Successful development of new approaches for the immunotherapy of cancer requires an understanding of complex, interdependent activities of early innate response elements with subsequent powerful adaptive immune responses. We are taking several novel approaches to maximize the host's ability to mount an effective antitumor response. These approaches include optimizing antigen presenting capability through the use of CD40, a TNF superfamily receptor that serves as a potent trigger for dendritic cells which provide a key interface between innate and adaptive responses. The potency of dendritic cell stimulation by agonist CD40 antibodies is enhanced when used in conjunction with IL-2 and the combination of agonist anti-CD40 plus IL-2 shows enhanced antitumor activity against metastatic kidney cancer in mice. Because CD40 is present on various hematopoietic-derived cells, endothelial cells, and some tumors themselves, we have also performed studies to definitively determine if the antitumor effects of CD40 stimulation and IL-2 were primarily mediated by CD40+ hematopoietic-derived cells. We have developed an approach where only tumor cells express functional CD40;specifically we have used models where CD40+ human tumors that respond only to anti-human CD40 were implanted in SCID mice that express only mouse CD40 and do not respond to the administration of the anti-human CD40. These two models have allowed us to show that tumor cells do respond to ligation with agonist CD40 and this response does contribute to limiting tumor growth. In addition, targeted disruption of other events in the tumor microenvironment may ultimately reveal new approaches for combining immunotherapy with other molecularly targeted strategies. In this regard, we have made expression vectors encoding the soluble forms of the VEGF receptors (VEGFRs), Flk-1 and Flt-1, linked to the constant region of human IgG1, and delivered these plasmids to Balb/c mice by highly efficient hydrodynamic injection, this strategy resulted in up to 0.1mg/ml of the appropriate gene products in mouse serum and successfully inhibited angiogenesis in VEGF-containing matrigel. Growth in vivo of a Balb/c mouse renal cancer was also blocked by the same treatment, particularly with soluble Flt-1. Overall, these types of complementary approaches seek to trigger several pathways through which the host can recognize or impair tumor growth, while also targeting the ability of tumor cells to enhance their own survival or inhibit antitumor host responses.
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