Crosstalk between TGF‐β1 and complement activation augments epithelial injury in pulmonary fibrosis

Abstract

The epithelial complement inhibitory proteins (CIPs) cluster of differentiation 46 and 55 (CD46 and CD55) regulate circulating immune complex-mediated complement activation in idiopathic pulmonary fibrosis (IPF). Our previous studies demonstrated that IL-17A mediates epithelial injury via transforming growth factor ␤1 (TGF-␤1) and down-regulates CIPs. In the current study, we examined the mechanistic role of TGF-␤1 in complement activationmediated airway epithelial injury in IPF pathogenesis. We observed lower epithelial CIP expression in IPF lungs compared to normal lungs, associated with elevated levels of complement component 3a and 5a (C3a and C5a), locally and systemically. In normal primary human small airway epithelial cells ( Idiopathic pulmonary fibrosis (IPF) is a disease of high mortality for which lung transplantation is considered the only definitive therapy. Its pathogenesis remains largely unknown (1), but emerging concepts point to repeated injury to bronchiole-like epithelial cells and hyperplastic type II alveolar epithelial cells lining areas of honeycomb fibrosis (1, 2). These injured epithelial cells produce key profibrotic factors, including transforming growth factor ␤ (TGF-␤), which is implicated in epithelial injury (3-5) and epithelial-tomesenchymal transition (EMT; refs. 6, 7).The complement system is an integral arm of innate and adaptive immunity. Early studies demonstrated evidence of circulating immune complexes (8) and complement activation (9) in patients with IPF. In experimental models of IPF, antifibrotic effects due to deletion of complete downstream complement factors (10), specifically complement component 5 (C5; ref. 11), were reported. C3a and C5a are implicated in autoimmune diseases (12), chronic lung transplant rejection (13), experimental allergic asthma (14), and Abbreviations: ATII, alveolar type II; C3a, complement component 3a; C3aR, complement component 3a receptor; C5a, complement component 5a; C5aR, complement component 5a receptor; CD46, cluster of differentiation 46; CD55, cluster of differentiation 55; CIP, complement inhibitory protein; E-CAD, E-cadherin; EMT, epithelial-mesenchymal transition; GAPDH, glyceraldehyde-3-phosphate dehydrogenase; IL-17A, interleukin-17A; IPF, idiopathic pulmonary fibrosis; p38MAPK, mitogen-activated protein kinase; PARP, poly(ADP-ribose) polymerase; PCR, polymerase chain reaction; RNAi, RNA interference; SABM, small airway basal medium; SAEC, small airway epithelial cell; siRNA, small interference RNA; SMAD7, mothers against decapentaplegic homolog 7; TGF-␤1, transforming growth factor ␤, isoform 1 4223 0892-6638/14/0028-4223 © FASEB wnloaded from www.fasebj.org by (158

Potential Mechanisms Underlying TGF-β-mediated Complement Activation in Lung Fibrosis

“…We had reported elevated local C3a and C5a in clinical [2] and experimental lung fibrosis [4]. Our prior studies indicate an induction of C3aR and C5aR expression in lung epithelial cells in response to TGF-β [2]; and in mesenchymal cells in response to C3a or C5a [4].…”

“…Antibodies used were against C3aR [3], and C5aR [3] were purchased from Novus Biologicals, Littleton, CO. CD46 [2] and DAF [3] antibodies were purchased from Santa Cruz Biotechnology. Densitometric analyses were performed with ImageJ 1.32j (NIH, Bethesda, MD).…”

“…Studies from our lab demonstrate two different inflammatory signaling cascades, i.e. IL-17A [1] and the complement cascade [2] which trigger TGF-β-related signaling pathways. Our recent report also suggests that IL-17A may drive fibrosis via the complement cascade [3].…”

“…The complement cascade has been implicated in IPF [2,4-6] as well as several other autoimmune diseases, including chronic lung allograft rejection [7], renal transplant [8] and allergic asthma [9]. The complement cascade is an integral arm of innate immunity with the components C3a and C5a being the major players in epithelial injury.…”

The cytokines of pulmonary fibrosis: Much learned, much more to learn

“…Oxidative stress and redox signaling [67][68][69] and changes in matrix stiffness [70,71] contribute to initiation and maintenance of fibrosis by engaging TGF-b. Epithelial injury and apoptosis [33,69,[72][73][74], coagulation disturbances [75][76][77], and defective autophagy [78] lead to fibrosis in a fashion dependent on multiple cytokines.…”