Targeted siRNA Silencing of Indoleamine 2, 3-Dioxygenase in Antigen-presenting Cells Using Mannose-conjugated Liposomes

Indoleamine 2, 3-dioxygenase (IDO) expression in dendritic cells (DCs) leads to the inhibition of T-cell activation, induction of T-cell apoptosis, and promotion of T-cell differentiation into regulatory T cells. All of these could promote tumor escapement of the host's immune surveillance system. We hypothesized that DC-targeted gene silencing of IDO would enhance antitumor immunity and thus restrain tumor growth. Mannose receptors are highly expressed in antigen-presenting cells (APCs) including DCs. In this study, we developed a novel APC-targeted small interfering RNA delivery system using mannosed liposomes (Man-lipo) with encapsulated IDO small interfering RNA (Man-lipo-siIDO), which preferentially knocked down IDO expression in draining lymph node and spleen of melanoma-bearing mice. Mice treated with Man-lipo-siIDO displayed a delayed time of onset of implanted murine melanomas, increased survival time, reduced tumor size, and increased reactivity of T cells from spleen and lymph nodes against melanoma antigens. The enhanced antitumor immunity may be linked to inhibition of apoptosis in CD8 and CD4 T cells as well as Treg cells in spleen and lymph nodes. This study is the first to demonstrate that Man-lipo-siIDO can preferentially targets APCs and efficiently silence IDO expression in vitro and in vivo; events expected to enhance antitumor immune reactions against melanoma xenografts. This study supports the hypothesis that Man-lipo-siIDO may possess the potential for development as an immune-targeting therapeutic anticancer agent.