Human epidermal growth factor receptor 2 (HER2) is overexpressed in a subset of cancers including 25% of breast cancers. Since combination therapy consisting of multiple therapeutic approaches is considered a promising regimen, we examined combination treatment modalities in a xenograft model in Balb/c mice injected with 4T1-HER2 cells. We used HER2/neu-loaded bone marrow-derived dendritic cells (BM-DC’s) along with anti-PD-L1 monoclonal antibody in a new combination immunotherapy model. The combination was composed of an active immunotherapy (i.e. BM-DC-based vaccine) designed to boost the immune response against target antigen and was augmented by using anti-PD-L1 mAb to prevent immune evasion by the xenografted tumors. The vaccine combination was further supported using a QS-21 saponin adjuvant and the immune response was evaluated. Mice treated with HER2/neu-loaded BM-DCs, combined with QS-21 and anti-PD-L1 mAb had significantly decreased tumor sizes and their splenocytes had enhanced cytotoxic activity, based on the lactate dehydrogenase (LDH) assay, compared to vaccine and adjuvant groups alone. The same vaccination group demonstrated a remarkable increase in IFN-γ secreting CD8+ T-cells analyzed by flow cytometry. ELISA data also revealed a significant increase in the serum anti-HER2 IgG1 response; in addition, there was significant splenocyte proliferation upon stimulation with antigen compared to vaccine and adjuvant groups. Consistently, a significant infiltration of CD4+, CD8+ immune cells in and around the tumors was observed. Our data suggest that the BM-DC + HER2/neu + QS-21 + anti-PD-L1 vaccine combination paradigm synergistically generates anti-tumor activity and immune responses against HER2 overexpressing breast cancer in mice.