Figure 1. Antigen cascade: An initial immune response (e.g., a PSA-specific cytotoxic T lymphocyte [CTL] following vaccination with a PSA specific vaccine) may encounter a PSA expressing tumor cell. If that CTL can kill the tumor cell in an immunologically relevant manner, as that tumor cell is dying it can be taken up by antigen-presenting cells (APCs). There, other tumor-associated antigens (depicted by prostate-specific membrane antigen [PSMA], prostate stem cell antigen [PSCA], prostatic acid phosphatase [PAP] and mucin-1 [MUC-1]) can be processed and presented via cross-presentation to other CTLs, a process that can activate these CTLs specific for these other antigens. Thus, while the initial vaccine may target one antigen, a broader antitumor immune response may ensue, and this dynamic response may continue to broaden and adapt to subsequent mutations.