Antibody:CD47 ratio regulates macrophage phagocytosis through competitive receptor phosphorylation

Cancer immunotherapies often modulate macrophage effector function by introducing either targeting antibodies that activate Fc gamma receptors (Fc gamma Rs) or blocking antibodies that disrupt inhibitory SIRP alpha-CD47 engagement. However, how these competing signals are integrated is poorly understood, raising questions about how to effectively titrate immune responses. Here, we find that macrophage phagocytic decisions are regulated by the ratio of activating ligand to inhibitory ligand over a broad range of absolute molecular densities. Using both endogenous and chimeric receptors, we show that activating:inhibitory ligand ratios of at least 10:1 are required to promote phagocytosis of model antibody-opsonized CD47-inhibited targets and that lowering that ratio reduces Fc gamma R phosphorylation because of inhibitory phosphatases recruited to CD47-bound SIRP alpha. We demonstrate that ratiometric signaling is critical for phagocytosis of tumor cells and can be modified by blocking SIRP alpha, indicating that balancing targeting and blocking antibodies may be important for controlling macrophage phagocytosis in cancer immunotherapy.

Automated summary

The study reveals that the phagocytic decisions of macrophages are regulated by the ratio of activating ligand to inhibitory ligand, and a ratio of at least 10:1 is required for promoting phagocytosis. Ratiometric signaling is crucial for phagocytosis of tumor cells and can be modified by blocking SIRP alpha.