Vaccine Trials for the Clinician: Prospects for Tumor Antigens

Recent insights in antigen presentation, the identification of human tumor antigens, and the demonstration of MHC class‐I‐restricted cytotoxic T lymphocyte (CTL) recognition of peptides encoded by tumor antigen have renewed the interest and enthusiasm for the development of cancer vaccines. Melanoma serves as a paradigm of an immunogenic human tumor, and several tumor antigens, including MAGE, MART‐1/Melan‐A and gp100, recognized by CTLs, have now been isolated. Candidate antigens for novel vaccine trials may include HLA class‐I‐binding tumor peptides that serve as CTL epitopes, whole tumor protein, or DNA‐based vaccines. Requirements for the use of peptides are that the patient's tumor presents the relevant CTL epitopes as used in the vaccine and expresses the appropriate MHC class‐I‐restricting molecule. Immunological monitoring may be facilitated when using peptide‐based vaccines. Because optimal presentation of tumor antigens may depend on provision of appropriate costimulatory signals, it may be more advantageous to administer professional antigen‐presenting cells (APCs), such as dendritic cells (DCs) pulsed with tumor peptide or protein, to cancer patients. Developments in molecular genetics have led to a new approach in vaccines consisting of cancer cells genetically engineered to express immunomodulatory molecules. This may result in increased antitumor responses to both gene‐modified as well as unmodified tumor cells. The therapeutic approach is extended to vaccination trials with recombinant viruses containing the genes encoding tumor antigens, minigenes containing multiple CTL epitopes, or double recombinant vectors engineered to express both the tumor antigen and immunostimulatory molecules. Clinical peptide, protein, and DNA‐based vaccine trials have recently been initiated. Thus far, exciting clinical remissions were obtained in melanoma patients following vaccination with HLA‐A1‐binding MAGE‐3 peptide and in B‐cell lymphoma patients immunized with autologous DCs pulsed with anti‐idiotype protein, i.e., the individual patient's unique tumor antigen. Also, following injection of foreign HLA‐B7 DNA into cutaneous melanoma metastases, T‐cell migration into treated lesions and enhanced cellular immunity at the site of the tumor were shown in some patients. These encouraging results suggest that effective new vaccines in cancer will be identified.