Rapamycin Blocks Fibrocyte Migration and Attenuates Bronchiolitis Obliterans in a Murine Model

Abstract

Background Fibrocytes are integral in the development of fibroproliferative disease. The CXCL12/CXCR4 chemokine axis has been shown to play a central role in fibrocyte migration and the development of bronchiolitis obliterans post lung transplantation. Inhibition of the mTOR (mammalian target of rapamycin) pathway with rapamycin has been shown to decrease expression of both CXCR4 and its receptor agonist, CXCL12. Thus, we hypothesize that rapamycin treatment would decrease fibrocyte trafficking into tracheal allografts and prevent bronchiolitis obliterans. Methods A total alloantigenic mismatch, murine heterotopic tracheal transplant model of bronchiolitis obliterans was used. Animals were either treated with rapamycin or dimethyl sulfoxide (DMSO) for 14 days post tracheal transplant. Fibrocyte levels were assessed via flow cytometry, and allograft neutrophil, CD3+ T-cell, macrophage, and smooth muscle actin levels were assessed via immunohistochemistry. Tracheal luminal obliteration was assessed on hematoxylin and eosin stains. Results Compared to DMSO controls, rapamycin-treated mice showed a significant decrease in fibrocyte levels in tracheal allografts. Fibrocytes levels in recipient’s blood showed a similar pattern, although not statistically significant. Furthermore, animals treated with rapamycin showed a significant decrease in tracheal allograft luminal obliteration compared to controls. Based on immunohistochemistry analyses, populations of α-SMA positive cells, neutrophils, CD3+ T-cells, and macrophages were all decreased in rapamycin-treated allograft versus DMSO controls. Conclusions Rapamycin effectively reduces recruitment of fibrocytes into tracheal allografts and mitigates development of tracheal luminal fibrosis. Further studies are needed to determine the cellular and molecular mechanisms that mediate the protective effect of rapamycin against bronchiolitis obliterans.

Local Origin of Mesenchymal Cells in a Murine Orthotopic Lung Transplantation Model of Bronchiolitis Obliterans

“…In heterotopic tracheal transplant models, mesenchymal cells have been shown to be derived from the recipient rather than the donor, 29 and the fibrocyte population is being investigated. 27,28 Similar findings of recipient derivation of mesenchymal cells was suggested in an intrapulmonary tracheal transplant model of BO using immunofluorescent staining. 56 The contrary finding of the predominant donor origin of collagen Ieexpressing cells in murine lung allografts in the present study could reflect the investigation of fibrosis in a whole-lung allograft rather than in an isolated trachea.…”

Rapamycin prevents bronchiolitis obliterans through increasing infiltration of regulatory B cells in a murine tracheal transplantation model

“…The full mechanism of rapamycin's immunosuppression is still under exploration. When we study the effects of rapamycin on BO development through inhibiting fibrocytes recruitment (10) and promoting epithelial progenitor cell regeneration(11), an unexpected finding was seen that rapamycin significantly reduced luminal obliteration, but markedly increased cellular infiltration in the allografts on days 14 and 28. We and others have reported previously that the cellular infiltration was peaked on day 7, then the inflammatory cells gradually decreased to base levels (6, 7, 27, 28).…”

“…Our previous reports showed that short course treatment of rapamycin, a macrocyclic triene antibiotic pro-drug, prevented development of BO through two different mechanisms in a HTT model: 1) reducing fibrocyte recruitment to the tracheal allografts(10); 2) protects against airway epithelium loss and promotes epithelial progenitor cells(11). During these studies, we appreciated that despite rapamycin significantly reduced BO development-; it simultaneously increased cell infiltration into the allografts.…”

Short-Course Rapamycin Treatment Preserves Airway Epithelium and Protects Against Bronchiolitis Obliterans

“…We utilized a murine HTT model of bronchiolitis obliterans, as previously described [16, 21-23]. Briefly, donor mice were anesthetized and their trachea were removed via a midline cervical incision.…”

“…Rapamycin has both immunosuppressive and anti-fibrotic properties [15]. We have shown that rapamycin treatment prevents secondary loss of the epithelium and mitigates the development of BO in the HTT model [16]. Rapamycin treatment also reduces intra-tracheal levels of fibrocytes [16].…”