Post-transcriptional (re)programming of B lymphocyte development: From bench to bedside?

Abstract

Hematopoiesis, a process which generates blood and immune cells, changes significantly during mammalian development. Definitive hematopoiesis is marked by the emergence of long-term hematopoietic stem cells (HSCs). Here, we will focus on the post-transcriptional differences between fetal liver (FL) and adult bone marrow (ABM) HSCs. It remains unclear how or why exactly FL HSCs transition to ABM HSCs, but we aim to leverage their differences to revive an old idea: in utero HSC transplantation. Unexpectedly, the expression of certain RNA-binding proteins (RBPs) play an important role in HSC specification, and can be employed to convert or reprogram adult HSCs back to a fetal-like state. Among other features, FL HSCs have a broad differentiation capacity that includes the ability to regenerate both conventional B and T cells, as well as innate-like or unconventional lymphocytes such as B-1a and marginal zone B (MzB) cells. This chapter will focus on RNA binding proteins, namely LIN28B and IGF2BP3, that are expressed during fetal life and how they promote B-1a cell development. Furthermore, this chapter considers a potential clinical application of synthetic co-expression of LIN28B and IGF2BP3 in HSCs.

Keywords: B lymphocyte development; Fetal hematopoiesis; Hematopoietic stem cells; In utero transplantation; Innate-like lymphocytes; MicroRNA; Post-transcriptional regulation; RNA-binding protein.

Figure 1.

Waves of hematopoiesis during mouse ontogeny. This over-simplified schema depicts some of the waves of hematopoiesis during mouse ontogeny. Primitive or HSC-independent hematopoiesis starts in the yolk sac, with erythron-myeloid progenitors giving rise to the first wave of erythrocytes and certain macrophage subsets, such as brain-resident microglial cells. Hematopoietic stem cells (HSCs) born in the aorta-gonad-mesonephros (AGM) region migrate to the fetal liver where they establish it as the primary site for definitive hematopoiesis prior to birth. These fetal liver HSCs eventually migrate to the bone marrow around the time of birth. The bone marrow remains the primary site for hematopoiesis in postnatal life.

Figure 2.

Let-7 miRNAs are regulated coordinately in adult vs. fetal progenitor B cells. Digital nanoString analysis was performed to systematically compare the miRNA expression profiles of pro-B cells from adult bone marrow versus fetal liver. The differentially expressed members of mature let-7 microRNAs are highlighted (red). The nanoString data are available from the Gene Expression Omnibus (GEO) database (http://www.ncbi.nlm.nih.gov/geo) under the accession number GSE35107. a.u., arbitrary units.

Figure 3.

Inverse relationship of LIN28B and let-7 miRNAs. LIN28B is highly expressed in FL HSCs and lowly expressed in adult BM HSCs. Conversely, levels of mature let-7 miRNAs are high in adult BM HSCs and low in FL HSCs. Indeed, these two factors comprise a double negative feedback loop wherein LIN28B represses the biogenesis of mature let-7 miRNAs post-transcriptionally, and in turn let-7 miRNAs can post-transcriptionally repress LIN28B expression by directly binding to evolutionarily conserved sites in its 3’ UTR.

Figure 4:

Multi-layered origins of mature B-1a cells. The majority of B-1a cells are thought to arise from hematopoietic progenitor cells (HPCs) in the yolk sac and fetal liver and then the mature B-1a cells can self-renew throughout life. Fetal-derived B-1a cells in mice are less diverse in part because TdT is not expressed prior to birth and thus they harbor V(D)J rearrangements that lack N-nucleotides. In contrast, mature B-2 cells harbor a diverse V(D)J repertoire, cannot self-renew, and require constant replenishment from adult BM HSPCs. As a second developmental pathway, TdT-expressing bone marrow progenitors can also generate B-1a cells during postnatal life albeit not as efficiently as fetal liver progenitors. As a result, N-nucleotides are incorporated during V(D)J recombination when such cells are developing. Finally, it has been shown that altering the BCR specificity on mature B-2 cells can promote conversion into B-1a cells (Graf et al., 2019). However, it is not known whether such a conversion of mature B cell fate can happen naturally.

Figure 5.

Lin28b may promote B-1a cell development via inhibition of let-7 and/or independently of let-7. Lin28b blocks the maturation of let-7 microRNAs, either by inhibiting processing by Drosha in the nucleus or by enhancing oligo-uridylation and Dis312-mediated degradation of pre-let-7 in the cytoplasm. The absence of mature let-7 microRNAs allows for expression of targets including Hmga2, Lin28b, Igf2bp1, Igf2bp3, Arid3a, Arid3b, and H19. Though the Lin28b-let-7 axis is well documented, Lin28b can also function independently of let-7 (Wang et al., 2019). Indeed, there may be additional let-7-independent pathways by which Lin28b may promote the development of B-1a cells. Only some examples are shown here.