Adjuvant treatment is still only working in a small percentage of breast cancer patients. Therefore, new strategies need to be developed. Immunotherapies are a very promising approach because they could successfully attack tumor cells in the stage of dormancy. To assess the feasibility of using an allogeneic approach for vaccination of breast cancer patients, we selected a CD80-transfected breast cancer cell line based on its immunogenic properties. Using CD80+ KS breast cancer cells and human leukocyte antigen (HLA)-A*02-matched peripheral blood mononuclear cells (PBMCs) of breast cancer patients in allogeneic mixed lymphocyte-tumor cell cultures (MLTCs), it was possible to isolate HLA-A*02-restricted cytotoxic T cells (CTLs). Furthermore, a genetically modified KS variant expressing influenza A matrix protein serving as a surrogate tumor-associated antigen (TAA) was able to stimulate flu peptide-specific T cells alongside the induction of alloresponses in MLTCs. KS breast cancer cells were demonstrated to express already known TAAs such as CEA, MUC-1, MAGE-1, MAGE-2, and MAGE-3. To further improve antigenicity, HER-2/neu was added to this panel as a marker antigen known to elicit HLA-A*02-restricted CTLs in patients with breast cancer. Thus, the antigen-processing and antigen-presentation capacity of KS cells was further demonstrated by the stimulation of HER-2/neu-specific CD8+ T cells in PBMCs of breast cancer patients in vitro. These results gave a good rationale for a phase I/II trial, where the CD80+ HER-2/neu-overexpressing KS variant is actually used as a cellular vaccine in patients with metastatic breast cancer. As a proof of principle, we present data from two patients where a significant increase of interferon-gamma (IFN-gamma) release was detected when postvaccination PBMCs were stimulated by allogeneic vaccine cells as well as by HLA-A*02-restricted HER-2/neu epitopes. In whole cell vaccine trials, monitoring is particularly challenging because of strong alloresponses and limited knowledge of TAAs. In this study, a panel of HER-2/neu epitopes, together with the quantitative real time (qRT)-PCR method to analyze vaccine-induced cytokines secreted by T cells, proved to be highly sensitive and feasible to perform an "immunological staging" following vaccination.