CD47 is a ubiquitously expressed cell surface marker responsible of maintaining self-tolerance under physiological conditions. Upon binding to its receptor signal regulatory protein alpha (SIRPα) on myeloid cells, CD47 triggers a negative signal that ultimately inhibits phagocytosis. The upregulation of CD47 is an escape mechanism frequently used by tumor cells to gain immune resistance. Thus, the blockade of the CD47-SIRPα interaction by high affinity monoclonal antibodies (mAbs) is a promising strategy to restore phagocytosis and enhance tumor cell clearance. However, CD47 expressed on healthy cells may act as potential site of toxicity and antigen sink for high affinity CD47 mAbs. We developed bifunctional antibody derivatives, herein referred to as local inhibitory monoclonal antibodies (licMABs), to restrict the blockade of the CD47-SIRPα innate immune checkpoint to tumor cells and therefore reduce systemic toxicities.
LicMABs are SIRPα-antibody fusion proteins that target a tumor antigen with high affinity and block the CD47-SIRPα interaction with the endogenous SIRPα domain, which has a physiologically low affinity to CD47. For proof or principal studies, we developed licMABs targeting CD33, a validated antigen for Acute Myeloid Leukemia (AML). The anti-tumor efficacy of SIRPα-αCD33 licMABs via antibody-dependent cellular cytotoxicity (ADCC) and antibody-dependent cellular phagocytosis (ADCP) mechanisms was evaluated in vitro. Furthermore, the potential toxicity of SIRPα-αCD33 licMABs on CD33-negative cells was addressed by preferential binding assays and ex vivo cytokine release studies in whole blood.
SIRPα-αCD33 licMABs mediated specific lysis of AML cell lines and primary, patient-derived AML cells by ADCC and induced a higher phagocytosis of AML cells than CD33 mAbs. Most importantly, SIRPα-αCD33 licMABs showed a potentially safe cytokine profile and preferentially bound to tumor cells co-expressing CD33 and CD47 even in an excess of healthy CD47-positive CD33-negative cells.
Our results demonstrate the high efficacy and potentially low toxicity of SIRPα-αCD33 licMABs and qualify these molecules as a novel and promising immunotherapy for the treatment of AML.
AG Hopfner, Gene Center, LMU.
DFG CRC1243, m4-Award of the Bavarian Ministry of Economic Affairs and Graduate School of Quantitative Biosciences Munich.
All authors have declared no conflicts of interest.