Friendly fire against neutrophils: Proteolytic enzymes confuse the recognition of apoptotic cells by macrophages

doi:10.1016/j.biochi.2007.09.008

Biochimie

Volume 90, Issue 2, February 2008, Pages 405-415

https://doi.org/10.1016/j.biochi.2007.09.008 Get rights and content

Abstract

Physiologically the only acceptable fate for almost all damaged or unwanted cells is their apoptotic death, followed by engulfment of the corpses by healthy neighbors or professional phagocytes. Efficient clearance of cells that have succumbed to apoptosis is crucial for normal tissue homeostasis, and for the modulation of immune responses. The disposal of apoptotic cells is finely regulated by a highly redundant system of receptors, bridging molecules and ‘eat me’ signals. The complexity of the system is reflected by the term: ‘engulfment synapse’, used to describe the interaction between a phagocytic cell and its target. In healthy humans, dying neutrophils are the most abundant and important targets for such recognition and engulfment. In inflammation the scope and importance of this complicated task is further increased. Paradoxically, despite growing evidence highlighting the priority of neutrophils clearance, the recognition of these cells by phagocytes is not as well understood as the recognition of other apoptotic cell types. New findings indicate that the interaction of phosphatidylserine (PS) on apoptotic neutrophils with its receptor on macrophages is not as critical for the specific clearance of neutrophil corpses it was previously believed. In this review we focus on recent findings regarding alternative, PS-independent “eat me” signals expressed on neutrophils during cell death and activation. Based on our own research, we emphasize the clearance of dying neutrophils, especially at the focus of bacterial infection; and the associated inflammatory reaction, which occurs in a highly proteolytic milieu containing both host and bacteria-derived proteinases. In these environments, eat-me signals expressed by neutrophils are drastically modified; arguing against the phospholipid-based detection of apoptotic cells, but supporting the importance of proteinaceous ligand(s) for the recognition of neutrophils by macrophages. In this context we discuss the effect of the gingipain R (Rgp) proteinases from Porphyromonas gingivalis on neutrophils interactions with macrophages. Since the recognition of apoptotic neutrophils is an important fundamental process, serving multiple functions in the regulation of immunity and homeostasis, we hypothesize that many pathogenic bacteria may have developed similar strategies to confuse macrophage-neutrophil interaction as a common pathogenic strategy.

Introduction

A paradox of immunity is that it can attack the host. In the case of innate immunity, this phenomenon can be explained by the immune system implementing the evolutionary archetypical function of phagocytosis used by primitive cells to prey on other cells. In the sophisticated immune system of multicellular organisms, phagocytosis no longer serves a feeding purpose, but has been adopted to eliminate invading pathogens and unwanted host cells. Our bodies are constantly patrolled by countless professional phagocytes, which in many ways resemble free living amoeba, and devour not only bacteria, but also neutral particles including charcoal, metal splinters, latex beads; and, most importantly, other cells. The fact that most of the phagocytic activity of a healthy organism is directed against its own phagocytes can be considered to be a nature's joke. However, as apoptosis is the only safe way to eliminate unwanted cells, including rapidly aging neutrophils, the timely and flawless removal of apoptotic cell corpses constitutes a major task. Apparently, in the process of evolution, phagocytes were gradually equipped with complex mechanisms of pattern recognition, allowing them not only to identify different pathogens (Pathogen-Associated Molecular Patterns; PAMPs), but also to recognize the molecular patterns of self-derived, dying cells (Apoptotic Cell-Associated Molecular Patterns; ACAMPs). Such recognition systems are highly redundant and overlapping. The same receptors participate in the recognition of PAMPs and ACAMPs as exemplified by the function of CD14, Toll-Like Receptors (TLRs) and scavenger receptors.

Over the last decade it has become clear that studying single receptor-ligand interactions is a limited approach towards delineating, in detail, the mechanism of apoptotic cell recognition, as this mechanism is constructed on the principle of modality. A novel term: ‘engulfment synapse’, has been proposed to depict the complex nature of the phagocyte-target cell contact site [1], [2]. On one hand, this complexity has resulted in a robust internalization system which cannot simply be blocked, inhibited, or hindered by mutations; but, on the other hand, it has implemented an extreme sensitivity and fragility to the specific recognition of particulate antigens. Even subtle changes in the composition of the ‘synapse’ may be translated to regulatory signals, resulting in gross, and sometimes contradicting, physiological effects [3].

There is no better example of macrophage-target cell interaction than that of the recognition and removal of apoptotic neutrophils, which are responsible for the safe disposal of 10¹¹ cells daily. The numbers of cleared cells increases even more during periods of inflammation. Regulation of cell homeostasis in the immune system involves constant generation of neutrophils, as well as removal of aging or damaged cells by apoptosis. Neutrophils are short living cells that in the absence of specific stimuli spontaneously enter apoptosis. Since neutrophils constitute the first line of defence against bacterial invasion, it is critical that the circulating population of cells is fully functional, ready to ingest and kill the interlopers. This state of permanent readiness is sustained by the prompt recognition and phagocytosis of apoptotic neutrophils by macrophages [4], [5], [6]. In the last decade, we have witnessed significant progress in understanding the mechanisms by which macrophages continuously monitor neutrophil viability and timely engulf dying cells. Here we discuss: (i) the controversial role of phosphatidylserine and its receptor in the phagocytosis of neutrophils; (ii) recent progress in understanding ‘engulfment synapses’ between macrophages and apoptotic neutrophils; (iii) proteinases produced by Porphyromonas gingivalis as efficient modifiers of recognition signals in periodontitis; and (iv) role of the impaired clearance of dead cells in the pathogenesis of bacterial diseases.

Section snippets

Recognition of apoptotic neutrophils–the fading glitter of phosphatidylserine?

Phagocytes recognize apoptotic cells via a repertoire of cell surface receptors that directly bind engulfment ligands exposed on the surface of targets [7], [8]. Although the molecular identity of the ‘eat me’ ligands is largely unknown, the best-studied marker of apoptosis is the exposure of phosphatidylserine (PS) to the outer leaflet of the plasma membrane [9], [10]. Several classes of receptors from engulfing cells have been implicated in binding directly to the exposed PS, or indirectly

Recognition signals on neutrophils–tagged for death

The importance of the fine balance between production of fresh neutrophils and the removal of apoptotic ones has been underestimated by modern immunology. Pathogenic, or opportunistic bacteria that colonise many niches within the human host perceive both neutrophils and macrophages as highly dangerous enemies due to their phagocytic and killing ability. During co-evolution with the immune system, some bacteria have acquired mechanisms that allow them to manipulate host pathways involved in the

Confusing ‘eat me’ signals and phagocytic receptors–an unconventional attack on innate, apoptotic immunity

A thorough understanding of the biological mechanisms behind the molecular strategy employed by P. gingivalis requires an appreciation of the hostile environment of the gingival pocket: a location crowded with professional phagocytes where the intruder attempts to survive and proliferate on the subgingival tooth surface. From the point of view of human war doctrines, such a beach-head position would be easily lost by the intruder. Colonizing pathogens in the dental plaque that are not able to

Acknowledgments

We are indebted to Dr. Ky-Anh Nguyen (Sydney, Australia) and Dr. Maria Mycielska (Imperial College London, UK) for critical reading of the manuscript and helpful discussions and to Dr. Lindsay N. Shaw from University of South Florida, Tampa, FL, USA, for language corrections. This work was supported by Grant 6 P04A 060 19 awarded to KG by the State Committee for Scientific Research (KBN, Warsaw, Poland).

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ReviewFriendly fire against neutrophils: Proteolytic enzymes confuse the recognition of apoptotic cells by macrophages

Abstract

Introduction

Section snippets

Recognition of apoptotic neutrophils–the fading glitter of phosphatidylserine?

Recognition signals on neutrophils–tagged for death

Confusing ‘eat me’ signals and phagocytic receptors–an unconventional attack on innate, apoptotic immunity

Acknowledgments

Trends Cell Biol.

Mol. Cell

Immunity

Immunol. Today

Trends Cell. Biol.

Curr. Biol.

J. Biol. Chem.

FEBS Lett.

Dev. Cell

Cell

Blood

J. Biol. Chem.

Exp. Cell Res.

J. Biol. Chem.

Curr. Biol.

Immunity

J. Biol. Chem.

Cell

Exp. Cell Res.

J. Biol. Chem.

FEBS Lett.

Blood

J. Biol. Chem.

Mol. Immunol.

Trends Microbiol.

Curr. Biol.

Curr. Opin. Microbiol.

FEBS Lett.

Phagocytic docking without shocking

Nature

Corpse clearance define the meaning of cell death

Nature

Exposure of phosphatidylserine on surface of apoptotic lymphocytes triggers specific recognition and removal by macrophages

J. Immunol.

A receptor for phosphatidylserine-specific clearance of apoptotic cells

Nature

Phagocytosis and clearance of apoptotic cells is mediated by MER

Nature

Thrombospondin cooperates with CD36 and the vitronectin receptor in macrophage recognition of PMNs undergoing apoptosis

J. Clin. Invest.

Role for the class A macrophage scavenger receptor in the phagocytosis of apoptotic thymocytes in vitro

Proc. Natl. Acad. Sci. USA

Vitronectin receptor-mediated phagocytosis of cells undergoing apoptosis

Nature

Alphavbeta5 integrin recruits the CrkII-Dock180-rac1 complex for phagocytosis of apoptotic cells

Nat. Cell Biol.

Human CD14 mediates recognition and phagocytosis of apoptotic cells

Nature

CD14-dependent clearance of apoptotic cells by human macrophages: the role of phosphatidylserine

Cell Death Differ.

Persistence of apoptotic cells without autoimmune disease or inflammation in CD14−/− mice

J. Cell Biol.

Surfactant protein A enhances alveolar macrophage phagocytosis of apoptotic neutrophils

J. Immunol.

C1q and mannose binding lectin engagement of cell surface calreticulin and CD91 initiates macropinocytosis and uptake of apoptotic cells

J. Exp. Med.

A hierarchical role for classical pathway complement proteins in the clearance of apoptotic cells in vivo

J. Exp. Med.

Early redistribution of plasma membrane phosphatidylserine is a general feature of apoptosis regardless of the initiating stimulus: inhibition by overexpression of Bcl-2 and Abl

J. Exp. Med.

C. elegans mitochondrial factor WAH-1 promotes phosphatidylserine externalization in apoptotic cells through phospholipid scramblase SCRM-1

Nat. Cell Biol.

Membrane phosphatidylserine distribution as a non-apoptotic signalling mechanism in lymphocytes

Nat. Cell Biol.

CD36 is required for phagocytosis of apoptotic cells by human macrophages that use either a phosphatidylserine receptor or vitronectin receptor (αvβ3)

J. Immunol.

Oxidized phosphatidylserine-CD36 interactions play an essential role in macrophage-dependent phagocytosis of apoptotic cells

J. Exp. Med.

Phosphatidylserine receptor is required for clearance of apoptotic cells

Science

The phosphatidylserine receptor has essential functions during embryogenesis but not in apoptotic cell removal

J. Biol.

Review
Friendly fire against neutrophils: Proteolytic enzymes confuse the recognition of apoptotic cells by macrophages

CD36 is required for phagocytosis of apoptotic cells by human macrophages that use either a phosphatidylserine receptor or vitronectin receptor (α_vβ₃)