Elevated Expression of Serum Amyloid A 3 Protects Colon Epithelium Against Acute Injury Through TLR2-Dependent Induction of Neutrophil IL-22 Expression in a Mouse Model of Colitis

Abstract

Induced expression of serum amyloid A (SAA) is a hallmark of many inflammatory diseases, but whether SAA exacerbates inflammation or protects tissues against injury remains unclear. In dextran sulfate sodium (DSS)-induced colitis, SAA3 is the predominant isoform of inducible SAA proteins that also include SAA1 and SAA2, and mice with genetic deletion of Saa3 exhibits increased production of proinflammatory cytokines, decreased expression of IL-22 along with aggravated epithelium disruption, and reduced colon length compared with wild-type littermates. Colonic neutrophils have been identified as a major source of IL-22 in these mice. Administration of exogenous SAA3 as recombinant protein to Saa3−/− mice improves neutrophil IL-22 production, colonic epithelial integrity, and secretion of the antimicrobial peptides Reg3β and Reg3γ. Stimulation of mouse bone marrow neutrophils with mouse SAA3 or human SAA1 leads to expansion of IL-22-producing neutrophils. Unlike previously reported IL-22 induction through IL-23, the SAA3-induced neutrophil IL-22 expression utilizes a TLR2-dependent mechanism that does not depend on IL-23. Adoptive transfer of the SAA3-treated neutrophils to Saa3−/− mice ameliorates DSS-induced colitis and improves colonic epithelial integrity. These findings suggest that in the DSS-induced mouse colitis model, SAA isoforms are expressed to different extent in colon and deletion of Saa3 renders these mice more susceptible to DSS-induced injury. The presence of SAA3 in the inflamed colon mucosal serves to protect epithelial barrier in part through expansion of IL-22-producing neutrophils. It is speculated that SAA3 stimulation of autologous neutrophils may have therapeutic potential for inflammatory bowel disease.

Serum amyloid A promotes LPS clearance and suppresses LPS ‐induced inflammation and tissue injury

“…Five peptides covering the entire lengths of mature SAA1 protein were prepared ( Fig 6A). In vitro characterization of these peptides in SAA1-LPSbinding assays identified Pep 2 (a.a. [32][33][34][35][36][37][38][39][40][41][42][43][44][45][46][47] as being most potent in disrupting the interaction between SAA1 and LPS, in binding assays with a fixed concentration of the peptides and variable concentrations of LPS ( Fig 6B) or a fixed concentration of LPS and variable concentrations of the peptides (Fig 6C). To examine its in vivo effect, Pep 2 or a control peptide with scrambled sequence was injected i.v.…”

“…A recent study reports that, under low pH conditions (pH 3.5-4.5), SAA1 forms soluble oligomers that undergo an ahelix to b-sheet conversion catalyzed by lipid vesicles [27]. Pep 2 (a.a. [32][33][34][35][36][37][38][39][40][41][42][43][44][45][46][47] that disrupts SAA1-LPS interaction overlaps with the second helix of mature SAA1, suggesting that helix II or a 3D structure containing helix II is involved in LPS binding. In addition, the sequence of Pep 2 partially overlaps with SAA fragments (e.g., a.a. 11-58) that were found recently to suppress LPSinduced inflammatory response [33].…”

Intestinal epithelial cells: at the interface of the microbiota and mucosal immunity

“…In addition to mechanosensing, a recent study demonstrated that several metabolites from a consortium of commensal spore‐forming bacteria (predominantly Clostridial species) promote 5‐HT biosynthesis by colonic enterochromaffin cells in colonized mice . In response to microbes, IECs also secrete a number of cytokines and effector molecules including interleukin‐25 (IL‐25) and SAA . These effectors regulate the development and function of intestinal immune cells, as described in the next section of this review.…”

“…37 In response to microbes, IECs also secrete a number of cytokines and effector molecules including interleukin-25 (IL-25) and SAA. 38,39 These effectors regulate the development and function of intestinal immune cells, as described in the next section of this review. Collectively, these recent findings indicate that a broad range of IEC functions are affected by sensing of intestinal microbes (Table 1); however, it is worth noting that many of these studies were performed in the context of pathogenic microbial infection.…”

Serum Amyloid A and Immunomodulation

“…TLR4 was found to mediate SAA-induced expression of iNOS and activation of the related signaling pathways [87]. Despite differences in primary and high-level structures between SAA and the microbial ligands for these receptors, the two TLRs mediate SAA functions both in transfected cells expressing the receptors and in vivo [48,71,79,[88][89][90][91][92]. The identification of the two TLRs as SAA receptors illustrates the possible roles for TLRs in detecting host-derived molecules as a mechanism for alerting immune cells upon exposure to environmental stress.…”