BACKGROUND: Secondary lymphedema is a common complication of cancer treatment and epidermal changes are recognised as histological hallmarks of secondary lymphedema; however, the role of keratinocytes in the pathophysiology of this disease remains unclear. METHODS: Hyperkeratosis, up-regulation of protease-activated receptor 2 (PAR2) and Th2-inducing cytokines were assessed in biopsy specimens from patients with unilateral breast cancer-related lymphedema (BCRL) and in a mouse model of lymphedema. PAR2 inhibition using global PAR2 knockout, keratinocyte-specific PAR2 KO and bone marrow chimera models, or keratinocyte proliferation inhibition using a topical formulation of Teriflunomide (TF), was analysed in mouse models of lymphedema. We also assessed the direct effects of patient-derived lymphedema lymph fluid (LF) on keratinocyte activation in vitro. RESULTS: Hyperkeratosis, expression of Th2-inducing cytokines and PAR2 were significantly increased in BCRL patient biopsies and mouse models. Keratinocytes play a primary role in the lymphedema development by producing T helper 2 (Th2)-inducing cytokines. Specifically, keratinocyte proliferation and PAR2 expression are early responses following lymphatic injury and regulate the expression of Th2-inducing cytokines, the migration of Langerhans cells and the infiltration of Th2-differentiated T cells into the skin. Deficiency of PAR2 or topical inhibition of thymic stromal lymphopoietin rescues secondary lymphedema by reducing Th2 inflammation. Inhibition of PAR2 activation with a small-molecule inhibitor, or the proliferation of the inhibitor TF, prevents activation of keratinocytes stimulated with lymphedema fluid. Finally, topical TF is highly effective in reducing swelling, fibrosis and inflammation and the overall pathology of lymphedema. CONCLUSIONS: Our findings suggest that lymphedema is a chronic inflammatory skin disease, and topically targeting keratinocyte inhibition may be a clinically effective therapy for this condition. KEY POINTS: Activated keratinocytes play a key role in the pathophysiology of secondary lymphedema through PAR2 by producing Th2-inducing cytokines that modulate skin inflammatory responses.
Park et al. (Mon,) studied this question.