Background: PSA-TRICOM (Prostvac) is a vector-based vaccine designed to generate a robust immune response (IR) against PSA–expressing tumor cells. To date, studies of Prostvac in patients with mCRPC have shown IR in peripheral blood but effects on prostate tumors are unknown. Methods: An open label phase 2 study of neoadjuvant Prostvac (NCT02153918) enrolled patients (pts) with localized prostate cancer undergoing radical prostatectomy (RP). Priming vaccination was given followed by boosts on days 15, 29 and 57 prior to RP (~day 64). The 1° objective evaluated increases in CD4 and CD8 cell infiltrate (RP tissue vs. baseline biopsies) by IHC. IR to tumor associated antigens (TAA) was measured using intracellular cytokine staining (ICS) in PBMC from baseline and ~day 63. Results: 27 pts (median age 64.8 years) enrolled. All pts received 4 pre-RP vaccine injections. Matched tissue IHC analysis and peripheral IR is available on 24 pts. TAA specific T cell peripheral IR to PSA, Muc-1 or Brachyury were observed in 12/24 (50%) pts post vaccine, with 25% of pts responding to each antigen. Table comparing peripheral IR vs. tumoral IR. Clinical trial information: NCT02153918 19 pts were evaluable for tissue RNA expression profiles pre/post vaccine and observed upregulation of genes such as RANKL (DC maturation), CCL25 (chemotaxis), and FLT3 (T-cell development) suggests vaccine-induced IRs. Conversely, increased expression of immunosuppressive genes (e.g., FOXP3, IL-10) suggests adaptive resistance post vaccination. Conclusions: Prostvac treatment is associated with a ≥2X increase in CD8 and CD4 intra/ peritumoral infiltrate in 17/24 and 21/24 pts, respectively. However only 12/24 had a TAA response in the peripheral blood. Anti-tumor IRs may become broader and more immunologically relevant over time (Ag spreading) and may be underestimated by peripheral IRs to TAA. These data better inform the immunodynamic effects of Prostvac which are incompletely represented by peripheral IR data. Work is ongoing to more fully characterize the IR with multispectral imaging, targeted RNASeq and TCRSeq analysis.