Efferocytosis in the tumor microenvironment

Abstract

Within the course of a single minute, millions of cells in the human body will undergo programmed cell death in response to physiological or pathological cues. The diminished energetic capacity of an apoptotic cell renders the cell incapable of sustaining plasma membrane integrity. Under these circumstances, intracellular contents that might leak into the surrounding tissue microenvironment, a process referred to as secondary necrosis, could induce inflammation and tissue damage. Remarkably, in most cases of physiologically rendered apoptotic cell death, inflammation is avoided because a mechanism to swiftly remove apoptotic cells from the tissue prior to their secondary necrosis becomes activated. This mechanism, referred to as efferocytosis, uses phagocytes to precisely identify and engulf neighboring apoptotic cells. In doing so, efferocytosis mantains tissue homeostasis that would otherwise be disrupted by normal cellular turnover and exacerbated further when the burden of apoptotic cells becomes elevated due to disease or insult. Efferocytosis also supports the resolution of inflammation, restoring tissue homesostasis. The importance of efferocytosis in health and disease underlies the increasing research efforts to understand the mechanisms by which efferocytosis occurs, and how a failure in the efferocytic machinery contributes to diseases, or conversely, how cancers effectively use the existing efferocytic machinery to generate a tumor-tolerant, immunosuppressive tumor microenvironment. We discuss herein the molecular mechanisms of efferocytosis, how the process of efferocytosis might support a tumor 'wound healing' phenotype, and efforts to target efferocytosis as an adjunct to existing tumor treatments.

Keywords: Apoptotic cell death; Efferocytosis; MerTK; Phosphatidyl serine, macrophage; Tumor microenvironment.

Fig. 1

The stages of efferocytosis: Find Me, Eat Me, and Post-Engulfment Signaling. a Apoptotic cells release nucleotides (ATP/UTP) and CX₃CL1 as “Find me” signals that biochemically attract monocytes by engaging the P2Y₂ and CX₃CR1 receptors, respectively. Lactoferrin has potent and specific anti-migratory effects on neutrophils and is released by apoptotic cells as a “Keep out” signal for inflammatory cells. b Phagocytes must preferentially identify apoptotic cells for efferocytosis while leaving healthy cells unengulfed. Apoptotic cells display PtdSer lipid on the outer leaflet of the cell membrane, which is recognized by phagocytes either directly through BAI1, TIM-4, or Stabilin 2 or indirectly through the αvβ3-MFG-E8 or MerTK-Gas6 bridging combinations. In addition to lower PtdSer outer-leaflet exposure, healthy cells express “Don’t eat me” signals such as CD47 and CD31 which prevent phagocyte activation and engulfment. c Apoptotic debris are engulfed via LC3-associated phagosomes which fuse with lysosomes and efficiently degrade cellular contents of the apoptotic cell. The production of fatty acids such as 25-hydroxysterol result in the activation of nuclear receptors and transcriptional regulators, PPAR and LXR, whose activation promotes transcription of pro-efferocytic machinery (MerTK, Gas-6, Rac1) and immunosuppressive cytokines (TGF-β1, IL-10, IL-13)

Fig. 2

Efferocytosis shapes a pro-tolerogenic tumor microenvironment. Intact efferocytosis leads to efficient clearance of apoptotic cells in the tumor microenvironment. Efferocytosis leads to M2-like macrophage polarization, production of wound healing cytokines (e.g., IL-10, IL-13, TGF-β), and recruitment of FOXP3+ regulatory T cells, promoting a tolerogenic and immunosuppressive tumor microenvironment. Secondary necrosis occurs when efferocytosis is blocked leading to the release of pro-inflammatory damage-associated molecular patterns (DAMPs). Pro-inflammatory DAMPs drive M1-like macrophage polarization, production of pro-inflammatory cytokines (e.g., TNF, IFN, IL-12), and recruitment of cytotoxic cells such as CD8+ T cells and natural killer cells that mediate anti-tumor immunity