Clearance of apoptotic and necrotic cells and its immunological consequences

Abstract

The ultimate and most favorable fate of almost all dying cells is engulfment by neighboring or specialized cells. Efficient clearance of cells undergoing apoptotic death is crucial for normal tissue homeostasis and for the modulation of immune responses. Engulfment of apoptotic cells is finely regulated by a highly redundant system of receptors and bridging molecules on phagocytic cells that detect molecules specific for dying cells. Recognition of necrotic cells by phagocytes is less well understood than recognition of apoptotic cells, but an increasing number of recent studies, which are discussed here, are highlighting its importance. New observations indicate that the interaction of macrophages with dying cells initiates internalization of the apoptotic or necrotic targets, and that internalization can be preceded by "zipper"-like and macropinocytotic mechanisms, respectively. We emphasize that clearance of dying cells is an important fundamental process serving multiple functions in the regulation of normal tissue turnover and homeostasis, and is not just simple anti- or pro-inflammatory responses. Here we review recent findings on genetic pathways participating in apoptotic cell clearance, mechanisms of internalization, and molecules involved in engulfment of apoptotic versus necrotic cells, as well as their immunological consequences and relationships to disease pathogenesis.

From regulation of dying cell engulfment to development of anti-cancer therapy

“…We showed that apoptotic cells are engulfed by a zipper-like mechanism of phagocytosis ( Figure 1), whereas necrotic cells are internalized by a macropinocytotic mechanism, such that parts of the cell are co-ingested together with extracellular fluid. 8,31,32 In contrast with the initial paradigm, PS exposure mediates recognition and engulfment not only of apoptotic cells, but also of necrotic cells 33,34 and of cells dying from autophagic cell death, 35 indicating that PS-mediated clearance may be a general mechanism irrespective of the way a cell dies. Accordingly, several macrophage receptor systems known to be involved in the engulfment of apoptotic cells also contribute to the uptake of necrotic cells.…”

“…For example, C1q, mannose-binding lectin, pentraxin-3, C3, C4, C-reactive protein (an acute phase protein), and thrombospondin bind predominantly to late apoptotic or secondary necrotic cells. 8 Great progress has been achieved during the last decade in unraveling molecules on the surface of apoptotic cells, but it is still not clear how non-apoptotic cells, namely necrotic and autophagic cells, are recognized by macrophages. We showed that apoptotic cells are engulfed by a zipper-like mechanism of phagocytosis ( Figure 1), whereas necrotic cells are internalized by a macropinocytotic mechanism, such that parts of the cell are co-ingested together with extracellular fluid.…”

“…8 One of the mechanisms for this protection is related to repelling signals mediated through CD31 (platelet endothelial cell adhesion molecule). CD31 on viable cells homotypically interacts with its identical counterpart on phagocytes, promoting detachment, and preventing engulfment of the viable cells by an active and temperaturedependent mechanism.…”

“…48 Necrotic cells release massive amount of high mobility group box 1 (HMGB-1) protein, which incites inflammatory responses of macrophages through pathways mediated by Toll-like receptor 2 (TLR2) and TLR4. 8,52 Whether HMGB-1 protein is specifically released by necrotic cells but not by apoptotic or secondary necrotic cells remains controversial. Apparently, in apoptosis and secondary necrosis HMGB-1 remains bound on the DNA.…”

Dual-promoter lentiviral system allows inducible expression of noxious proteins in macrophages

“…Electroporation has been used to transfer plasmid DNA into monocytic cell lines or macrophages with high efficiency (Weir and Meltzer, 1993;Hume et al, 2001). Usually, electroporation causes significant cell death and release of intracellular contents, which is known to induce macrophage responses (Krysko et al, 2006). Moreover, both transfection and electroporation with plasmid DNA containing unmethylated CpG (cytosine followed by guanine) dinucleotides may activate macrophages through Toll-like receptor (TLR)-9 to produce proinflammatory cytokines (Stacey et al, 1996;Sester et al, 1999;Jiang et al, 2006).Thus, the existing methods of plasmid delivery perturb the normal physiological status of the macrophage population and complicate interpretation of the experimental data.…”