Immune responses to human tumors: development of tumor vaccines

Strong evidence has been accumulated demonstrating that tumor cells in humans and animal are recognized in general as non-self by the immune system and they are able to induce an immune response which often leads to their elimination. In humans, this evidence includes: (a) The development of T-cell lines and clones with antitumor activity (cytotoxic or helper) which is restricted to autologous tumor cells or to cells expressing the same tumor peptide/HLA epitope; (b) the presence of oligoclonal T cells infiltrating many tumors; (c) the identification and molecular cloning of tumor antigens and of peptides derived from these antigens, which elicit HLA-restricted immune responses. Their discovery provided the ultimate proof for the presence of specific immune responses in human tumors. The availability for the first time of molecularly cloned tumor antigens permitted the development of peptide or recombinant tumor vaccines. Although significant progress has been made and tumor peptide vaccines capable of eliciting biological responses in more than 50% of the patients and objective clinical responses in 10 to 42% of the patients have been reported, certain major problems remain and need to be resolved in order to develop effective tumor vaccines. These problems emanate from the following mechanisms that the tumor cells are employing to avoid detection and destruction by the immune system: (i) Down-regulation of HLA class I expression on the surface of tumor cells; (ii) Down-regulation of tumor antigen expression or selection of negative tumor variants; (iii) Expression of naturally occurring altered peptide ligands by tumor cells; (iv) Lack of costimulatory molecules on tumors cells; (v) Production of immunosuppressive cytokines, such as TGF-beta and IL-10; (vi) Induction of lymphocyte apoptosis by tumor cells using the Fas/Fas L pathway; (vii) Down-regulation or absence of CD3 zeta (zeta) transcripts or protein in tumor-infiltrating lymphocytes (TIL), and others. The selection of optimal tumor antigens for vaccine development is another issue that requires attention. Lineage specific or differentiation antigens appear to be better candidates for the development of tumor vaccines because they are expressed in all tumor cells. Methods for antigen presentation, such as those using dendritic cells, also play a critical role in the development of tumor vaccines. In addition to the progress towards the development of tumor vaccines, substantial progress has been made in developing advanced methods of adoptive immunotherapy based on TIL. This approach can be effective when an immune response can not be elicited in vivo. The progress made towards the development of tumor vaccines and approaches for adoptive immunotherapy has been substantial. Additional studies need to be carried out to develop new and effective tumor vaccines and adoptive immunotherapy methods.