Neutrophils M2 Markers and Efferocytosis of Apoptotic Amyloid A in the Induction of Macrophage Ex Vivo and In Vitro Effect of Serum

Macrophages affect the magnitude and duration of inflammatory response in a functionally heterogeneous manner. The phenotype of macrophages is maintained through a reversible homeostatic mechanism. A number of determinants that modulate macrophage plasticity have been identified, although the precise mechanisms are not fully understood. In this study, we report that stimulation of isolated human blood monocytes and mouse bone marrow–derived macrophages with human serum amyloid A (SAA), a major acute-phase protein, leads to induced expression of macrophage M2 markers, including IL-10, Ym1, Fizz-1, MRC1, IL-1Rn, and CCL17. The same effect was observed with macrophages exposed to SAA in peritoneal cavity. SAA also increases arginase 1 activity and enhances macrophage efferocytosis of apoptotic neutrophils in mouse macrophages. The induction of M2 markers requires MyD88 and the activation of multiple signaling pathways, but it is independent of Stat6. SAA induces IFN regulatory factor (IRF)4 expression and increases its DNA-binding activity. Silencing IRF4 by small interfering RNA abrogates SAA-induced expression of the M2 markers. These results suggest a potential role for SAA to alter macrophage phenotype and modulate macrophage functions through an MyD88-dependent mechanism that involves IRF4-mediated transcription. M acrophages are dynamic and heterogeneous innate immune cells that are essential for the initiation and resolution of inflammation. These cells display different phenotypes and respond to environmental cues in a polarized manner (1, 2). Polarized macrophages have been broadly classified into two groups, M1 macrophages and M2 macrophages (3, 4). Macrophages stimulated with LPS and IFN-g assume an M1 proinflammatory phenotype, which is characterized by high levels of inducible NO synthase expression, increased capacity of Ag presentation, and elevated production of proinflammatory cyto-kines, including TNF-a, IL-1b, and IL-12 (5). The M2 macro-phages, which are induced by exposure to IL-4, IL-13, TGF-b, and glucocorticoids, produce anti-inflammatory cytokines, including IL-10, TGF-b, IL-1R antagonist (IL-1Rn), CCL17, and, in mice, are found in inflammatory zone-1 (Retnla, Fizz-1) (3), chitinase 3–like 3 (Chi3l3, Ym1), and arginase 1 (Arg1) (6). Additionally, the M2 macrophages express high levels of CD206 (mannose receptor C type 1 [MRC1]) and CD163 (1, 6, 7). The different cytokines, surface markers, and metabolic enzymes expressed by these macrophage subsets reflect the diverse functions of the M1 and M2 macrophages. The M2 macrophages are often seen in the late phase of inflammation and are thought to play a role in the resolution of inflammation, tissue repair, and remodeling (8). Neutrophils are the first cells recruited to the …