Induction of broad repertoire of melanoma associated antigen-specific CD4+ T cells by dendritic cell vaccine loaded with killed allogeneic melanoma cells in patients with metastatic melanoma

Abstract

3029 Background: Tumor-specific CD4+ T cells play an important role in antitumor immunity. CD4+ T cells recognizing melanoma associated antigens can be spontaneously generated in patients with metastatic melanoma as demonstrated by the infiltration of such cells at tumor sites; however, they are often dysfunctional. We analyzed tumor-specific CD4+ T cell repertoires in the blood of metastatic melanoma patients, and examined whether dendritic cell (DC) vaccine loaded with killed allogeneic melanoma cells administered to metastatic melanoma patients can induce and/or improve melanoma-specific CD4+ T cell immunity. Methods: PBMCs were obtained from ten metastatic melanoma patients who received at least four monthly DC vaccine injections (GM-IL-4 DC vaccine loaded with killed allogeneic melanoma cells, matured with CD40L and TNFa). Melanoma-specific T cell repertoires at baseline and after vaccinations were assessed with EPIMAX approach using overlapping peptide libraries spanning MART-1, gp100, and TRP-1 antigens. The frequencies of CD4+ T cells responding to the identified peptides were assessed by intracellular cytokine detection assay with flow cytometry. Results: At baseline, four out of ten patients displayed gp100-specific CD4+ T cells which secreted IFNg and/or IL-2, while none displayed MART-1-specific CD4+ T cells. Nine gp100 CD4+ T cell epitopes were identified in total. The T cells lacked proliferative capacity (seven out of nine gp100-specific T cells) in spite of cytokine secretion in response to peptide stimulation. We found PD-1 expression on specific CD4+ T cells which lacked the proliferative capacity. Analysis of post-vaccine showed the presence of proliferative gp100-specific CD4+ T cells in four patients. One of these patients experienced a complete response and another one a long-term survival. The frequencies of specific CD4+ T cells secreting IL-2 and/or IFNg were increased at post-vaccine. Functional MART-1 and TRP-1-specific CD4+ T cells were also induced in both patients. Conclusions: DC vaccine loaded with killed tumor cells can prime broad repertoire of tumor-specific CD4+ T cells as well as revert dysfunction of pre-existing specific T cells. No significant financial relationships to disclose.