Malignant melanoma initiating cells (MMICs) are minority subpopulations in which clinical virulence resides as a consequence of unlimited self-renewal capacity, resulting in inexorable tumor progression and potential metastasis. Our laboratories have recently identified human MMICs and shown them to express the targetable biomarker and multidrug resistance transporter, ABCB5 (Nature, Jan 17, 2008). In this study, proof of principle of immune-mediated MMIC destruction and consequent inhibition of tumor growth was demonstrated. More recently, we have shown that MMICs employ mechanisms to thwart endogenous anti-tumor immunity via the B7-2 and PD1 pathways (Cancer Res, Jan 15, 2010). This proposal seeks to further advance these findings in a translationally-relevant manner with the goal of accelerating progress toward clinical application of anti- melanoma immune therapies specifically targeting MMICs. The specific aims of this proposal are: (1) Characterization of ABCB5+ MMIC response to immunotherapy in human patients and assessment/prediction of MMIC therapeutic response; (2) In vivo dissection of antitumor immunity pathway interactions with MMIC in a novel humanized xenotransplantation model, melanoma to hu-PBMC NOD-scid IL2r3null mice; and (3) Preclinical immunomodulatory/MMIC-targeted combination therapies. This initiative should enhance the rapid development and refinement of targeted immunotherapies directed against MMICs, and thus holds great promise for rapid evolution to clinical testing.