AML-327 Novel Anti–B7-H3 Blocking Antibody Enhances NK Cell–Mediated Cytotoxicity and Improves Outcomes in AML-Bearing Mice

Context: B7-H3 is an immune checkpoint molecule that is overexpressed in various human malignancies. Several monoclonal antibodies (mAbs) targeting B7-H3 have shown promising results against solid tumors. However, B7-H3's role in AML remains unexplored. Objective: i) To analyze the prognostic significance of B7-H3 in AML. ii) To investigate the immunomodulatory role of B7-H3 in AML in vitro and in vivo. Design: We analyzed B7-H3 expression in 100 AML patients and 20 healthy donors by flow cytometry and tested for associations with clinical features. To investigate the role of B7-H3 in immunomodulation, B7-H3 was knocked down in AML cell lines using shRNA or using anti–B7-H3 blocking mAbs (T1-A5, HEK1B3A3, and 58B1A2). A human–mouse chimeric (chT-1A5) antibody was generated, and its binding site on B7-H3 protein was characterized by epitope mapping. Finally, we treated mice bearing AML cells with anti–B7-H3 antibodies and measured human AML engraftment with flow cytometry and bioluminescence imaging. Results: Expression of B7-H3 was significantly higher in AML patients than in healthy donors (p<0.01) and in CD34+ AML cells than in CD34^- cells (p<0.01). Clinically, we observed that high B7-H3 expression was associated with a poor prognosis. Furthermore, we observed that inhibition of B7-H3 expression or blocking of its activity in AML cells using a novel monoclonal antibody (T-1A5) significantly enhanced natural killer (NK) cell–mediated cytotoxicity in vitro and in vivo. Moreover, we observed that the human–mouse chimera of this antibody (ChT-1A5) induced dose-dependent cell-mediated cytotoxicity (ADCC) in B7-H3+ primary AML cells but not in normal hematopoietic cells, suggesting the specify of this antibody for AML cells. Epitope mapping using peptides derived from the B7-H3 protein identified the FG loop region as the binding site for the T1-A5 antibody. Finally, treatment with T-1A5 or chT-1A5 in combination with human NK cells significantly reduced leukemia burden and extended survival in AML xenograft and patient-derived xenograft models. Conclusions: Our data indicate that B7-H3 is overexpressed in AML and that an anti–B7-H3 antibody (T1-A5) not only blocks B7-H3's immunomodulatory function but also induces ADCC in AML cells in vitro and in vivo.