The Role of Langerin⁺ CD8α⁺ Dendritic Cells in Tumour Immunotherapy

The immune system has the potential to selectively target and eliminate tumours cells. However, the induction of an immunosuppressive environment by factors released by tumours cells, or by the tumour stroma, in combination with difficulties in differentiating between healthy and malignant cells, contributes to inefficient or disabled anti-tumour immune responses. A variety of different immunotherapeutic approaches are being developed to tip the balance in favour of anti-tumour immunity. Many of these approaches are designed to stimulate improved activity of T cells with specificity for tumour-associated antigens. This thesis explores how T cell-mediated responses are initiated and maintained in immunotherapy, with an emphasis on the role of antigen presentation by resident dendritic cells (DCs). An animal model was used in which a DC subset in the spleen that expresses the cell marker langerin could be selectively ablated during the course of therapy. As these DCs have been shown to be uniquely capable of acquiring circulating antigens and cellular debris, and have a heightened capacity for cross-priming CD8⁺ T cells, it was hypothesised that the function of these cells could play a significant role in determining the outcome of immunotherapies. A model of adoptive T cell therapy was examined in mice challenged with an intravenously administered lymphoma that formed tumour foci in a variety of locations in the body. Treating established tumours by adoptively transferring in vitro activated effector CD8⁺ T cells significantly increased their symptom-free survival. The protection received by this therapy was dependent on a stimulus being provided by endogenous langerin⁺ CD8α⁺ DCs to the transferred T cells. In the absence of langerin⁺ CD8α⁺ DCs, the proportion and number of transferred anti-tumour CD8⁺ T cells was lower in the blood and spleen. However, no obvious differences in phenotype and function could be defined. Langerin⁺ CD8α⁺ DCs therefore contribute to the maintenance of an effective CD8⁺ T cell-based immunotherapy and the role of endogenous DCs should be taken into consideration during the design of immunotherapies. To investigate the role of langerin⁺ CD8α⁺ DCs in initiating effector T cell responses, a novel whole-cell vaccine was developed for the treatment of acute myeloid leukaemia (AML). This vaccine exploited the stimulatory functions of invariant natural killer T cells, and was therefore administered intravenously to access the large invariant natural killer T cell compartment of the spleen. The vaccine completely protected mice from developing leukaemia when challenged with AML cells after vaccination, with CD4⁺ and CD8⁺ T cells mediating protection. The immune response generated by the vaccine was shown to be completely dependent on langerin⁺ CD8α⁺ DCs. In hosts with established tumours; however, the vaccine was ineffective. This may have been partially due to a reduced function of langerin⁺ CD8α⁺ DCs as their activation phenotype was significantly reduced in the presence of established AML; however, non-specific T cells could still be stimulated via these DCs. Reduced vaccine efficacy was associated with increased number and/or function of suppressor cells, including regulatory T cells and myeloid derived suppressor cells within the host. In addition, in leukemic hosts, the proportion of T cells in the spleen was reduced, and the function of AML-specific CD4⁺ T cells, but not CD8⁺ T cells, was impaired. Driving AML-bearing hosts into remission with chemotherapy prior to vaccination enabled the vaccine to protect the host from subsequent AML challenge. Langerin⁺ CD8α⁺ DCs are therefore responsible for initiating the vaccine-induced immune response in this model and their suppression may have contributed to the inefficacy of the vaccine in the presence of established tumours.