There have been great advances in clinical immunotherapies against cancer over the past two decades. Most recently, immunotherapies directed against immuno-inhibitory molecules such as CTLA4 and PD1 have led to major therapeutic breakthroughs, rapidly becoming an integral part of clinical practice. Yet we still lack complete understanding of how these therapies work, how to increase their efficacy, and how to limit their often life-threatening side effects. Furthermore, we lack comprehensive understanding of the immune landscape in cancer. Antitumour responses incorporate a large number of cell types with a complex range of differentiation states, which cannot be readily determined in patients. Mouse models are more suitable to explore specific mechanisms of antitumour immunity.
The work in this thesis utilises tumour-specific T cells and B cells from five different transgenic mouse lines, to explore how their interactions influence murine tumour growth. A model of CD4+ T cell mediated antitumour immunity was established, in which regulatory T cell, CD8+ T cell or B cell populations could be co-transferred into tumour-bearing hosts to observe how their interactions influenced tumour clearance. In this model, CD4+ T cells and regulatory T cells recognise tumour antigen indirectly on antigen presenting cells within the tumour bed, rather than the malignant cells themselves, whereas B cells and CD8+ T cells are capable of direct antigen recognition.
Indirect antigen recognition was sufficient to drive potent tumour rejection by naive CD4+ T cells adoptively transferred into immunodeficient mice bearing subcutaneous tumours. Tumour clearance was associated with extensive T cell proliferation and Th1 cytokine production, whereas delayed relapse observed in some animals was associated with conversion of CD4+ T cells into immunosuppressive regulatory T cells. Poor tumour clearance was observed when naive CD4+ T cells were co-transferred into tumour-bearing mice together with tumour-specific natural regulatory T cells.
The role of B cells and antibodies in antitumour immunity is still not fully understood. In some human cancers, the presence of tumour-infiltrating B cells and tumour-specific antibodies correlates with prolonged survival, whereas both positive and negative effects have been reported in animal models. The primary immune role of B cells is to produce antibodies, but they can also influence T cell function via antigen presentation and, in some contexts, immune regulation. Here we describe the multifaceted role of tumour-specific B cells in an in vivo model, addressing their role in antibody production, antigen presentation and immune suppression, and identifying the contexts in which they function to inhibit or promote tumour growth. The number of lung metastases was significantly decreased in the presence of tumour-specific antibodies, with the amount of IgG2a/c correlating most closely with protection. In contrast, antibody was not protective against subcutaneous tumours, even in a prophylactic setting. In short-term experiments, B cells were capable of presenting tumour-derived antigen to CD4+T cells, although less efficiently than myeloid antigen presenting cells. The efficiency of B cell antigen-presentation was increased if B cells were activated, or when memory CD4+T cells were used in place of naive CD4+T cells. In long-term experiments, the presence of tumour-specific B cells reduced the number of high affinity tumour-specific CD4+ T cells, which in turn decreased their ability to eradicate subcutaneous tumours. Failure to eradicate tumours was also associated with an increased proportion of induced Foxp3+ regulatory T cells within the tumour-specific CD4+ T cell population. However, the absolute number of induced regulatory T cells was independent of number of tumour-specific B cells and tumour growth.
Finally, four novel transplantable tumour lines were generated in order to address the cooperative role of CD4+ T cells and CD8+ T cells in anti-tumour immune responses. Recognition of tumour-derived antigen by transgenic CD4+ and CD8+ T cells was confirmed.
In summary, a novel animal model was established to study cooperation between tumour antigen-specific CD4+ T cells, regulatory T cells, B cells and/or CD8+ T cells. For the first time, the dual antitumour and immunosuppressive functions of B cells were characterised within a single model. The results reported in this thesis advance our understanding of lymphocyte populations in antitumour immunity and can easily be adapted to study the effects of immune checkpoint inhibitors in a pre-clinical setting.