Antibody Cross-Linking of CD14 Activates MerTK and Promotes Human Macrophage Clearance of Apoptotic Neutrophils: the Dua
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ORIGINAL ARTICLE
Antibody Cross-Linking of CD14 Activates MerTK and Promotes Human Macrophage Clearance of Apoptotic Neutrophils: the Dual Role of CD14 at the Crossroads Between M1 and M2c Polarization Gaetano Zizzo1,2,3 and Philip L. Cohen1,2,3
Abstract— Mer receptor tyrosine kinase (MerTK) is key for efficient phagocytosis of apoptotic neutrophils (ANs) and homeostasis of IL-10 production by human antiinflammatory M2c monocytes/macrophages. We asked whether stimulation of M2c surface receptors contributes in turn to MerTK activation. For this purpose, human monocytes/ macrophages were differentiated under M1, M2a, and M2c polarizing conditions. The effects of antibody-mediated cross-linking of M2c receptors (i.e., CD14, CD16, CD32, CD163, CD204) on MerTK phosphorylation and phagocytosis of ANs were tested. MerTK expression was also studied by flow cytometry and western blot in the presence of LPS and in M2c-derived microvesicles (MVs). Antibody cross-linking of either CD14 or CD32/FcγRII led to Syk activation and MerTK phosphorylation in its two distinct glycoforms (175–205 and 135– 155 kDa). Cross-linked CD14 enhanced efferocytosis by M2c macrophages and enabled M1 and M2a cells to clear ANs efficiently. In M1 conditions, LPS abolished surface MerTK expression on CD14bright cell subsets, so disrupting the anti-inflammatory pathway. In M2c cells, instead, MerTK was diffusely and brightly co-expressed with CD14, and was also detected in M2c macrophage-derived MVs; in these conditions, LPS only partially downregulated MerTK on cell surfaces, while the smaller MerTK glycoform contained in MVs remained intact. Altogether, cooperation between CD14 and MerTK may foster the clearance of ANs by human monocytes/macrophages. CD14 stands between M1-related LPS co-receptor activity and M2c-related MerTK-dependent response. MerTK interaction with CD32/FcγRII, its detection in M2c MVs, and the differential localization and LPS susceptibility of MerTK glycoforms add further new elements to the complexity of the MerTK network. KEY WORDS: Mer receptor tyrosine kinase (MerTK); CD14; CD32 (FcγRII); human macrophages; apoptotic neutrophils (ANs); microvesicles (MVs).
1
Temple Autoimmunity Center, Temple University, 3500 N. Broad Street, Philadelphia, PA 19140, USA 2 Section of Rheumatology, Department of Medicine, Lewis Katz School of Medicine at Temple University, 3322 N. Broad Street, Philadelphia, PA 19140, USA 3 To whom correspondence should be addressed at Section of Rheumatology, Department of Medicine, Lewis Katz School of Medicine at Temple University, 3322 N. Broad Street, Philadelphia, PA 19140, USA. E-mails: [email protected]; [email protected]
INTRODUCTION Dying neutrophils, if not rapidly removed, can release large amounts of danger signals, nucleic acids, and other antigenic material, so triggering the development of chronic inflammation and autoimmunity [1]. Macrophage-expressed Mer receptor tyrosine kinase (MerTK), through its ligands growth arrest-specific 6
0360-3997/18/0000-0001/0 # 2018 Springer Scie
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