Plasmacytoid dendritic cell — the interferon cell that KMT2A-rearranged leukaemia occasionally becomes, and is more often mistaken for
also called pDC · plasmacytoid dendritic cell · BPDCN cell · CD123+CD4+CD56+ blast
The plasmacytoid dendritic cell is a rare white cell whose whole job is to detect viruses and flood the body with interferon, the alarm signal that makes you feel ill during a viral infection. A KMT2A-rearranged leukaemia can occasionally take on this cell's appearance, and more often it imitates it closely enough to be misdiagnosed.
The same thing, for a scientist
The plasmacytoid dendritic cell is the professional type I interferon producer of the immune system, identified in blood by CD123-high, CD4-positive, CD56-variable, HLA-DR-positive, lineage-negative phenotype and by a plasma-cell-like morphology. Its malignant counterpart, blastic plasmacytoid dendritic cell neoplasm (BPDCN), is rare and has few recurrent cytogenetic lesions. KMT2A enters this compartment in two distinguishable ways and the graph should keep them apart. **Genuinely**: a paediatric BPDCN with KMT2A-MLLT1 confirmed by FISH and multiplex RT-PCR [E1], and a KMT2A-rearranged acute leukaemia in which a single blast population showed **both** blastic plasmacytoid dendritic cell and monocytic differentiation [E2]. **Spuriously**: a KMT2A-rearranged case with CD4, CD56, CD117, CD123 and monocytic CD64 that looked morphologically and immunophenotypically like BPDCN but was **MPO- and muramidase-positive** and was diagnosed as acute monocytic leukaemia under WHO 5th-edition criteria [E3]. The literature here is case-report-level: the intersection of KMT2A terms with plasmacytoid dendritic terms returns **9 PubMed records** against 2,919 for KMT2A terms alone and 5,404 for plasmacytoid dendritic terms alone.
What a pDC is and what it would be doing if it were healthy
Most dendritic cells are shape-shifting sentinels that swallow material, migrate to a lymph node and show fragments of it to T cells. The plasmacytoid dendritic cell is different. It looks like a plasma cell — round, with an eccentric nucleus and abundant rough endoplasmic reticulum — and it is built for one purpose: sensing viral nucleic acid through TLR7 and TLR9 in its endosomes and responding by producing type I interferon at a rate no other cell approaches, up to a thousand-fold more per cell than any other leukocyte.
It is rare, well under one percent of blood mononuclear cells, and is identified by high CD123 (IL-3 receptor alpha), CD4, HLA-DR and often CD56, with no myeloid, B or T lineage markers.
That marker set is the origin of the diagnostic problem described below: CD123 is also high on AML blasts, CD4 is expressed by monocytes, and CD56 appears on many myeloid neoplasms.
A modern caveat: unbiased single-cell RNA-seq of human blood redefined pDCs, identifying a new subset that shares properties with pDCs but potently activates T cells, alongside six dendritic-cell and four monocyte subtypes in total 4. The 'pDC' of the diagnostic literature is a flow-cytometric gate that single-cell data resolve into more than one thing.
What is actually reported in KMT2A-rearranged disease
A confirmed paediatric case. BPDCN with a KMT2A-MLLT1 rearrangement, karyotype 48,XX,+8,t(11;19)(q23;p13.3),+19 in all 20 metaphases, KMT2A rearrangement in 83 percent of interphase nuclei by FISH, and the fusion confirmed by multiplex RT-PCR. The authors note that only one adult case had previously been reported and call for larger series 1.
A hybrid blast. A KMT2A-rearranged acute leukaemia in which a single blast population showed both blastic plasmacytoid dendritic cell and monocytic differentiation. The authors present it as evidence about the cell of origin of plasmacytoid dendritic cells — which is exactly the kind of statement this graph cares about, because it places the pDC programme within reach of the same cell that is running a monocytic programme 2.
And a mimic. A 35-year-old with blastic cells showing perinuclear microvacuoles and hand-mirror forms, dim CD45, CD4, CD56, CD117, CD123 and monocytic CD64, with FISH showing a KMT2A rearrangement to an unknown partner on chromosome 19 — and MPO and muramidase positive, giving a final diagnosis of acute monocytic leukaemia. The case is published specifically to illustrate why the WHO 5th edition tightened BPDCN criteria 3.
The honest summary is that the KMT2A-pDC link is real but rests on single cases, and that at least one published KMT2A 'BPDCN-like' presentation turned out to be monocytic leukaemia. Given that cell-type-monocyte is the dominant identity in this disease and that CD123, CD4 and CD56 are shared, the prior should favour monocytic whenever MPO has not been excluded.
Why this belongs in the graph rather than in a rejection list
Three things keep this above the threshold for an entry:
- Neonatal leukaemia includes BPDCN. The standard review of neonatal leukaemia lists blastic plasmacytoid dendritic cell neoplasms among the presentations, alongside AML with monocytic/monoblastic characteristics, B-ALL and mixed phenotype — in a population whose commonest lesion is t(4;11)/KMT2A-AFF1 5.
- CD123 is a live therapeutic target. Tagraxofusp targets CD123 and is approved in BPDCN; CD123 is also expressed on AML blasts and on the GMP. Any CD123-directed agent entering KMT2A-rearranged disease inherits this cell's biology.
- The hybrid blast is mechanistically informative. A single blast running both pDC and monocytic programmes is a direct observation of the lineage promiscuity this graph studies, in a third direction beyond the B-to-myeloid switch it usually discusses.
| ontology | CL:0000784 |
|---|---|
| location | Blood, lymph node T-cell zones, bone marrow; skin in BPDCN |
| normal function | professional type I interferon producer, sensing viral nucleic acid through endosomal TLR7 and TLR9 |
markers
[ "CD123 (IL3RA) high", "CD4+", "CD56 variable", "HLA-DR+", "CD303/BDCA-2", "CD304/BDCA-4", "MPO negative (MPO positivity excludes BPDCN)" ]
lineage potential
[ "terminal; type I interferon production" ]
kmt2a relevance
{
"evidence_level": "case reports only",
"confirmed_cases": "paediatric BPDCN with KMT2A-MLLT1 (FISH + multiplex RT-PCR); a KMT2A-rearranged acute leukaemia with hybrid pDC and monocytic differentiation in one blast population",
"mimicry_hazard": "a KMT2A-rearranged CD4/CD56/CD123-positive case was MPO and muramidase positive and was diagnosed as acute monocytic leukaemia under WHO 5th edition criteria",
"literature_size": "9 PubMed records at the intersection, against 2919 for KMT2A terms and 5404 for plasmacytoid dendritic terms (eutils, 2026-09-19)",
"therapeutic_hook": "CD123 is high on pDCs, on AML blasts and on the GMP, so any CD123-directed agent inherits this cell's biology"
}atlas refs
[
{
"label": "Single-cell redefinition of human blood dendritic cells",
"pmid": "28428369",
"title": "Single-cell RNA-seq reveals new types of human blood dendritic cells, monocytes, and progenitors.",
"authors": "Villani AC, Satija R, Reynolds G, et al.",
"venue": "Science",
"year": 2017,
"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": "KMT2A-MLLT1 in paediatric BPDCN",
"pmid": "26164398",
"title": "KMT2A (MLL)-MLLT1 rearrangement in blastic plasmacytoid dendritic cell neoplasm.",
"authors": "Yang N, Huh J, Chung WS, et al.",
"venue": "Cancer Genet",
"year": 2015,
"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."
}
]- CD123 is expressed on normal plasmacytoid dendritic cells, on normal GMP and on AML blasts, so a CD123-directed therapeutic removes the body's principal antiviral alarm cell along with the leukaemia, and the resulting interferon deficit would be hardest on an immunosuppressed infant.
- If a KMT2A-rearranged monocytic leukaemia can present with a BPDCN-like immunophenotype, a trial enrolling on immunophenotype rather than on fusion status will misallocate patients in both directions.
- A third available lineage destination beyond B and myeloid would mean that blocking the known escape route does not close the others.
- How many reported KMT2A-rearranged BPDCN cases survive reclassification under WHO 5th edition criteria with MPO and muramidase assessed? A systematic re-review of the nine-record literature would settle whether the association exists at all.
- In the hybrid case, was the dual pDC-monocytic phenotype present in single cells or in two populations within one gate? Single-cell molecular profiling of such a case would be the first direct evidence that one blast can run both programmes.
- Does CD123-directed therapy such as tagraxofusp have activity in KMT2A-rearranged monocytic AML, where CD123 is also expressed? The target is shared even though the diagnosis is not.
- Is the pDC programme reachable from the monocytic state under therapeutic pressure, giving a third lineage-escape route alongside B-to-myeloid switch?
- E1weak confidence 2 of 5: Preliminary, small, indirect, or not yet peer reviewed.paper
A paediatric case of blastic plasmacytoid dendritic cell neoplasm carried a KMT2A-MLLT1 rearrangement, with karyotype 48,XX,+8,t(11;19)(q23;p13.3),+19 in all 20 metaphases analysed, KMT2A rearrangement in 83 percent of interphase cells by FISH, and the fusion confirmed by multiplex RT-PCR; only one adult case had previously been reported.
KMT2A (MLL)-MLLT1 rearrangement in blastic plasmacytoid dendritic cell neoplasm · Yang N, Huh J, Chung WS, Cho MS, Ryu KH, Chung HS · Cancer Genetics 2015What this evidence cannot show
A single case report; the authors themselves call for study of a larger number of cases. BPDCN diagnostic criteria in 2015 predate the WHO 5th edition tightening, so the diagnosis was made under looser criteria than current practice. Read: abstract only.
Evidence that argues against this (1)
- A KMT2A-rearranged case with a near-identical immunophenotype — dim CD45, CD4, CD56, CD117, CD123 — turned out to be acute monocytic leukaemia once MPO and muramidase were assessed, and was published expressly to illustrate why the WHO 5th edition tightened BPDCN criteria. Under current criteria a proportion of historically reported KMT2A-rearranged BPDCN would be reclassified as monocytic AML. Read: abstract only. Jhajj B et al. 2023
- E2weak confidence 2 of 5: Preliminary, small, indirect, or not yet peer reviewed.paper
In a KMT2A-rearranged acute leukaemia, a single blast population showed both blastic plasmacytoid dendritic cell and monocytic differentiation, a hybrid immunophenotype the authors present as informative about the cell of origin of plasmacytoid dendritic cells; blastic plasmacytoid dendritic cell differentiation in KMT2A-rearranged acute leukaemia is described as extremely rare.
Monocytic and blastic plasmacytoid dendritic cell differentiation in acute leukemia with KMT2A rearrangement · Xue O, Curry CV, Fang H, Bueso-Ramos CE, Medeiros LJ, Wang W · eJHaem 2023What this evidence cannot show
A single case; 'a single blast population' showing both programmes is an immunophenotypic inference from flow cytometry rather than a single-cell molecular demonstration, and a bimodal population within one gate would look the same. Read: abstract only.
Evidence that argues against this (1)
- The pDC gate itself is not a single entity. Unbiased single-cell RNA-seq of human blood identified a new subset sharing properties with pDCs but potently activating T cells, thereby redefining pDCs, and resolved four monocyte subtypes. A 'hybrid pDC-monocytic' blast defined by surface markers may be occupying a normal state that the CD123/CD4/CD56 gate simply does not distinguish from either. Read: abstract only. Villani AC et al. 2017
- E3moderate confidence 3 of 5: One strong study, or several smaller consistent ones.paper
A KMT2A-rearranged case (unknown partner on chromosome 19) presented with blastic cells showing perinuclear microvacuoles and hand-mirror forms, dim CD45, CD4, CD56, CD117, CD123 and monocytic CD64, but expressed MPO and muramidase and was finally diagnosed as acute monocytic leukaemia; the case is reported to illustrate the utility of the WHO 5th edition's tightened BPDCN criteria given the overlapping features of AML and BPDCN.
A Case of Acute Myeloid Leukemia Mimicking Blastic Plasmacytoid Dendritic Cell Neoplasm: Utility of the Proposed Upcoming WHO-5 Diagnostic Criteria · Jhajj B, Henrie R, El-Khalidy Y, Moshref Razavi H · Case Reports in Hematology 2023What this evidence cannot show
A single 35-year-old adult case, not paediatric, with a KMT2A partner that was never identified. It demonstrates that misclassification is possible, not how often it happens. Read: abstract only.
Evidence that argues against this (1)
- That AML can mimic BPDCN does not mean every reported KMT2A-rearranged BPDCN is misclassified. One paediatric case had the KMT2A-MLLT1 fusion confirmed by karyotype, FISH at 83 percent of interphase nuclei and multiplex RT-PCR, and a separate case documented pDC and monocytic differentiation coexisting in one blast population rather than one being mistaken for the other. The mimicry argument raises the evidential bar; it does not clear the field. Read: abstract only. Xue O et al. 2023
- E4strong confidence 4 of 5: Replicated in peer-reviewed work by more than one group.paper
Unbiased single-cell RNA sequencing of approximately 2400 human blood cells identified six dendritic cell and four monocyte subtypes, revealed a new subset sharing properties with plasmacytoid dendritic cells but potently activating T cells, thereby redefining pDCs, and characterised the relationship between blastic plasmacytoid dendritic cell neoplasia cells and healthy dendritic cells.
Single-cell RNA-seq reveals new types of human blood dendritic cells, monocytes, and progenitors · Villani AC, Satija R, Reynolds G, Sarkizova S, Shekhar K, Fletcher J, et al., Hacohen N · Science 2017What this evidence cannot show
Roughly 2400 cells by 2017 single-cell methods, from healthy blood; the BPDCN comparison is limited and does not address KMT2A-rearranged disease. Read: abstract only.
Evidence that argues against this (1)
- A redefined research taxonomy has not displaced the diagnostic one. BPDCN diagnosis in practice remains a flow-cytometric and morphological exercise governed by WHO criteria, and the WHO 5th edition tightened those criteria on immunophenotypic and morphological grounds rather than by adopting single-cell subsets. A graph entry that treats the single-cell taxonomy as authoritative would be describing a different object from the one clinical reports describe. Read: abstract only. Khoury JD et al. 2022
- E5moderate confidence 3 of 5: One strong study, or several smaller consistent ones.review
Neonatal leukaemia presents in two-thirds of patients as acute myeloid leukaemia, frequently with monocytic/monoblastic characteristics, with most other cases being acute lymphoblastic leukaemia (particularly B lineage) and some being mixed phenotype or blastic plasmacytoid dendritic cell neoplasms; the most frequently observed cytogenetic/molecular abnormality is t(4;11)(q21.3;q23.3)/KMT2A-AFF1.
Neonatal leukaemia · Roberts I, Fordham NJ, Rao A, Bain BJ · British Journal of Haematology 2018What this evidence cannot show
A narrative review; BPDCN is listed among neonatal presentations without a frequency and without stating whether any neonatal BPDCN case carried a KMT2A rearrangement. The association of BPDCN with KMT2A in neonates is therefore not established by this source. Read: abstract only.
Evidence that argues against this (1)
- The review lists BPDCN and KMT2A-AFF1 as separate features of the same rare population without linking them, so co-occurrence in a review's list is not evidence of association. The only direct KMT2A-BPDCN evidence remains case-level, and the total PubMed intersection of KMT2A terms with plasmacytoid dendritic terms is 9 records against 2,919 and 5,404 for the two halves separately, which is the profile of a sporadic rather than a recognised association. Oncofusion Knowledge Graph project search 2026
shared CD123/CD4/CD56 phenotype makes monocytic leukaemia the main differential and the main misdiagnosis
- arises in →KMT2A::MLLT1 (MLL-ENL), t(11;19)(q23;p13.3)
paediatric BPDCN with KMT2A-MLLT1 confirmed by FISH and RT-PCR
- related to →Lineage switch in KMT2A-rearranged leukemia — the same clone changes what kind of blood cell it is
a third possible lineage destination alongside B and myeloid
- ← related toMonoblast — the immature monocyte that names FAB M5a, the dominant morphology of congenital KMT2A leukaemia
a single KMT2A-rearranged blast population showed both monocytic and pDC differentiation
the candidate closest to the rejection threshold, written with weak confidence
REQUIRED IN (imperfect node match): relB-null mice have a reduced population of THYMIC dendritic cells and impaired cellular immunity; this graph has no thymic or conventional DC node, so the edge is drawn weakly and the discrepancy is stated (murine germline knockout; PMID 7834753)
- ← co-occurs withPre-pro-B cell (murine Hardy Fraction A) - the stage before pro-B, with germline immunoglobulin genes, and the stage RYBP loss actually removes
PURITY WARNING, not a lineage relationship: pDCs were defined as a Ly-6C+ CD11c-low subset of the same B220+CD19-CD43+CD24-low Fraction A gate, so a marker-only pre-pro-B phenotype is a mixture
the two DC lineages gene-relb had to be forced to choose between: RelB is required for the conventional myeloid-related subset and no cited source assigns it a pDC requirement. cell-type-plasmacytoid-dendritic-cell is not superseded, it is simply not RELB's cell