NG2+ periarteriolar pericyte — the cell that holds blood stem cells asleep, and the one MLL-AF9 leukaemia destroys
also called NG2+ pericyte · CSPG4+ periarteriolar cell · arteriolar niche cell
Wrapped tightly around the small arteries of the bone marrow is a rare kind of support cell that keeps nearby blood stem cells in a deep resting state. When these cells are removed, the stem cells start dividing and the long-term stem-cell pool shrinks — and in a mouse model of this leukaemia, the disease destroys exactly these cells.
The same thing, for a scientist
NG2/CSPG4-positive pericytes ensheath small marrow arterioles and are the stromal population whose depletion induces HSC cycling and reduces long-term repopulating HSCs. Three-dimensional whole-mount imaging with computational association analysis placed quiescent HSCs specifically at these arterioles, which are preferentially found in endosteal marrow, and showed that pharmacological or genetic activation of the HSC cell cycle **redistributes HSCs from NG2+ periarteriolar to LEPR+ perisinusoidal niches** — making arteriolar residence a correlate of the quiescent state rather than a fixed address [E1]. The KMT2A-relevant finding is direct: in MLL-AF9 AML the mesenchymal compartment expands towards osteoblast-primed MSPCs **at the expense of HSC-maintaining NG2+ periarteriolar niche cells** [E2]. Independent pO2 measurement complicates the picture by showing that the endosteal region, where these arterioles are enriched, is **less** hypoxic than deep peri-sinusoidal marrow because it is arterially perfused [E3]. This is the niche cell most directly tied to `cell-state-quiescence`, the state this graph invokes to explain measurable residual disease.
Why this cell is the one that matters for dormancy
cell-state-quiescence records that dormancy in patient-derived ALL xenografts is reversible on removal from the in vivo niche: dormant, drug-resistant cells re-enter cycle and regain drug sensitivity once taken out. That entry names the niche as the cause and does not say which part of it.
1 is the strongest candidate answer available in normal biology. Quiescent HSCs sit at arterioles ensheathed by NG2+ pericytes; depleting those pericytes induces cycling; and activating the cycle moves HSCs away from arterioles to sinusoids. The association runs both directions, which is what distinguishes it from a static co-localisation.
Whether leukaemic cells use the same address is not established for KMT2A-rearranged disease in any species. The one piece of evidence pointing that way is indirect and unfavourable: MLL-AF9 AML destroys NG2+ periarteriolar cells 2. If the leukaemia eliminates the compartment that enforces quiescence, the simplest reading is that the leukaemia is not using it — which argues against a periarteriolar sanctuary for residual disease and in favour of the metabolic, mesenchymal support documented in cell-type-car-cell and cell-type-nestin-msc. That reading should be stated as a hypothesis and not as a finding, and it is carried in hyp-niche-remodelling-precedes-relapse-kmt2a.
Naming hazard: in this disease NG2 is mostly a blast marker, not a niche marker
This must be said explicitly because it is the kind of ambiguity that produces a wrong entry three cycles later.
Intersecting NG2/CSPG4/pericyte terms with the KMT2A disease half returns 19 PubMed records. Almost all of them are about NG2 on the leukaemic blast, not about NG2+ niche pericytes: NG2 positivity is a diagnostic immunophenotypic hallmark of KMT2A-rearranged B-ALL, it is a direct transcriptional target of MLL-AF4, it drives CNS infiltration, it confers glucocorticoid resistance by downregulating NR3C1, it is a CAR-T target, and it partners with ITGA4 to drive aggressiveness 4. Exactly one of the 19 concerns the periarteriolar niche cell — the MLL-AF9 sympathetic-neuropathy paper cited here as 2.
So NG2 names two different cells in this project: the pericyte in the normal-niche literature, and the blast in the KMT2A literature. The graph covers the blast side in concept-flow-cytometry-immunophenotype-kmt2a-r, concept-cns-involvement-at-diagnosis and mechanism-itga4-niche-adhesion-kmt2a; this entry covers the pericyte and nothing else.
The practical consequence is a question nobody has asked: an NG2-directed therapeutic would hit both. CSPG4-specific CAR T cells for high-risk childhood BCP-ALL have been described. Such an agent would engage the leukaemic blast and the HSC-quiescence-maintaining periarteriolar pericyte at the same time. Whether that is an on-target toxicity that would collapse the patient's stem-cell niche, or an unexpected two-for-one, is unknown and is written up as an open question below.
The oxygen problem
The standard picture has the quiescent stem cell in a hypoxic niche. Direct two-photon pO2 measurement in live mice says the opposite about where: marrow is uniformly low in oxygen (<32 mmHg) and the lowest values (about 9.9 mmHg) are deep peri-sinusoidal, with the endosteal region less hypoxic precisely because it is perfused by the small arteries these NG2+ cells wrap 3.
So a quiescent HSC at an arteriole is sitting in the best-oxygenated part of an already low-oxygen organ. Either the link between quiescence and hypoxia is weaker than assumed, or the relevant variable is something else that tracks with arterial perfusion. This entry records the tension rather than resolving it; see concept-marrow-oxygen-gradient.
| ontology | CL:0000669 (pericyte) |
|---|---|
| naming hazard | NG2/CSPG4 marks this niche pericyte AND the KMT2A-rearranged leukaemic blast. Of 19 PubMed records intersecting NG2/CSPG4/pericyte terms with the KMT2A disease half, nearly all concern the blast; one concerns the pericyte. An NG2-directed therapeutic, such as a CSPG4 CAR-T, would engage both. |
| location | ensheathing small arterioles, preferentially in endosteal bone marrow |
| function | maintains HSC quiescence; depletion induces HSC cycling and reduces long-term repopulating HSCs |
| state association | quiescent HSCs at NG2+ arteriolar sites; cycling HSCs redistribute to LEPR+ perisinusoidal sites |
markers
[ "NG2 / CSPG4", "alpha-SMA (subset)", "LEPR negative", "CD45 negative" ]
kmt2a relevance
{
"finding": "lost in MLL-AF9 AML marrow, displaced by osteoblast-primed MSPC expansion",
"interpretation_flagged_as_hypothesis": "if the leukaemia eliminates the quiescence-enforcing compartment, a periarteriolar sanctuary for residual KMT2A-r disease is unlikely; carried as a hypothesis, not a finding"
}atlas refs
[
{
"label": "Arteriolar niches maintain HSC quiescence",
"pmid": "24107994",
"title": "Arteriolar niches maintain haematopoietic stem cell quiescence.",
"authors": "Kunisaki Y, Bruns I, Scheiermann C, et al.",
"venue": "Nature",
"year": 2013,
"title_provenance": "Title/authors/venue backfilled from PubMed esummary on 2026-09-20 because this citation stored a bare PMID with no title, which made it unscoreable by verify-citations.py. BACKFILLING IS NOT VERIFICATION: it confirms the PMID resolves to a real paper, not that this is the right paper for the claim. That still requires reading."
},
{
"label": "NG2+ periarteriolar cells lost in MLL-AF9 AML",
"pmid": "25017722",
"title": "Acute myelogenous leukemia-induced sympathetic neuropathy promotes malignancy in an altered hematopoietic stem cell niche.",
"authors": "Hanoun M, Zhang D, Mizoguchi T, et al.",
"venue": "Cell Stem Cell",
"year": 2014,
"title_provenance": "Title/authors/venue backfilled from PubMed esummary on 2026-09-20 because this citation stored a bare PMID with no title, which made it unscoreable by verify-citations.py. BACKFILLING IS NOT VERIFICATION: it confirms the PMID resolves to a real paper, not that this is the right paper for the claim. That still requires reading."
}
]- If this compartment enforces quiescence in leukaemic as well as normal cells, restoring it would deepen dormancy and make residual disease harder to kill, not easier.
- Depleting NG2+ pericytes to force leukaemic cells into cycle would simultaneously force the patient's own HSCs into cycle and shrink the long-term repopulating pool they need to recover counts.
- NG2/CSPG4 is expressed well outside the marrow, including on oligodendrocyte precursors, so any NG2-directed agent carries central nervous system liability in a disease already treated with CNS-directed therapy.
- Do dormant KMT2A-rearranged blasts sit at arterioles? The xenograft dormancy result that this graph relies on never localised the dormant cells, and imaging them against NG2+ vessels is a tractable experiment.
- Are NG2+ cells lost or merely NG2-negative in leukaemic marrow? The distinction decides whether the compartment could be restored.
- Does the human infant marrow have periarteriolar NG2+ pericytes in the quantity mouse marrow does, and is arteriolar density different in infants?
- Why is the quiescence-maintaining niche the best-oxygenated part of the marrow? Either hypoxia is not the mechanism of niche-imposed quiescence, or the measurement and the biology are being connected wrongly.
- An NG2/CSPG4-directed therapeutic would engage the leukaemic blast and this niche pericyte simultaneously. Does a CSPG4 CAR-T collapse the HSC-quiescence niche as an on-target toxicity, and has anyone measured periarteriolar pericytes after such treatment?
- E1strong confidence 4 of 5: Replicated in peer-reviewed work by more than one group.paper
Quiescent HSCs associate specifically with small arterioles preferentially found in endosteal bone marrow; these arterioles are ensheathed exclusively by rare NG2/CSPG4-positive pericytes distinct from sinusoid-associated LEPR-positive cells; pharmacological or genetic activation of the HSC cell cycle redistributes HSCs from NG2+ periarteriolar to LEPR+ perisinusoidal niches; and conditional depletion of NG2+ cells induces HSC cycling and reduces functional long-term repopulating HSCs in the bone marrow.
Arteriolar niches maintain haematopoietic stem cell quiescence · Kunisaki Y, Bruns I, Scheiermann C, Ahmed J, Pinho S, Zhang D, et al. · Nature 2013What this evidence cannot show
Mouse whole-mount calvarium and long-bone imaging with computational association modelling; the causal arm is diphtheria-toxin-mediated NG2+ depletion, which also removes pericytes from vessels generally and could act through vascular changes rather than through a direct HSC signal. NG2/CSPG4 is expressed on other cell types including oligodendrocyte precursors and some smooth muscle. Read: abstract only.
Evidence that argues against this (1)
- A haematopoietic cell, the megakaryocyte, has an equally direct claim on the same function: megakaryocytes lie adjacent to HSCs non-randomly and their selective in vivo depletion causes specific loss of HSC quiescence and marked expansion of functional HSCs, mediated by CXCL4. Two independent depletion experiments in two unrelated cell types both abolish HSC quiescence, so neither population can be the sole enforcer of the state. Read: abstract only. Bruns I et al. 2014
- E2strong confidence 4 of 5: Replicated in peer-reviewed work by more than one group.paper
In an MLL-AF9 mouse AML model, disease development expands phenotypic mesenchymal stem and progenitor cells primed for osteoblastic differentiation at the expense of the HSC-maintaining NG2-positive periarteriolar niche cells, alongside disruption of sympathetic nerves and loss of Nestin-positive niche-cell quiescence.
Acute myelogenous leukemia-induced sympathetic neuropathy promotes malignancy in an altered hematopoietic stem cell niche · Hanoun M, Zhang D, Mizoguchi T, Pinho S, Pierce H, Kunisaki Y, et al. · Cell Stem Cell 2014What this evidence cannot show
Mouse myeloid MLL-AF9 model; the NG2+ loss is quantified by imaging and flow phenotype in leukaemic marrow, and the abstract does not state whether the cells die or lose the marker. No lymphoid or infant arm. Read: abstract only.
Evidence that argues against this (1)
- A separate study in the same MLL-AF9 driver reports that nestin+ BMSC numbers are NOT reduced while bulk stroma is, and that what changes is their function. Loss of one perivascular mesenchymal marker and preservation of another in the same disease means 'leukaemia destroys the niche' is too coarse a summary; some compartments are eliminated and others repurposed, and which of those happens to the NG2+ subset is asserted from one report. Read: abstract only. Forte D et al. 2020
- E3strong confidence 4 of 5: Replicated in peer-reviewed work by more than one group.paper
Direct in vivo measurement of local oxygen tension shows the endosteal region of mouse bone marrow, where the HSC-quiescence-maintaining arterioles are enriched, is LESS hypoxic than deep peri-sinusoidal marrow (which reaches about 9.9 mmHg, 1.3 percent), because it is perfused by small, often Nestin-positive arteries.
Direct measurement of local oxygen concentration in the bone marrow of live animals · Spencer JA, Ferraro F, Roussakis E, Klein A, Wu J, Runnels JM, et al. · Nature 2014What this evidence cannot show
Mouse calvarial marrow; local extracellular pO2 is not intracellular hypoxic signalling. The arteriolar/endosteal overlap is anatomical and the entry does not claim the measured region and the NG2+ niche are exactly co-extensive. Read: abstract only.
Evidence that argues against this (1)
- Metabolic imaging in MLL-AF9 AML describes the most leukemogenic, glycolytic leukaemia-initiating cells homing to and dividing symmetrically in a HYPOXIC ENDOSTEAL niche, the opposite spatial assignment. Both cannot be right about where the low-oxygen compartment is. Read: abstract only. Hao X et al. 2019
- E4strong confidence 4 of 5: Replicated in peer-reviewed work by more than one group.paper
In KMT2A-rearranged B-ALL, NG2 (CSPG4) - a direct transcriptional target of KMT2A fusions whose expression is associated with poor prognosis, early relapse and glucocorticoid resistance - forms a biological axis with the alpha-4 integrin subunit ITGA4: NG2 promotes proliferation and migration of KMT2A-r B-ALL cells and is associated with Rho GTPase activity in an ITGA4-dependent manner, combined genetic ablation of both genes delayed disease onset and prolonged survival in immunodeficient mice, and natalizumab, an approved anti-ITGA4 antibody, delayed leukaemia progression and potentiated standard-of-care chemotherapy in patient-derived xenografts.
NG2-ITGA4 axis regulates Rho GTPases and leukemic aggressiveness in KMT2A-r B-ALL and is targetable with natalizumab · Rubio-Gayarre A, Vinyoles M, Tejedor JR, Guerrero-Murillo M, Fernandez-Fuentes N, Caracuel-Peramos R, Plaza-Calonge MC, Martinez-Moreno A, et al. · Blood 2026What this evidence cannot show
Cited in this entry solely to establish that NG2 in the KMT2A literature denotes the leukaemic blast rather than the niche pericyte; the therapeutic claim is developed in `mechanism-itga4-niche-adhesion-kmt2a`. 2026 publication, one group, PDX in immunodeficient mice with murine stroma. Read: abstract only.
Evidence that argues against this (1)
- NG2 on the blast is not a single-function marker and its causal weight is contested even within the group that studies it: serial xenotransplantation of primary MLL-AF4-positive leukaemias showed NG2 is a MALLEABLE marker that does NOT enrich for leukaemia-initiating or CNS-initiating cells, even though NG2 was strongly upregulated on blasts infiltrating extramedullary sites and CNS and NG2 blockade almost abolished engraftment. A marker that is functionally required but does not mark the functional cell is an awkward therapeutic target. Read: abstract only. Prieto C et al. 2018
SAME MOLECULE, TWO CELLS: gene-cspg4 is NG2 on the leukaemic blast; cell-type-ng2-periarteriolar-cell is NG2 on the marrow pericyte. An NG2-directed therapeutic engages both.
periarteriolar compartment of the niche
depletion induces HSC cycling; quiescent HSCs sit at NG2+ arterioles, cycling HSCs at sinusoids
MLL-AF9 AML expands osteoblast-primed MSPCs at the expense of NG2+ periarteriolar cells
arterioles ensheathed by NG2+ pericytes are enriched in endosteal marrow
NG2/CSPG4 names both this leukaemic axis and the niche pericyte; an NG2-directed agent would engage both
the normal marrow cell type that carries the same antigen
the normal NG2-bearing cell type already in this graph