Bifunctional Phagocytic Synapse Enhancers Remodel the Tumor Microenvironment to Overcome Immunosuppression.

Current immunotherapies often fail in immunologically "cold", macrophage-rich tumor microenvironments (TMEs). Multi-targeting approaches that modulate innate-adaptive immune activation represent a promising frontier in cancer immunotherapy. Here, we developed phagocytic synapse enhancers (PSEs), a class of modular immune engagers comprising a high-affinity PD-L1 binder coupled to a macrophage-stimulating peptide, tuftsin. PSEs strengthened effector-target cell interactions and enhanced tumor phagocytosis by bridging tumor PD-L1 to macrophage neuropilin-1 (NRP1), bypassing the classical FcγR-dependent pathways. The PSEs also acted in cis on PD-L1+ macrophages, accelerating the endocytosis and lysosomal compartmentalization of surface PD-L1, therefore stripping the immunosuppressive checkpoint from the local microenvironment. Beyond physical clearance, PSEs reprogrammed macrophage phenotype and triggered a robust pro-inflammatory cytokine and chemokine response. The lead molecule, longPSE, and its half-life-extended variant fused to the albumin binding domain, ABD- longPSE, showcased superior efficacy than the macrophage enhancer magrolimab in a syngeneic tumor model of colorectal cancer and an orthotopic model of pancreatic cancer. PSE treatment remodeled the TME by inducing phenotypic changes in the lymphoid and myeloid compartments, together with a reduction of tumor-associated macrophages and regulatory T cells. These findings establish PSEs as bifunctional molecules that complement innate and adaptive immune modulation. The bifunctional design offers a versatile approach for next-generation immunotherapies and provides a blueprint for a plug-and-play platform of immune engagers targeting diverse cancer-associated pathways.