epitope (see presentation by Ronald Schwartz, above). Adjuvants containing helper epitopes can be very important, but when peptides are pulsed into dendritic cells, helper epitopes seem to be less critical. Consequently, researchers would be able to immunize numerous patients, each with a different mutation of p53 requiring a unique peptide—challenge enough—without also having to attach a helper epitope to each unique peptide.
Tumor Immunotherapeutic Experiments. To test the ability of the mutant peptide vaccine to treat a mouse with an established tumor, researchers injected tumor cells, waited 8 days until they could see or palpate tumor nodules that were 2 to 4 millimeters in diameter, and then immunized with peptidepulsed dendritic cells either (1) a single time or (2) repeatedly every 4 or 5 days. The single immunization did not change the rate of growth of established tumors, but multiple immunizations significantly inhibited tumor growth. This protection lasts as long as immunizations continued; when immunization stopped, the tumors eventually began growing again. The animals were not cured, but the tumors were suppressed as long as researchers kept boosting their immunity.
Based on these results, researchers have started a clinical trial with human subjects. They performed a biopsy of the patient’s tumor to determine if there was a mutation of p53 or ras, synthesized the corresponding peptide, pulsed it onto autologous PBMCs, and then reinfused the cells into the patient to immunize. This trial is still at an early stage, and it would be premature to report any results, but in most cases the patients had bulky tumors and extensive prior treatment with chemotherapy, which has left them with very poor immune systems (e.g., many are unable to make a CTL response to flu). However, researchers have shown the safety of this approach, and a few patients with less severe disease they have seen hints of either cytokine or CTL responses. This clinical trial is now moving into a new stage involving patients with less tumor bulk, or no tumor bulk, in whom investigators expect to see a better response because the immune system is more intact. This new stage may also immunize with larger numbers of cells, or with purified dendritic cells.
Fusion Proteins as Target Antigens. A similar approach has been tried in a different disease system found in Ewing’s sarcoma and alveolar rhabdomyosarcoma (AR). These two pediatric sarcomas involve chromosomal translocations that create fusion proteins. For example, in about 90 percent of AR patients, there is a fusion between the PAX3 and FKHR genes, both of which are transcriptional regulators. In a chromosomal translocation between chromosomes 2 and 13, the DNA-binding domains of PAX3 are juxtaposed with the activation domain of FORCO, creating an aberrant transcription factor that is believed to be causative in this sarcoma. However, this break point also creates a potential neoantigenic determinant.
Researchers therefore asked whether there was a similar type of fusion in Ewing’s sarcoma, which involves translocations between chromosomes 11 and