A Mutated Prostatic Acid Phosphatase (PAP) Peptide-Based Vaccine Induces PAP-Specific CD8(+) T Cells with Ex Vivo Cytotoxic Capacities in HHDII/DR1 Transgenic Mice

SIMPLE SUMMARY: Treatments for castrate (hormone)-resistant prostate cancer (CRPC) remain limited and are not curative. Although the first (and only) FDA-approved vaccine for CRPC (PROVENGE(®), Sipuleucel-T) has been shown to improve the overall survival of patients, it is not curative and its cost prevents its widespread use. PROVENGE(®) induces immunity to prostatic acid phosphatase (PAP), a protein which is highly expressed in prostate cancer (PCa). Herein, we have developed a new PAP-based vaccine for PCa and demonstrated the presence of circulating PAP-specific CD8(+) T cells that are responsive to this vaccine in patients with PCa. We have also shown that this new PAP sequence-derived peptide containing a modified amino sequence, in association with a strong adjuvant called CAF(®)09, induces strong immune responses and cytotoxic potential in a murine HHDII/DR1 transgenic model. ABSTRACT: Background: Current treatments for castrate (hormone)-resistant prostate cancer (CRPC) remain limited and are not curative, with a median survival from diagnosis of 23 months. The PAP-specific Sipuleucel-T vaccine, which was approved by the FDA in 2010, increases the Overall Survival (OS) by 4 months, but is extremely expensive. We have previously shown that a 15 amino accid (AA) PAP sequence-derived peptide could induce strong immune responses and delay the growth of murine TRAMP-C1 prostate tumors. We have now substituted one amino acid and elongated the sequence to include epitopes predicted to bind to several additional HLA haplotypes. Herein, we present the immunological properties of this 42mer-mutated PAP-derived sequence (MutPAP42mer). Methods: The presence of PAP-135-143 epitope-specific CD8(+) T cells in the blood of patients with prostate cancer (PCa) was assessed by flow cytometry using Dextramer™ technology. HHDII/DR1 transgenic mice were immunized with mutated and non-mutated PAP-derived 42mer peptides in the presence of CAF(®)09 or CpG ODN1826 (TLR-9 agonist) adjuvants. Vaccine-induced immune responses were measured by assessing the proportion and functionality of splenic PAP-specific T cells in vitro. Results: PAP-135-143 epitope-specific CD8(+) T cells were detected in the blood of patients with PCa and stimulation of PBMCs from patients with PCa with mutPAP42mer enhanced their capacity to kill human LNCaP PCa target cells expressing PAP. The MutPAP42mer peptide was significantly more immunogenic in HHDII/DR1 mice than the wild type sequence, and immunogenicity was further enhanced when combined with the CAF(®)09 adjuvant. The vaccine induced secretory (IFNγ and TNFα) and cytotoxic CD8(+) T cells and effector memory splenic T cells. Conclusions: The periphery of patients with PCa exhibits immune responsiveness to the MutPAP42mer peptide and immunization of mice induces/expands T cell-driven, wild-type PAP immunity, and therefore, has the potential to drive protective anti-tumor immunity in patients with PCa.