Hematopoietic stem cell (HSC)
transplantation is an established therapy
for regenerating the immunohematopoietic
system of cancer patients whose bone
marrow (BM) has been eradicated by the
radiation and chemotherapy used to kill
their tumor. The most common sources of
HSCs are the BM, where the majority of
stem cells reside under steady-state
conditions, and the peripheral blood,
following treatment of patients with
growth factors that mobilize stem and
progenitor cells into the circulation. Stem
cell transplantation is not a physiological
process. There are no selective pressures
that enforce the evolution of mechanisms
regulating engraftment of intravenously
transplanted HSCs into radiation-ablated
marrow. Clinicians and scientists have
therefore puzzled over why HSC
transplantation should even work.
It is well known that HSCs traffic from
the yolk sac to the fetal liver during
embryogenesis, and then to the BM shortly
before birth. This process is mediated by
specific interactions between adhesion
molecules present on the surface of
hematopoietic cells and receptors
expressed by mesenchymal cells in the
marrow microenvironment. However, much
less is known about the trafficking patterns
of adult HSCs, and whether the small
number of stem cells that circulate in the
peripheral blood of normal individuals
contribute in any meaningful way to
hematopoiesis in nonablated marrow.
To examine the physiological role of
blood-borne HSCs, Wright
et al
. [1] created
parabiotic mice, in which the circulatory
systems of two genetically distinguishable
(CD45.1 and CD45.2) but otherwise normal
animals was joined by surgery. Cross
circulation was established within several
days, resulting in the stable 50% chimerism
of each parabiont with partner-derived
(CD45 congenic) leukocytes. Parabiosed
mice were then maintained for 7–39 weeks
before separation and analysis of cross
engraftment by circulating HSCs. All mice
exhibited low but significant levels (1–5%)
of partner-derived granulocytes in the
peripheral blood 7 weeks after separation.
Given that granulocytes are short-lived cells
(~1 day), their sustained presence provides
evidence of continual production from
engrafted, blood-derived long-term
repopulating HSCs. More direct evidence of
engraftment by circulating stem cells was
provided by the detection of partner-derived
Thy-1
ten
Lin
−
Sca-1
+
c-kit
+
cells, a phenotype
characteristic of primitive HSCs, in the BM
up to 22 weeks after separation.
This study shows that HSCs exit the BM,
circulate for short periods of time and then
reseed the marrow in unperturbed adult
animals and provides a physiological
rationale for the efficacy of stem cell
transplantation as a clinical therapy. A more
complete description of the developmental
potential of blood-borne HSCs awaits further
investigation. Nevertheless, this study
provides important insights into the
dynamic nature of HSC distribution in
unconditioned individuals and opens the
way for future molecular or pharmacological
manipulation of this process. For example, if
one could modulate the distribution patterns
of HSCs
in vivo
without using cytokines or
agents that stimulate stem and progenitor
cell proliferation and differentiation, as is
currently the case, then it might be possible
to direct hematopoietic cells to locations that
would spare them from the toxic effects of
organ-targeted radiation or chemotherapy.
Or, they could be directed to injured or
diseased non-hematopoietic organs that
might provide the stimulus for lineage-
switching and tissue regeneration, as
evidenced by recent studies of stem cell
plasticity. The intrinsic ability of HSCs to
‘gowith the flow’ clearly provides exciting
opportunities for future advances in stem
cell-based therapies.
1 Wright, D.E.
et al.
(2001) Physiological migration
of hematopoietic stem and progenitor cells.
Science
294, 1933–1936
Stephen J.Szilvassy
szilvas@pop.uky.edu
TRENDS in Biotechnology
Vol.20 No.3 March 2002
http://tibtech.trends.com 0167-7799/02/$ – see front matter © 2002 Elsevier Science Ltd. All rights reserved.
94
News&Comment
Journal Club
Hematopoietic stem cells go with the flow
Inter-kingdom sex: the mating of bacterial and mammalian cells
How does horizontal gene transfer, the
movement of DNA between organisms that
are not closely related, occur? The answer
to this question depends on which
organisms you are thinking about. But, until
recently, if you asked most biologists what
mechanism could be responsible for the
transfer of DNA from bacteria to higher
eukaryotes, the least likely answer would
have been conjugation – the transfer of DNA
from donor to recipient via a cellular bridge.
As the process of conjugation involves
contact between cells, the process is often
thought of as sexual in nature. Indeed,
bacteriologists frequently refer to the DNA
donor as male and the recipient as female.
The idea of eukaryote–prokaryote
conjugation thus has undertones of
inter-kingdom sex.
Now, a paper by Waters [1] presents
evidence for the conjugative transfer of DNA
from bacterial (
Escherichia coli
) to Chinese
Hamster ovary CHO K1 cells, in a carefully
constructed set of experiments using
plasmid shuttle vectors, containing genes
encoding antibiotic resistance, which can
move between bacteria and mammalian
cells and replicate within them. This was a
crucial element in the experiments: the
movement of genes between the cells
occurred only occasionally, and addition of
antibiotics to the medium meant that the
very small number of cells that picked up
these genes could be selected because the
antibiotics killed those cells that did not
receive the resistance genes.
Initial experiments investigated the
transfer of DNA from one
E. coli
strain
(DH5α) to another (HB101). The bacterial
donor (DH5α) also had a plasmid that
contained genes coding for the conjugative
apparatus. Not surprisingly, DNA transfer
was highly efficient but was not possible
when any of the genes specifying
conjugation were disrupted. A similar set of
experiments was conducted using
E. coli
cells and CHO K1 cells. In those mating
experiments in which the genes for
conjugative transfer were disrupted, no
CHO K1 cells were antibiotic resistant,
implying that they did not receive DNA