CD4+ T cell fate in multiple myeloma

Abstract

The average age of onset of multiple myeloma (MM) is in the 7th decade coinciding with the development of immunosenescence. As a consequence, lymphocyte recovery after lymphopenia-inducing anti-MM therapies (most notably autologous stem cell transplant, ASCT) is reliant on homeostatic proliferation of peripheral T cells rather than replenishment by new thymic emigrants. It has been shown that there is a significant reduction in circulating naïve T cells with a reciprocal expansion of antigen-experienced cells from newly diagnosed MM (NDMM) to relapsed/refractory disease (RRMM). This results in a reduced T cell receptor repertoire and the accumulation of senescence-associated secretory phenotype cytotoxic T cells, which retain IFNg production but lose proliferative potential. A reduction in CD4:8 ratio is characteristic of MM disease progression, due to high IL-15 levels in lymphopenic states preferentially driving expansion of CD8+ memory T cells.

The first section of this thesis describes changes that occur in the T cell population in MM from diagnosis and throughout treatment, and compares with age-matched healthy donors (HD) and RRMM. ASCT and lymphopenia-inducing therapies in MM have a largely irrecoverable effect on the CD4+ T cell population, with a permanent reduction in CD4:8 ratio and loss of naïve T cells. Furthermore, there is a substantial and sustained increase in the proportion of PD-1 expressing T cells post-ASCT. Regulatory T cells increase with treatment in NDMM and RRMM patients. Utilising bisulfite sequencing, it is demonstrated that these cells are peripherally- rather than thymically-derived, which has implications for the stability of the regulatory T cell population.

In the second part of this thesis, RNA-Seq is utilised to investigate differences in CD4+ T cells from NDMM, RRMM and HD. These results demonstrated upregulation of mitochondrial biogenesis and metabolic pathways involved with mitochondrial respiration. Using qPCR of key rate-limiting enzymes in metabolism, it is shown that enzymes associated with fatty acid beta-oxidation were significantly increased in T cells in RRMM compared with HD, and this was associated with high mitochondrial mass in all differentiation subsets. Mitochondrial membrane potential was elevated in CD4+ T cells from RRMM compared with HD and this appeared to be mainly due to the central memory (TCM) T cell subset, suggesting that TCM remain a metabolically active subset, although reactive oxygen species (ROS) levels are dampened compared to HD. The relevance of these findings for dendritic cell vaccination and chimeric antigen receptor (CAR)-T cell production is discussed, both of which are being pursued in the immunotherapy of MM.

The third part of this thesis evaluates how well the Vk*MYC mouse model of MM and in vitro PBMC co-culture systems represent T cell pathology in human MM, and their utility in assessing immune therapies and for personalised medicine. The shortcomings of these model systems is outlined, and how they might be utilised in the future is discussed.

The thesis concludes with a re-evaluation of how we could treat MM by fitting treatment to immune profile and disease state, in order to best utilise immune therapies and avoid further detrimental effects to the immune system.