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Multiple myeloma (MM) is an essentially incurable malignancy associated with profound cellular and soluble immune deficiencies. Despite recent progresses in the treatment of MM, the prognosis remains frequently poor due to the difficulty in targeting MM progenitor cells, which are responsible for disease relapse. Immunotherapy, and in particular the employment of Natural Killer (NK) cells, offers the potential to target and eliminate MM cells within the bone marrow stromal sanctuaries, where they appear to be better protected against conventional therapeutic interventions. However, these strategies have so far provided limited clinical benefit, possibly reflecting the various escape mechanisms employed by MM cells to avoid immune recognition. The work presented in this thesis aims to further elucidate the mechanisms underlying MM-induced inhibition of NK cells and to investigate the therapeutic potential of immunomodulatory strategies to reverse this inhibition. Initially, the effect of co-culturing MM cell lines and healthy donor (HD) peripheral blood mononuclear cells (PBMCs) on NK cell phenotype and function was analysed. The results demonstrate that MM cell lines are able to impair NK cell cytolytic activity. This inhibition, which correlates with the downregulation of activating receptors such as NKG2D, NKp30, and DNAM-1, was shown to be the product of direct and contact-dependent interactions between MM and NK cells, without the need for other peripheral blood components. Importantly, the analysis of NK cells isolated from MM patients show that they display the same suppressed phenotype and activity as healthy donor NK cells co-cultured in the presence of MM cells lines, thereby suggesting that the detected suppression of NK cell activation by MM cells is a function of MM cells rather than any inherent defect in NK cells isolated from MM patients. In the light of this immunosuppressive effect, MM cells genetically-modified with a self-inactivating lentiviral vector encoding CD80 (B7.1) and IL-2 were tested for their ability to enhance and recover NK cell functional competence. Our results show that the in vitro co-culture of healthy donor or MM patient PBMCs with CD80/IL-2-modified MM cells is able to expand NK and T cell numbers, and to induce a significant increase in the fraction of NK cells expressing activating receptors such as NKp44, NKG2D, NKp30, and CD69, when compared to unmodified MM cells. More importantly for potential therapeutic applications, stimulated NK cells from healthy donors show increased cytolytic activity. These data suggest that the stimulation of PBMCs with CD80/IL-2-modified MM cells may be able to overcome the immune suppressive functions of unmodified MM cells and to stimulate NK and T cell mediated responses. Therefore, vaccination with CD80/IL-2-modified MM cells may represent a potential strategy for NK cell recovery and stimulation and, possibly, for the induction of a broad ranging immunological responses against multiple myeloma cells.