COOH-terminal SAA1 peptides fail to induce chemokines but synergize with CXCL8 and CCL3 to recruit leukocytes via FPR2.

A natural leukocyte chemoattractant was isolated from bovine serum by an established four-step purification procedure. Based on its relative molecular mass of 7287 and NH2 -terminal sequence, the protein was identified as a carboxy-terminal peptide of the acute phase protein serum amyloid A (SAA) 1. This SAA1(46-112) fragment and its human equivalent SAA1(47-104) were chemically synthesized. Unlike intact SAA1α, these SAA fragments failed to directly chemoattract neutrophils and monocytes, to induce chemokines and to stimulate downstream ERK signaling in monocytes. However, the SAA fragments potently synergized with CCL3 to induce monocyte migration and with CXCL8 to stimulate neutrophil shape change and chemotaxis. Unlike intact SAA1α, SAA1(46-112) did not induce CXCL6 ex vivo, but provoked a cooperative intraperitoneal neutrophil recruitment in mice when co-injected with CXCL6 into the peritoneal cavity. Moreover, SAA1(47-104) desensitized the synergy between intact SAA1α and CXCL8 in neutrophil chemotaxis, suggesting that this peptide binds formyl peptide receptor (FPR) 2. This was evidenced by a complete blockade of synergy between the COOH-terminal SAA1 fragments and CXCL8 or CCL3 in neutrophil and monocyte chemotaxis, respectively, by the FPR2 antagonist WRW4 Thus, SAA1 is degraded into fragments lacking chemokine-inducing capacity, whilst keeping synergy with cytokine-induced chemokines to sustain limited inflammation.