Background: PSA-TRICOM is a vector-based therapeutic cancer vaccine designed to generate a targeted anti-tumor immune response against prostate-specific antigen (PSA)–expressing tumor cells. Early clinical trials have evaluated the immunologic impact of this vaccine and demonstrated promising clinical activity. PSA-TRICOM is being evaluated in a phase III trial in metastatic castration resistant prostate cancer (mCRPC). Methods: We recently conducted a broad overview of both published and new data which analyzed the immune responses to PSA-TRICOM. Immune responses included ELISPOT for antigen-specific immune response and flow-cytometry analysis of peripheral immune cells. Results: 104 patients (pts) with prostate cancer were tested for T-cell responses and 59 out of 104 (57%) demonstrated a greater than or equal to 2-fold increase in PSA-specific T cells 4 weeks after vaccine. The responders had a median 5-fold increase relative to pre-vaccine levels. For most pts PSA-specific immune responses (likely memory cells) seen 28 days following the most recent vaccine are quantitatively similar to levels of circulating influenza-specific T cells in the same pts. In addition, 19 out of 28 pts (68%) evaluated demonstrated immune responses to tumor-associated antigens not present in the vaccine (antigen spreading). Since PSA-TRICOM is designed to generate a cellular (TH1 immune response), it is not surprising that 2 out of 349 pts (<1.0%) demonstrated evidence of PSA antibody induction following vaccine. This suggests that post-vaccine PSA kinetics were not affected by PSA antibodies. Conclusions: PSA-TRICOM has demonstrated the ability to generate immune responses. Despite these findings, it is important to note that systemic immune response to PSA may underestimate the true therapeutic immune response since it does not measure cells that trafficked to tumor or antigen spreading. Furthermore, while the entire PSA gene is the vaccine, only one epitope of PSA is specifically evaluated in the T-cell responses. Further immune analysis continues in an ongoing phase III of PSA-TRICOM in mCRPC (NCT01322490), accruing worldwide, and two trials combining PSA-TRICOM with enzalutamide (biochemical recurrence/ NCT01875250 and mCRPC/ NCT01867333) currently accruing at NCI. Clinical trial information: multiple trials.