BACKGROUND: Adoptive cell transfer-based immunotherapy holds promise for treating advanced cancer. However, a key challenge remains: generating sufficient numbers of lymphocytes capable of recognizing and targeting a broad range of cancer antigens. We recently developed Autologous Lymphoid Effector Cells Specific Against Tumor (ALECSAT), a novel procedure for selecting, expanding and maturing polyclonal lymphocytes from peripheral blood with the capacity to target cancer cells. In this single-center phase Ib trial, we evaluated the safety, tolerability, and preliminary efficacy of ALECSAT in combination with standard carboplatin and gemcitabine in patients with locally advanced or metastatic triple-negative breast cancer (mTNBC). METHODS: This clinical study enrolled 15 patients with mTNBC. The patients received three ALECSAT doses, administered every 28 days. Subsequently, ALECSAT doses were given at 6-week intervals. Carboplatin and gemcitabine were administered on days 1 and 8 in 3-week cycles. The cell composition of ALECSAT preparations was analyzed using flow cytometry. Additionally, patient-derived xenograft (PDX) mouse models were generated and treated with ALECSAT to assess treatment responses. RESULTS: 14 patients with mTNBC, who had received one to four prior treatment lines, were treated with 1-10 doses of ALECSAT. The combination of ALECSAT with carboplatin and gemcitabine was well tolerated and demonstrated a favorable safety profile. Common adverse events (AEs), including fatigue, nausea, and hematological abnormalities, were consistent with the known toxicity profiles of carboplatin and gemcitabine. Notably, grade ³3 AEs were predominantly hematological, with manageable durations of neutropenia and thrombocytopenia. Among treated patients, one achieved a complete response, four had partial responses, five had stable disease, and four had progressive disease. The objective response rate was 36% (95% CI 12.8% to 64.9%). Median progression-free survival was 4.3 months (95%_CI 1.6 to 7.0), while median overall survival was 8.7 months (95%_CI 5.1 to 12.4). A positive correlation was observed between the total number of administered ALECSAT cells (particularly CD8+T cells) and time to progression. Additionally, ALECSAT treatment outcomes in patients correlated with responses observed in their corresponding PDX models. CONCLUSION: ALECSAT, in combination with carboplatin and gemcitabine, was safe, well tolerated, and demonstrated promising antitumor activity in mTNBC. These findings support further investigation in larger clinical trials. TRIAL REGISTRATION NUMBER: NCT00891345.