To understand why cancer vaccine-induced T cells often fail to eradicate tumors, we studied immune responses in mice vaccinated with gp100 melanoma peptide in incomplete Freund’s adjuvant (IFA), commonly used in clinical cancer vaccine trials. Peptide/IFA vaccination primed tumor-specific CD8+ T cells, which accumulated not in tumors but at the persisting, antigen-rich vaccination site. Once there, primed T cells became dysfunctional and underwent antigen-driven, Interferon-γ (IFN-γ) and Fas ligand (FasL)-mediated apoptosis, resulting in hyporesponsiveness to subsequent vaccination. Provision of anti-CD40 antibody, Toll-like receptor 7 (TLR7) agonist and interleukin-2 (IL-2) reduced T cell apoptosis but did not prevent vaccination site sequestration. A non-persisting vaccine formulation shifted T cell localization towards tumors, inducing superior anti-tumor activity while reducing systemic T cell dysfunction and promoting memory formation. Persisting peptide/IFA vaccine depots can induce specific T cell sequestration, dysfunction and deletion at vaccination sites; short-lived formulations may overcome these limitations and result in greater therapeutic efficacy of peptide-based cancer vaccines.

Type I IFNs, including IFN-α, enhance Ag presentation and promote the expansion, survival, and effector function of CD8+ CTL during viral infection. Because these are ideal characteristics for a vaccine adjuvant, we examined the efficacy and mechanism of exogenous IFN-α as an adjuvant for antimelanoma peptide vaccination. We studied the expansion of pmel-1 transgenic CD8+ T cells specific for the gp100 melanocyte differentiation Ag after vaccination of mice with gp10025–33 peptide in IFA. IFN-α synergized with peptide vaccination in a dose-dependent manner by boosting relative and absolute numbers of gp100-specific T cells that suppressed B16 melanoma growth. IFN-α dramatically increased the accumulation of gp100-specific, IFN-γ-secreting, CD8+ T cells in the tumor through reduced apoptosis and enhanced proliferation of Ag-specific CD8+ T cells. IFN-α treatment also greatly increased the long-term maintenance of pmel-1 CD8+ T cells with an effector memory phenotype, a process that required expression of IFN-α receptor on the T cells and IL-15 in the host. These results demonstrate the efficacy of IFN-α as an adjuvant for peptide vaccination, give insight into its mechanism of action, and provide a rationale for clinical trials in which vaccination is combined with standard-of-care IFN-α therapy for melanoma.