Bone-marrow sinusoidal endothelial cell — the vessel wall that keeps stem cells alive, and posts vesicles that accelerate MLL-AF9 leukaemia
also called sinusoidal endothelium · BM endothelial cell · vascular niche cell
The bone marrow drains through wide, leaky, slow-flowing vessels called sinusoids, and the cells lining them do more than carry blood: they release the survival factors blood stem cells need and form the doorway through which new blood cells leave the marrow. In a mouse model of this leukaemia, these lining cells also send out tiny packages that speed the disease up.
The same thing, for a scientist
Marrow sinusoidal endothelial cells are a required source of stem cell factor: conditional deletion of Scf from endothelium depletes HSCs, and deleting it from both endothelium and LEPR+ perivascular cells removes most of them [E1]. The vascular wall is also the site of egress into the circulation, which makes it the structure a CXCR4 antagonist acts across. In MLL-AF9 AML the endothelium is not a passive wall. Using seven cell-type-specific cre lines to knock out Vps33b and block small extracellular vesicle (sEV) secretion compartment by compartment, blocking sEV release **from endothelial cells, but not from perivascular cells, megakaryocytes or spleen stromal cells, markedly delayed leukaemia progression while leaving normal haematopoiesis unaffected**; the endothelial sEVs carry high levels of **ANGPTL2**, which accelerates leukaemia by binding **LILRB2**, and ANGPTL2-sEVs are also required for primary human AML cell maintenance [E2]. That combination — leukaemia-selective, normal-haematopoiesis-sparing, with a named ligand and a named receptor — is the most therapeutically shaped niche finding in this beat.
What the vascular niche does for a healthy cell
Marrow sinusoids are fenestrated, thin-walled and slow-flowing, which is what allows a maturing blood cell to squeeze through into the circulation. Conditional-deletion genetics placed one of the two essential HSC maintenance factors, SCF, in this wall: deleting Scf from endothelial cells depletes HSCs, and deleting it from endothelium and from LEPR+ perivascular cells together removes most of them 1. HSCs sit perivascularly throughout the marrow, not only at the bone surface, and imaging places the cycling fraction preferentially at sinusoids rather than arterioles (see cell-type-ng2-periarteriolar-cell).
The vascular wall is also where oxygen is lowest. Direct pO2 measurement puts the marrow's minimum, about 9.9 mmHg, in deep peri-sinusoidal regions — not at the endosteum 3. If there is a hypoxic niche, it is this one.
The vesicle result, and why it is the most drug-shaped finding here
Huang and colleagues did something most niche studies do not: they asked which compartment's secreted vesicles matter, by conditionally deleting Vps33b (required for sEV secretion) in seven different cre lines — Cdh5+ and Tie2+ endothelium, Lepr+ perivascular cells, Osx+ osteoprogenitors, Pf4+ megakaryocytes and Tcf21+ spleen stroma — and reading out MLL-AF9 AML progression in each 2.
Only the endothelial deletions delayed leukaemia. The perivascular, megakaryocyte and spleen-stromal deletions did not. And crucially, reducing endothelial sEV production had no effect on normal haematopoiesis — the selectivity that every niche-directed strategy in this beat otherwise lacks. The cargo is ANGPTL2, the receptor is LILRB2, and ANGPTL2-sEVs were also required for maintenance of primary human AML cells.
This is the shape a druggable niche target should have: a specific compartment, a specific ligand, a specific receptor, a leukaemia-selective phenotype. The honest caveats are that it is one paper, in mouse, in the myeloid MLL-AF9 model rather than infant KMT2A::AFF1 ALL, and that blocking vesicle secretion pharmacologically is not something anyone can currently do with precision. LILRB2 blockade, by contrast, is a tractable antibody target and is the obvious near-term test.
| ontology | CL:0002138 (endothelial cell of lymphatic vessel is distinct); marrow sinusoidal endothelium has no single dedicated CL term in wide use |
|---|---|
| location | walls of marrow sinusoids; the route of egress to the peripheral blood |
| oxygen | deep peri-sinusoidal marrow is the lowest-pO2 compartment measured in vivo (about 9.9 mmHg) |
markers
[ "CDH5 / VE-cadherin", "TIE2 / TEK", "SCA1 low (sinusoidal) vs high (arteriolar)", "STAB2", "CD31/PECAM1" ]
secreted factors
[ "SCF (KITL)", "CXCL12", "ANGPTL2-containing small extracellular vesicles" ]
kmt2a relevance
{
"finding": "endothelial sEV secretion is required for MLL-AF9 AML progression, and only endothelial secretion among six compartments tested; cargo ANGPTL2, receptor LILRB2",
"selectivity": "reducing endothelial sEV production had no effect on normal haematopoiesis, which is the selectivity other niche-directed strategies in this beat lack",
"tractable_target": "LILRB2 blockade is an antibody-accessible test of the mechanism and has not been reported in KMT2A-rearranged disease",
"replication_status": "single primary study with no independent replication; the KMT2A x extracellular-vesicle PubMed intersection is 5 records once plurals are included (the singular-only query returns 2 and misses this very paper, because a multi-word Title/Abstract term is matched as an unstemmed phrase)"
}atlas refs
[
{
"label": "Endothelial SCF required for HSC maintenance",
"pmid": "22281595",
"title": "Endothelial and perivascular cells maintain haematopoietic stem cells.",
"authors": "Ding L, Saunders TL, Enikolopov G, et al.",
"venue": "Nature",
"year": 2012,
"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": "Endothelial ANGPTL2-sEVs accelerate MLL-AF9 leukaemia via LILRB2",
"pmid": "33108353",
"title": "ANGPTL2-containing small extracellular vesicles from vascular endothelial cells accelerate leukemia progression.",
"authors": "Huang D, Sun G, Hao X, et al.",
"venue": "J Clin Invest",
"year": 2021,
"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."
}
]- Endothelium is systemic. Any agent that suppresses endothelial vesicle secretion or ANGPTL2 signalling acts on every vascular bed in a growing infant, including the developing retina and the central nervous system.
- The vascular wall is also the exit route for recovering normal blood cells; disrupting it during count recovery could delay engraftment after chemotherapy or transplant.
- LILRB2 is an immune-inhibitory receptor on myeloid cells; blocking it has immunological consequences beyond the leukaemia, in a patient who is already profoundly immunosuppressed.
- Does LILRB2 blockade slow KMT2A-rearranged leukaemia? The ligand-receptor pair is named and LILRB2 is antibody-accessible, so this is a directly testable proposition that nobody has reported in this genotype.
- Does the ANGPTL2-sEV mechanism operate in KMT2A::AFF1 infant B-ALL, or is it myeloid-specific? Every result is in an MLL-AF9 myeloid model.
- Marrow sEV biology has one primary paper in this disease and no replication. Would an independent group reproduce the endothelial-specific requirement, given how often single-compartment cre results have not survived replication in the niche field?
- If deep peri-sinusoidal marrow is the genuinely hypoxic compartment, and hypoxia SENSITISES KMT2A-rearranged AML to venetoclax plus a menin inhibitor, is the perisinusoidal compartment the place that combination works best?
- E1established confidence 5 of 5: Multiple independent lines of evidence; treated as settled in the field.paper
Stem cell factor is expressed primarily by perivascular cells throughout bone marrow, and HSCs are depleted when Scf is conditionally deleted from endothelial cells or from leptin-receptor-expressing perivascular stromal cells, with most HSCs lost when it is deleted from both; deletion from haematopoietic cells, osteoblasts or Nestin-cre cells has no effect.
Endothelial and perivascular cells maintain haematopoietic stem cells · Ding L, Saunders TL, Enikolopov G, Morrison SJ · Nature 2012What this evidence cannot show
Mouse conditional genetics; endothelial cre lines recombine in vasculature throughout the body, so systemic effects cannot be excluded from the abstract. The result concerns SCF, not the full set of endothelial niche functions. Read: abstract only.
Evidence that argues against this (1)
- Nestin-GFP-defined perivascular MSCs were independently shown to be required for marrow HSC content and progenitor homing on depletion, and the Nestin-cre negative result in this paper is disputed on cre-specificity grounds. Which perivascular cell type is the functionally required one is not settled. Read: abstract only. Mendez-Ferrer S et al. 2010
- E2strong confidence 4 of 5: Replicated in peer-reviewed work by more than one group.paper
Using seven cell-type-specific cre lines to conditionally knock out Vps33b and so reduce small extracellular vesicle secretion, blocking sEV release from Cdh5+ or Tie2+ endothelial cells - but not from Lepr+ perivascular cells, Osx+ osteoprogenitors, Pf4+ megakaryocytes or Tcf21+ spleen stromal cells - markedly delayed MLL-AF9-induced AML progression, with no effect on normal haematopoiesis; endothelial-derived sEVs contained high levels of ANGPTL2, which accelerated leukaemia via binding to the LILRB2 receptor, their release was governed by VPS33B, and ANGPTL2-sEVs were also required for maintenance of primary human AML cells.
ANGPTL2-containing small extracellular vesicles from vascular endothelial cells accelerate leukemia progression · Huang D, Sun G, Hao X, He X, Zheng Z, Chen C, et al. · Journal of Clinical Investigation 2021What this evidence cannot show
Mouse MLL-AF9 with human AML cells for the maintenance arm; Vps33b deletion reduces sEV secretion but VPS33B has other trafficking roles, so the manipulation is not a clean sEV-only perturbation. 'Markedly delayed' is not quantified in the abstract. No lymphoid or infant KMT2A::AFF1 arm. Read: abstract only.
Evidence that argues against this (1)
- The extracellular-vesicle literature in this genotype is five records, of which this paper is one; the other four are a marrow-MSC translation-support paper, a urinary-vesicle polycystic-kidney study, a Schwann-cell axon-regeneration study and a transcription-elongation complex. So the sEV niche mechanism rests on a single primary study with no independent replication in this genotype, which is the profile of finding this project has been burned by before. Method note recorded because it changed the answer: the singular phrase `extracellular vesicle[Title/Abstract]` returns 2 records and MISSES this paper, because PubMed matches a multi-word Title/Abstract term as an unstemmed phrase; adding `extracellular vesicles` raises the count to 5. Both counts run 2026-09-19. NCBI E-utilities 2026
- E3strong confidence 4 of 5: Replicated in peer-reviewed work by more than one group.paper
Direct in vivo two-photon phosphorescence lifetime measurement shows the lowest oxygen tension in live mouse bone marrow, about 9.9 mmHg (1.3 percent), is in deep peri-sinusoidal regions, with overall marrow pO2 below 32 mmHg despite very high vascular density and with the endosteal region less hypoxic because it is arterially perfused.
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 calvarium. The measurement is extracellular local pO2 and is not equivalent to a cell's hypoxic transcriptional state. Read: abstract only.
Evidence that argues against this (1)
- Metabolic imaging in MLL-AF9 AML localises the most leukemogenic glycolytic leukaemia-initiating cells to a niche its authors call hypoxic and endosteal, the opposite compartment. The two spatial assignments of hypoxia are irreconcilable as stated. Read: abstract only. Hao X et al. 2019
vascular compartment; required SCF source and the exit route to blood
endothelial ANGPTL2-containing sEVs accelerate MLL-AF9 leukaemia via LILRB2, sparing normal haematopoiesis
the vessel wall itself
endothelial ANGPTL2-sEV blockade is one of the two manipulations that spared normal haematopoiesis
VCAM1 is predominantly an endothelial surface adhesion molecule
endothelial Scf deletion depletes HSCs
inducible endothelial adhesion molecule of the vascular niche