SCIENCE sciencemag.org 13 MARCH 2020 • VOL 367 ISSUE 6483 1206-B
STEPS TOWARD STABLE EX VIVO
HSC EXPANSION
To test this hypothesis, I took a reduction-
ist approach to optimize media conditions
that we had previously shown could main-
tain mouse HSCs for 7 days and supported
approximately twofold functional expan-
sion ( 11 ). The results were dramatic. Using
our new “cleaned-up” culture condition, we
were able to grow HSCs for more than 2
months ex vivo and achieve ~900-fold ex-
pansion of functional HSCs ( 10 ).
The biggest factor in establishing optimal
ex vivo HSC expansion conditions was the
removal of serum albumin. As a major blood
component, serum albumin has long been
considered an essential supplement for he-
matopoietic cell cultures. However, the use
of serum or serum albumin introduces nu-
merous biologically active impurities ( 11 ).
In our HSC cultures, even recombinant se-
rum albumin contained contaminants that
HSCs were highly sensitive to and that in-
hibited HSC self-renewal.
I discovered that serum albumin can
be entirely replaced with the synthetic
polymer polyvinyl alcohol within our HSC
media. The use of this biologically inert al-
bumin replacement to remove these HSC
differentiation-inducing contaminants dra-
matically improved HSC expansion ( 10 ).
This substitution—combined with the right
levels of stem cell factor, thrombopoietin, and
fibronectin—enabled us to grow bulk popu-
lations of HSCs in long-term cultures and
to expand single HSCs clonally. Using clone-
splitting assays, we were even able to defini-
tively prove ex vivo HSC self-renewal ( 10 ).
This new HSC culture system also pro-
vided an important clinical insight. When
large numbers of ex vivo–expanded HSCs
(generated from just 50 freshly isolated
HSCs) were transplanted into noncondi-
tioned recipient mice, durable engraftment
was achieved. We are now working to dem-
onstrate the efficacy of this intervention in
combination with CRISPR/Cas9–based au-
tologous HSC gene correction.THE NEXT STEPS
Our mouse HSC expansion system offers a
valuable technology with which to uncover
new biological mechanisms and to develop
therapeutic approaches for regenerative med-icine and cell therapy. Additionally, our recent
progress in expanding mouse HSCs has also
provided an important roadmap with which
to achieve stable ex vivo human HSC expan-
sion, a route that will, I hope, ultimately lead
to safer clinical HSCT therapies. jREFERENCES AND NOTES- M. Evans, Nat. Rev. Mol. Cell Biol. 12 , 680 (2011).
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(2018). - D. P. Dever, M. H. Porteus, Curr. Opin. Hematol. 24 , 481
(2017). - Y. Ta y a et al., Science 354 , 1152 (2016).
- A. C. Wilkinson, M. Morita, H. Nakauchi, S. Yamazaki,
Exp. Hematol. 63 , 12 (2018). - A. Czechowicz, D. Kraft, I. L. Weissman, D. Bhattacharya,
Science 318 , 1296 (2007). - D. Bhattacharya, D. J. Rossi, D. Bryder, I. L. Weissman,
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ACKNOWLEDGMENTS
I thank my supportive mentors and collaborators, in particular
H. Nakauchi, S. Yamazaki, K. Loh, D. Dever, and R. Ishida. I also
thank my research funders, The Leukemia and Lymphoma
Society, Bloodwise, and the Japan Society for the Promotion
of Science.10.1126/science. aba6108Published by AAAS