Defining novel targets for immunotheraphy of human cancer
Cancer cells frequently exhibit defects in apoptosis, contributing to increased survival and resistance to chemotherapy. This can result from decreased expression of pro-apoptotic proteins such as Bax, through abnormal proteasomal degradation. The central hypothesis of this project is that this abnormal proteasomal degradation in cancer cells will result in the generation of peptides that bind to MHC class I molecules, and displayed at the cell surface for CD8+ T cell recognition. If this is correct, then T cells directed against Bax peptides should be able to recognize and kill human cancer cells. To test this hypothesis, candidate HLA-A2 binding peptide epitopes were identified from Bax and T cell immunogenicity tested using IFNy ELISpot assays. Positive T cell responses were detected against Bax peptides in 10/16 healthy donors. The specificity of a CD8+ T cell clone (KSIVB17) derived from one donor, was mapped to two similar peptide epitopes, Baxi36-144 (IMGWTLDFL) and Bax135144 (TIMGWTLDFL). In addition to recognition of peptide pulsed target cells, this clone was also able to recognise primary tumour cells from CLL (chronic lymphocytic leukaemia) patients, and kill cervical carcinoma and osteosarcoma cell lines. T cell reactivity correlated with abnormal proteasomal degradation of Bax in cancer cells. Attempts to generate Bax-specific T cells in CLL patients demonstrated that naive and memory T cell responses were compromised compared to healthy donors. This compromised T cell immunity appeared to result from increased expression of immunosuppressive molecules, increased frequency of Treg cells and a skewed memory T cell phenotype in CLL patients. Overall, this research supports the novel concept that proteins such as Bax, which are abnormally degraded in cancer cells, can be targets for CD8+ T cells. However use of Bax in immunotherapy will require strategies to overcome suppression of T cell responses in CLL and other cancers.