Background: CD40L/IFN-γ matured Dendritic Cells (DCs) produce IL-12 and are potent antigen-presenting cells for naïve resting T cells. We sought to determine the magnitude and kinetics of CD8+ T cell growth in patients receiving autologous CD40L/IFN-γ matured DC and identify biomarkers associated with clinical outcome. Methods: A phase I clinical trial (NCT00683670) incorporating CD40L/IFN-γ for the ex vivo maturation of autologous DCs pulsed with three well characterized gp100 melanoma antigen derived peptides (G154, G209-2M, G280-9V) was initiated with enrollment from 2008-11 at a single center. HLA-A*0201+ individuals with treatment naïve metastatic melanoma were immunized every 3 weeks by intravenous infusion for six doses after a single dose of cyclophosphamide (300 mg/m² iv). CT imaging was performed at baseline, week 9 and 18 for clinical assessment using RECIST. Responding patients were eligible for maintenance doses every 2-4 months. PBMC were taken weekly for immune monitoring by tetramer analysis and functional assays. DC preparations were characterized to assess for biomarkers of response. Results: 10 patients were screened. Among the 7 treated patients, there were 3 confirmed responses (independently verified), including one durable CR >3 years and 2 PR. Three patients had rapid disease progression and received only 3 doses. Four patients (1 CR, 2 PR, 1 PD) received 6 or more vaccine doses. No SAEs were noted. There was no correlation between tumor volume and response. Using pre-specified immune response criteria, 6 (86%) treated patients developed CD8+ T cell immunity to all three peptides as assessed by tetramer analysis. The vaccine-induced T cells from all 6 individuals were polyfunctional and killed gp100+, HLA-A2+ human melanoma targets in a standard ⁵¹Cr release assay. IL-12 production by DCs correlated with TTP (p=0.0198, likelihood ratio test) but not OS (p=0.08). Conclusions: Weekly immune monitoring reveals the rapid onset of CD8+ T cell immunity against gp100 among the responder patients. This is the first DC vaccine clinical trial in melanoma to demonstrate a correlation of IL-12 production and TTP.