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.