Macrophages, being completely unrelated to T-cells on the hematologic family tree, have a plethora of different functions that make them attractive candidates for CAR modification in cases where T-cells encounter changes. For example, while T-cells poorly infiltrate solid tumors, macrophages are often summoned to them by cytokines released by the tumor; the actively migrate into the tumor against a pressure gradient while T-cells largely lack this capacity. While T-cells kill primarily through inducing apoptosis by granzyme and perforin secretion, to which tumor cells have variably levels of innate and acquired resistance, macrophages can theoretically dis pose of their targets by direct phagocytosis. Additionally, they have been known to exert cytotoxic effects through antibody dependent cytotoxicity (ADCC). Unfortunately, macrophages are usually drawn to the tumor microenvironment to provide a “healing” role; they sense destruction and arrive to reduce inflammation and promote recovery. This role generally has the effect of promoting tumor growth, spread, and metastasis. Since they are highly plastic, they can change from this reparative, pro-tumor role to an inflammatory, anti-tumor role and back again, depending on their stimuli.
Engineering macrophages with a CAR is an appealing way to genetically instruct them to maintain an inflammatory anti-tumor phenotype, as well as to infiltrate and phagocytose tumor. Macrophages modified with a CAR containing a signaling domain from Megf10 or FcRɣ have both been found to achieve target-specific phagocytosis. A CAR macrophage targeting the extracellular matrix protein CD147 successfully reduced tumor collagen deposition and improved T-cell infiltration. CAR macrophages expressing a CD3z-based CAR phagocytosed tumor in vitro and reduced tumor burden in vivo, expressed proinflammatory cytokines and chemokines, converted bystander M2 macrophages to M1, resisted the effects of immunosuppressive cytokines, and activated anti-tumor T-cells. Challenges to macrophage therapy include genetically modifying the cells in large numbers, and potentially finding ways to increase the magnitude of their direct anti-tumor efficacy in vivo, as studies thus far have shown tumor reduction but not elimination. Given the number of T-cell supportive functions CAR macrophages have, they may function synergistically with CAR T-cells; however, these studies have yet to be reported. One Phase I clinical trial is currently open utilizing CAR macrophages for HER2 expressing tumors (NCT04660929).
