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

Interleukin 17A (IL-17A) and complement (C') activation have each been implicated in the pathogenesis of idiopathic pulmonary fibrosis (IPF). We have reported that IL-17Ainduces epithelial injury via TGF-beta inmurine bronchiolitis obliterans; that TGF-b and the C' cascade present signaling interactions inmediating epithelial injury; and that the blockade of C' receptorsmitigates lung fibrosis. In the present study, we investigated the role of IL-17A in regulating C' 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 C' components (C' 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 C' components and the synthesis of active C' 3a (C3a) in normal primary human alveolar type II epithelial cells (AECs). Wild-type mice subjected to IL-17A neutralization and IL-17A knockout (il17a(-/-)) mice were protected against bleomycin (BLEO)-induced fibrosis and collagen deposition. Further, BLEO-injured il17a(-/-) mice had diminished levels of circulating Krebs Von Den Lungen 6 (alveolar epithelial injurymarker), local caspase-3/7, andlocalendoplasmic reticular stress-relatedgenes. BLEO-induced local C' activation [C3a, C5a, and terminal C' complex (C5b-9)] was attenuated in il17a(-/-) mice, and IL-17Aneutralization prevented the loss of epithelial C' inhibitors (C' receptor-1 related isoform Y and decay accelerating factor), and an increase in local TUNEL levels. RNAi-mediated gene silencing of il17a in fibroticmice arrested the progression of lung fibrosis, attenuated cellular apoptosis (caspase-3/7) and lung deposition of collagen and C' (C5b-9). Compared tonormals, plasma fromIPF patients showed significantlyhigher hemolytic activity. Our findings demonstrate that limiting complement activation by neutralizing IL-17A is a potential mechanism in ameliorating lung fibrosis.