In solid tumours, the abundance of macrophages is typically associated with a poor prognosis. However, macrophage clusters in tumour-cell nests have been associated with survival in some tumour types. Here, by using tumour organoids comprising macrophages and cancer cells opsonized via a monoclonal antibody, we show that highly ordered clusters of macrophages cooperatively phagocytose cancer cells to suppress tumour growth. In mice with poorly immunogenic tumours, the systemic delivery of macrophages with signal-regulatory protein alpha (SIRPα) genetically knocked out or else with blockade of the CD47–SIRPα macrophage checkpoint was combined with the monoclonal antibody and subsequently triggered the production of endogenous tumour-opsonizing immunoglobulin G, substantially increased the survival of the animals and helped confer durable protection from tumour re-challenge and metastasis. Maximizing phagocytic potency by increasing macrophage numbers, by tumour-cell opsonization and by disrupting the phagocytic checkpoint CD47–SIRPα may lead to durable anti-tumour responses in solid cancers. Durable anti-tumour responses can be triggered by maximizing the cooperative phagocytic potency of macrophages through the disruption of the CD47–SIRPα macrophage checkpoint and by delivering a tumour-opsonizing monoclonal antibody.

Macrophages are abundant in solid tumours and typically associate with poor prognosis, but macrophage clusters in tumour nests have also been reported as beneficial even though dispersed macrophages would have more contacts with cancer cells. Here, by maximizing both phagocytic activity and macrophage numbers, we discover cooperative phagocytosis by low entropy clusters in rapidly growing engineered immuno-tumouroids. The results fit the calculus of proliferation-versus-engulfment, and rheological measurements and molecular perturbations provide a basis for understanding phagocytic disruption of a tumour’s cohesive forces in soft cellular phases. The perturbations underscore the utility of suppressing a macrophage checkpoint in combination with an otherwise ineffective tumour-opsonizing monoclonal antibody, and the approach translates in vivo to tumour elimination that durably protects mice from re-challenge and metastasis. Adoptive transfer of engineered macrophages increases the fraction of mice that eliminate tumours and potentially overcomes checkpoint blockade challenges in solid tumours like insufficient permeation of blocking antibodies and on-target, off-tumour binding. Finally, anti-cancer IgG induced in vivo are tumour-specific but multi-epitope and contribute to a phagocytic feedback that drives macrophage clustering in vitro . Given that solid tumours remain challenging for immunotherapies, durable anti-tumour responses here illustrate unexpected advantages in maximizing net phagocytic activity.

Solid tumors can be challenging for immunotherapies, but it may be possible to improve outcomes by overcoming physical barriers that limit antibody permeation and immune cell function. We show that B16 melanoma tumors with knockout of the macrophage checkpoint ligand CD47 can be cured by treating with pro-phagocytic, tumor-opsonizing IgG. Cured mice generate de novo IgG that also opsonizes B16 by binding to multiple antigens, thereby facilitating a ‘phagocytic feedback’ to protect survivors from tumor rechallenge and limit antigen escape. Similar treatment of B16 tumors displaying wild-type levels of CD47 or with only 50% of CD47 blocked by a nanobody is ineffective, demonstrating the potency of the checkpoint. As a more translatable approach to complete disruption of CD47 signaling from tumors, adoptive transfer of marrow phagocytes saturated ex vivo with antibody against the macrophage checkpoint receptor SIRPα also extends survival and cures a fraction of mice. For physical insight, we characterized B16 tumors by pipette aspiration rheology and found the tumor interior to be soft and elastic with little fibrillar matrix. In contrast, the more fibrous periphery stiffens with age as tumors become larger and rounder. Cohesiveness among tumor cells and adhesion to matrix is expected to oppose phagocytosis by macrophages, underscoring the importance of the physical microenvironment of solid tumors targeted by macrophage checkpoint blockade.