Background/Purpose: Scleroderma is a systemic autoimmune disease in which thrombosis and fibrosis contribute to skin pathology. Plasminogen activator inhibitor-1 (PAI-1) is the major inhibitor of pro-fibrinolytic plasminogen activators uPA and tPA and its expression has been shown to be increased in the skin of scleroderma patients. The purpose of this study was to evaluate the contribution of PAI-1 inhibition of PAs to pathological changes in the skin in an animal model that recapitulates both the fibrosis and occlusive vasculopathy of human scleroderma.

Methods: B10D2 splenocytes were injected into BALB/c Rag-2-/- mice resulting in a model of graft-vs-host disease-induced skin fibrosis (GVHD). A monoclonal antibody that selectively prevents the binding of PAI-1 to its target PAs was administered ip two times per week beginning either at time of engraftment or at week 3 post-engraftment. Effect of PAI-1 neutralization was assessed on clinical skin score, gene expression and histological changes in the skin. In addition, the effect of blocking PAI-1 on MMP-1 activation was evaluated in vitro in human dermal microvascular endothelial cells (HMVECs).

Results: In this model fibrosis peaks at week 6 post-graft, following the inflammation peak at week 4. PAI-1 expression is increased in the skin beginning at week 2.. Prophylactic neutralization of PAI-1 significantly reduced the clinical skin score in a dose-dependent manner as early as week 2 post-graft. Clinical benefits were associated with normalization of fibrinolysis genes (plasmin, PAI-1- uPA, uPAR, KLK6) and this correlated with resolution of vascular injury markers (VCAM-1, MMP-12), T cell infiltration and TNF-α levels in skin, as assessed over the inflammatory phase of the disease (week 1-4). Administration of the antibody in a therapeutic regimen also resulted in significant reduction of the clinical skin score. Clinical benefits were associated with normalization of fibrinolysis along with resolution of skin fibrosis over the chronic phase of the disease (week 4-6) as shown by reduction of dermal thickness and extracellular collagen which correlated with reduction of expression of pro-fibrotic cytokines (TGF-β, IL-13) and matrix turnover regulators (TIMP-1). The matrix turnover component of PAI-1 inhibition was further supported where neutralization of PAI-1 in cultured dermal HMVECs decreased MMP-1 activation in a concentration-dependent manner.

Conclusion: These data suggest that PAI-1 plays a key role in both the skin vasculopathy and fibrosis observed in this murine model of human scleroderma and that inhibition of the binding of PAI-1 to PAs resolves fibrosis via two distinct plasmin-based mechanisms: (1) directly via reducing MMP activation and (2) indirectly via reducing infiltration of profibrotic cytokine-secreting inflammatory cells following thrombolysis-based resolution of vascular injury and activation.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/neutralization-of-plasminogen-activator-inhibitor-1-resolves-skin-fibrosis-and-vascular-injury-in-a-murine-model-of-human-scleroderma/