(604c) Immunotherapy Combining IgG Opsonization and Macrophage Checkpoint Disruption Represses Growth of Cohesive Tumor Cell Clusters in Vitro and Solid Tumors in Mice

Solid tumors pose significant challenges for immunotherapies including those employing phagocytic macrophages as effector cells. Macrophages and giant cells composed of macrophages are abundant in many neoplasms, but it is unknown whether they can phagocytose target cancer cells that adhere to one another or whether phagocytosis can outcompete proliferation. We show that macrophages can eliminate melanoma cells in vitro from rapidly proliferating clusters formed on non-adhesive materials. However, both IgG opsonization and disruption of the macrophage checkpoint ligand CD47 by CRISPR knockout or antibody blockade were required to achieve elimination. Micropipette aspiration rheology of cell clusters and freshly isolated mouse tumors revealed viscoelasticity and cell cohesion in both tissues. Aggregation of phagocytic macrophages within clusters and the nonlinear dependence of the cluster growth rate on macrophage number suggest that macrophages may exhibit cooperativity in engulfing solid tumor targets, which resembles the foreign body response to biomaterials in some ways. Anti-tumor IgG administered intravenously to mice eliminated a fraction of CD47 knockout tumors in vivo but had no effect on wild-type tumors. Growth suppression of wild-type tumors was achieved by adoptive transfer of marrow-derived phagocytes treated ex vivo to block the checkpoint receptor SIRPα. Thus, our in vitro and in vivo models utilizing gene-edited cells, monoclonal antibodies for opsonization and checkpoint blockade, and cell therapy begin to define the parameters required for successful immunotherapy of solid tumors including (i) macrophage density, (ii) inhibitory checkpoint disruption, and (iii) IgG-opsonization.