Melanoma antigen gp100 (280-288)

Degradation of the tumor antigen epitope gp100(280-288) (YLEPGPVTA) was investigated in the presence of cultured human fibroblasts, and acellular supernatants obtained from these cells; the possible effect of substrate degradation on in vitro immunorecognition was also addressed. In the presence of fibroblasts, gp100(280-288) was degraded to free amino acids with a half-life of less than 4 min; hydrolysis data support the hypothesis that substrate degradation was mainly caused by the activity of cell-expressed amino- and carboxypeptidases. Gp100(280-288) was also degraded in the presence of acellular supernatants: under these conditions, the hydrolysis pattern was similar to that observed in the presence of whole cells, but degradation kinetics was slower. As a result of these phenomena, immunorecognition of gp100(280-288)-specific cytotoxic T lymphocyte (CTL) clones was severely hampered, and was totally suppressed after 90 min. In conclusion, the high activity of fibroblast-expressed proteases, and the presence of wide-scope soluble enzymes, may explain, at least in part, the low activity of peptide-based antineoplastic vaccines, as well as the transient effectiveness of subcutaneously administered peptides in general.

A melanoma vaccine composed of HLA-A2-restricted peptide YLEPGPVTA (gp100(280)), with or without a modified T-helper epitope from tetanus toxoid AQYIKANSKFIGITEL, has been evaluated in a Phase I trial to assess safety and immunological response. The vaccines were administered s.c. in either of two adjuvants, Montanide ISA-51 or QS-21, to 22 patients with high-risk resected melanoma (stage IIB-IV). Local and systemic toxicities were mild and transient. We detected CTL responses to the gp100(280) peptide in peripheral blood in 14% of patients. Helper T-cell responses to the tetanus helper peptide were detected in 79% of patients and had a Th1 cytokine profile. One patient with a CTL response to gp100 had a recurrence in a lymph node 2 years later; her nodes contained CD8+ cells reactive to gp100(280) (0.24%), which proliferated in response to peptide. The overall survival of patients is 75% (95% confidence interval, 57-94%) at 4.7 years follow-up, which compares favorably with expected survival. Four of 14 patients who completed at least six vaccines subsequently developed metastases, all of which were solitary and surgically resectable. They remain alive and clinically free of disease at last follow-up. Data from this trial demonstrate immunogenicity of the gp100(280) peptide and suggest that immune responses may persist long-term in some patients. The frequency and magnitude of the CTL response may be improved with more aggressive vaccination regimens. Although this Phase I study was not intended to evaluate clinical benefit, the excellent survival of patients on this protocol suggests the possibility of a benefit that should be assessed in future studies.

Dendritic cells (DCs) pulsed with unfractionated tumor cell lysates or defined tumor peptides provide potent vaccines which elicit strong antitumor immunity. In this study, we generated DCs from the 2-h adherent progenitor cells obtained from the peripheral blood of melanoma patients. These DCs were able to capture biotinylated melanoma tumor cell lysates. We examined the efficacy of immunogens composed of DCs loaded either with the melanoma peptide gp100 [amino acids 280-288 (DC/gp100)] or with lysates from melanoma tumor cells (DC/lysates) in inducing cytotoxic T-cells from autologous PBLs of HLA-A2 melanoma patients. After four to five weekly stimulations of bulk PBLs with DC/gp100 or DC/lysates, the cultures were enriched with CD3+ T-cells and exhibited one of three phenotypic and functional patterns: (1) Predominant expression of CD8+ and MHC class I-restricted CTLs which displayed strong lytic activity against melanoma cells and T2 cells loaded with the gp100 peptide, (2) mixed CD4+/CD8+ phenotype and weak lytic activity, or (3) nonlytic and predominantly CD4+ cultures. Interestingly, T-cell cultures from each patient exhibited similar phenotypes and lytic activities whether the stimulant was DC/gp100 or DC/cell lysates. Our study demonstrates that DCs pulsed with soluble melanoma peptides or cell lysates are capable of inducing CD8+ CTLs from autologous PBLs of some, but not all, melanoma patients. The function and phenotype of the generated T-cell cultures are governed by DCs since both antigens (the gp100 peptide and melanoma lysates), when presented by a given DC preparation, induced similar T-cell cultures. In summary, it may be difficult to predict the nature of the cellular responses elicited by DC/tumor antigen vaccines from patient to patient.