Binding of inhibitory checkpoints to CD18 in cis hinders anti-cancer immune responses

SIRP is an inhibitory receptor on macrophages that limits phagocytosis and anti-tumor activity of macrophages by "trans" interacting with CD47 on tumor cells. Herein, we found that a large component of SIRPs inhibitory function occurred independently of CD47-binding and phosphatase signaling. This function resulted from a direct interaction between SIRP and CD18 ({beta}2 integrin) in "cis" at the surface of macrophages, involving SIRP amino acids distinct from those implicated in the SIRP-CD47 interaction. The cis interaction prevented activation of CD18, which is necessary for phagocytosis. Combined blockade of SIRP-CD18 and SIRP-CD47 was essential for maximizing phagocytosis and suppression of tumor growth in vivo. Similar cis interactions between CD18 and other inhibitory checkpoints, including PD-1, were also observed. Thus, in addition to mediating effects when engaged by ligands in trans, inhibitory checkpoints suppress immune cell activation through a mechanism targeting CD18 in cis. This dual mode of action should be considered when developing blockers of inhibitory checkpoints for immunotherapy. One-Sentence SummaryIn addition to being engaged in "trans" by ligands on tumor cells, inhibitory receptors, such as SIRP and PD-1, hinder anti-cancer immune responses by "cis" interacting with {beta}2 integrin CD18.