Abstract
The postnatal thymus is an efficient microenvironment for T cell specification and differentiation. B cells are also present in the thymus and have been recently shown to impact T cell selection, however, the mechanisms controlling B cell development in the thymus are largely unknown. In Foxn1lacZ mutant mice, down-regulation of Foxn1 expression in thymic epithelial cells beginning 1 week after birth caused a dramatic reduction of T progenitors and an increase of B cell progenitors. This time point is coincident with the switch from fetal to adult-type hematopoietic stem cells (HSCs), which is regulated by the Lin28-Let7 system. We hypothesize that the thymic environment might regulate this process to suppress fetal-type B cell development in the thymus. In this study we show that in the Foxn1lacZ thymus, although the down-regulation of Lin28 in thymocytes was normal, up-regulation of Let-7 was impaired. The failure to up-regulate Let-7 caused a transient increase of Arid3a in B precursors, which is known to promote fetal-type B cell fate. Over-expression of Lin28a in HSCs also reduced Let-7 and promoted Arid3a expression in BM and thymic B progenitors, increasing B cell production in the thymus. The level of Let-7 in thymic B progenitors was up regulated by in vitro co-culture with IL15, Vitamin-D3, and retinoic acid, thus down-regulating Arid3a to promote B cell differentiation. All of these signals were produced in thymic epithelial cells (TECs) related to Let-7 expression in thymic B progenitors, and down-regulated in Foxn1lacZ mutants. Our data show that signals provided by TEC control thymic B cell development by up-regulating Let-7, suppressing Arid3a expression in intrathymic progenitor B cells to limit their proliferation during the neonatal to adult transition.
Highlights
Hematopoietic stem cells (HSCs) undergo a developmental program change during ontogeny including changes in hematopoiesis sites, self-renewal activities, gene expression profiles, lineage biases, and differential intrinsic properties and differentiation potentials [1,2,3]
To test whether the thymic microenvironment controls the lineage potential of thymic seeding progenitors (TSPs) during the fetal to adult HSC phenotypic switch, we analyzed whether the increased B cells in the Z/Z thymus in the perinatal period were of the B-1a or B-2 type
Consistent with these data, Arid3a expression was increased, peaking at day 28 in Z/Z B progenitors at 3.4-fold higher than in controls (Fig 1F). These results indicated that the progenitor B cells in Foxn1lacZ thymus retained a more fetal like profile compared to the same cells in +/Z controls
Summary
Hematopoietic stem cells (HSCs) undergo a developmental program change during ontogeny including changes in hematopoiesis sites, self-renewal activities, gene expression profiles, lineage biases, and differential intrinsic properties and differentiation potentials [1,2,3]. Two distinguishable properties of HSCs have been defined as specific characteristics of fetal (FL-HSCs) and adult (BM-HSCs) [3,4]. The switch from fetal to adult type HSC profiles has been. Thymic epithelial cell-derived signals control B development by regulating Let-7 and Arid3a
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