Immune complex-loaded dendritic cells are superior to soluble immune complexes as antitumor vaccine

Dendritic cells (DCs) play an important role in the induction of T cell responses. FcγRs, expressed on DCs, facilitate the uptake of complexed Ag, resulting in efficient MHC class I and MHC class II Ag presentation and DC maturation. In the present study, we show that prophylactic immunization with DCs loaded with Ag-IgG immune complexes (ICs) leads to efficient induction of tumor protection in mice. Therapeutic vaccinations strongly delay tumor growth or even prevent tumors from growing out. By depleting CD4+ and CD8+ cell populations before tumor challenge, we identify CD8+ cells as the main effector cells involved in tumor eradication. Importantly, we show that DCs that are preloaded in vitro with ICs are at least 1000-fold more potent than ICs injected directly into mice or DCs loaded with the same amount of noncomplexed protein. The contribution of individual FcγRs to Ag presentation, T cell response induction, and induction of tumor protection was assessed. We show that FcγRI and FcγRIII are capable of enhancing MHC class I-restricted Ag presentation to CD8+ T cells in vitro and that these activating FcγRs on DCs are required for efficient priming of Ag-specific CD8+ cells in vivo and induction of tumor protection. These findings show that targeting ICs via the activating FcγRs to DCs in vitro is superior to direct IC vaccination to induce protective tumor immunity in vivo.