Monomethyl-auristatin E-based anti-CD137 (4-1BB) antibody drug conjugates are not suitable tools to deplete activated T lymphocytes

CD137 (4-1BB) is a surface marker selectively expressed on activated T cells, whose induction depends on antigen recognition, and is readily internalized upon ligand recognition. These factors make it an attractive target for antibody-drug conjugates (ADCs). Selective elimination of CD137 + T cells could represent a therapeutic strategy in autoimmune diseases, allograft rejection, and graft-versus-host disease. In this study, we aimed to generate and evaluate anti-human and anti-mouse CD137 ADCs for their ability to kill CD137-expressing cells. ADCs were produced from monoclonal antibodies by stochastic conjugation of active esters to lysine residues. The generated ADCs bound effectively and specifically to CD137 on stable Jurkat and HEK-293T transfectants, and were efficiently internalized. In transfectants expressing high CD137 surface levels, selective cytotoxicity was observed upon incubation with ADCs. Human and mouse T-cell blasts were generated in vitro with anti-CD3/anti-CD28-coated beads to evaluate killing using flow cytometry. However, despite testing different drug-to-antibody ratios (DARs), the ADCs failed to kill activated T-cell blasts of either human or murine origin, and they did not ameliorate experimental graft-versus-host disease in mice. These findings demonstrate that although anti-CD137 ADCs can be successfully engineered to bind and internalize into CD137 + cells, the evaluated approaches did not achieve therapeutic elimination of pathogenic T lymphocytes.