75 Background: The treatment options for cancer are ever-evolving & now include vaccines targeting tumor-specific immunogenic peptides to induce tumor cytolysis. Folate binding protein (FBP) is a tumor-associated antigen (TAA) highly expressed in most endometrial & ovarian cancers (Ca), shielded from the normal immune system. The most promising FBP peptides are E39 & an attenuated form, E39' (aka J65). In our most recent phase I/IIa trial we evaluated an E39+GM-CSF inoculation series at 3 doses (VG) vs controls (CG) as well as booster vaccination with E39 or E39' after the initial inoculation series. Here we present the immunologic data from this phase I/IIa trial. Methods: Patients (pts) with ovarian or endometrial Ca who were disease-free after standard of care therapy but at risk for recurrence were enrolled. HLA-A2+ pts were vaccinated in a 3+3 dose escalation of 100mcg, 500mcg, & 1000mcg. The remaining pts received 1000mcg inoculations. Vaccine was given every 3-4 weeks for a total of 6 inoculums. Pts were then offered to participate in booster vaccination with 9 pts randomized to receive E39 & 9 pts E39'. FBP expression level was measured on the resected disease. Overall immunologic response & by subgroups was measured by delayed type hypersensitivity (DTH) & ELISPOT in the VG. Results: A total of 29 pts were vaccinated. Mean DTH prior to the initial dose of E39 was 5.74mm & after final dose was 10.33mm (p = 0.018). Mean overall ELISPOT change over time through 18 months was +69.1 when compared to baseline (p = 0.675). At 18 months, mean ELISPOT increased by 97.1 in 1000mcg vs -57.0 in < 1000mcg dosed pts (p = 0.047) & by +255.33 in pts with > average initial DTH vs -0.75 in < average initial DTH (p = 0.004) when compared to baseline. Immunological analyses were not significantly different between FBP hi/lo expression or E39 vs E39’ booster (p > 0.05). Conclusions: E39 demonstrated significant overall immunogenicity on in vivo testing as measured by DTH. Ex vivo analysis (ELISPOT) suggests that E39 is more immunogenically efficacious in pts with > average initial DTH & those who are optimally dosed (1000mcg). FBP expression level & E39 vs E39’ use in booster inoculations did not significantly impact in vivo or ex vivo immunogenicity. Clinical trial information: NCT01580696.