Extracellular Histones Inhibit Efferocytosis

Arnaud Friggeri,Sami Banerjee,Na Xie,Mourad Zerfaoui,Gang Liu,Hervé Dupont,Andressa De Freitas,Huachun Cui,Edward Abraham

doi:10.2119/molmed.2012.00005

Abstract

The uptake and clearance of apoptotic cells by macrophages and other phagocytic cells, a process called efferocytosis, is a major component in the resolution of inflammation. Increased concentrations of extracellular histones are found during acute inflammatory states and appear to contribute to organ system dysfunction and mortality. In these studies, we examined the potential role of histones in modulating efferocytosis. We found that phagocytosis of apoptotic neutrophils or thymocytes by macrophages was significantly diminished in the presence of histones H3 or H4, but not histone H1. Histone H3 demonstrated direct binding to macrophages, an effect that was diminished by preincubation of macrophages with the opsonins growth arrest-specific gene 6 (Gas6) and milk fat globule-epidermal growth factor (EGF) 8 (MFG-E8). Incubation of histone H3 with soluble α(v)β₅ integrin and Mer, but not with α(v)β₃, diminished its binding to macrophages. Phagocytosis of apoptotic cells by alveolar macrophages in vivo was diminished in the presence of histone H3. Incubation of histone H3 with activated protein C, a treatment that degrades histones, abrogated its inhibitory effects on efferocytosis under both in vitro and in vivo conditions. The present studies demonstrate that histones have inhibitory effects on efferocytosis, suggesting a new mechanism by which extracellular histones contribute to acute inflammatory processes and tissue injury.

Highlights

The uptake and clearance of apoptotic cells by macrophages and other phagocytic cells, a process called “efferocytosis,” is a major component in the resolution of inflammatory states [1,2]
Histone H3 decreased the ability of macrophages to ingest apoptotic thymocytes (Figure 1B), indicating that the inhibitory effect of histone H3 on efferocytosis is a general phenomenon and not specific for apoptotic neutrophils
To rule out this possibility, macrophages were incubated with increasing doses of histone H3, as used in the efferocytosis assays, and cytotoxicity was determined by measuring LDH levels in the media

Summary

Introduction

The uptake and clearance of apoptotic cells by macrophages and other phagocytic cells, a process called “efferocytosis,” is a major component in the resolution of inflammatory states [1,2]. Engulfment of apoptotic cells decreases local inflammation through enhancing release of antiinflammatory cytokines and suppressing production of proinflammatory mediators by the phagocytic cells [3,4]. Engulfment of apoptotic cells requires the recognition of “eat-me” signals on the surface of the dying cell through interaction with specific receptors expressed on the phagocyte [3,4,5]. One of the best characterized eat-me signals is phosphatidylserine, a phospholipid situated on the inner leaflet of the cell membrane of viable cells that is exposed on the cell surface during the early stages of apoptosis.

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Conclusion