Although the anti-HER2 antibody, trastuzumab, has been the mainstay of therapy for HER2+ breast cancer (BC), its clinical benefit remains heterogeneous among HER2+ patients and metastatic HER2+ BC remains generally incurable. The final success of HER2 antibody therapy relies on the induction of anticancer immunity, but this is limited by immunosuppressive mechanisms posed by the tumor microenvironment (TME), resulting in treatment resistance, metastasis, and ultimately lethal BC. Elevated adenosine level is considered a major immunosuppressive mechanism in the TME, causing resistance to trastuzumab by suppressing innate and adaptive antitumor immunity induced by the antibody. Adenosine deaminase (ADA) catalyzes the deamination of adenosine and is capable of reversing immunosuppressive activities of adenosine. However, systemic ADA administration causes toxicity in normal tissues, where adenosine protects the host from excessive immune activation. We hypothesize that targeted anchoring of ADA on HER2+ BC cell surface reprograms immunometabolism in the TME, enhancing the efficacy and safety of HER2-directed immunotherapy. We have developed a highly cancer specific targeting approach to deliver ADA to the surface of HER2+ BC cells. Targeted ADA delivery with an anti-HER2 antibody led to localization of ADA on the cell surface of HER2+ BC cells, and the surface-anchored ADA was capable of depleting adenosine. In tumor-bearing syngeneic mice, HER2-targeted delivery resulted in 5.6-fold increase in tumoral ADA level compared to free ADA administration. Targeted ADA delivery led to TME reprograming into an immunostimulatory landscape and resulted in immune-mediated tumor regression. In the proposed studies, we will elucidate the mechanisms of TME reprograming by surface ADA anchoring (Aim 1) and will develop an approach to enhance surface ADA anchoring by combination with a tyrosine kinase inhibitor (Aim 2). This project can be directly translated into a targeted cancer immunotherapy to treat patients with HER2+ BC. By addressing lack of disease specificity, a main obstacle to the development of next-generation immunotherapeutic regimens, we will contribute to the use of precision medicine for cancer immunotherapy.