PROJECT SUMMARY/ABSTRACT Secondary lymphedema is a common and dreaded disease that occurs in patients treated for a variety of solid tumors. An estimated 20–40% of Americans treated surgically for breast cancer, melanoma, gynecological or urologic tumors go on to develop lymphedema. Once the disease develops, it is usually progressive and causes significant functional impairment, recurrent infections, and decreased quality of life. Current treatments are palliative, aiming to treat symptoms and prevent progression rather than cure the underlying anatomic abnormalities. Thus, development of novel treatments for this disease is an important goal. Recent studies have shown that infiltration of T helper 2 (Th2)-differentiated CD4+ cells is an important mediator of lymphedema development by regulating fibroadipose tissue deposition, preventing formation of collateral lymphatics, decreasing lymphatic pumping, and causing lymphatic leakiness/dysfunction. However, the cellular mechanisms that activate these Th2 inflammatory responses remain unknown. In preliminary studies, we have found that the epidermis may play an important role in regulating Th2 inflammatory responses in lymphedema. For example, we have found that the expression of Th2-inducing cytokines— cytokines known to coordinate Th2 inflammation in other chronic skin disorders—is significantly increased by keratinocytes in lymphedematous tissues of patients with unilateral upper extremity lymphedema. We have also found that the expression of serine proteases, an important mechanism regulating Th2-inducing cytokine expression in other skin disorders, is also increased. Using mouse models, we have found that epidermal changes, including decreased expression of skin barrier proteins and of Th2-inducing cytokines, precedes infiltration of Th2 differentiated CD4+ cells. Based on these preliminary studies, as well as the pathophysiology of other Th2-mediated chronic skin disorders, we will test the hypothesis that impaired skin barrier function and skin pH regulate the expression of Th2-inducing cytokines by keratinocytes following lymphatic injury. Our study is innovative because the role of the epidermis has not been elucidated. This gap in our knowledge is important as epidermal changes are a prominent finding in lymphedema. We will test our hypothesis by analyzing skin barrier function, pH, and Th2-inducing cytokine expression in patients with unilateral breast cancer-related lymphedema. In other studies, we will use animal models to analyze the cellular mechanisms that regulate epidermal Th2-inducing cytokine expression and how these changes regulate the pathophysiology of lymphedema.