Riehl, D. R.; Sharma, A.; Roewe, J.; Murke, F.; Ruppert, C.; Eming, S. A.; Bopp, T.; Kleinert, H.; Radsak, M. P.; Colucci, G.; Subramaniam, S.; Reinhardt, C.; Giebel, B.; Prinz, I.; Guenther, A.; Strand, D.; Gunzer, M.; Waisman, A.; Ward, P. A.; Ruf, W.; Schäfer, K.; Bosmann, M.
Significance
Lung fibrosis is a progressive, lethal disease with limited treatment options available that do not provide cure. Hence, a better understanding of the inflammatory pathophysiology and molecular mechanisms of tissue remodeling is mandatory. In this study, we uncover that externalized histones from neutrophil extracellular traps act profibrotic by shifting the balance of critical cytokines. Externalized histones activate platelets to release TGFβ1, which subsequently antagonizes antifibrotic Interleukin-27 (IL-27) production from macrophages via multiple intracellular signaling pathways. The importance of these mechanisms is highlighted by the observation that blocking monoclonal antibodies against externalized histones protect from excessive collagen matrix deposition in injured lungs.
Abstract
Externalized histones erupt from the nucleus as extracellular traps, are associated with several acute and chronic lung disorders, but their implications in the molecular pathogenesis of interstitial lung disease are incompletely defined. To investigate the role and molecular mechanisms of externalized histones within the immunologic networks of pulmonary fibrosis, we studied externalized histones in human and animal bronchoalveolar lavage (BAL) samples of lung fibrosis. Neutralizing anti-histone antibodies were administered in bleomycin-induced fibrosis of C57BL/6 J mice, and subsequent studies used conditional/constitutive knockout mouse strains for TGFβ and IL-27 signaling along with isolated platelets and cultured macrophages. We found that externalized histones (citH3) were significantly (P < 0.01) increased in cell-free BAL fluids of patients with idiopathic pulmonary fibrosis (IPF; n = 29) as compared to healthy controls (n = 10). The pulmonary sources of externalized histones were Ly6G+CD11b+ neutrophils and nonhematopoietic cells after bleomycin in mice. Neutralizing monoclonal anti-histone H2A/H4 antibodies reduced the pulmonary collagen accumulation and hydroxyproline concentration. Histones activated platelets to release TGFβ1, which signaled through the TGFbRI/TGFbRII receptor complex on LysM+ cells to antagonize macrophage-derived IL-27 production. TGFβ1 evoked multiple downstream mechanisms in macrophages, including p38 MAPK, tristetraprolin, IL-10, and binding of SMAD3 to the IL-27 promotor regions. IL-27RA-deficient mice displayed more severe collagen depositions suggesting that intact IL-27 signaling limits fibrosis. In conclusion, externalized histones inactivate a safety switch of antifibrotic, macrophage-derived IL-27 by boosting platelet-derived TGFβ1. Externalized histones are accessible to neutralizing antibodies for improving the severity of experimental pulmonary fibrosis.