The lack of reproducible, quantitative assays for T-cell responses has been a limitation in the development of
cancer vaccines to elicit T-cell immunity. We utilized the Elispot assay, which allows a quantitative and functional assessment of T cells directed against specific
peptides after only brief in vitro incubations. CD8(+) T-cell reactivity was determined with an
interferon (IFN)-gamma Elispot assay detecting T cells at the single cell level that secrete IFN-gamma. We studied both healthy individuals and patients with
melanoma. Healthy
HLA-A*0201-positive individuals showed a similar mean frequency of CD8(+) cells recognizing a
tyrosinase peptide, YMDGTMSQV, when compared with
melanoma patients prior to immunization. The frequencies of CD8(+) cells recognizing the
tyrosinase peptide remained relatively constant over time in healthy individuals. Nine
HLA-A*0201-positive patients with stage IV metastatic
melanoma were immunized intradermally with the
tyrosinase peptide together with the immune adjuvant
QS-21 in a
peptide dose escalation study with 3 patients per dose group. Two patients demonstrated a significant increase in the frequency of CD8(+) cells recognizing the
tyrosinase peptide during the course of immunization, from approx. 1/16,000 CD8(+) T cells to approx. 1/4,000 in the first patient and from approx. 1/14,000 to approx. 1/2,000 in the second patient. These results demonstrate that modest expansion of
peptide-specific CD8(+) T cells can be generated in vivo by immunization with
peptide plus
QS-21 in at least a subset of patients with
melanoma.