Complement in Cancer and Cancer Immunotherapy

Recently, there has been an increase of interest in the use of biological or immune-based therapies for patients with malignancies. This has been informed by the deeper understanding of the crosstalk between the host immune system and malignant tumours, as well as the potential advantages of immunotherapy—high specificity and less toxicity compared to standard approaches. The particular emphasis of this article is on the role of the complement system in tumour growth and antibody-based cancer immunotherapy. The functional consequences from overexpression of complement regulators by tumours and the development of strategies for overcoming this are discussed in detail. This review discusses these issues with a view to inspiring the development of new agents that could be useful for the treatment of cancer.     

Cancer genomics

Almost all cells in the human body contain a complete copy of the genome with an estimated number of 25,000 genes. The sequences of these genes make up about three percent of the genome and comprise the inherited set of genetic information. The genome also contains information that determines when and where in the organism a given gene is active. This is the epigenetic information. Genomics is the study of DNA sequences and the epigenetic information of gene regulation.     

Cancer Immunotherapy

This review summarises the evolution of recent major advances in cancer immunotherapy, using metastatic melanoma as a model. The first true clinical progress with immunotherapy developed from the application of recombinant DNA technology for the large scale production of immunostimulant cytokines. Clinical trials demonstrated that the systemic administration of recombinant high-dose bolus intravenous interleukin-2 (IL-2; 720 000 IU/kg every 8 hours) mediated objective tumour progression in 20% of patients with metastatic renal cancer and in 17% of patients with metastatic melanoma, with complete responses of 9% and 7%, respectively. The use of adoptive immunotherapy (the transfer of immune cells with anti-tumour activity to the tumour-bearing host) focused interest on T lymphocyte-mediated tumour recognition. Clinical trials described the systemic administration of lymphokine activated killer (LAK) cells and subsequently tumour infiltrating lymphocytes (TIL) to patients with advanced cancer. Although able to kill tumour targets in vitro, LAK cells did not prove useful for the treatment of patients with metastatic melanoma and renal cancer. A randomised trial, in which IL-2 was administered alone or with LAK cells, failed to show a difference in response rate or survival. In contrast, the treatment of 86 patients with metastatic melanoma using TIL plus IL-2 resulted in a 34% objective response rate, which included patients who had previously failed treatment with high-dose IL-2 alone. The focus on cellular immune responses, combined with rapid biotechnological advances, resulted in the identification of tumour specific antigens, such as MART-1 and gp100, that could be recognised by autologous TIL. This provided fundamental evidence of the existence of melanoma-associated antigens that were recognised in vivo by effector cells of the immune system. In vitro studies demonstrated immunodominant epitopes from MART-1 and gp100 that could induce in vitro-specific cytotoxic T lymphocyte reactivity. To enhance in vitro immunogenicity, single amino acid substitutions were made to identify peptides with higher affinity for HLA-A*0201. Modified peptides from gp100 were compared with the parental peptide for increased immunogenicity based on their ability to induce anti-tumour lymphocytes in vitro. From these studies, a candidate peptide was identified (G9-209-2M) which had increased immunogenic reactivity in vitro. Clinical trials demonstrated that the modified G9-209-2M peptide was more effective. Unfortunately, objective tumour regression was still low. However, when high-dose IL-2 was combined with G9-209-2M objective clinical responses increased to 42%. Efforts to find better ways to immunise against self antigens are ongoing and involve further peptide immunisations, as well as recombinant viral vectors, adjuvant cytokine therapy and cellular adjuvants such as dendritic cells.     

Cancer vaccines

1993 represents the 100th anniversary of William Coley's first report of tumor regressions induced by immune system activation in response to bacterial toxins. While many subsequent cancer vaccine trials have yielded tantalizing results, active immunotherapy has not yet become an established modality of cancer therapy. Newer molecular vaccine approaches based on rational immunological principles have resulted in improved systemic antitumor effects in animal models. Ultimately the genetic definition of tumor-specific antigens will allow the development of targeted antigen-specific vaccines for cancer therapy.