TRAMP PCa cells retrovirally transduced to express human PSMA (TRAMP-PSMA+HHD−) and/or HHD (TRAMP-PSMA+HHD+) or (TRAMP-PSMA−HHD+) were used as targets. The PSMA27, PSMA663, and PSMA711-specific CTLs demonstrated high levels of cytolytic activity (over 75% specific lysis) against target cells loaded with the respective PSMA peptide (Fig. 3A–C). The PSMA27 and PSMA663-specific CTLs were also able to specifically and effectively kill the target cells endogenously
expressing human PSMA and HHD (approximately 60 and 75% specific lysis, respectively, Fig. 3A and B). This confirms the processing and presentation of both PSMA27 and PSMA663 peptides from the protein backbone. However, despite displaying
high cytotoxic capacity against peptide-loaded cells, the PSMA711-specific CTLs were unable to kill the human PSMA and HHD-expressing cells RG7204 cell line (Fig. 3C). These observations were confirmed in multiple experiments and indicate that the PSMA711 peptide may be poorly processed and presented. As strong ex vivo CD8+ T-cell responses were generated against p.DOM-PSMA27 and p.DOM-PSMA663 constructs (Fig. 1B), and these CTLs were able to specifically lyse target cells which expressed PSMA endogenously (Fig. 3A and B), the in vivo MG-132 price cytotoxicity of these CTLs was investigated. Testing of the ability of the CTLs induced in HLA-A*0201 transgenic mice to kill tumor cells in vivo is hampered by the fact these mice lack expression of endogenous mouse MHC class I (H-2b) antigens. This means that the H-2b antigens expressed by the TRAMP PCa cell line will be immunogenic, preventing their long-term survival in standard tumor challenge experiments. We therefore used two approaches: the first was to demonstrate that the induced CD8+ T cells could kill peptide-loaded autologous target cells (splenocytes; Fig. 4A–C). For this, mice were immunized with p.DOM-PSMA27, p.DOM-PSMA663, or p.DOM control vaccine and boosted 28 days later with electroporation. Eight days after boosting, CFSE-labeled HHD splenocytes loaded with PSMA27, PSMA663, or control peptide
were injected as target cells. Representative flow cytometry plots are shown in Fig. 4A. Mice vaccinated with p.DOM-PSMA27 had a significantly reduced ratio of surviving CFSEhi PSMA27-loaded Sulfite dehydrogenase cells in respect to CFSElo control cells, with ∼33% fewer cells persisting compared with those in control mice (p=0.0011, Fig. 4B). The level of lysis of target cells observed in individual mice correlated with IFN-γ ELISpot responses detected in vitro (p=0.0049, Fig. 4B). After p.DOM-PSMA663 vaccination, the effect was even greater, with an approximately 50% reduction in the survival of PSMA663-positive cells in the vaccinated group compared with controls (p=0.0076, Fig. 4C). Again the level of lysis of target cells correlated with IFN-γ ELISpot responses (p<0.0001, Fig. 4C).