Abstract LB-322: A modified HER2/neu peptide vaccine (2L9V-GP2) demonstrates enhanced CD8+ T cell recognition and stimulation in breast cancer patients at high risk for recurrence

Abstract

Abstract Introduction: The affinity between tumor-associated antigens and MHC Class I complexes directly affects the degree of CD8+ T cell (CTL) response. GP2 (IISAVVGIL, p654-662) is a HER2/neu-derived peptide, that has poor binding affinity to HLA-A2 (A2) and HLA-A3 (A3). We conducted a preclinical study to determine if amino acid substitutions in the GP2 peptide, predicted to increase binding affinity to A2 and A3, enhance CTL recognition and stimulation. Methods: Based on computer modeling algorithms (BIMAS, SYFPEITHI), two GP2 peptide variants were predicted to have higher binding affinity to A2: SPA-12(2L9V-GP2) and SPA-13(2L5L9V-GP2). Freshly isolated PBMCs were obtained from randomly chosen, disease-free, node positive (NP) and high risk node negative (NN) breast cancer (BCa) patients, who had completed standard adjuvant therapy and are enrolled in the E75 vaccine booster trial. Peptide-specific CD8+ T cells were measured by HLA-A2: IgG dimer (A2 pts) and by interferon-γ ELISPOT (A2, A3 pts) assays. Groups were compared by t-test (dimer) and Mann-Whitney U (ELISPOT) tests. Results: Thirty-five A2 (21 NP, 14 NN) and thirteen A3 (13 NP) samples were included in the study. Among A2 pts, the mean level of SPA-12-specific CTLs was greater than the GP2- (2.8±0.3% vs. 1.7±0.1%, p<0.001) and SPA-13-specific CTLs (2.8±0.3% vs. 1.1±0.1%, p<0.001). The level of SPA-13-specific CTLs was less than GP2-specific CTLs (p<0.001). For both A2 and A3 pts, the median number of IFN- producing cells was greater in response to SPA-12 (144, range 0-799 spots/105 cells) compared to GP2 (88, range 0-432 spots/105 cells, p=0.03) and SPA-13 (0, range 0-344 spots/105 cells, p<0.001). Again, the response to SPA-13 was less than GP2 (p=0.001). In addition, the NP A2 pts demonstrated a more robust CD8+ T cell response to SPA-12 compared to the NNs, both by HLA-A2: IgG dimer (3.2±.5 vs 2.2±0.2, p=0.05) and ELISPOT assay (223, range 0-799 vs 65, range 0-548 p=0.003). Nodal status did not have any significant bearing on the responses to GP2 and SPA-13. Conclusion: The substituted sequences SPA-12 (2L9V-GP2) and SPA-13 (2L5L9V-GP2), both have higher predicted binding affinity to A2 and A3 than GP2. While SPA-12 showed significantly increased recognition and functional CTL responses compared to GP2, SPA-13 demonstrated lower responses. These data suggest that SPA-12 may be a more effective cancer vaccine than GP2, particularly in NP BCa patients. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr LB-322.