Disease-driven T cell activation predicts immune responses to vaccination against melanoma. 
Tumor vaccines may induce activation and expansion of specific CD8 T cells which can subsequently destroy tumor cells in cancer patients.  This phenomenon can be observed in approximately 5-20% of vaccinated melanoma patients.  We searched for factors associated with T cell responsiveness to peptide vaccines.  Peptide antigen-specific T cells were quantified and characterized ex vivo before and after vaccination.  T cell responses occurred primarily in patients with T cells that were already pre-activated before vaccination.  Thus, peptide vaccines can efficiently boost CD8 T cells that are pre-activated by endogenous tumor antigen.  Our results identify a new state of T cell responsiveness and help to explain and predict tumor vaccine efficacy.