Lymphedema is the accumulation of protein rich fluid in tissues, and in the US, occurs most commonly as a surgical complication of cancer treatment. Despite the fact that 3-5 million Americans suffer from lymphedema, its etiology is unknown and treatment remains palliative. Based on our laboratory data as well as the clinical presentation of lymphedema we hypothesize that the pathology of lymphedema is secondary to progressive fibrosis and is mediated by chronic T-helper (CD4+) cell inflammation. The objective of this proposal is to determine how lymphatic fluid stasis causes tissue fibrosis and lymphatic dysfunction. Our long-term objective is to develop therapies to treat or prevent lymphedema by disrupting the cycle of stasis, fibrosis, and lymphatic dysfunction. This approach is innovative since previous efforts have attempted to treat lymphedema by augmenting lymphatic repair and regeneration using cytokines. These cytokines, however, can also cause tumor metastasis or recurrence thereby limiting the applicability of this approach in cancer patients. This proposal isrelevant since lymphedema is a common and morbid complication of cancer treatment for which there is no proven preventative methods and treatment is palliative. We plan to achieve our objectives using 3 specific aims. Aim 1: Determine the cellular sources of profibrotic cytokines. This aim will test the working hypothesis that CD4+ T-cells are the primary source of profibrotic cytokines using transgenic mice, antibody depletion, and adoptive transfer experiments in a mouse-tail and axillary dissection model. Aim 2: Determine how the expression of profibrotic cytokines is regulated. This aim will test the working hypothesis is that lymphatic stasis causes Th2 cell inflammation and IL4 and IL13 expression and that these cytokines interact to cause fibrosis and lymphatic dysfunction. We will use clinical specimens from patients with lymphedema before and after medical or surgical treatment as well as our mouse tail model. Aim 3: Determine how profibrotic cytokines regulate lymphatic function and fibrosis. This aim will test the working hypothesis that impaired lymphatic function secondary to fibrosis is a consequence of indirect effects of T-cells and Th2 cytokines on the lymphatic system rather than down-regulation of lymphangiogenic cytokine expression.