B cells are a vital component of humoral immunity. Upon activation, their immunoglobulin B cell receptor (BCR) is secreted as antibody to neutralise pathogens or mediate the immune response. A diverse repertoire of immunoglobulin molecules is therefore essential for immune surveillance. As we age however, antibody diversity declines, as does our ability to mount a sufficient humoral immune response to vaccines and infections.

The process of generating the BCR, and its diversity therein, begins in the bone marrow. B cell progenitors undergo tightly-regulated recombination of different VH, DH, and JH gene segments of the immunoglobulin heavy chain (IgH) gene loci, followed by recombination of the Ig light chain loci, to produce the heavy (μ/Mu) and light (κ/λ) chain proteins that make up an immunoglobulin BCR (IgM). Signalling via the interleukin-7 receptor (IL7R) and the preBCR are required for successful progenitor B cell development in mice. There has also been mounting evidence implicating insulin-like growth factor 1 receptor (IGF1R) signalling to progenitor B cell development at the Pro-B and Pre-B cell stages.

Aged progenitor B cells exhibit reduced output, both in terms of cell number and immunoglobulin repertoire diversity. These defects are suggestive of disrupted differentiation, and correlate with downregulation of several IGF1R signalling components, which may also be regulated at multiple levels. However, given the inherent heterogeneity of the ageing process, it has not yet been fully elucidated that disruption of the IGF1R signalling pathway is a hallmark of ageing progenitor B cells.

I therefore hypothesise that, despite their heterogeneity, progenitor B cells from all aged individuals have impaired IGF1R signalling, and that this suppressed response to IGF1 contributes to their reduced biological output.

In this thesis, I aim to characterise the intrinsic defects of aged progenitor B cells through transcriptomic, V-DJH repertoire, and cell cycle analyses. To achieve this, I developed an innovative analytical approach for grouping individual aged mice based on the transcriptomic variability of their Pre-B cell populations, and demonstrate how this reflects the severity of their aged phenotype. To explore the role of IGF1 – the cognate ligand of IGF1R – in early B cell development, I developed a novel in-vitro system for differentiating bone marrow haematopoietic progenitors into B cells that preferentially undergo V(D)J recombination to express IgM in response to IGF1. With this system, it has been possible to demonstrate aged B cell developmental defects in response to IGF1, and highlight the B cell intrinsic nature of these aged defects through mixed chimeric cultures of young and aged cells. Utilisation through this in-vitro system of small molecule inhibitors to the histone H3K27 demethylase JMJD3, and the methyltransferase Ezh2, has also provided evidence to support a hypothesis of polycomb-mediated IGF1R signalling suppression in ageing B cell progenitors. Finally, I provide scope for clinical translation from these aged murine findings by demonstrating similar aged defects, including downregulation of IGF1R transcripts, in older human bone marrow-derived B cells.