Immunotherapy of pancreatic carcinoma with siRNA targeting TGF-beta

Abstract

Increasing evidence suggests that immune responses are important in the control of tumors. Pancreatic cancers are often infiltrated with lymphocytes, which correlates with a better prognosis. Continued tumor growth in the presence of a lymphocytic infiltration indicates a failure of immune control. TGF-beta is a highly active immunosuppressive cytokine and high serum levels correlate with a poor prognosis in pancreatic cancer, making it an interesting target for immunotherapy. The ubiquitously expressed cytosolic helicase retinoic acid-inducible protein I (RIG-I) is an immunoreceptor that signals the presence of viral RNA and activates the immune system via type I IFN. RIG-I detects RNA with a triphosphate group at the 5' end. Modification of siRNA as 5'-triphosphat-RNA (3p-siRNA) allows combining gene silencing and RIG-I activation. Here, we describe immunotherapy of murine pancreatic cancer with 3p-siRNA targeting TGF-beta. The 3p-siRNA reduced TGF-beta production by pancreatic carcinoma cells. In addition, RIG-I activation in tumor cells resulted in up-regulation of MHC-I expression, secretion of type I IFN and IP-10. As a consequence, tumor cells were sensitized to T cell-mediated killing. Moreover, 3p-siRNA induced tumor cell apoptosis via the intrinsic pathway. In vivo, 3p-siRNA against TGF-beta induced a Th1-type cytokine profile, with high levels of IL-12 and type I IFN. Systemic therapy slowed down tumor growth and prolonged survival in mice with orthotopic pancreatic tumors. Bi-functional RNA molecules that combine siRNA activity and activation of the innate immune system are promising candidates for the treatment of pancreatic cancer.