Donor-Derived B Cells Produce Potent AML-Specific Antibodies That Recognize Novel Tumor-Specific Antigens and Mediate Graft-Versus-Leukemia Immunity

Unleashing the tumor-specific immune response by immunotherapies such as checkpoint inhibitors or allogeneic stem cell transplantation can result in long lasting tumor regression. While the role of T cells in such graft versus leukemia (GvL) immune responses has been established, the contribution of B cells to GvL responses is less clear. Using SEREX and other techniques, the presence of antibodies directed against established tumor antigens following allogeneic hematopoietic stem cell transplantation (HSCT) has been demonstrated. The function of these antibodies remains to be established though. Here we tested the hypothesis that B cells contribute to GvL.

We selected three patients with high-risk AML who mounted potent GvL responses after allogeneic HSCT. Of these patients we established antibody-producing clonal B cell lines following transduction of memory B cells from the peripheral blood of these patients with Bcl-6 and Bcl-xL and screened those for producing antibodies specifically binding to surface antigens on AML cell lines and AML blasts. A number of antibodies were identified that recognized primary AML blasts isolated from newly diagnosed AML patients, but did not bind to healthy bone marrow, peripheral blood mononuclear cells or tissues such as liver, skin and colon. Target identification analyses revealed two novel, AML-specific surface proteins as primary targets of these antibodies. These newly identified AML-specific antigens are widely expressed on different types of AML, including blasts of the last 17 AML patients that entered our clinic of whom diagnosis material was made available for this project.

Strikingly, antibody binding to one of these targets induced direct cell death of cultured AML cell lines and of primary AML blasts. Cytotoxicity of the antibodies was rapid, occurred both at 37^oC and 4^oC and could be prevented by Cytochalasin D, an actin polymerization inhibitor that stabilizes the cytoskeleton. This, in combination with the observation that cell death could not be prevented by apoptosis inhibitors indicated that the tumor cells were killed through a necrotic pathway like oncosis. Other antibodies induced antibody-dependent cell-mediated cytotoxicity (ADCC) and complement-dependent-cytotoxicity (CDC).

Together these data indicate that tumor selective antibodies are elicited following allogeneic HSCT in AML patients with a strong GvL response. The direct and indirect cytotoxic activities against tumor cells of these antibodies suggest that they contribute to the GvL response.