Regression of Allograft Airway Fibrosis: The Role of MMP-Dependent Tissue Remodeling in Obliterative Bronchiolitis after Lung Transplantation

Obliterative bronchiolitis after lung transplantation is a chronic inflammatory and fibrotic condition of small airways. The fibrosis associated with obliterative bronchiolitis might be reversible. Matrix metalloproteinases (MMPs) participate in inflammation and tissue remodeling. MMP-2 localized to myofibroblasts in post-transplant human obliterative bronchiolitis lesions and to allograft fibrosis in a rat intrapulmonary tracheal transplant model. Small numbers of infiltrating T cells were also observed within the fibrosis. To modulate inflammation and tissue remodeling, the broad-spectrum MMP inhibitor SC080 was administered after the allograft was obliterated, starting at post-transplant day 21. The allograft lumen remained obliterated after treatment. Only low-dose (2.5 mg/kg per day) SC080 significantly reduced collagen deposition, reduced the number of myofibroblasts and the infiltration of T cells in association with increased collagenolytic activity, increased MMP-2 gene expression, and decreased MMP-8, MMP-9, and MMP-13 gene expression. In in vitro experiments using cultured myofibroblasts, a relatively low concentration of SC080 increased MMP-2 activity and degradation of type I collagen. Moreover, coculture with T cells facilitated persistence of myofibroblasts, suggesting a role for T-cell infiltration in myofibroblast persistence in fibrosis. By combining low-dose SC080 with cyclosporine in vivo at post-transplant day 28, partial reversal of obliterative fibrosis was observed at day 42. Thus, modulating MMP activity might reverse established allograft airway fibrosis by regulating inflammation and tissue remodeling.Chronic allograft dysfunction after lung transplantation is manifested by obliterative bronchiolitis (OB), a fibroproliferative obstructive lesion in small airways, and its clinical correlate, bronchiolitis obliterans syndrome (BOS).^1,2 Once the fibrotic process of OB is initiated, conventional immunosuppression is usually ineffective.³ The traditional pathological perspective is that fibrosis is the end result of damage: scar tissue, with no possibility of return to the pre-existing structure.⁴ However, increasing evidence suggests that fibrosis still undergoes dynamic remodeling and is potentially a reversible process. For example, the resolution of liver fibrosis is well documented both clinically and experimentally. In animal experiments, up-regulation or overexpression of matrix metalloproteinases (MMPs) capable of degrading interstitial type I and type III collagen (including MMP-1,⁵ MMP-8,⁶ MMP-13,⁷and MMP-2 and MMP-14^8,9) is associated with the regression of liver fibrosis. Pulmonary fibrosis has also been shown to be conditionally reversible.¹⁰One possible mechanism rendering fibrosis unlikely to resolve is the aberrant persistence of myofibroblasts, an active form of fibroblasts positive for α-smooth muscle actin (α-SMA), which leads to production of extracellular matrix (ECM) in excess of MMP-dependent ECM degradation.¹¹ Unresolved inflammation can be an important contributor to this mechanism.¹⁰ Accumulating evidence suggests that chronic fibrotic conditions are mediated by complex interactions between immune and nonimmune cells, in which the persistence of a relatively low grade of inflammation continuously stimulates resident stromal cells^12,13 and provides survival signals to myofibroblasts.¹⁴ For instance, the resolution of liver fibrosis encountered in alcohol-induced and virus-related fibrosis occurs only after remedy of the underlying cause.^15,16 Moreover, in experimental models of fibrosis, reversal of fibrosis has occurred in one-hit injury models such as bleomycin-induced pulmonary fibrosis,¹⁷ in which the initial tissue injury leads to fibrosis but the tissue injury or inflammation is not continuous.^8,9Along those lines, OB after lung transplantation is a fibrotic and chronic inflammatory condition¹⁸ in which myofibroblasts persist.¹⁹ The intrapulmonary tracheal transplant model of OB is a unique animal model in which persistent alloantigen from the donor trachea within the pulmonary milieu causes continuous alloantigen-induced inflammation and results in robust fibrosis in the allograft lumen.²⁰ We have previously demonstrated that myofibroblasts expressing high levels of collagen and MMP-2 and MMP-14 play a central role in the remodeling of established allograft airway fibrosis.²⁰ Given that MMPs also play important but complex roles in the trafficking of immune responsive cells,²⁰ MMPs involved in both tissue remodeling and inflammation may play key roles in the reversal of fibrosis.We therefore hypothesized that allograft airway fibrosis is a potentially reversible process involving MMPs. Here, we demonstrate expression patterns of MMPs in established human OB lesions and describe the roles of MMPs in the remodeling of collagen matrix, myofibroblasts, and immune responsive cells using in vivo and in vitro models with SC080, a general MMP inhibitor. Finally, we demonstrate for the first time reversibility of allograft airway fibrosis by combining immunosuppression with a low dose of SC080.