PURPOSE:Immune-based therapy for metastatic breast cancer has had limited success, particularly in molecular subtypes with low somatic mutations rates. Strategies to augment T cell infiltration of tumors include vaccines targeting established oncogenic drivers like the genomic amplification of HER2. We constructed a vaccine based on a novel alphaviral vector encoding a portion of HER2 (VRP-HER2). EXPERIMENTAL DESIGN:In preclinical studies, mice were immunized with VRP-HER2 before or after implantation of hHER2+ tumor cells and HER2-specific immune responses and anti-tumor function were evaluated. We tested VRP-HER2 in a Phase I clinical trial where subjects with advanced HER2-overexpressing malignancies in cohort 1 received VRP-HER2 every 2 weeks for a total of three doses. In cohort 2, subjects received the same schedule concurrently with a HER2-targeted therapy. RESULTS:Vaccination in preclinical models with VRP-HER2 induced HER2-specific T cells and antibodies while inhibiting tumor growth. VRP-HER2 was well tolerated in patients and vaccination induced HER2-specific T cells and antibodies. Although a phase I study, there was one partial response and two patients with continued stable disease. Median OS was 50.2 months in cohort 1 (n=4) and 32.7 months in cohort 2 (n=18). Perforin expression by memory CD8 T cells post-vaccination significantly correlated with improved PFS. CONCLUSIONS:VRP-HER2 increased HER2-specific memory CD8 T cells and had anti-tumor effects in preclinical and clinical studies. The expansion of HER2-specific memory CD8 T cells in vaccinated patients was significantly correlated with increased PFS. Subsequent studies will seek to enhance T cell activity by combining with anti-PD-1.