P317: anti-leukemia activity of cd70-directed immunotherapy in b cell precursor acute lymphoblastic leukemia

Background: B-cell precursor acute lymphoblastic leukemia (BCP-ALL) is the most frequent pediatric malignancy. Although mostly treated successfully, relapse remains a major clinical problem associated with poor prognosis. However, therapy-related toxicities limit further chemotherapy intensification emphasizing the need for novel and alternative treatment strategies without increasing overall toxicity. Previously, we found that rapid leukemia development of patient BCP-ALL samples transplanted onto immunodeficient mice is strongly indicative of poor patient outcome. Further characterization revealed highly upregulated transcript levels of the gene coding for CD70, a surface molecule and member of the tumor necrosis factor family involved in B- and T-cell regulation. Interestingly, CD70 has been reported to be highly expressed on hematologic malignancies and solid tumors, but only at low levels on normal hematopoietic cells suggesting it as a specific target for directed immunotherapy. Aims: We studied CD70 expression profiles in different cohorts of BCP-ALL samples and evaluated CD70 as immunotherapeutic target using CD70-directed antibodies in vitro, ex vivo and in vivo. Methods: Protein and transcript expression were analyzed in primary patient, patient-derived xenograft (pdx) BCP-ALL and non-leukemia hematopoietic cells and associated with outcome data. Anti-leukemia activities and mechanisms of anti-CD70 antibodies were investigated and characterized in vitro and by transplantation of ex vivo antibody-loaded leukemia cells and in vivo treatment experiments. Results: Investigating large cohorts of diagnostic specimens BCP-ALL patients, high CD70 transcript but also surface protein expression was found and associated with early patient relapse and inferior outcome, confirming our previous observation. Therefore, we further evaluated CD70 as target for directed immunotherapy. Anti-CD70 monoclonal antibodies displayed antibody dependent cell mediated cytotoxicity against primary BCP-ALL cells and cell lines with CD70 expression in vitro. Next, we addressed anti-leukemia activity of anti-CD70 immunotherapy in vivo. Transplantation of CD70-positive ALL pdx samples loaded with anti-CD70 mAb onto NOD/SCID mice resulted in clearly reduced leukemia development with lower infiltration in spleen, bone marrow and lower percentages of ALL cells in peripheral blood as compared to control IgG loaded cells. Interestingly, depletion of NK-cells in recipient NOD/SCID mice or using NK-cell-deficient NSG mice abrogated in vivo anti-leukemia activity, indicating that anti-CD70 activity is mediated by NK-cell induced cytotoxicity on CD70 expressing ALL cells. Moreover, we also investigated anti-CD70 activity in a pre-clinical model of manifest ALL and observed a dose-dependent, significant leukemia reduction in treated animals. Also here, the anti-leukemia effect was abolished upon NK-cell depletion, further highlighting the role of NK-cells in mediating anti-leukemia activity of anti-CD70 immunotherapy. However, using a modified antibody without ADDC function in a pdx sample expressing CD70 and its signaling partner CD27, we also observed some leukemia reduction in vivo, suggesting an additional mechanism by blocking CD70-CD27 signaling. Summary/Conclusion: We show increased CD70 surface expression on BCP-ALL cells associated with inferior outcome. Using CD70-directed antibodies, we characterize CD70 as immunotherapeutic target in vitro, ex vivo and in vivo showing anti-leukemia activity including effective reduction of leukemia loads in a pre-clinical patient-derived xenograft model of CD70-positive BCP-ALL. Keywords: Acute lymphoblastic leukemia, Immunotherapy, Antibody targeting