Metastasis of malignant melanoma accounts for the majority of skin cancer related deaths. The 5-year survival rate for metastatic melanoma patients is less than 14%. As incidence of malignant melanoma continues to rapidly rise, understanding the mechanisms underlying the development of distal metastasis is imperative to the improvement of patient survival. Type 1 interferon (IFN1) adjuvant therapy remains the standard of care for the post-surgical adjuvant treatment in melanoma patients who are at high risk for developing metastasis. However, many patients failed to elicit response to IFN1 treatment and progressed to develop distal metastases resulting in eventual lethal outcome. Resistance to IFN1 treatment points to impaired IFN1 signaling in the melanoma cells and/or the surrounding stroma, thereby contributing to disease progression. However, our ability to circumvent this problem is severely undermined by a limited understanding of mechanisms responsible for IFN1 evasion. This problem represents a gap in our knowledge and ability to improve patients' prognosis. Studies proposed here are designed to close this gap and may expand our understanding of the efficacy of IFN1 as a therapeutic agent, improve identification of patients likely to benefit from IFN1 as an adjuvant therapy, and possibly identify novel targets The anti-tumorigenic effects of IFN1 require stable expression of its receptor chain IFNAR1. Our laboratory has shown that melanoma cells release factors that promote the ubiquitin-mediated proteolysis of IFNAR1. Melanoma development induced in our Ifnar1S526A knock-in mice (SA) mice, whose IFNAR1 is insensitive to ubiquitin-mediated degradation, was substantially delayed and failed to develop metastasis. Intriguingly, these tumors expressed low levels of ?v integrin known to contribute to the metastatic process. We have generated a highly metastatic IFNAR1-null melanoma cell line (termed PVMM) that was able to form subcutaneous tumors and progress to pulmonary metastasis in wild-type animals but not in SA mice. These findings lead to the overall hypothesis that IFNAR1 downregulation within the cancer cell, the seed, and microenvironment, the soil, are required to promote melanoma metastasis. To test this hypothesis, I will carry out the following aims. Aim 1, I will determine the cell autonomous role o IFNAR1 signaling (the seed) in melanoma metastasis. I will modulate Ifnar1 expression to examine the effects of IFNAR1 on cell functions that define metastasis, such as adhesion, migration, and invasion. I will also investigate whether modulation of Ifnar1 exerts changes in ?v expression and ?v-mediated metastatic functions. Results of this aim will provide a mechanism by which IFNAR1 degradation contributes to melanoma metastasis and identify ?v as a possible target in IFN1-insensitive patients. Aim 2, I will determine the role of IFNAR1 signaling within stromal cells (the soil) in suppressing melanoma metastasis. I will use WT and SA mice to demonstrate the role of IFNAR1 expression in stromal cells in affecting the metastatic functions of melanoma cells in vitro and in vivo. The completion of the proposed studies will elucidate mechanisms by which endogenous IFN1 counteracts the metastatic process and determine the role of downregulation of IFNAR1 in ability of melanoma to metastasize. Furthermore, elucidation of these mechanisms should help to identify novel targets for adjuvant therapy, and stratify patients for improved use of IFN1 therapy or an alternate regimen.