NKG2D, an activating receptor expressed on NK cells and T cells, is critically involved in tumor immunosurveillance. In this study, we explored the potential therapeutic utility of the NKG2D ligand ULBP2 for the treatment of colon carcinoma. To this end we designed a fusion protein consisting of human ULBP2 and an antibody-derived single chain targeting the tumor carcinoembryonic antigen (CEA). The bispecific recombinant fusion protein re-directed NK cells towards malignant cells by binding to both, tumor cells and NK cells, and triggered NK cell-mediated target cell killing in vitro. Moreover, tumor growth was significantly delayed in a syngeneic colon carcinoma mouse model in response to immunoligand treatment. The anti-tumor activity could be attributed to the stimulation of immune cells with an elevated expression of the activation marker CD69 on NK, T and NKT cells and the infiltration of CD451 immune cells into the solid tumor. In summary, it was demonstrated that immunoligands provide specific tumor targeting by NK cells and exert anti-tumor activity in vitro and in vivo. This technology represents a novel immunotherapeutic strategy for solid tumors with the potential to be further developed for clinical applications.Colorectal cancer is fatal in advanced stages of the disease. Curative treatment options are commonly reserved for tumors not yet metastasized or with resectable metastasis. Colorectal cancer is susceptible to chemotherapy from the onset, but due to therapeutic pressure cancer cells usually become resistant to the standard therapies. In addition, the standard therapies are frequently associated with severe side effects. Thus, there is a high demand for targeted therapies. Most recently developed antibody-based immunotherapies including cetuximab, bevacizumab and the fully human antibody panitumumab have not only added new impulses to colorectal cancer therapy, but essentially improved clinical outcome. 1 They have shown to overcome resistance to conventional chemotherapy and have clearly demonstrated that colorectal carcinoma is accessible to antibody therapy.The potential of immune modulation as well as the role of antibody therapy to enhance a cell-based immunotherapy is currently evaluated. 2-4 Increasing attention has been drawn onto NK cells, which are part of the innate immune system and can attack malignant cells without prior antigen stimulation. 5,6 Numerous ways of tumor cell recognition by NK cells have been described. NK cell cytotoxicity against tumors can be triggered by cells lacking expression of self-recognized MHC class I molecules. This mechanism has been referred to as the "missing-self"-hypothesis and is controlled through a group of inhibitory receptors on the NK cell surface. 7 In addition, an anti-tumor immune response can be induced by the upregulation of ligands for triggering NK cell receptors on the surface of tumor cells. 8 This mechanism has been referred to as "induced-self" and provokes an effective immune response even in the presence of inhibitory signalling....