The rate of melanoma has continued to rise for over 30 years. Once distant metastasis occurs, the 5 year survival rate for melanoma is only 16%. Until the age of 45, incidence rates of malignant melanoma are higher in woman than in men, but by the age of 60, rates in men are more than double those in women and by the age of 80, they are almost triple, indicating a gender disparity for melanoma. It is estimated that there will be 75,000 new cases of melanoma and nearly 10,000 deaths in 2015, with two thirds of these deaths expected to be males. The role of Innate immunity and the continuously decreasing testosterone levels with age in males have not been explored and the reason for this gender disparity remains unknown. To investigate the role of androgens in this gender disparity we used a B16 colonization and metastasis mouse model. We observed that 8 week old female mice had a significantly greater tumor burden than male mice. While ovariectomy did not affect tumor burden, castrated male mice had an increased tumor burden in the lung, as well as reduced neutrophil infiltration and natural killer (NK) cell activation, suggesting that these two cell types play a protective role and that Innate Immunity play a key role. Neutrophil depletion resulted in increased tumor burden and reduced NK cell activation, indicating a crosstalk between these two cell types in this model. In addition to the B16 cells, we have now established working metastatic models using ?YUMM? cell lines obtained from genetically engineered mice with human melanoma mutations. The impact that testosterone levels has on Innate immunity, on neutrophils, NK cells, and possibly other mononuclear cells in melanoma patients is unknown. Based on these preliminary data and the reported presence of Androgen Receptor (AR) on bone marrow cells, we hypothesize that testosterone may interact directly with neutrophils, which in turn impacts NK cell function, to elicit a protective immune response against melanoma colonization and metastasis in the lungs. We propose this as the mechanism for the increased lung metastasis in female mice compared to male mice. To test this hypothesis we propose the following two specific AIMs for this exploratory R21 application: Aim1) To Determine the mechanism by which testosterone modulates the innate immune system and melanoma tumor burden in the lung and Aim 2) To Investigate the efficacy of testosterone treatment in reducing melanoma tumor cell lung colonization/metastasis The successful completion of this proposal will elucidate the mechanism by which androgens mediate colonization and tumor burden in the lungs of B16 or YUMM melanoma injected mice through the interaction with immune cells. The ultimate goal is to identify a subset of patients that may be good candidates for testosterone replacement therapy to increase the ability of the immune system to prevent and fight the metastasis of melanoma. Further, the focus on gender disparity may lead to new diagnostic tools and therapeutic modalities for prevention and treatment of metastatic melanoma by identifying an unknown role for hormones in an immunogenic cancer.