Macrophages show higher levels of engulfment after disruption of cis interactions between CD47 and the checkpoint receptor SIRP alpha

The macrophage checkpoint receptor SIRP alpha signals against phagocytosis by binding CD47 expressed on all cells - including macrophages. Here, we found that inhibiting cis interactions between SIRP alpha and CD47 on the same macrophage increased engulfment ('eating') by approximately the same level as inhibiting trans 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 SIRP alpha, and linearly increased binding of soluble CD47 in trans, consistent with cis-trans competition. Many cell types express both SIRP alpha and CD47, including mouse melanoma 816 cells, and CRISPR-mediated deletions modulate B16 phagocytosis, consistent with cis-trans competition. Additionally, soluble SIRP alpha binding to human CD47 displayed on Chinese hamster ovary (CHO) cells was suppressed by SIRP alpha co-display, and atomistic computations confirm SIRP alpha bends and binds CD47 in cis. Safety and efficacy profiles for CD47-SIRP alpha blockade might therefore reflect a disruption of both cis and trans interactions.