Fibrotic lung diseases increase with age. Previously we determined that senescence increases tissue expression of fibronectin EDA (Fn-EDA) and decreases fibroblast expression of Thy-1, and that fibrocytes contribute to fibrosis following bleomycin-induced lung injury in mice. In this study we hypothesized that fibroblasts lacking Thy-1 expression produce an extracellular matrix that promotes fibrocyte retention and myofibroblast transdifferentiation, thereby promoting fibrogenesis. Young and old mice were treated with bleomycin intratracheally; fibrocytes in the bone marrow, blood, and lungs were quantified, and lung fibroblast Thy-1 expression was assessed. Bone marrowderived fibrocytes were cultured on matrices derived from Thy-1(+) or Thy-1(?) fibroblasts ± the pro-fibrotic cytokine TGFβ1. Older mice had more fibrocytes in their bone marrows at baseline and more fibrocytes in their lungs following bleomycin treatment. In parallel, lung fibroblasts in older mice had lower expression of Thy-1 at baseline that increased transiently 7 days after bleomycin treatment but then rapidly waned such that 14 days after bleomycin treatment Thy-1 expression was again markedly lower. Fibrocytes cultured on matrices derived from Thy-1(?) fibroblasts + TGFβ1 had increased gene expression for collagen type 1, fibronectin, Fn-EDA, and α-smooth muscle actin. In parallel, whereas the matrices derived from Thy-1(?) fibroblasts stimulated phosphorylation of Akt in cultured fibrocytes, the matrices derived from Thy-1(+) fibroblasts induced apoptosis. These findings suggest that senescence increases fibrocyte recruitment to the lung following injury and that loss of Thy-1 expression by lung fibroblasts promotes fibrocyte retention and myofibroblast transdifferentiation that renders the “aging lung” susceptible to fibrosis.
 Nathan, S.D., Shlobin, O.A., Weir, N., Ahmad, S., Kaldjob, J.M., Battle, E., Sheridan, M.J. and du Bois, R.M. (2011) Long-term course and prognosis of idiopathic pulmonary fibrosis in the new millennium. CHEST Journal, 140, 221-229.
 Meltzer, E.B. and Noble, P.W. (2008) Idiopathic pulmonary fibrosis. Orphanet Journal of Rare Diseases, 3, 8.
 Mora, A.L. and Rojas, M. (2008) Aging and lung injury repair: A role for bone marrow derived mesenchymal stem cells. Journal of Cellular Biochemistry, 105, 641-647. http://dx.doi.org/10.1002/jcb.21890
 Lee, J.S., Collard, H.R., Raghu, G., Sweet, M.P., Hays, S.R., Campos, G.M., Golden, J.A. and King Jr., T.E., (2010) Does chronic microaspiration cause idiopathic pulmonary fibrosis? The American Journal of Medicine, 123, 304-311.
 Faner, R., Rojas, M., Macnee, W. and Agusti, A. (2012) Abnormal lung aging in chronic obstructive pulmonary disease and idiopathic pulmonary fibrosis. American Journal of Respiratory and Critical Care Medicine, 186, 306-313. http://dx.doi.org/10.1164/rccm.201202-0282PP
 Crosby, L.M. and Waters, C.M. (2010) Epithelial repair mechanisms in the lung. American Journal of Physiology: Lung Cellular and Molecular Physiology, 298, L715-L731. http://dx.doi.org/10.1152/ajplung.00361.2009
 Zhang, Y. and Kaminski, N. (2012) Biomarkers in idiopathic pulmonary fibrosis. Current opinion in pulmonary medicine, 18, 441-446.
 Strieter, R.M., Keeley, E.C., Hughes, M.A., Burdick, M.D. and Mehrad, B. (2009) The role of circulating mesenchymal progenitor cells (fibrocytes) in the pathogenesis of pulmonary fibrosis. Journal of Leukocyte Biology, 86, 1111-1118.
 Bellini, A. and Mattoli, S. (2007) The role of the fibrocyte, a bone marrow-derived mesenchymal progenitor, in reactive and reparative fibroses. Laboratory Investigation, 87, 858-870.
 Rojas, M., Xu, J., Woods C.R., Mora, A.L., Spears, W., Roman, J. and Brigham, K.L. (2005) Bone marrow-derived mesenchymal stem cells in repair of the injured lung. American Journal of Respiratory Cell and Molecular Biology, 33, 145-152.
 Hagood, J.S., Prabhakaran, P., Kumbla, P., Salazar, L., MacEwen, M.W., Barker, T.H, Ortiz, L.A., Schoeb, T., Siegal, G.P., Alexander, C.B., et al., (2005) Loss of fibroblast thy-1 expression correlates with lung fibrogenesis. American Journal of Pathology, 167, 365-379.
 Phipps, R.P., Penney, D.P., Keng, P., Quill, H., Paxhia, A., Derdak, S. and Felch, M.E. (1989) Characterization of two major populations of lung fibroblasts: Distinguishing morphology and discordant display of thy 1 and class ii mhc. American Journal of Respiratory Cell and Molecular Biology, 1, 65-74.
 Derdak, S., Penney, D.P., Keng, P., Felch, M.E., Brown, D. and Phipps, R.P. (1992) Differential collagen and fibronectin production by Thy 1+ and Thy1- lung fibroblast subpopulations. The American Journal of Physiology, 263, L283-L290.
 Zhou, Y., Hagood, J.S. and Murphy-Ullrich, J.E. (2004) Thy-1 expression regulates the ability of rat lung fibroblasts to activate transforming growth factor-beta in response to fibrogenic stimuli. American Journal of Pathology, 165, 659-669.
 Ramirez, G., Hagood, J.S., Sanders, Y., Ramirez, R., Becerril, C., Segura, L., Barrera, L., Selman, M. and Pardo, A. (2011) Absence of thy-1 results in tgf-beta induced mmp-9 expression and confers a profibrotic phenotype to human lung fibroblasts. Laboratory Investigation, 91, 1206-1218.
 Sueblinvong, V., Neujahr, D.C., Mills, S.T., Roser-Page, S., Ritzenthaler, J.D., Guidot, D., Rojas, M. and Roman, J. (2012) Predisposition for disrepair in the aged lung. The American Journal of the Medical Sciences, 344, 41-51.
 Beckett, T., Loi, R., Prenovitz, R., Poynter, M., Goncz, K.K., Suratt, B.T. and Weiss, D.J. (2005) Acute lung injury with endotoxin or no2 does not enhance development of airway epithelium from bone marrow. Molecular Therapy, 12, 680-686.
 Sechler, J.L., Takada, Y. and Schwarzbauer, J.E. (1996) Altered rate of fibronectin matrix assembly by deletion of the first type iii repeats. The Journal of Cell Biology, 134, 573-583. http://dx.doi.org/10.1083/jcb.134.2.573
 Sueblinvong, V., Loi, R., Eisenhauer, P.L., Bernstein, I.M., Suratt, B.T., Spees, J.L. and Weiss, D.J. (2008) Derivation of lung epithelium from human cord bloodedrived mesenchymal stem cells. American Journal of Respiratory and Critical Care Medicine, 177, 701-711.
 Fries, K.M., Blieden, T., Looney, R.J., Sempowski, G.D., Silvera, M.R., Willis, R.A. and Phipps, R.P. (1994) Evidence of fibroblast heterogeneity and the role of fibroblast subpopulations in fibrosis. Clinical Immunology and Immunopathology, 72, 283-292.
 Hardie, W.D., Glasser, S.W. and Hagood, J.S. (2009) Emerging concepts in the pathogenesis of lung fibrosis. The American Journal of Pathology, 175, 3-16.
 Willis, B.C. and Borok, Z. (2007) Tgf-beta-induced emt: Mechanisms and implications for fibrotic lung disease. American Journal of Physiology: Lung Cellular and Molecular Physiology, 93, L525-L534.
 Andersson-Sjoland, A., de Alba, C.G., Nihlberg, K., Becerril, C., Ramirez, R., Pardo, A., Westergren-Thorsson, G. and Selman, M. (2008) Fibrocytes are a potential source of lung fibroblasts in idiopathic pulmonary fibrosis. The International Journal of Biochemistry & Cell Biology, 40, 2129-2140.
 Moeller, A., Gilpin, S.E., Ask, K., Cox, G., Cook, D., Gauldie, J., Margetts, P.J., Farkas, L., Dobranowski, J., Boylan, C., et al., (2009) Circulating fibrocytes are an indicator for poor prognosis in idiopathic pulmonary fibrosis. American Journal of Respiratory and Critical Care Medicine, 179, 588-594.
 Yan, X., Liu, Y., Han, Q., Jia, M., Liao, L., Qi, M. and Zhao, R.C. (2007) Injured microenvironment directly guides the differentiation of engrafted flk-1(+) mesenchymal stem cell in lung. Experimental Hematology, 35, 1466-1475.
 Williams, D.A., Rios, M., Stephens, C. and Patel, V.P. (1991) Fibronectin and vla-4 in haematopoietic stem cell-microenvironment interactions. Nature, 352, 438-441. http://dx.doi.org/10.1038/352438a0
 Tenney, R.M. and Discher, D.E. (2009) Stem cells, microenvironment mechanics, and growth factor activation. Current Opinion in Cell Biology, 21, 630-635.
 Pek ,Y.S., Wan, A.C. and Ying, J.Y. (2010) The effect of matrix stiffness on mesenchymal stem cell differentiation in a 3d thixotropic gel. Biomaterials, 31, 385-391.
 Muro, A.F., Chauhan, A.K., Gajovic, S., Iaconcig, A., Porro, F., Stanta, G. and Baralle, F.E. (2003) Regulated splicing of the fibronectin eda exon is essential for proper skin wound healing and normal lifespan. The Journal of Cell Biology, 162, 149-160.
 Sanders, Y.Y., Pardo, A., Selman, M., Nuovo, G.J., Tollefsbol, T.O., Siegal, G.P. and Hagood, J.S. (2008) Thy-1 promoter hypermethylation: A novel epigenetic pathogenic mechanism in pulmonary fibrosis. American Journal of Respiratory Cell and Molecular Biology, 39, 610-618. http://dx.doi.org/10.1165/rcmb.2007-0322OC
 Koumas, L., Smith, T.J. and Phipps, R.P. (2002) Fibroblast subsets in the human orbit: Thy-1- and thy-1- subpopulations exhibit distinct phenotypes. European Journal of Immunology, 32, 477-485.
 Koumas, L. and Phipps, R.P. (2002) Differential cox localization and pg release in thy-1(+) and thy-1(-) human female reproductive tract fibroblasts. American journal of Physiology Cell Physiology, 283, C599-C608.
 Downward, J. (2004) Pi 3-kinase, akt and cell survival. Seminars in Cell & Developmental Biology, 15, 177-182.