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Y. Hirata et al.
Transfer of these CD150high Treg and CD150high nonTreg, but not of CD150low Treg or CD150neg-low nonTreg, reversed the increase of HSC in CD73 KO mice. Additionally, phar- macological activation of A2AR, highly expressed by HSC, reversed the increase of HSC in CD73 KO mice. Taken together, these results suggest that CD73 of CD150high Treg and CD150high nonTreg regulates HSC qui- escence and abundance via A2AR.
To the best of our knowledge, this is the first study showing the role of conventional T cells in HSC regula- tion. This work is complemented by our recent study7 showing that CD39 on CD150high Treg played a critical role in maintaining HSC quiescence. As both CD150high Treg and CD150high nonTreg frequently displayed an effec- tor memory T-cell phenotype,7 the observations of our current and previous studies7 suggest that BM CD4 mem- ory T cells and memory Treg coordinate each other to generate extracellular adenosine via CD39 and CD73, maintaining HSC quiescence. As the BM is known to be a site to which memory T cells frequently home and in which they are maintained,9,10 memory T cells and Treg generated following infection may play important roles in protecting BM HSC from oxidative and inflammatory stresses, controlling hematopoiesis. As CD150high Treg fre- quently localized adjacent to HSC, a future histological analysis is warranted to identify the spatial distribution of CD150high nonTreg (CD3+CD4+NK1.1-FoxP3-) with respect to HSC (CD150+CD48-CD41-Lin-), although such a study is technically challenging because of the requirement of multiple colors.
Our study does not rule out the possibility that HSC are regulated by other adenosine receptors or by P2 receptors that bind ATP metabolized by CD39. HSC expression of CD39, but not of CD73, may reflect the possibility that tight control of ATP/adenosine ratios is required for the maintenance of HSC quiescence. Indeed, a previous study showed that global P2YR deletion abrogated the radiore- sistance of HSC.16 However, under normal conditions,
P2YR KO mice did not show significant alteration of HSC numbers,16 suggesting that the observed phenotypes in CD73 KO mice and FoxP3cre CD39fl/wt mice under normal conditions were not attributable to P2YR.
CD150high nonTreg and CD150high Treg are likely to gen- erate adenosine in concert with various CD39+ or CD73+ BM cell populations, including HSC and the following two niche constituents: CD31+ vasculature and CD140a+CD140b+CD45- mesenchymal cells which exclu- sively overlap with lepr+ perivascular cells.2,3 Nevertheless, these two niche constituents are unlikely to be the major source of adenosine, because the frequencies of CD39high and CD73high cells in these mesenchymal cells were com- parable to those in CD150low Treg and transfer of these lat- ter cells did not alter HSC numbers in CD73 KO mice. Indeed, our additional study using leprcre CD39fl/wt mice showed that conditional deletion of CD39 in lepr+ cells did not alter HSC number or reconstituting potential of BM cells (Online Supplementary Figure S4A-C). This observation further supports the important role of CD150high Treg and CD150high nonTreg in adenosine-mediated HSC regulation.
In summary, this work showed that CD150high Treg and CD150high nonTreg maintain HSC quiescence via CD73. An examination of the roles of adenosine and memory T cells in human hematopoiesis and transplantation is war- ranted.
Acknowledgments
This work was supported by NIH NHLBI R01HL129506 (JF), an ASH Junior Faculty Scholar Award (JF), a Schaefer Research Scholar Award (JF) and an Uehara Memorial Foundation Research Fellowship Award (YH). Research report- ed in this publication was performed in the CCTI Flow Cytometry Core, supported in part by the Office of the Director, National Institutes of Health under award S10OD020056. The content is solely the responsibility of the authors and does not necessarily represent the official views of the National Institutes of Health.
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