Serum amyloid A (SAA) is, like C-reactive protein (CRP), an acute phase protein and can be used as a diagnostic, prognostic or therapy follow-up marker for many diseases. Increases in serum levels of SAA are triggered by physical insults to the host, including infection, trauma, inflammatory reactions and cancer. The order of magnitude of increase in SAA levels varies considerably, from a 10- to 100-fold during limited inflammatory events to a 1000-fold increase during severe bacterial infections and acute exacerbations of chronic inflammatory diseases. This broad response range is reflected by SAA gene duplications resulting in a cluster encoding several SAA variants and by multiple biological functions of SAA. SAA variants are single-domain proteins with simple structures and few post-translational modifications. SAA1 and SAA2 are inducible by inflammatory cytokines, whereas SAA4 is constitutively produced. We review here the regulated expression of SAA in normal and transformed cells and compare its serum levels in various disease states. At low concentrations (10-100 ng/ml), early in an inflammatory response, SAA induces chemokines or matrix degrading enzymes via Toll-like receptors and functions as an activator and chemoattractant through a G protein-coupled receptor. When an infectious or inflammatory stimulus persists, the liver continues to produce more SAA (> 1000 ng/ml) to become an antimicrobial agent by functioning as a direct opsonin of bacteria or by interference with virus infection of host cells. Thus, SAA regulates innate and adaptive immunity and this information may help to design better drugs to treat specific diseases.

Levels of serum amyloid A (SAA), a major acute phase protein in humans, are increased up to 1000-fold upon infection, trauma, cancer or other inflammatory events. However, the exact role of SAA in host defense is yet not fully understood. Several pro- and anti-inflammatory properties have been ascribed to SAA. Here, the regulated production of SAA by cytokines and glucocorticoids is discussed first. Secondly, the cytokine and chemokine inducing capacity of SAA and its receptor usage are reviewed. Thirdly, the direct (via FPR2) and indirect (via TLR2) chemotactic effects of SAA and its synergy with chemokines are unraveled. Altogether, a complex cytokine-SAA-chemokine network is established, in which SAA plays a key role in regulating the inflammatory response.

Cell migration depends on the ability of leukocytes to sense an external gradient of chemotactic proteins produced during inflammation. These proteins include chemokines, complement factors, and some acute phase proteins, such as serum amyloid A. Serum amyloid A chemoattracts neutrophils, monocytes, and T lymphocytes via its G protein-coupled receptor formyl peptide receptor 2. We demonstrate that serum amyloid A1α more potently chemoattracts neutrophils in vivo than in vitro. In contrast to CD14(+) monocytes, no rapid (within 2 h) induction of interleukin-8/CXC chemokine ligand 8 or macrophage-inflammatory protein-1α/CC chemokine ligand 3 was observed in purified human neutrophils after stimulation of the cells with serum amyloid A1α or lipopolysaccharide. Moreover, interleukin-8/CXC chemokine ligand 8 induction in monocytes by serum amyloid A1α was mediated by toll-like receptor 2 and was inhibited by association of serum amyloid A1α with high density lipoprotein. This indicates that the potent chemotactic response of neutrophils toward intraperitoneally injected serum amyloid A1α is indirectly enhanced by rapid induction of chemokines in peritoneal cells, synergizing in a paracrine manner with serum amyloid A1α. We observed direct synergy between IL-8/CXC chemokine ligand 8 and serum amyloid A1α, but not lipopolysaccharide, in chemotaxis and shape change assays with neutrophils. Furthermore, the selective CXC chemokine receptor 2 and formyl peptide receptor 2 antagonists, SB225002 and WRW4, respectively, blocked the synergy between IL-8/CXC chemokine ligand 8 and serum amyloid A1α in neutrophil chemotaxis in vitro, indicating that for synergy their corresponding G protein-coupled receptors are required. Additionally, SB225002 significantly inhibited serum amyloid A1α-mediated peritoneal neutrophil influx. Taken together, endogenous (e.g., IL-1β) and exogenous (e.g., lipopolysaccharide) inflammatory mediators induce primary chemoattractants such as serum amyloid A that synergize in an autocrine (monocyte) or a paracrine (neutrophil) fashion with secondary chemokines induced in stromal cells.

Serum amyloid A (SAA) is an acute phase protein that is upregulated in inflammatory diseases and chemoattracts monocytes, lymphocytes, and granulocytes via its G protein-coupled receptor formyl peptide receptor like 1/formyl peptide receptor 2 (FPRL1/FPR2). Here, we demonstrated that the SAA1α isoform also chemoattracts monocyte-derived immature dendritic cells (DCs) in the Boyden and μ-slide chemotaxis assay and that its chemotactic activity for monocytes and DCs was indirectly mediated via rapid chemokine induction. Indeed, SAA1 induced significant amounts (ࣙ5 ng/mL) of macrophage inflammatory protein-1α/CC chemokine ligand 3 (MIP-1α/CCL3) and interleukin-8/CXC chemokine ligand 8 (IL-8/CXCL8) in monocytes and DCs in a dosedependent manner within 3 h. However, SAA1 also directly activated monocytes and DCs for signaling and chemotaxis without chemokine interference. SAA1-induced monocyte migration was nevertheless significantly prevented (60-80% inhibition) in the constant presence of desensitizing exogenous MIP-1α/CCL3, neutralizing anti-MIP-1α/CCL3 antibody, or a combination of CC chemokine receptor 1 (CCR1) and CCR5 antagonists, indicating that this endogenously produced CC chemokine was indirectly contributing to SAA1-mediated chemotaxis. Further, anti-IL-8/CXCL8 antibody neutralized SAA1-induced monocyte migration, suggesting that endogenous IL-8/CXCL8 acted in concert with MIP-1α/CCL3. This explained why SAA1 failed to synergize with exogenously added MIP-1α/CCL3 or stromal cell-derived factor-1α (SDF-1α)/CXCL12 in monocyte and DC chemotaxis. In addition to direct leukocyte activation, SAA1 induces a chemotactic cascade mediated by expression of cooperating chemokines to prolong leukocyte recruitment to the inflammatory site. Additional supporting information may be found in the online version of this article at the publisher's web-site Keywords: Chemokines IntroductionOver the past decades, a large number of endogenous inflammatory mediators that chemoattract leukocytes in vitro and provoke Correspondence: Prof. Jo Van Damme e-mail: jo.vandamme@rega.kuleuven.be leukocyte migration upon in vivo administration have been identified. These include complement factors (such as C5a or anaphylatoxin) present in the blood circulation, and also chemotactic cytokines (in particular chemokines) produced either locally in * These authors contributed equally to this work.C 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.eji-journal.eu 102Mieke Gouwy et al. Eur. J. Immunol. 2015. 45: 101-112 specific tissues or secreted systemically in response to infection and inflammation. Several cytokines, which present chemotactic activity, are able to provoke leukocyte recruitment in vivo, but fail to stimulate directed cell migration in vitro. For example, the inflammatory cytokine interleukin-1β (IL-1β) induces leukopenia/leukocytosis upon intravenous injection and elicits neutrophil accumulation upon local administration [1]. More detailed studies revealed that IL-1β is by itself not chemotactic for leukocytes, but fun...