Macrophages use different internalization mechanisms to clear apoptotic and necrotic cells

Abstract

The present study characterized two different internalization mechanisms used by macrophages to engulf apoptotic and necrotic cells. Our in vitro phagocytosis assay used a mouse macrophage cell line, and murine L929sAhFas cells that are induced to die in a necrotic way by TNFR1 and heat shock or in an apoptotic way by Fas stimulation. Scanning electron microscopy (SEM) revealed that apoptotic bodies were taken up by macrophages with formation of tight fitting phagosomes, similar to the 'zipper'-like mechanism of phagocytosis, whereas necrotic cells were internalized by a macropinocytotic mechanism involving formation of multiple ruffles directed towards necrotic debris. Two macropinocytosis markers (Lucifer Yellow (LY) and horseradish peroxidase (HRP)) were excluded from the phagosomes containing apoptotic bodies, but they were present inside the macropinosomes containing necrotic material. Wortmannin (phosphatidylinositol 3 0 -kinase (PI3K) inhibitor) reduced the uptake of apoptotic cells, but the engulfment of necrotic cells remained unaffected. Our data demonstrate that apoptotic and necrotic cells are internalized differently by macrophages.

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.…”

“…80 Recently it has been shown that apoptotic cell preparations of allogeneic origin containing phytohaemagglutinin or anti-CD3/ CD28 activated T cells, but not resting T cells, are able to induce activation of DCs and presentation of alloantigens. 81 Recognition and internalization 8,31,32 of necrotic cells differs from that of apoptotic cells, and promotes pro-inflammatory macrophage responses. 82 Ucker and colleagues elaborated further the differences between apoptotic and necrotic cell clearance with respect to inflammatory responses.…”

Apaf-1-independent programmed cell death in mouse development

“…41 Conversely, different mechanisms have been proposed to clear apoptotic and necrotic cells in mammalian system. 42 Apoptotic cells expose phosphatidylserine, which is recognized by specific receptors or opsonins for engulfment. 12 Because necrotic cells also expose phosphatidylserine at a late stage, 38 it is possible that similar receptors or opsonins mediate the engulfment of apoptotic and necrotic cells.…”

Far upstream element-binding protein-1, a novel caspase substrate, acts as a cross-talker between apoptosis and the c-myc oncogene

“…Apoptosis involves a complex series of cellular events in which the constituents of a doomed cell are dismantled and packaged into smaller bodies for easy removal by neighboring cells or macrophages (Krysko et al, 2006). At the core of the death process is a family of cytosolic cysteine proteases, designated caspases, which display specificity for aspartic acid residues (Stennicke and Salvesen, 1999;Denault and Salvesen, 2002).…”