IL-17A deficiency mitigates bleomycin-induced complement activation during lung fibrosis

Abstract

Interleukin 17A (IL-17A) and complement (C9) activation have each been implicated in the pathogenesis of idiopathic pulmonary fibrosis (IPF). We have reported that IL-17A induces epithelial injury via TGF-b in murine bronchiolitis obliterans; that TGF-b and the C9 cascade present signaling interactions in mediating epithelial injury; and that the blockade of C9 receptors mitigates lung fibrosis. In the present study, we investigated the role of IL-17A in regulating C9 in lung fibrosis. Microarray analyses of mRNA isolated from primary normal human small airway epithelial cells indicated that IL-17A (100 ng/ml; 24 h; n = 5 donor lungs) induces C9 components (C9 factor B, C3, and GPCR kinase isoform 5), cytokines (IL8, -6, and -1B), and cytokine ligands (CXCL1, -2, -3, -5, -6, and -16). IL-17A induces protein and mRNA regulation of C9 components and the synthesis of active C9 3a (C3a) in normal primary human alveolar type II epithelial cells (AECs). Wild-type mice subjected to IL-17A neutralization and IL-17A knockout (il17a2/2) mice were protected against bleomycin (BLEO)-induced fibrosis and collagen deposition. Further, BLEO-injured il17a2/2 mice had diminished levels of circulating Krebs Von Den Lungen 6 (alveolar epithelial injury marker), local caspase-3/7, and local endoplasmic reticular stress-related genes. BLEO-induced local C9 activation [C3a, C5a, and terminal C9 complex (C5b-9)] was attenuated in il17a2/2 mice, and IL-17A neutralization prevented the loss of epithelial C9 inhibitors (C9 receptor-1 related isoform Y and decay accelerating factor), and an increase in local TUNEL levels. RNAi-mediated gene silencing of il17a in fibrotic mice arrested the progression of lung fibrosis, attenuated cellular apoptosis (caspase-3/7) and lung deposition of collagen and C9 (C5b-9). Compared to normals, plasma from IPF patients showed significantly higher hemolytic activity. Our findings demonstrate that limiting complement activation by neutralizing IL-17A is a potential mechanism in ameliorating lung fibrosis.