Engineering macrophages to eat cancer: from "marker of self" CD47 and phagocytosis to differentiation

Abstract

The ability of a macrophage to engulf and break down invading cells and other targets provides a first line of immune defense in nearly all tissues. This defining ability to "phagos" or devour can subsequently activate the entire immune system against foreign and diseased cells, and progress is now being made on a decades-old idea of directing macrophages to phagocytose specific targets, such as cancer cells. Engineered T cells provide precedence with recent clinical successes against liquid tumors, but solid tumors remain a challenge, and a handful of clinical trials seek to exploit the abundance of tumor-associated macrophages instead. Although macrophage differentiation into such phenotypes with deficiencies in phagocytic ability can raise challenges, newly recognized features of cancer cells that might be manipulated to increase the phagocytosis of those cells include ≥1 membrane protein, CD47, which broadly inhibits phagocytosis and is abundantly expressed on all healthy cells. Physical properties of the target also influence phagocytosis and again relate-via cytoskeleton forces-to differentiation pathways in solid tumors. Such pathways extend to mechanosensing by the nuclear lamina, which is known to influence signaling by soluble retinoids that can regulate the macrophage SIRPα, the receptor for CD47. Here, we highlight some of those past, present, and rapidly emerging efforts to understand and control macrophages for cancer therapy.

Keywords: cytoskeleton; mechanobiology; solid tumors.

Figure 1

Anti‐cancer Mϕ and CD47. (A) Timeline of adoptive Mϕ transfer and CD47 studies converging on anti‐CD47–focused Mϕ therapies. (B) Inhibition of cancer cell engulfment because of recognition of CD47 by a nonphagocytic phenotype, despite the presence of a pro‐phagocytic Ab. Addition of anti‐CD47–blocking Ab and a more phagocytic phenotype can drive engulfment. The actomyosin cytoskeleton has a key role in phagocytosis and in linking the microenvironment to influence the phenotype. (C) Ab modification (blocking SIRPα and loading Fc receptor with targeting Ab) of marrow Mϕs, followed by systemic injection, could be an effective method for adoptive Mϕ cancer therapy. In circulation, antibody‐primed Fc receptor plus anti‐SIRPα blocked Mϕ (A’PB Mϕ) could, in principle, migrate into tumors, phagocytose cancer cells, and then either exit the tumor or continue to destroy tumor cells.

Figure 2

Tool kit for studying the effect of CD47 inhibition on tumor growth. (A) Growth curve of syngeneic, orthotopic B16F10 tumors in C57 mice show the effects that si‐mCD47 and clodronate liposomes have on tumor growth. Data adapted from Wang et al. [53]. (B) Analysis of how long untreated orthotopic B16F10 tumors in C57 mice take to reach 100 mm² when challenged with either 200,000 or 500,000 cells. Data are adapted from Alvey [unpublished results] and studies, Wang et al. [53], Bencherif et al. [63], Ali et al. [110], and Sockolosky et al. [57]; *P ≤ 0.05. (C) Growth curves of orthotopic B16F10 tumors in C57 mice treated with a combination of anti‐CD47 nanobodies and an Ab that binds tyrosinase‐related protein 1 (Trp1). Data adapted from Sockolosky et al. [57]. i) Log‐scale growth highlights differences in tumor sizes between treatment conditions near the start of the treatment. ii) Normalizing growth data to d 5 gives a different interpretation from the reported conclusions in (i): anti‐TRP1 has only a small effect, but a combination with the anti‐CD47 nanobody can significantly reduce tumor growth.

Figure 3

Stiff matrix regulation of Lamin‐A. (A) RNA‐seq reads per million for Lamin‐A and Lamin‐B in tissue Mϕ from [1], plotted as a function of tissue stiffness measurements in Swift et al. [103]. (B) Ratio of RNA reads for lamin‐A: lamin‐B in Mϕs, including tumor‐associated Mϕs isolated from human tumor xenografts per Lavin et al. [1] and Swift et al. [103]. Subcutaneous A549 tumors were engrafted in NSG mice and allowed to grow to 80 mm² before tumor stiffness was measured and Mϕ were profiled.

Figure 4

Targeting the physical properties and molecular interactions at the cell surface determines the efficiency of human RBC engulfment by human Mϕ. (Adapted from Sosale et al. [39]). (A) Phagocytosis increases with IgG opsonization and with cross‐linker–based rigidification of hRBCs. Phagocytosis of rigid, opsonized RBCs is independent of hCD47 inhibition in contrast to “soft,” native RBCs whose uptake is enhanced by an hCD47‐blocking Ab. A “sphering” treatment, which gives a rounded and rigid hRBC, shows reduced uptake relative to the discocytes. (B) Time‐lapse images of rigidified hRBC discocytes show rapid engulfment and lack of deformation by the Mϕ. (C) Surface interactions combine kinetically with physical properties of a candidate target in the calculus that determines phagocytic uptake.