Macrophages show higher levels of engulfment after disruption of <i>cis</i> interactions between CD47 and the checkpoint receptor SIRPα

Abstract

The macrophage checkpoint receptor SIRPα signals against phagocytosis by binding CD47 expressed on all cells - including macrophages. Here, we found that inhibiting <i>cis</i> interactions between SIRPα and CD47 on the same macrophage increased engulfment ('eating') by approximately the same level as inhibiting <i>trans</i> interactions. Antibody blockade of CD47, as pursued in clinical trials against cancer, was applied separately to human-derived macrophages and to red blood cell (RBC) targets for phagocytosis, and both scenarios produced surprisingly similar increases in RBC engulfment. Blockade of both macrophages and targets resulted in hyper-phagocytosis, and knockdown of macrophage-CD47 likewise increased engulfment of 'foreign' cells and particles, decreased the baseline inhibitory signaling of SIRPα, and linearly increased binding of soluble CD47 in <i>trans</i>, consistent with <i>cis-trans</i> competition. Many cell types express both SIRPα and CD47, including mouse melanoma B16 cells, and CRISPR-mediated deletions modulate B16 phagocytosis, consistent with <i>cis-trans</i> competition. Additionally, soluble SIRPα binding to human CD47 displayed on Chinese hamster ovary (CHO) cells was suppressed by SIRPα co-display, and atomistic computations confirm SIRPα bends and binds CD47 in <i>cis</i> Safety and efficacy profiles for CD47-SIRPα blockade might therefore reflect a disruption of both <i>cis</i> and <i>trans</i> interactions.