Targeting MIC Shedding to Revive Host NKG2D-mediated Immune Response in Prostate Cancer NKG2D-mediated tumor rejection has been well demonstrated in experimental animal models. In humans, the system is not effective due to tumor shedding of the human NKG2D ligands, the MHC class I chain-related family of molecules MICA and MICB (collectively termed MIC). Strong evidence has demonstrated that tumor shedding of MIC results in multiple negative effects on NKG2D-mediated immunity and suggested that it is one of the mechanisms by which tumors escape immune destruction and progress. The mechanisms by which tumors shed MIC are not fully understood, although a diverse group of enzymes have been shown to be involved. However, the functional complexity of these enzymes may not make it clinically feasible to use inhibitors to target MIC shedding for cancer therapy. Our long-term goal is to define optimal strategies to inhibit MIC shedding and ultimately to harness NKG2D-mediated anti-tumor immunity as potential therapies for prostate cancer and other MIC-positive tumors as well. In our accomplished studies, we have shown that preventing MIC shedding resulted in prostate tumor rejection in vivo. Recently we have defined an 11-aa motif (shedding motif) in the a3 domain of MIC that is critical for regulating MIC shedding and generated a single chain antibody (scFv) that inhibits MIC shedding by targeting the shedding-motif. In this proposal, we specifically hypothesize that targeting MIC shedding in association with amplification of NKG2D- mediated immune responses by IL-15 agonists can attenuate prostate cancer progression. The experimental focus of proposal is to elicit the mechanisms by which the 11-aa shedding-motif is a therapeutic target and to evaluate the therapeutic impacts of targeting MIC shedding with our novel antibody in association with amplification of NKG2D-mediated immune response. Our specific Aims are: 1) to elucidate the mechanisms by which the shedding-motif regulating MIC shedding and is a therapeutic target to inhibit MIC shedding; 2) to define the impact of persistent tumor cell surface MIC stimulation on NKG2D function in NK cells and the impact of IL-15 agonist in this context; 3) To evaluate the therapeutic impact of antibody-mediated inhibition of MIC shedding combined with IL-15 agonists in prostate tumorigenesis and progression. If we show that inhibiting MIC shedding with our antibody in combination of IL-15 agonist can successfully harness host anti- tumor immune responses in animal models, the treatment strategy can be readily translated into clinical trials for prostate cancer. In addition, the reagents can be further engineered for the clinical application. Furthermore, as shedding of MIC was evident in many malignancies, the outcomes of this proposed research will have broad clinical implications for cancer therapy.