Rationale: Neutrophilic inflammation is an important pathologic feature of chronic obstructive pulmonary disease (COPD) and infectious exacerbations of COPD. Serum amyloid A (SAA) promotes neutrophilic inflammation by its interaction with lung mucosal ALX/FPR2 receptors. However, little is known about how this endogenous mediator regulates IL-17A immunity.

Objectives: To determine whether SAA causes neutrophilic inflammation by IL-17A–dependent mechanisms.

Methods: The relationship between SAA and neutrophils was investigated in lung sections from patients with COPD and a chronic mouse model of SAA exposure. A neutralizing antibody to IL-17A was used to block SAA responses in vivo, and a cell-sorting strategy was used to identify cellular sources.

Measurements and Main Results: SAA mRNA expression was positively associated with tissue neutrophils in COPD (P < 0.05). SAA predominately promoted expression of the T_H17 polarizing cytokine IL-6, which was opposed by 15-epi-lipoxin A₄, a counter-regulatory mediator, and ALX/FPR2 ligand. SAA-induced inflammation was markedly reduced by a neutralizing antibody to IL-17A in vivo. Cellular sources of IL-17A induced by SAA include CD4⁺ T cells, γδ T cells, and an Epcam⁺CD45⁻ population enriched for epithelial cells. SAA promotes expression of IL-17A in γδ T cells and this innate cell proportionally expressed higher levels of IL-17A transcript than CD4⁺ T cells or epithelial cells.

Conclusions: The SAA–IL-17A axis represents an important innate defense network that may underlie persistent neutrophilic airway inflammation in COPD and modulating the ALX/FPR2 receptor represents a novel approach to targeting aberrant IL-17A–mediated lung immunity.