(526c) Macrophages Aggregate and Cooperate to Phagocytose Cohesive Tumor Cell Targets in Engineered Immuno-Tumoroids and Solid Tumors

Macrophages are abundant in many solid tumors and are potential effector cells for monoclonal antibody-based cancer immunotherapies. This effect is dependent on Fc-mediated phagocytosis of cancer cells, but how macrophages engulf target cells that adhere to one another as in solid tumors is unknown. To address this, we engineered ‘immuno-tumoroids’ comprising soft but cohesive mixtures of melanoma cells with macrophages. Macrophages phagocytosed the cohesive melanoma cells only when an antitumor monoclonal antibody was supplied, and phagocytosis was maximized when signaling from the macrophage immune checkpoint ligand CD47 was abrogated by either CRISPR/Cas9 knockout or antibody blockade. Under the conditions for maximum phagocytosis, macrophages showed a striking spatial segregation within tumoroids. Although such segregation might be expected to hinder phagocytosis by limiting interactions between macrophages and target cells, we instead observed increased target engulfment by aggregated macrophages leading to tumoroid elimination. The results fit a phenomenological model of proliferation opposed by cooperative phagocytosis (i.e., Hill-like). Importantly, the same approaches for eliminating tumoroids in vitro also durably cured melanoma in immunocompetent mice when macrophage numbers were maximized by adoptive transfer of marrow-derived donor cells or engineered macrophages. Macrophage aggregates have previously been associated with increased survival in some human solid tumors, and our work provides a basis for engineering phase separating macrophages for maximum phagocytosis of cancer cells.