Designing Strategies to Improve the T Cell Mediated Immunotherapy of Mouse Tumours.
The adoptive transfer of activated dendritic cells (DC) loaded with tumour antigen or tumour specific T cells improves weak anti-tumour responses, however, without treatments to relieve suppression, these therapies will continue to fall short of their full potential. The aim of this thesis was to understand the role of hypoxia-induced increases in adenosine and of CD4+ CD25+ Foxp3+ regulatory T cells (Treg) in the suppression of anti-tumour immune responses and to design strategies to abrogate these mechanisms. These aims were investigated using the B16.OVA murine melanoma model because the OVA specific CD4+ (OTII) and CD8+ (OTI) T cell transgenic mice allowed detailed investigation of Ag specific T cell responses. Recent studies have shown that the inhibition of adenosine signalling in activated CD8+ T cells can improve the anti-tumour activity of these cells. To investigate these findings using the B16.OVA model, tumour-bearing mice were given activated OTI T cells and the adenosine receptor inhibitor caffeine. Caffeine treatment did not improve the anti-tumour response, possibly because this response was suppressed due to the increased frequency of myeloid derived suppressor cells observed in mice that received T cells. To determine whether the defective function of tumour infiltrating DC (TIDC) in tumours is due to suppression by Treg, mice were treated with the anti-CD25 monoclonal antibody PC61 to deplete Treg and challenged with tumours. PC61 treatment caused a delay in tumour growth but did not affect DC frequency, or expression of the DC activation markers CD40, CD86 and MHC II in tumours or lymph nodes. DC function was tested using in vitro and in vivo T cell proliferation assays and was found to be unaffected by PC61 treatment. Studies in RAG1-/- mice, which lack Treg, also showed no improvement in DC activation status or function. These results show that Treg do not suppress TIDC in the B16.OVA model. It is well known, however, that Treg suppress T cell responses and it has been suggested that Treg may mediate some of this suppression by using the perforin-granzyme pathway to cause T cell death. To investigate this possibility, naive, perforin sufficient OTI T cells were transferred into normal and perforin knockout (PKO) mice, with or without PC61 treatment. To stimulate an OTI T cell response, mice also received OVA-loaded DC. Depletion of both normal and PKO Treg resulted in decreased death and increased proliferation of the transferred cells, increased expression of IFN-y and TNF-a, and improved in vivo target cell killing by the transferred cells. These findings indicate that perforin expression by Treg is not required to suppress T cell responses or cause T cell death. In conclusion, the results of this thesis were consistent with the observation that there are multiple suppressive mechanisms in tumours and that there is substantial redundancy of these mechanisms. Depletion of Treg was found to improve the anti-tumour response, however, suppression of the DC was still evident, demonstrating that the neutralisation of a single suppressive mechanism may not be sufficient to treat aggressive, late stage cancers such as melanoma.