The macrophage checkpoint receptor SIRPα signals against phagocytosis by binding CD47 expressed on all cells – including macrophages. Here, inhibiting cis interactions between SIRPα and CD47 on the same macrophage increases eating approximately the same as inhibiting trans interactions. Antibody blockade of CD47, as pursued in clinical trials against cancer, is applied separately to human-derived macrophages and to red blood cell (RBC) targets for phagocytosis, and both scenarios produce surprisingly similar increases in RBC engulfment. Blockade of both macrophages and targets results in hyper-phagocytosis, and knockdown of macrophage-CD47 likewise increases eating of ‘foreign’ cells and particles, decreases SIRPα’s baseline inhibitory signaling, and linearly increases binding of soluble-CD47 in trans, consistent with cis-trans competition. Many cell types express both SIRPα and CD47, including mouse melanoma B16 cells, and CRISPR-mediated deletions modulate B16 phagocytosis consistent with cis-trans competition. Additionally, soluble SIRPα binding to human-CD47 displayed on Chinese hamster ovary (CHO) cells is suppressed by SIRPα co-display, and atomistic computations confirm SIRPα bends and binds CD47 in cis. Safety and efficacy profiles for CD47-SIRPα blockade might therefore reflect a disruption of both cis and trans interactions.
© 2020. Published by The Company of Biologists Ltd.