Preventative Cancer Vaccine-Elicited Human Anti-MUC1 Antibodies Have Multiple Effector Functions

Background/Objectives: Mucin-1 (MUC1) is a transmembrane glycoprotein that is overexpressed and hypoglycosylated in premalignant and malignant epithelial cells compared to normal cells, creating a target antigen for humoral and cellular immunity. Healthy individuals with a history of advanced colonic adenomas and at high risk for colon cancer were enrolled in a clinical trial to evaluate the feasibility of using a MUC1 peptide vaccine to prevent colon cancer. Anti-MUC1 antibodies elicited by this vaccine were cloned using peripheral blood B cells and sera collected two weeks after a one-year booster. Twelve of these fully human monoclonal antibodies (mAb) were tested for binding to MUC1+ target cells, and three with the highest binding were further evaluated for various effector functions important for tumor rejection. Methods: Immune cells were incubated together with target cells expressing variations in the number, distance, and membrane anchoring properties of the MUC1 epitope in the presence of each mAb. Results: All three mAbs mediated antibody-dependent cytokine release (ADCR), antibody-dependent cellular cytotoxicity (ADCC), and antibody-dependent cellular phagocytosis (ADCP). Two also mediated antibody-dependent trogocytosis/trogoptosis (ADCT). None were capable of complement-dependent cytotoxicity (CDC). Conclusions: ADCP and ADCT functions were more efficient when antibodies bound epitopes proximal to and anchored to the membrane, providing insight for future therapeutic antibody validation strategies.