Background: Patients with breast cancers (BCs) harboring low expression of hormone receptors (HRs) and human epidermal growth factor receptor-2 (HER2) have poor outcomes. Results from the KEYNOTE-522 trial showed that activation of the immune system using a PD1/PD-L1-targeted approach improves outcomes of patients with these high-risk tumors. Antigen-presenting cells (eg, dendritic cells [DCs]) are pivotal for robust cytotoxic responses due to broader activation of the adaptive immune system (ie, CD4⁺ and CD8⁺ Th1) against tumor-associated antigens (ie, HER2 or HER3) expressed by high-risk BCs. Methods: DecipHER is a dose-escalation, dose-expansion phase 1 trial designed to assess the safety and the preliminary efficacy of autologous, HER2- and HER3-primed DCs in combination with KEYNOTE 522 regimen in 30 patients. Patients with clinical stage cT1cN1/2 or cT2-4cN0-2, HR 20, HER2-negative BCs are eligible. Patients with inflammatory BC cancer and uncontrolled immune mediated diseases are excluded. After collection through aphaeresis, autologous DCs will be primed ex vivo against 6 HER2 and 8 HER3 immunogenic peptides. Participants will receive alternating ultrasound-guided intratumoral HER2 and HER3 DC injections administered twice a week for 8 doses in total starting 2 weeks prior to neoadjuvant therapy with KEYNOTE 522 regimen. The dose-escalation phase of the study has 3 planned cohorts (10-20, 30-50, 80-100 million DCs) and follows a 3+3 design (maximum n=18). The 12 additional patients enrolled will be treated at the maximum tolerated dose (MTD) in the dose-expansion cohort. The MTD will be defined as the highest dose level at which < 2 of 6 patients experience dose-limiting toxicities (grade ≥ 3 non-hematologic, ; grade ≥ 3 hematologic toxicity thought be at least possibly related to DCs ; any grade 4 nausea, vomiting, or diarrhea [or grade 3 if duration > 3 days]) during the 5 weeks following treatment initiation with DCs. Secondary endpoints include the absolute risk of adverse events, clinical and pathological responses, and recurrence free survival. Tumor tissue, blood and stool samples will be collected for correlative analyses. The study is open at H. Lee Moffitt Cancer Center. Clinical trial information: NCT05504707.