Pre-pro-B cell (murine Hardy Fraction A) - the stage before pro-B, with germline immunoglobulin genes, and the stage RYBP loss actually removes
also called pre-proB cell · prepro-B cell · Hardy Fraction A · Fr. A · B220+ CD43+ BP-1- HSA-low B-lineage precursor · B-2 progenitor (in the B-1/B-2 framing)
Before a mouse B cell becomes a pro-B cell it passes through an earlier step called pre-pro-B, in which its antibody genes have not been cut and rejoined at all. This entry exists because a gene in this graph, RYBP, was shown to be needed for exactly that step, and the graph had no word for it.
The same thing, for a scientist
Pre-pro-B is the earliest B-lineage-committed fraction in the murine Hardy scheme: B220-positive, IgM-negative, CD43(S7)-positive, BP-1-negative, heat-stable-antigen-low, and - the defining functional property - with immunoglobulin genes still in **germline configuration**, whereas the succeeding Fractions B and C are pro-B stages with increasing D-J but no V-D-J rearrangement [E1]. The graph needed the term because `gene-rybp` currently points at `cell-type-pro-b` with the mismatch stated in its own prose: conditional Rybp inactivation in adult mouse haematopoiesis reverts the B-1-to-B-2 progenitor ratio, and **the increased B-1 progenitors correlate with a LOSS OF PRE-PROB CELLS, the B-2 progenitors** [E2]. That is a claim about this stage, not about pro-B. The distinction is not pedantry in this disease: the human fetal counterpart of the earliest B-restricted progenitor, the CD10-negative PreProB-progenitor, shares a transcriptomic signature with CD10-negative B-progenitor infant ALL blasts and is the best marker-level match this graph has between a normal fetal cell and infant KMT2A-rearranged immunophenotype [E3] - and that human node (`cell-type-fetal-preprob-cd10-negative`) is a **different cell** from this one, human rather than murine, fetal rather than adult, defined by CD19/CD34/CD10 rather than by the Hardy fractions. Both are needed; neither substitutes for the other. The one thing a reader must carry away is that murine Fraction A is **not a pure B-lineage population** - plasmacytoid dendritic cells were rigorously defined as a Ly-6C+ CD11c-low subset of B220+CD19- CD43+ CD24-low bone marrow Fraction A [E4] - so a phenotype attributed to 'pre-pro-B cells' by surface markers alone may belong to a cell that is not in the B lineage at all.
The definition, and the functional criterion that makes it real
Hardy's resolution of B220+ IgM- bone marrow into fractions by CD43 (S7), BP-1 and heat-stable antigen is the scheme the murine literature still uses. Fractions A, B and C are all CD43-positive and sit before the CD43-negative pre-B cells. What separates Fraction A from B and C is not a marker but the Ig locus: Fraction A cells have germline immunoglobulin genes, Fractions B and C are pro-B stages with increasing D-J and no V-D-J joining 1.
That matters here because 'arrested at pro-B' is a statement about a cell that has begun rearranging, and 'lost the pre-pro-B compartment' is a statement about the cells upstream of any rearrangement. Those are different developmental claims and they license different models of a fusion's cell of origin.
The claim this node was written for
In mice, conditional inactivation of Rybp in adult haematopoiesis reverts the B-1-to-B-2 B-cell progenitor ratio - that is, it shifts the balance from the acquired-immunity (mostly adult) precursors toward the innate, predominantly fetal B-1 precursors. The increased numbers of B-1 progenitors correlated with a loss of pre-proB cells, the B-2 progenitors. RYBP-deficient LKS populations and isolated common lymphoid progenitors gave rise to increased numbers of B-1 progenitors in vitro. Rybp inactivation did not change global H2A monoubiquitination and did not interact genetically with Ring1A or Ring1B deletion, so the authors conclude the role is most likely independent of RYBP's Polycomb affiliation 2.
gene-rybp records this correctly in prose and then had to attach it to cell-type-pro-b, because that was the nearest node that existed. This entry replaces that approximation. The cell-state-fetal-program-retention link in that entry is unaffected and remains the more interesting half of the result: losing RYBP produces a fetal-programme-retaining state in a fusion-free animal, which is the same phenomenology this graph attributes to infant KMT2A-rearranged disease reached by a completely different route.
Why this is not the same node as the human fetal PreProB progenitor
cell-type-fetal-preprob-cd10-negative already exists and is a human CD19+CD34+CD10-negative fetal progenitor, about one third of fetal B-progenitors and 0.53% (SD 0.24) of adult bone marrow, transcriptionally distinct from its adult counterpart and sharing an ontogeny-related programme with CD10-negative B-progenitor infant ALL blasts 3.
They are related concepts and not interchangeable:
| this node | cell-type-fetal-preprob-cd10-negative | |
|---|---|---|
| species | mouse | human |
| defined by | Hardy fractions: B220+ IgM- CD43+ BP-1- HSA-low, germline Ig | CD19+ CD34+ CD10-negative |
| ontogeny | adult bone marrow in the cited experiment | fetal liver then fetal bone marrow |
| lineage purity | NOT pure - contains pDC precursors and NK-marker cells 4 | B-lineage only in the cited assays |
Cell Ontology assigns CL:0002045 'fraction A pre-pro B cell' to the murine scheme, and the human node's own entry records that it had to borrow that term for want of a human-specific one. Writing this node gives CL:0002045 the entry it actually names and frees the human node from a term that was never about it.
The purity problem, stated because it invalidates marker-only claims
Plasmacytoid dendritic cells competent to make type I interferon were rigorously defined as a Ly-6C+ CD11c-low subset of the B220+CD19- CD43+ CD24-low bone marrow Fraction A; otherwise similar Ly6C-negative cells in the same gate express the NK markers DX5 and NK1.1. Those pDCs are present in the bone marrow of RAG1-/-, IL-7Ra-/- and Pax5-/- mice, which is about as clear as evidence gets that they are not B-lineage 4.
So any result reported for 'Fraction A' or 'pre-pro-B cells' on surface markers alone is a result for a mixture. A claim about this cell type should say whether the population was resolved further - by CD19, by Ly-6C, by Ig locus configuration, or by single-cell transcriptomics. This graph carries cell-type-plasmacytoid-dendritic-cell separately, and the overlap between these two nodes at the flow-cytometry gate is a real hazard rather than a curiosity.
| ontology | CL:0002045 |
|---|---|
| ontology note | CL:0002045 is 'fraction A pre-pro B cell' and is defined on the MURINE Hardy scheme, which is this node. cell-type-fetal-preprob-cd10-negative had to borrow CL:0002045 for want of a human-specific term and says so in its own entry; that borrowing is now visible rather than silent. |
| species | mouse |
| location | adult murine bone marrow (the compartment measured in the Rybp experiment); the fetal-liver equivalent is not what this node describes |
| position in hierarchy | CLP -> pre-pro-B (Fr. A) -> pro-B (Fr. B, Fr. C; D-J rearrangement begins) -> pre-B (CD43-negative; V-D-J complete, pre-BCR) |
| purity warning | The classical Fraction A gate is a mixture. pDCs were defined as a Ly-6C+ CD11c-low subset of it and are present in RAG1-/-, IL-7Ra-/- and Pax5-/- marrow (PMID 15728131). Any claim attributed to 'pre-pro-B cells' on surface markers alone is a claim about a mixed population; resolve by CD19, Ly-6C, Ig locus configuration or single-cell transcriptomics before transferring it. |
| shown for fusion | NO - nobody has looked. No KMT2A fusion has been expressed in, or measured in, a defined murine pre-pro-B/Fraction A population. The RYBP result that requires this node is in a fusion-free animal. |
| why this node exists | gene-rybp's only measured haematopoietic cell-type requirement is the loss of pre-proB cells on Rybp inactivation (PMID 26711264), and the entry had to attach it to cell-type-pro-b with the mismatch stated in prose because no pre-pro-B node existed. |
markers
[ "B220+ (CD45R)", "IgM-", "CD43/S7+", "BP-1-", "heat-stable antigen (CD24) low", "CD19- in the classical Fraction A gate", "immunoglobulin loci in GERMLINE configuration - the defining functional criterion" ]
lineage potential
[ "B-lineage for the CD19-competent B-lymphoid cells in the fraction", "the marker-defined fraction also contains plasmacytoid dendritic cell precursors and NK-marker-positive cells, which are NOT B-lineage" ]
relation to human nodes
{
"cell-type-fetal-preprob-cd10-negative": "the human fetal CD19+CD34+CD10-negative PreProB-progenitor - related concept, different species, different defining markers, different ontogenetic window. Not interchangeable with this node.",
"cell-type-pro-b": "the succeeding stage and the one KMT2A::AFF1 disease is classified at; gene-rybp previously pointed here for want of this node",
"cell-type-clp": "the upstream progenitor from which RYBP-deficient cells gave rise to increased B-1 progenitors in vitro"
}- If the pre-pro-B compartment is where a fetal-versus-adult B programme is decided, then any therapy that transiently depletes early B progenitors in an infant - and induction chemotherapy does exactly that - is perturbing the compartment in which the disease's defining fetal phenotype is set. Nothing has measured what the compartment looks like on recovery.
- A cell-of-origin argument built on this stage is an argument for a target cell that is essentially absent from adult marrow (0.53% in the human counterpart), which makes it very hard to obtain human material for validation and very easy to keep the hypothesis alive without testing it.
- The Fraction A gate contains plasmacytoid dendritic cell precursors. Any immunotherapy or depleting antibody chosen on 'pre-pro-B' markers in a mouse model will also hit the interferon-producing compartment, and the toxicity would not be predicted from the B-lineage framing.
- Is the pre-proB loss on Rybp inactivation a loss of B-lineage cells or a loss of the pDC/NK component that shares the Fraction A gate? Re-sorting on CD19 and Ly-6C, or scoring Ig locus configuration, would separate them, and neither was done.
- Does a KMT2A fusion do anything in a pre-pro-B cell? No fusion has been introduced into, or assayed in, a defined Fraction A population. Given that the human fetal PreProB progenitor is this graph's best marker-level match to infant KMT2A-rearranged blasts, the murine experiment - Mll-AF4 in sorted Fraction A versus sorted Fraction B/C - has an obvious design and has not been done.
- Is the murine Fraction A the right model for the human CD10-negative PreProB progenitor at all? The two are defined by non-overlapping marker sets and the human one carries a fetal ontogeny-related programme absent from its own adult counterpart, so the cross-species mapping is an assumption this graph currently makes without evidence.
- RYBP's effect was reported as Polycomb-independent - no change in global H2AUb and no genetic interaction with Ring1A or Ring1B. If RYBP holds the B-2 programme by a non-Polycomb mechanism, what is that mechanism, and does it touch anything the KMT2A fusion does?
- E1established confidence 5 of 5: Multiple independent lines of evidence; treated as settled in the field.paper
IN NORMAL MOUSE BONE MARROW, B220+ IgM- B-lineage cells resolve by CD43(S7), BP-1 and heat-stable antigen into fractions in which the CD43-negative fraction is pre-B cells and the CD43-positive Fractions A, B and C precede them; the immunoglobulin genes of Fraction A are in germline configuration whereas Fractions B and C are pro-B stages with increasing D-J but no V-D-J rearrangement, the ordering Fr. A to Fr. B to Fr. C to pre-B is supported by phenotypic change in stromal culture, and the proliferative response to IL-7 is restricted to the CD43-positive stages.
Resolution and characterization of pro-B and pre-pro-B cell stages in normal mouse bone marrow. · Hardy RR, Carmack CE, Shinton SA, Kemp JD, Hayakawa K · J Exp Med 1991What this evidence cannot show
Mouse, and a 1991 flow-cytometry scheme built on three antibodies; the Ig-configuration evidence is bulk PCR on sorted fractions, not single-cell. The fractions were explicitly called 'provisional' by the authors and the scheme has been repeatedly refined since. It is cited here as the definition the murine literature uses, which is what makes the RYBP claim readable, not as a modern characterisation of the compartment. Read: abstract only.
Evidence that argues against this (1)
- Fraction A as defined by these markers is not a B-lineage population. Plasmacytoid dendritic cells were rigorously defined as a Ly-6C+ CD11c-low subset of B220+CD19- CD43+ CD24-low bone marrow Fraction A, with otherwise similar Ly6C-negative cells expressing the NK markers DX5 and NK1.1, and those pDCs are present in RAG1-/-, IL-7Ra-/- and Pax5-/- marrow. A marker-defined Fraction A phenotype can therefore belong to a cell outside the B lineage entirely. Read: abstract only. Pelayo R et al. 2005
- E2strong confidence 4 of 5: Replicated in peer-reviewed work by more than one group.paper
IN MICE, conditional inactivation of Rybp in adult haematopoiesis reverts the B-1-to-B-2 B-cell progenitor ratio toward the innate, predominantly fetal B-1 precursors, and the increased numbers of B-1 progenitors correlated with a LOSS OF PRE-PROB CELLS, the B-2 progenitors; RYBP-deficient LKS populations and isolated common lymphoid progenitors gave rise to increased B-1 progenitors in vitro, while Rybp inactivation did not change global H2A monoubiquitination and did not interact genetically with Ring1A or Ring1B deletion.
Role of Polycomb RYBP in Maintaining the B-1-to-B-2 B-Cell Lineage Switch in Adult Hematopoiesis. · Cales C, Pavon L, Starowicz K, Perez C, Bravo M, Ikawa T, et al., Vidal M · Mol Cell Biol 2015What this evidence cannot show
Murine conditional knockout in normal adult haematopoiesis: no fusion, no leukaemia, no human cells. The pre-proB loss is reported as a correlation with the B-1 progenitor increase rather than as an independently quantified primary endpoint in the abstract, and the pre-proB population is identified by the standard marker scheme, which is impure (see E1 counter-evidence). The authors' own conclusion is that the role is most likely independent of RYBP's Polycomb affiliation, which weakens any read-across to this graph's PRC1 biology. Read: abstract only.
Evidence that argues against this (1)
- The stage assignment depends on a marker-defined population that is known to contain non-B cells. Since B220+CD19-CD43+CD24-low bone marrow Fraction A includes pDC precursors and NK-marker-positive cells, a reported loss of 'pre-proB cells' by flow cytometry could in principle be a loss of a non-B-lineage component of the same gate. Nothing in the Rybp work resolves the population by Ig locus configuration or by CD19, which is the measurement that would settle it. Read: abstract only. Pelayo R et al. 2005
- E3strong confidence 4 of 5: Replicated in peer-reviewed work by more than one group.paper
IN HUMAN FETAL TISSUE, two distinct CD19+ B-progenitors exist - an adult-type CD10-positive ProB-progenitor and a CD10-negative PreProB-progenitor - with PreProB-progenitors forming almost one third of fetal B-progenitors but only 0.53% (SD 0.24) of adult bone marrow; single-cell transcriptomics and functional assays place fetal PreProB upstream of ProB as the first B-lymphoid-restricted progenitor of human fetal life, and fetal PreProB-progenitors display a distinct ontogeny-related gene expression pattern absent from adult PreProB-progenitors that is shared with CD10-negative B-progenitor infant acute lymphoblastic leukaemia blasts.
Discovery of a CD10-negative B-progenitor in human fetal life identifies unique ontogeny-related developmental programs. · O'Byrne S, Elliott N, Rice S, Buck G, Fordham N, Garnett C, et al., Roy A · Blood 2019What this evidence cannot show
This is the HUMAN cell and is cited here only to establish that the murine and human 'pre-pro-B' concepts are related but distinct, and that the human one already has its own node. The human population is defined by CD19/CD34/CD10, not by the Hardy fractions, and the transcriptomic sharing with infant ALL blasts is a similarity claim, not a demonstration of cell of origin. Identifiers re-fetched from PubMed for this entry rather than copied from cell-type-fetal-preprob-cd10-negative. Read: abstract only.
Evidence that argues against this (1)
- A shared transcriptomic signature between a normal fetal progenitor and a leukaemic blast does not establish that the progenitor is the cell of origin, and in this disease the blast's phenotype is known to be instructed by the fusion rather than inherited from the target cell: a retroviral MLL-Af4 in human CD34-positive cells generates a model that recapitulates the immunophenotypic and molecular features of t(4;11) pro-B ALL, which is an argument that the fusion imposes the phenotype on whatever cell it lands in. Read: abstract only. Lin S et al. 2016
- E4strong confidence 4 of 5: Replicated in peer-reviewed work by more than one group.paper
IN MURINE BONE MARROW, plasmacytoid dendritic cells competent to make type I interferon were rigorously defined as a Ly-6C-positive, CD11c-low subset of the B220+CD19- CD43+ CD24-low Fraction A; otherwise similar Ly6C-negative cells in the same fraction express the NK markers DX5 and NK1.1, and pDCs are present in the bone marrow of RAG1-/-, IL-7Ra-/- and Pax5-/- mice.
Derivation of 2 categories of plasmacytoid dendritic cells in murine bone marrow. · Pelayo R, Hirose J, Huang J, Garrett KP, Delogu A, Busslinger M, Kincade PW · Blood 2005What this evidence cannot show
Mouse bone marrow. The point being taken from this paper is narrow and negative - that the Fraction A gate is impure - rather than its positive claims about pDC ontogeny, which belong in cell-type-plasmacytoid-dendritic-cell. Read: abstract only.
Evidence that argues against this (1)
- Impurity of the gate does not make the stage unreal. The Ig-locus criterion is independent of surface markers: Fraction A cells have germline immunoglobulin genes while Fractions B and C carry increasing D-J rearrangement, and that is a property of the B-lineage cells in the gate regardless of what else shares it. A population can be operationally contaminated and developmentally genuine at the same time. Read: abstract only. Hardy RR et al. 1991
Rybp inactivation in adult mouse haematopoiesis causes LOSS of pre-proB cells, the B-2 progenitors, with a reciprocal increase in B-1 progenitors. A requirement, not a residency claim: no RYBP expression measurement in this cell is cited
the immediately preceding stage: Fraction A has germline immunoglobulin genes, Fractions B and C are pro-B with increasing D-J. gene-rybp pointed at pro-B for want of this node
- analogous to →Fetal CD19+CD34+CD10-negative PreProB progenitor — a cell that exists before birth and almost not at all after
the human fetal counterpart concept - CD19+CD34+CD10-negative rather than Hardy-fraction-defined, fetal rather than adult. Related and NOT interchangeable; the human node borrowed CL:0002045, which is this node's term
upstream progenitor; isolated RYBP-deficient CLPs gave rise to increased B-1 progenitors in vitro
- co-occurs with →Plasmacytoid dendritic cell — the interferon cell that KMT2A-rearranged leukaemia occasionally becomes, and is more often mistaken for
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