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SIRPA-Inhibited, Marrow-Derived Macrophages Engorge, Accumulate, and Differentiate in Antibody-Targeted Regression of
Cory M Alvey1, Kyle R Spinler2, Jerome Irianto2
1Molecular & Cell Biophysics Lab, University of Pennsylvania, Philadelphia, PA 19104, USA; Graduate Group in Pharmacological Sciences, University of Pennsylvania, Philadelphia, PA 19104, USA.
Abstract:
Marrow-derived macrophages are highly phagocytic, but whether they can also traffic into solid tumors and engulf cancer cells is questionable, given the well-known limitations of tumor-associated macrophages (TAMs). Here, SIRPα on macrophages from mouse and human marrow was inhibited to block recognition of its ligand, the "marker of self" CD47 on all other cells. These macrophages were then systemically injected into mice with fluorescent human tumors that had been antibody targeted. Within days, the tumors regressed, and single-cell fluorescence analyses showed that the more the macrophages engulfed, the more they accumulated within regressing tumors. Human-marrow-derived macrophages engorged on the human tumors, while TAMs were minimally phagocytic, even toward CD47-knockdown tumors. Past studies had opsonized tumors in situ with antibody and/or relied on mouse TAMs but had not injected SIRPα-inhibited cells; also, unlike past injections of anti-CD47, blood parameters remained normal and safe. Consistent with tumor-selective engorge-and-accumulate processes in vivo, phagocytosis in vitro inhibited macrophage migration through micropores that mimic features of dense 3D tissue. Accumulation of SIRPα-inhibited macrophages in tumors favored tumor regression for 1-2 weeks, but donor macrophages quickly differentiated toward non-phagocytic, high-SIRPα TAMs. Analyses of macrophages on soft (like marrow) or stiff (like solid tumors) collagenous gels demonstrated a stiffness-driven, retinoic-acid-modulated upregulation of SIRPα and the mechanosensitive nuclear marker lamin-A. Mechanosensitive differentiation was similarly evident in vivo and likely limited the anti-tumor effects, as confirmed by re-initiation of tumor regression by fresh injections of SIRPα-inhibited macrophages. Macrophage motility, phagocytosis, and differentiation in vivo are thus coupled.
Insights
Inhibiting CD47 recognition on marrow macrophages enabled them to engulf tumor cells, leading to tumor regression. However, macrophage differentiation limited long-term anti-tumor effects.
Area of Science:
- Immunology
- Cancer Biology
- Cell Biology
Background:
- Marrow-derived macrophages are phagocytic but their tumor infiltration is limited.
- Tumor-associated macrophages (TAMs) exhibit poor phagocytic activity against cancer cells.
- The "marker of self" CD47 on cells is recognized by SIRPα on macrophages, inhibiting phagocytosis.
Purpose of the Study:
- To investigate if inhibiting SIRPα-CD47 interaction allows marrow macrophages to traffic into and clear solid tumors.
- To assess the anti-tumor efficacy and safety of systemically injected SIRPα-inhibited macrophages.
- To understand the mechanisms of macrophage behavior and differentiation within the tumor microenvironment.
Main Methods:
- Inhibiting SIRPα on mouse and human marrow macrophages to block CD47 recognition.
- Systemic injection of these modified macrophages into mice bearing fluorescent human tumors.
- Single-cell fluorescence analysis, in vitro phagocytosis assays, and collagen gel stiffness experiments.
- Analysis of macrophage differentiation markers (SIRPα, lamin-A) in vitro and in vivo.
Main Results:
- SIRPα-inhibited macrophages effectively infiltrated tumors, phagocytosed cancer cells, and induced tumor regression.
- Accumulation of phagocytic macrophages correlated with tumor regression, while TAMs remained minimally phagocytic.
- Macrophage phagocytosis in vitro reduced migration through dense matrices, mimicking tumor tissue.
- Donor macrophages differentiated into non-phagocytic TAMs in vivo, limiting sustained anti-tumor effects.
- Tumor stiffness and retinoic acid modulated SIRPα expression and macrophage differentiation via mechanosensitive pathways.
Conclusions:
- Blocking the SIRPα-CD47 axis enables marrow macrophages to overcome tumor defenses and exhibit potent anti-cancer activity.
- Macrophage differentiation driven by the tumor microenvironment, particularly stiffness, limits therapeutic efficacy.
- Repeated administration of SIRPα-inhibited macrophages can re-initiate tumor regression, suggesting potential therapeutic strategies.
- Macrophage motility, phagocytosis, and differentiation are coupled processes influenced by the tumor microenvironment.
