Background: Early stage HER2 positive breast cancer (BC) patients (pts) are commonly treated with neoadjuvant HER2-targeted therapy. Optimizing neoadjuvant therapy by improving efficacy while reducing toxicity is a critical unmet need. IT DC 1 therapy combined with an IgG1 antibody mediating ADCC drives strong anti-HER-2 CD4 Th1 responses and complete tumor regression in preclinical models. We initiated a phase I/II neoadjuvant study to explore the efficacy and immune stimulation effects of an initial 6 weeks of immunotherapy phase (IP) using IT DC1 vaccine plus trastuzumab (H) and pertuzumab (P) (as source of IgG1) followed by 12 weeks of paclitaxel (T) and HP. Methods: Early stage HER2+ BC pts with tumor ≥ 1cm were eligible. Treatment included IT DC1 weekly x6 followed by paclitaxel 80 mg/m² IV weekly x 12. Trastuzumab IV every (q) 3 weeks (8 mg/kg loading dose, then 6 mg/m²) and pertuzumab IV q 3 weeks (840 mg loading dose, then 420 mg) x 6 cycles starting from day 1. Two dose levels (DL) of IT DC1 (DL1 = 50 million and DL2 = 100 million cells) were evaluated including 6 pts in each DL. Once the optimal DL is determined based on toxicities, additional 22 patients will be enrolled in expansion phase 2 trial. The primary end point of the phase 1 is the safety, immune responses (measured by ELISPOT). MRI breast was performed at the baseline, post-immunotherapy and post-chemotherapy. Here we are reporting the safety and immune correlates in phase 1 study. Results: Twelve pts were enrolled between 10/2021 and 10/2022. Median age was 57 (range 34-74). Nine pts had hormone receptor positive disease with clinical stage I/II/III (5/5/2) and 4 pts had node positive disease. All pts completed IP and 10 patients completed THP chemotherapy as of 2/1/2023. Radiologic response per MRI breast on pre and post-IP showed 6 partial response, 3 complete response and 3 stable disease. Nine pts underwent surgery and 6 out of 9 pts had pCR (RCB 0). The most frequently observed toxicities were chills (50%), diarrhea (42%), nausea (42%), fatigue (42%), and headache (42%). ELISPOT demonstrated a significant decrease of HER2- specific T cell response following 6 weeks of DC1 IT injections in DL2 compared to DL1. Conclusions: The addition of IT DC1/HP to neoadjuvant chemotherapy was well tolerated with manageable toxicities. The 100 million DC1 led to a significant decrease in peripherally circulating anti-HER2 T cell response at week 6 following IP and improved radiologic tumor responses which correlates with a higher likelihood of pCR. Data from another IT DC1 trial suggests decreased circulating HER2 responsive T cells are due to increased trafficking of reactive T cells out of circulation into the tumor possibly leading to an enhanced anti-tumor effect. Updated results including correlative biomarkers will be presented at the meeting. Clinical trial information: NCT05325632.