GMF-KO in the MCs reduces the synergistic effects of MC and Calpain1 (calcium-activated cysteine protease enzyme)-dependent dopaminergic neuronal loss that reduces motor behavioral impairments in MPTP-treated mouse. Administration of MPTP increase in calpain-mediated proteolysis in nigral dopaminergic neurons further resulting in motor decline in mice. We found that MPTP administered WT mice exhibits oxidative stress due to significant increases in the levels of malondialdehyde, superoxide dismutase and reduction in the levels of reduced glutathione and glutathione peroxidase activity as compared with both MC-KO and GMF-KO mice. The number of TH-positive neurons in the ventral tegmental area, substantia nigra and the fibers in the striatum were significantly reduced while granulocyte macrophage colony-stimulating factor (GM-CSF), MC-Tryptase, GFAP, IBA1, Calpain1 and intracellular adhesion molecule 1 expression were significantly increased in WT mice. Similarly, tyrosine hydroxylase, dopamine transporters and vesicular monoamine transporters 2 proteins expression were significantly reduced in the SN of MPTP treated WT mice. The motor behavior as analyzed by rotarod and hang test was significantly reduced in WT mice as compared with both the MC-KO and GMF-KO mice. We conclude that GMF-dependent MC activation enhances the detrimental effect of astro-glial activation-mediated oxidative stress and neuroinflammation in the midbrain, and its inhibition may slowdown the progression of PD.

Deciphering the differentiation trajectory from hematopoietic stem cells to mast cells

Hematopoietic stem cells differentiate into all types of blood cells, including peripheral tissue-resident mast cells. The early mast cell differentiation takes place in the bone marrow, after which the progenitor cells enter the circulation and mature once reaching their target organ. Early results from single-cell culture experiments and colony-forming assays have produced the classic hierarchical tree model of hematopoiesis. The introduction of high-throughput, single-cell RNA sequencing is now revolutionizing our understanding of the differentiation process, questioning the classic tree-based models. By integrating the results from early cell culture experiments with single-cell transcriptomics, we present a differentiation landscape model of hematopoiesis and discuss it with focus on mast cells. The review also describes how the hematologic neoplasm systemic mastocytosis can be used to model human hematopoiesis using naturally occurring cell barcoding by means of the common KIT D816V mutation.

Single-cell analysis reveals the KIT D816V mutation in haematopoietic stem and progenitor cells in systemic mastocytosis

Background

Systemic mastocytosis (SM) is a haematological disease characterised by organ infiltration by neoplastic mast cells. Almost all SM patients have a mutation in the gene encoding the tyrosine kinase receptor KIT causing a D816V substitution and autoactivation of the receptor. Mast cells and CD34⁺ haematopoietic progenitors can carry the mutation; however, in which progenitor cell subset the mutation arises is unknown. We aimed to investigate the distribution of the D816V mutation in single mast cells and single haematopoietic stem and progenitor cells.

Methods

Fluorescence-activated single-cell index sorting and KIT D816V mutation assessment were applied to analyse mast cells and >10,000 CD34⁺ bone marrow progenitors across 10 haematopoietic progenitor subsets. In vitro assays verified cell-forming potential.

Findings

We found that in SM 60–99% of the mast cells harboured the KIT D816V mutation. Despite increased frequencies of mast cells in SM patients compared with control subjects, the haematopoietic progenitor subset frequencies were comparable. Nevertheless, the mutation could be detected throughout the haematopoietic landscape of SM patients, from haematopoietic stem cells to more lineage-primed progenitors. In addition, we demonstrate that FcεRI⁺ bone marrow progenitors exhibit mast cell-forming potential, and we describe aberrant CD45RA expression on SM mast cells for the first time.

Interpretation

The KIT D816V mutation arises in early haematopoietic stem and progenitor cells and the mutation frequency is approaching 100% in mature mast cells, which express the aberrant marker CD45RA.

Characterization of CD34+ hematopoietic cells in systemic mastocytosis: Potential role in disease dissemination

BACKGROUND

Recent studies show that most systemic mastocytosis (SM) patients, including indolent SM (ISM) with (ISMs+) and without skin lesions (ISMs-), carry the KIT D816V mutation in PB leukocytes. We investigated the potential association between the degree of involvement of BM hematopoiesis by the KIT D816V mutation and the distribution of different maturation-associated compartments of bone marrow (BM) and peripheral blood (PB) CD34⁺ hematopoietic precursors (HPC) in ISM and identified the specific PB cell compartments that carry this mutation.

METHODS

The distribution of different maturation-associated subsets of BM and PB CD34⁺ HPC from 64 newly diagnosed (KIT-mutated) ISM patients and 14 healthy controls was analyzed by flow cytometry. In 18 patients, distinct FACS-purified PB cell compartments were also investigated for the KIT mutation.

RESULTS

ISM patients showed higher percentages of both BM and PB MC-committed CD34⁺ HPC vs controls, particularly among ISM cases with MC-restricted KIT mutation (ISM_MC ); this was associated with progressive blockade of maturation of CD34⁺ HPC to the neutrophil lineage from ISM_MC to multilineage KIT-mutated cases (ISM_ML ). Regarding the frequency of KIT-mutated cases and cell populations in PB, variable patterns were observed, the percentage of KIT-mutated PB CD34⁺ HPC, eosinophils, neutrophils, monocytes and T cells increasing from ISMs-_MC and ISMs+_MC to ISM_ML patients.

CONCLUSION

The presence of the KIT D816V mutation in PB of ISM patients is associated with (early) involvement of circulating CD34⁺ HPC and multiple myeloid cell subpopulations, KIT-mutated PB CD34⁺ HPC potentially contributing to early dissemination of the disease.

KIT signaling is dispensable for human mast cell progenitor development

Human hematopoietic progenitors are generally assumed to require stem cell factor (SCF) and KIT signaling during differentiation for the formation of mast cells. Imatinib treatment, which inhibits KIT signaling, depletes mast cells in vivo. Furthermore, the absence of SCF or imatinib treatment prevents progenitors from developing into mast cells in vitro. However, these observations do not mean that mast cell progenitors require SCF and KIT signaling throughout differentiation. Here, we demonstrate that circulating mast cell progenitors are present in patients undergoing imatinib treatment. In addition, we show that mast cell progenitors from peripheral blood survive, mature, and proliferate without SCF and KIT signaling in vitro. Contrary to the prevailing consensus, our results show that SCF and KIT signaling are dispensable for early mast cell development.