Background: A central mechanism for the anti-tumor activity of Trastuzumab, a HER2 monoclonal antibody, against HER2+ tumors is induction of antibody dependent cell cytotoxicity (ADCC) mediated by NK cells. We are conducting a first-in-human trial of Trastuzumab followed by infusion of expanded, activated autologous NK cells in refractory HER2+ MBC (ClinicalTrials.gov: NCT02030561) to test the hypothesis that NK cell infusions will augment Trastuzumab-mediated ADCC and increase immune cell infiltration in tumor. Pre- and post-infusion tumor biopsies were obtained in a subset of patients to determine degree of NK cell infiltration in tumor and histopathological and immunological effects after infusion. Methods: HER2+ MBC patients underwent apheresis to harvest NK cells for ex vivo expansion and activation. NK cells (10⁷/kg), expressing high levels of the antibody receptor CD16, were infused 24 hours post-Trastuzumab. Histology analysis and immunohistochemistry with CD56, CD3, CD20 and cleaved caspase-3 to identify NK, T, B, and apoptotic cells respectively, was performed in pre- and post-infusion biopsies. Studies of CD4 and CD8 to further classify T cell infiltrates, and CD16 to assess NK cell functionality are underway. Results: Analysis of 7 paired tumor biopsies collected before and 7-14 days after NK cell infusion showed absolute increase in lymphocyte infiltration (mean cell count/5 HPF: 204 vs 265 in pre- vs post- biopsy, p = 0.109). Most infiltrating lymphocytes were CD3+ T cells (74.28±12.72% vs 80 ±10% in pre vs post) and CD56+ NK cell number in the immune infiltrate were increased post infusion (mean cell count/5 HPF: 4.57±3.46 vs 20.57±13.83 in pre vs post; p = 0.009). Increased tumor apoptosis was observed post NK cell infusion (mean apoptotic cell count/5 HPF: 3.14±2.48 vs 5.86±6.72 in pre vs post, p = 0.27). Conclusions: Histopathology analysis of tumors from HER2+ MBC patients demonstrated significant increase in NK and T cells in tumor following Trastuzumab and NK cell infusion, suggesting that this combination might also augment recruitment of T lymphocytes into tumor, further enhancing anti-tumor activity. Clinical trial information: NCT02030561