Lung Collagens Perpetuate Pulmonary Fibrosis via CD204 and M2 Macrophage Activation

Abstract

Idiopathic pulmonary fibrosis is characterized by abundant collagen production and accumulation of alternatively activated macrophages (M2) in the lower respiratory tract. Mechanisms as to how alveolar macrophages are activated by collagen breakdown products are unknown. Alveolar macrophages were obtained by bronchoalveolar lavage from 30 patients with idiopathic pulmonary fibrosis (IPF) and 37 healthy donors (HD). Alveolar macrophages were cultured in the presence of collagen type I, III, IV and V monomers w/wo a neutralizing antibody against scavenger receptor I class A (CD204). Culture supernatants were assayed for the M2 markers CCL18, CCL2, and interleukin-1 receptor antagonist (IL-1ra) by ELISA. Furthermore, expression of phospho-Akt was measured using ELISA and expression of CD204 by RT-PCR and flow cytometry. Stimulation with collagen type I and III monomers significantly up-regulated CCL18, IL-1ra production of alveolar macrophages. Furthermore, expression of CCL2 and CD204 were up-regulated by collagen type I exposure. In addition, collagen type I stimulation increased pospho-Akt expression. Collagen type I effects were abrogated by neutralizing antiCD204 and a non-selective Phosphatidylinositide 3-kinase inhibitor (LY294002). Spontaneous CD204 expression of alveolar macrophages was significantly increased in patients with IPF. In conclusion, our findings demonstrate that monomeric collagen type I via CD204 induces phospho-Akt expression shifting alveolar macrophages to the profibrotic M2 type. Innate immune responses induced by collagen monomers might perpetuate pulmonary fibrosis.

Characterization of Distinct Macrophage Subpopulations during Nitrogen Mustard–Induced Lung Injury and Fibrosis

“…By 28 days, macrophages staining positively for these M2 markers were enlarged, vacuolated, and clustered in thickened alveoli surrounding areas of fibrosis, supporting their profibrotic activity (29,30). Epidemiological evidence, as well as experimental models of idiopathic and chemical-induced fibrosis, have shown that CD163 and CD206 M2 macrophages contribute to fibrogenesis in the lung, kidney, and peritoneum (31)(32)(33)(34)(35). Our findings suggest that they may play a similar role in lung fibrosis after NM exposure; however, this remains to be investigated.…”

Mechanisms of Lung Fibrosis Resolution

“…95 When macrophages from the lungs of healthy controls or IPF patients were cultured and stimulated with collagen monomers, the IPFderived macrophages increased expression of profibrotic markers and cytokines, consistent with a classification of the alternative activated state. 96 Although macrophages can clearly promote fibrogenesis, depletion of macrophages during the recovery phase of both toxic liver injury and the lung bleomycin fibrosis models delayed the resolution of fibrosis. 97,98 These findings support that functionally distinct subpopulations of macrophages also play critical roles in recovery.…”

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

“…Despite the abundance of convincing reports associating M2 with fibrosis, mechanistic studies have not been conclusive on the exact role of these cells in fibrosis. M2 and M2-like macrophages are thought to be profibrotic through production of several profibrotic cytokines, notably CCL18 [161][162][163], thus enhancing and prolonging the profibrotic effect of Th2 cytokines. Contrary to this notion, mice lacking arginase 1, a hallmark molecule of M2, are either not protected against [164] or demonstrate exacerbated fibrosis [165].…”