Improving immunotherapy for high grade glioma
Glioblastoma multiforme (GBM) is a malignant primary brain tumour that is almost always fatal. Conventional treatment modalities are limited by toxicity. T cell-based immunotherapy is a promising alternative that has the potential to specifically target tumour cells. The author of this thesis was a principal investigator for a recently completed Phase I clinical trial in which patients with recurrent GBM were treated with surgery, dendritic cell-based immunotherapy and chemotherapy. In addition to conducting the trial in collaboration with others, the author used peripheral blood mononuclear cells from trial participants to assess anti-tumour immune responses before and after treatment. A broad correlation was observed between clinical outcome and anti-tumour immunity, with sustained progression-free survival occurring in two patients with baseline responses that persisted or increased after treatment. However, the overall clinical benefit was modest. For progress to be made, there is a need to develop a more potent vaccine. With this in mind, a novel “Glioma-Gal” vaccine was devised and tested in an orthotopic mouse model of glioma, This simple vaccine consisted of irradiated autologous tumour cells pulsed with the glycolipid alpha-galactosylceramide, an immunoadjuvant that induces invariant Natural Killer T cells to licence endogenous dendritic cells. The vaccine was shown to be effective in a therapeutic setting when accompanied by depletion of regulatory T cells. Mechanistically, vaccine efficacy was dependent on CD4 T cells and the mediastinal lymph node was shown to be an important site of T cell priming. It was further shown that components of the immune system necessary for the vaccine to work were present and competent in a cohort of GBM patients. The final chapters explore the idea of enhancing the therapeutic benefit of this vaccine by targeting certain tumour cell subsets or phenotypes. Cancer stem cells (CSC) are proposed to be a subset of tumour cells with a unique capacity for initiating and maintaining tumours. Eliminating these cells may therefore be both necessary and sufficient to achieve cure. Using the same mouse model, a variety of methods were assessed for their ability to isolate or enrich for a CSC subset. Of these, culture in serum-free medium in the presence of certain growth factors was shown to enrich for a more stem cell-like phenotype. However, a vaccine constructed from these stem cell-like cells was not more effective than the standard vaccine. Next, the author tested the hypothesis that a vaccine manipulated to target chemoresistant cells would be more effective than standard vaccine when used in combination with chemotherapy. However, the modified vaccine showed no advantage over standard vaccine in this model. In the course of these experiments, synergy was observed between the vaccine and the chemotherapy agent doxorubicin. The mechanism responsible for this supra-additive effect remains undetermined but is most likely due to an immunomodulatory effect of low dose doxorubicin. The Glioma-Gal vaccine design holds promise but more studies are needed to realise the full potential of this approach. The data presented in the thesis did not support targeting CSC or chemoresistant cells as ways of achieving this. In contrast, combining the vaccine with immunomodulation was effective and merits further exploration.