Synergistic induction of macrophage tumoricidal activity

Background: An important function of the innate immune system is immune surveillance of cancer which consists of detecting and eliminating transformed cells. Macrophages and dendritic cells (DCs) are two main types of innate immune cells involved in anti-tumor immunity. Method: We used an in vitro growth inhibition assay to analyze the inhibited growth of cancer cells by activated macrophages or DCs. Nitric oxide (NO) production was quantified by Griess assay. Combination of cytokine interferon (IFN)-γ and various Toll-like receptor (TLR) ligands were tested as activators. The macrophage cell line J774.A1 and the DC cell line D2SC/1 were used as effector cells. Target cells were the MOPC315 plasmacytoma and the Lewis Lung carcinoma (LLC). The TLR4 ligand lipopolysaccharides (LPS) was used as a positive control. Results: IFNγ or a TLR ligand alone was not sufficient to render macrophages or DCs cytotoxic to cancer cells. Macrophages and DCs were shown to require two signals to become tumoricidal and secrete NO. TLR ligands Pam3, Poly(I:C), flagellin, CL264, and CpG ODN were identified as molecules being able to render macrophages and DCs tumoricidal, when given in combination with IFN. Cancer cell killing may be mediated by NO secretion. Conclusions: Induction of tumoricidal activity in macrophages and DCs requires two signals. Five TLR ligands other than LPS were identified as being able to synergize with IFNγ to render macrophages and DCs cytotoxic to cancer cells. These molecular combinations may serve as a basis for the development of novel immunotherapy protocols for cancer