Combined TLR Stimulation With Pam3Cys and Poly I:C Enhances Flt3-Ligand Dendritic Cell Activation for Tumor Immunotherapy

The cytokines granulocyte macrophage colony-stimulating factor (GM-CSF) and interleukin (IL)-4 are frequently used for generating dendritic cells (DCs) for therapeutic vaccination against cancer. These in vitro DCs share several characteristics with inflammatory monocyte-derived DCs in vivo. In contrast, culture of bone marrow cells in Flt3-ligand (Flt3L) generates a heterogeneous population of DCs, which comprise conventional DCs (cDCs) and plasmacytoid DCs similar to the steady-state populations found in vivo. Although previous studies have identified combinations of toll-like receptor ligands (TLR-Ls) that induce optimal activation of GM-CSF/IL-4 DCs in vitro, the conditions for optimal activation of Flt3L-DCs have not been established. In this study, we show that various combinations of the TLR-Ls Pam3Cys, Poly I: C, lipopolysaccharide, and CpG all increased Flt3L-DC maturation, but only the combination of Pam3Cys/Poly I: C showed a trend to enhanced production of IL-12p70 and tumor necrosis factor-a by cDCs. Pam3Cys/Poly I: C-treated cDCs also displayed enhanced capacity to present antigen to CD4+ T cells, and cross-present to CD8+ T cells, increasing T-cell proliferation in vitro. Within a prophylactic vaccination setting, cDCs activated with Pam3Cys/Poly I: C conferred tumor protection in mice. However, the numbers of cDCs required for protection were higher than the numbers of optimally activated GM-CSF/IL-4 DCs required for a similar effect. Our results show that combined TLR stimulation can enhance both the phenotypic and functional properties of Flt3L-DCs, but even under conditions of optimal activation these cells are not superior in activity to GM-CSF/IL-4 DCs in vivo.