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Switching ON Fetal B Lymphopoiesis

Kristiansen, Trine LU (2018) In Lund University, Faculty of Medicine Doctoral Dissertation Series 2018(73).
Abstract
B-1a cells are innate-like lymphocytes that develop primarily during fetal and neonatal life, whereas adult bone marrow (BM) hematopoietic stem cells (HSCs) preferentially give rise to follicular B-2 cells. Functioning at the interface of the innate and adaptive immune systems, B-1a cells provide a non-redundant first line of defense prior to the temporally delayed establishment of a B-2 cell response. The underlying causes for the developmental attenuation in B-1a potential remain poorly resolved. HSCs undergo a functional switch in neonatal mice hallmarked by a decrease in self-renewing divisions and entry into quiescence. The timing of this switch around 3 weeks of age correlates with the change in B cell output from B-1a potent to... (More)
B-1a cells are innate-like lymphocytes that develop primarily during fetal and neonatal life, whereas adult bone marrow (BM) hematopoietic stem cells (HSCs) preferentially give rise to follicular B-2 cells. Functioning at the interface of the innate and adaptive immune systems, B-1a cells provide a non-redundant first line of defense prior to the temporally delayed establishment of a B-2 cell response. The underlying causes for the developmental attenuation in B-1a potential remain poorly resolved. HSCs undergo a functional switch in neonatal mice hallmarked by a decrease in self-renewing divisions and entry into quiescence. The timing of this switch around 3 weeks of age correlates with the change in B cell output from B-1a potent to predominantly B-2 restricted.
We hypothesized that the cellular basis for this developmental attenuation in B-1a cell output is a consequence of a shift in stem cell state during ontogeny. Using cellular barcoding for in vivo single-cell resolution analyses, we found that fetal liver definitive HSCs gave rise to both B-1a and B-2 cells. To directly assess whether a developmental shift in HSC state can lead to a selective loss in B-1a potential on a per cell basis, we performed longitudinal comparison of repopulation potential by following barcoded founder cells across serial transplantations. Whereas B-1a potential diminished over time, B-2 output was maintained. B-1a potential could be reinitiated in a subset of adult HSCs by ectopic expression of the RNA binding protein LIN28B, a key regulator of fetal hematopoiesis. This coincided with the clonal reversal to a fetal-like elevated self-renewal and repopulation potential. These results anchor the attenuation of B-1a cell output to fetal HSC behavior and demonstrate that the developmental decline in regenerative potential represents a reversible HSC state.
While these data made clear that developmentally restricted hematopoietic origins cannot fully account for the postnatal decline in B-1a output, the underlying mechanism for the positive selection and output of B-1a cells remains elusive. Recent studies showed that ectopic expression of Lin28b in adult pro-B cells was sufficient to potentiate fetal-like B-1a cell output. This led us to next hypothesize that Lin28b may play an important role during the latter part of B lymphopoiesis to potentiate the positive selection of B-1a cells early in life. We showed that CD5 levels of B-1 cells are developmentally set in the immature B cell stage and correlates with self-reactivity. Genetic perturbation studies show that Lin28b is necessary and sufficient for efficient positive selection of B-1a cells and potentiates neonatal immature B cell CD5 expression in a dose dependent fashion. Importantly, our results uncouple positive selection from specific B cell receptor identities, implicating the heterochronic RNA-binding protein LIN28b as the missing link that regulates the developmental attenuation in B-1a cell output through relaxing the permissiveness of B cell selection. Our findings shed light on the unique ability of B-1a cells to escape tolerance and undergo T cell like positive selection.
Finally, with ongoing investigations of developmental changes in chromatin accessibility between fetal and adult HSCs we have started to dissect the layers in regulation of a fetal HSC state. Interestingly, we find that regulation of the fetal HSC transcriptome relies more on a post-transcriptional layer compared to adult HSCs. This is consistent with the fetal specific expression pattern of the post-transcriptional regulator Lin28b.
Collectively this thesis work has elucidated fetal HSC state and Lin28b associated mechanisms in the attenuation of B-1a cell output during the transition from fetal to adult B lymphopoiesis. (Less)
Please use this url to cite or link to this publication:
author
supervisor
opponent
  • professor Cumano, Ana, Paris
organization
publishing date
type
Thesis
publication status
published
subject
keywords
B-1 B cells , Lin28, Fetal hematopoietic stem cells
in
Lund University, Faculty of Medicine Doctoral Dissertation Series
volume
2018
issue
73
pages
75 pages
publisher
Lund University: Faculty of Medicine
defense location
Segerfalksalen, BMC A10, Sölvegatan 17 i Lund
defense date
2018-05-24 09:00:00
ISSN
1652-8220
ISBN
978-91-7619-639-7
language
English
LU publication?
yes
additional info
ISSN: 1652-8220 Lund University, Faculty of Medicine Doctoral Dissertation Series 2018:73
id
39628366-ea68-464d-b9d1-a05076883779
date added to LUP
2018-05-02 12:29:50
date last changed
2020-03-13 10:32:06
@phdthesis{39628366-ea68-464d-b9d1-a05076883779,
  abstract     = {B-1a cells are innate-like lymphocytes that develop primarily during fetal and neonatal life, whereas adult bone marrow (BM) hematopoietic stem cells (HSCs) preferentially give rise to follicular B-2 cells. Functioning at the interface of the innate and adaptive immune systems, B-1a cells provide a non-redundant first line of defense prior to the temporally delayed establishment of a B-2 cell response. The underlying causes for the developmental attenuation in B-1a potential remain poorly resolved. HSCs undergo a functional switch in neonatal mice hallmarked by a decrease in self-renewing divisions and entry into quiescence. The timing of this switch around 3 weeks of age correlates with the change in B cell output from B-1a potent to predominantly B-2 restricted. <br/>We hypothesized that the cellular basis for this developmental attenuation in B-1a cell output is a consequence of a shift in stem cell state during ontogeny. Using cellular barcoding for in vivo single-cell resolution analyses, we found that fetal liver definitive HSCs gave rise to both B-1a and B-2 cells. To directly assess whether a developmental shift in HSC state can lead to a selective loss in B-1a potential on a per cell basis, we performed longitudinal comparison of repopulation potential by following barcoded founder cells across serial transplantations. Whereas B-1a potential diminished over time, B-2 output was maintained. B-1a potential could be reinitiated in a subset of adult HSCs by ectopic expression of the RNA binding protein LIN28B, a key regulator of fetal hematopoiesis. This coincided with the clonal reversal to a fetal-like elevated self-renewal and repopulation potential. These results anchor the attenuation of B-1a cell output to fetal HSC behavior and demonstrate that the developmental decline in regenerative potential represents a reversible HSC state. <br/>While these data made clear that developmentally restricted hematopoietic origins cannot fully account for the postnatal decline in B-1a output, the underlying mechanism for the positive selection and output of B-1a cells remains elusive. Recent studies showed that ectopic expression of Lin28b in adult pro-B cells was sufficient to potentiate fetal-like B-1a cell output. This led us to next hypothesize that Lin28b may play an important role during the latter part of B lymphopoiesis to potentiate the positive selection of B-1a cells early in life. We showed that CD5 levels of B-1 cells are developmentally set in the immature B cell stage and correlates with self-reactivity. Genetic perturbation studies show that Lin28b is necessary and sufficient for efficient positive selection of B-1a cells and potentiates neonatal immature B cell CD5 expression in a dose dependent fashion. Importantly, our results uncouple positive selection from specific B cell receptor identities, implicating the heterochronic RNA-binding protein LIN28b as the missing link that regulates the developmental attenuation in B-1a cell output through relaxing the permissiveness of B cell selection. Our findings shed light on the unique ability of B-1a cells to escape tolerance and undergo T cell like positive selection.<br/>Finally, with ongoing investigations of developmental changes in chromatin accessibility between fetal and adult HSCs we have started to dissect the layers in regulation of a fetal HSC state. Interestingly, we find that regulation of the fetal HSC transcriptome relies more on a post-transcriptional layer compared to adult HSCs. This is consistent with the fetal specific expression pattern of the post-transcriptional regulator Lin28b. <br/>Collectively this thesis work has elucidated fetal HSC state and Lin28b associated mechanisms in the attenuation of B-1a cell output during the transition from fetal to adult B lymphopoiesis.},
  author       = {Kristiansen, Trine},
  isbn         = {978-91-7619-639-7},
  issn         = {1652-8220},
  language     = {eng},
  number       = {73},
  publisher    = {Lund University: Faculty of Medicine},
  school       = {Lund University},
  series       = {Lund University, Faculty of Medicine Doctoral Dissertation Series},
  title        = {Switching ON Fetal B Lymphopoiesis},
  url          = {https://lup.lub.lu.se/search/ws/files/42490856/TAK_Kappa.pdf},
  volume       = {2018},
  year         = {2018},
}