Conversion of mature B cells into T cells by dedifferentiation to uncommitted progenitors

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Conversion of mature B cells into T cells by
dedifferentiation to uncommitted
progenitors César Cobaleda, Wolfram Jochum
Meinrad Busslinger Presented by Li-Chen
Wu 10-05-2007
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Introduction
Stem cells leading to various blood lines. The
basic marrow stem cell differentiates during
early fetal development into two types of stem
cells, the lymphoid (which produces lymphocytes)
and the myeloid (which produces all the other
blood cells). Platelets (thrombocytes) are not
true blood cells, but are cytoplasmic fragments
of the megakaryocytes. T-cells are lymphoid
cells that differentiate via action of the thymus
gland. All the cell lines except erythrocytes
(red blood cells) and megakaryocytes are involved
with immune responses.
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T cells originate from the common lymphoid
progenitor cells in the bone marrow. They
migrate as immature precursor T cells via the
bloodstream into the thymus, which they
populate as thymocytes. The thymocytes go
through a series of maturation steps including
distinct changes in the expression of cell
surface receptors, such as CD4 and CD8, and the
rearrangement of their antigen receptor TCR. More
than 98 of the thymocytes die during
maturation by apoptosis (). In humans, the
vast majority of peripheral blood T cells
expresses TCRs consisting of a and ß chains (aß
T cells). A small group of peripheral T cells
bears an alternative TCR composed of ? and d
chains (?/d T cells). aß and ?d T cells diverge
early in T cell development. The aß T cells are
responsible for the classical helper or
cytotoxic T cell responses.
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Pax5 is the master-regulator of B cell lineage
commitment. It is expressed throughout B cell
development where is plays a dual role in
activating B cell specific genes while repressing
genes associated with the stem cell,
or alternative lineage, fates. Conditional Pax5
deletion results in retrodifferentiation of B
lymphocytes to an uncommitted progenitor cell
stage. As in its absence B cell progenitors, that
are blocked at the pro-B cell stage, maintain a
broad lympho-myeloid differentiation potential.
The uncommitted Pax5-/- Pro-B cells are able to
differentiate into several hematopoietic cell
types, either in vitro in the presence of
indicated cytokines or in vivo after
transplantation into recipient mice.
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Generation of a Conditional Pax5 Allele
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Spectral karyotype analysis of Pax5 ?/-
progenitor cell tumours
Tumours of Cd19-cre Pax5 F/ mice, were treated
with colcemid followed by staining of metaphase
chromosome spreads.
A specific translocation or high genomic
instability is not required for lymphoma
development in Cd19-cre Pax5 F/ mice.
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The expression of the indicated cell surface
proteins on mature B cells and tumour cells was
examined by flow cytometric analysis of lymph
node cells.
The Cd19-cre Pax5 fl/ lymphomas may correspond
to Pax5-deficient progenitor cell tumours.
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All tumours contained a single, in-frame
rearranged and expressed Igh allele ---- Clonal
origin
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The progenitor cell lymphomas in Cd19-cre
Pax5fl/ mice must originate at least from late
pre-B cells or, more probably, from immature or
mature B cells.
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Eµ-bcl2-36 mice A strain of transgenic mice that
carry a human bcl-2 cDNA under the control of
5lgh enhancer (Eµ) which exhibited abnormalities
in B lymphoid cells, Also displayed T lymphoid
abnormalities.
In vivo Pax5-deleted mature B cells were purified
as LinCD25IgMIgD cells from the lymph nodes
of Cd19-cre Pax5fl/ Eµ -bcl2 Ly5.2 mice
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These Ly5.2c-KitB220 pro-B cells were
identified as Pax5-deficient progenitors,
because they did not express the Pax5 target gene
Cd19.
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All Rag2/ Ly5.1 mice showed robust
reconstitution of T-cell development after
transplantation with in vitro or in vivo
Pax5-deleted B cells. Consistent with the fact
that Pax5-deficient progenitors from the bone
marrow efficiently seed the thymus to initiate
T lymphopoiesis.
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PCR analysis of Igh, Igk and Igl rearrangements
confirmed the B-cell origin of the Ly5.2
thymocytes. Southern Blot shown that sorted
Ly5.2 double-positive thymocytes carried
polyclonal Tcrb rearragements like normal T
cells.
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Serial transplantation experiments demonstrated
that the Ly5.2 pro-B cells isolated from a
primary reconstituted Rag2/ Ly5.1 mouse homed
back to the bone marrow, and reconstituted T-cell
development in the thymus and spleen after
injection into secondary Rag2/ c ?  /
recipients.
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The loss of Pax5 allows mature B cells to develop
into T cells in the thymus by dedifferentiation
to an uncommitted progenitor cell stage in the
bone marrow.
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The spleen of the reconstituted Tcra/ mice
contained polyclonal TCR-ß T cells carrying
wild-type Tcra and deleted Pax5 alleles. These
donor-derived T cells efficiently induced the
development of germinal centres and PNAFas
germinal-centre B cells.
-- The B-cell-derived T cells are fully functional
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Conclusion
1. The loss of a single transcription factor,
Pax5, allows mature B cells to
dedifferentiate in vivo to uncommitted
haematopoietic progenitors and to develop
into progenitor cell lymphomas. 2. Pax5
inactivation also induces the dedifferentiation
of committed pro-B cells. 3. The loss of
Pax5 in the context of strong BCR signaling
results in forward differentiation of
mature B cells to plasma cells, whereas
Pax5 inactivation in the absence of BCR signaling
initiates the reversal of differentiation
to uncommitted progenitors. 4. Pax5 can function
as a tumour suppressor or oncoprotein in the
generation of different lymphoid malignancies in
analogy to a similar dual role of Notch1 and
c-Fos in the development of distinct
cancers.