During oncogenesis, tumors develop mechanisms to avoid rejection by the immune system. Recent studies have identified CD47 as an anti-phagocytic “don't eat me” signal tat cancer cells employ to inhibit macrophage-mediated destruction. Here, we modified the 14 kDa binding domain of human SIRPα, the receptor for CD47, for use as a CD47 antagonist. Using in vitro evolution via yeast surface display, we engineered high-affinity SIRPα variants with up to a 50,000-fold increase in affinity for human CD47 relative to wild-type SIRPα. As high-affinity SIRPα monomers, the variants potently antagonized CD47 on cancer cells, but to our surprise, they did not induce macrophage phagocytosis on their own. Instead, the high-affinity SIRPα monomers exhibited remarkable synergy with all tumor-specific monoclonal antibodies tested by increasing phagocytosis in vitro and enhancing anti-tumor responses in vivo. This novel “one-two punch” directs immune responses against tumor cells while lowering the threshold for macrophage activation, thereby providing a universal method for augmenting the efficacy of therapeutic anti-cancer antibodies.
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