Emerging evidence suggests the immune system has a role in preventing cancer, and in advanced cancer evidence of immune dysfunction is widespread. This project focused on cytotoxic T lymphocyte antigen 4 (CTLA4), a key negative regulator of T cell activation found on dedicated regulatory T cells (Treg) and activated T lymphocytes, and asked whether modulation of immune control with anti-CTLA4 blockade led to significant anti-tumour activity. Clinical and laboratory investigation of anti-CTLA4 blockade using tremelimumab in a phase II trial of second-line therapy in advanced oesophageal and gastric adenocarcinomas was combined with an attempt to establish a suitable pre-clinical model based on therapeutic vaccination against the tumour associated antigen (TAA) 5T4.Eighteen patients received tremelimumab. Most drug-related toxicity was mild but there was a single death due to bowel perforation. Four patients had stable disease with clinical benefit; one achieved a partial response after eight cycles (25.4 months) and remains well on study after four years.Markers of regulatory phenotype, forkhead box protein 3 (FoxP3) and CTLA4, doubled transiently in CD4+CD25high lymphocytes in the first month after tremelimumab before returning to baseline. In contrast, CTLA4 increased in CD4+CD25low/negative lymphocytes throughout the cycle of treatment. Post-treatment expanded Treg expressed FoxP3 without interleukin-2 and their defining suppressive function was not abolished despite prolonged anti-CTLA4 blockade. De novo proliferative responses to TAA 5T4 (8 of 18 patients) and carcinoembryonic antigen (CEA; 5 of 15) were detected. Patients with a post-treatment CEA proliferative response had median survival of 17.1 months compared to 4.6 months for non-responders (p=0.002). Baseline interleukin-2 release after T lymphocyte activation was higher in patients with clinical benefit and toxicity.Heterologous mouse 5T4 (m5T4) vaccination showed some evidence of weak therapeutic benefit, but all tumour models investigated had rapidly lethal kinetics. Specific m5T4 immune responses could be detected by serum antibody ELISA and IFN-gamma ELISPOT assays in naive animals but were lower frequency than published responses to h5T4, and were further attenuated in tumour-bearing animals. The addition of anti-CTLA4 blockade did not result in significant augmentation of m5T4 specific immunity after vaccination in non tumour-bearing animals and combination treatment was ineffective as therapy in this autologous model.Results are discussed in the context of emerging immunotherapeutics in melanoma and prostate cancer. In the absence of supportive data from the model system it would not be appropriate to pursue combination heterologous 5T4 vaccine with anti-CTLA4 blockade, but in view of the unusual durability of the best response to tremelimumab, and in vitro evidence of enhanced proliferative responses to relevant TAA, further investigation of drug activity may be warranted in metastatic gastric and oesophageal second-line treatment.