Phagocytosis of Necrotic Debris at Sites of Injury and Inflammation

Abstract

Clearance of cellular debris is required to maintain the homeostasis of multicellular organisms. It is intrinsic to processes such as tissue growth and remodeling, regeneration and resolution of injury and inflammation. Most of the removal of effete and damaged cells is performed by macrophages and neutrophils through phagocytosis, a complex phenomenon involving ingestion and degradation of the disposable particles. The study of the clearance of cellular debris has been strongly biased toward the removal of apoptotic bodies; as a result, the mechanisms underlying the removal of necrotic cells have remained relatively unexplored. Here, we will review the incipient but growing knowledge of the phagocytosis of necrotic debris, from their recognition and engagement to their internalization and disposal. Critical insights into these events were gained recently through the development of new in vitro and in vivo models, along with advances in live-cell and intravital microscopy. This review addresses the classes of "find-me" and "eat-me" signals presented by necrotic cells and their cognate receptors in phagocytes, which in most cases differ from the extensively characterized counterparts in apoptotic cell engulfment. The roles of damage-associated molecular patterns, chemokines, lipid mediators, and complement components in recruiting and activating phagocytes are reviewed. Lastly, the physiological importance of necrotic cell removal is emphasized, highlighting the key role of impaired debris clearance in autoimmunity.

Keywords: apoptosis; cell death; cell debris; inflammation; necrosis; phagocytosis; “eat-me”; “find-me”.

Figure 1

A comparison of apoptotic and necrotic “find-me” signals. (Left) Apoptosis is characterized by cell shrinkage, membrane blebbing, DNA fragmentation and nuclear condensation. As cells undergo apoptosis, “find-me” signals such as lysophoshatidylcholine (LPC), CX3CL1, ICAM3, and sphingosine 1-phosphate (S1P) are secreted, exposed on the outer leaflet of the plasma membrane, and/or released via apoptotic bodies or exosomes. Pannexin 1 (PANX1) is an important membrane channel involved in formation of membrane protrusions and ATP/UTP release during apoptosis. LPC, lysophosphatidylcholine; S1P, sphingosine-1-phosphate. (Right) Necrosis is considered to be an uncontrolled form of cell death characterized by nuclear and organellar swelling, plasma membrane rupture and leakage of intracellular contents, which many fall into the category of damage-associated molecular patterns (DAMPs or danger signals). “Find-me” signals released by necrotic cells include mitochondria-derived formylated peptides, as well as molecules released from the cytosol such as H₂O₂, ATP/UTP, leukotriene B4 (LTB4), and CXC/CC chemokines. LTB4 can also be released via sealed extracellular vesicles. The chemotactic complement components C3a and C5a are generated after complement activation on the surface of necrotic cells.

Figure 2

Apoptotic and necrotic “eat-me” signals, and their respective phagocytic receptors. (Left) As cells undergo apoptosis, they expose “eat-me” signals on their surface. The best studied eat-me signals for apoptotic cells is phosphatidylserine (PS). PS can either be bound directly by macrophage receptors such as BAI1, TIM 1/2/4, Stabilin-2, CD300, and TREM2. Alternatively, bridging molecules such as ProS, Gas6, MFG-E8, TSP1, and ICAM function to connect macrophage receptors (MerTK/TAM, integrin a_vβ_3/5, CD36, and CD14) to the apoptotic surface. (Right) Necrotic cells also expose “eat-me” signals on their surface to engage professional phagocytes. Necrotic cells share some exposed “eat-me” signals, such as PS, with apoptotic cells, although the means of exposure likely differ. Other “eat-me” signals are unique to necrotic cells, such as deposition of C1q, MBL (Mannose-binding lectin), C3b, and C4 as well as IgG/IgM opsonization, and the subsequent involvement of integrin CD11b/CD18 and Fcγ receptors.