Extracellular Chromatin Triggers Release of Soluble CEACAM8 Upon Activation of Neutrophils.

Matthieu Ribon,Patrice Decker,Bernhard B Singer,Luca Semerano,Julie Mussard

doi:10.3389/fimmu.2019.01346

Abstract

Increased concentrations of extracellular chromatin are observed in cancer, sepsis, and inflammatory autoimmune diseases like systemic lupus erythematosus (SLE) or rheumatoid arthritis (RA). In SLE and RA, extracellular chromatin may behave as a danger-associated molecular pattern (DAMP). Polymorphonuclear neutrophils (PMN) are described as typical pro-inflammatory cells but possess also immunoregulatory properties. They are activated in SLE and RA but surprisingly remain moderately studied in these diseases, and especially the disease-associated stimuli triggering PMN activation are still not completely characterized. PMN express plasma membrane carcinoembryonic antigen-related cell adhesion molecule (CEACAM) 8 (CD66b) and secrete a soluble form of CEACAM8 after activation. Soluble CEACAM8 has in turn immunoregulatory functions. However, few natural stimuli inducing soluble CEACAM8 secretion by PMN have been identified. Here we demonstrate for the first time that extracellular chromatin triggers secretion of soluble CEACAM8 by primary human PMN. Priming of PMN was not required. Secretion was associated with activation of PMN. Similar induction of soluble CEACAM8 release was observed with purified mono-nucleosomes as well as long chromatin fragments and occurred in a time-dependent and concentration-dependent manner. Results indicate that chromatin induces both neo-synthesis of soluble CEACAM8 and release of soluble CEACAM8 through degranulation. In addition, we report the presence of soluble CEACAM8 at high concentration in the synovial fluid of RA patients. Thus, we describe here a novel mechanism by which a natural DAMP, with inflammatory properties in SLE and RA, induces soluble CEACAM8 secretion by activated PMN with potential immunoregulatory consequences on other immune cells, including PMN.

Highlights

Systemic lupus erythematosus (SLE) and rheumatoid arthritis (RA) are two inflammatory autoimmune and rheumatic diseases of unknown etiology and triggered by a combination of genetic and environmental factors as well as immune dysregulation
Secretion of neo-synthesized soluble CEACAM8 is already detectable after 6 h (Supplementary Figure 2) but differences between non-stimulated and stimulated Polymorphonuclear neutrophils (PMN) are amplified after 14 h, because soluble CEACAM8 is accumulated in supernatants over time after stimulation without increasing spontaneous secretion in non-stimulated PMN
Chromatin-induced soluble CEACAM8 secretion was associated with PMN activation, as shown by plasma membrane CD66b (CEACAM8) and CD11b up-regulation (Figures 2A–E) and IL-8 secretion (Figure 2F)

Summary

Introduction

Systemic lupus erythematosus (SLE) and rheumatoid arthritis (RA) are two inflammatory autoimmune and rheumatic diseases of unknown etiology and triggered by a combination of genetic and environmental factors as well as immune dysregulation. In RA patients, cell-free chromatin is detected in the synovial fluid of inflamed joints [3] and deposits in affected joints where they form immune complexes [4] In these patients, chromatin might be released by polymorphonuclear neutrophils (PMN) recruited into inflamed tissues and dying after activation, or part of it might derive from neutrophil extracellular traps (NET) released (NETosis) upon activation [5]. Chromatin triggers activation of dendritic cells from healthy donors (HD) and SLE patients [6] It activates PMN from HD, SLE, and RA patients [7] in a Toll-like receptor (TLR) 9-independent manner [8], leading to secretion of proinflammatory cytokines as well as interferon (IFN)-α, a key cytokine in SLE, and induction of NET [9]. We observed that once recognized, chromatin is endocytosed by PMN

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Conclusion