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

Abstract

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. Here 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 is independent of Stat6. SAA induces IRF4 expression and increases its DNA-binding activity. Silencing IRF4 by siRNA 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 a MyD88-dependent mechanism that involves IRF4-mediated transcription.

Emerging functions of serum amyloid A in inflammation

“…Unlike most inflammatory cytokines, rhSAA has pleotropic effects in vitro, as it not only induces the expression of proinflammatory cytokines but also those exhibiting anti-inflammatory properties, such as IL-1rn and IL-10 [34]. As reported at the 2014 Joint Meeting of Society for Leukocyte Biology & International Endotoxin and Innate Immunity Society, human SAA1 are able to skew macrophages toward the M2 phenotype, including an enhanced ability to clear apoptotic neutrophils (efferocytosis), and induced expression of a set of secreted and cell-surface proteins, collectively termed the M2 markers [37]. More recent studies have shown that rhSAA stimulates the expression of IL-33 that modulates different functions of immune cells [36].…”

Serum amyloid A1: Structure, function and gene polymorphism

“…These studies identified chemotactic activity of the rhSAA for phagocytes (Badolato et al, 1994) as well as its ability to induce proinflammatory cytokines ex vivo (Patel et al, 1998; Furlaneto and Campa, 2000; He et al, 2003; Cai et al, 2007; Sun et al, 2015). These activities are mediated by several receptors for rhSAA including FPR2 (FPR2/ALX, formerly FPRL1) (Su et al, 1999; Ye et al, 2009), TLR2 (Cheng et al, 2008) and TLR4 (Sandri et al, 2008).…”

“…It was also reported that the THP-1-derived macrophages responded to rhSAA1 with the upregulation of 55 genes, many of which being proinflammatory cytokines induced by the chimeric rhSAA (Leow et al, 2012). A recent study has shown that rhSAA and rhSAA1 are both capable of promoting macrophage switch to the M2 phenotype, characterized by the induced expression of the M2 cytokines (Sun et al, 2015). This latter study suggests that SAA1 may be a homeostatic regulator of inflammation.…”

“…At least 6 functional receptors for SAA, including the receptor for advanced glycation end products (RAGE), the formyl peptide receptor 2 (FPR2), the scavenger receptor class B type I (SR-BI), the Toll-like receptors 2 (TLR2) and 4 (TLR4) and the purinoceptor P2X7 receptor, have been identified to date (recently reviewed in (Malle et al, 2009; Sun et al, 2015)). These receptors mediate various functions of SAA1 and possibly other forms of SAA proteins.…”

Cutting Edge: A Cullin-5–TRAF6 Interaction Promotes TRAF6 Polyubiquitination and Lipopolysaccharide Signaling

“…Bone marrow-derived macrophages (BMDMs) were prepared from wild-type (WT) and Cul-5 +/2 mice, as described previously (14). Culture medium of BMDMs treated with LPS was collect at the time points indicated in the figure legends.…”