TLR9-dependent systemic interferon-β production by intravenous injection of plasmid DNA/cationic liposome complex in mice

Background The type I interferon (IFN) response to DNA/cationic liposome complex, or lipoplex, has been reported in cultured cells, but little is known about the response in vivo. Studies of the pro‐inflammatory cytokine response to lipoplex have shown the importance of the unmethylated CpG dinucleotide (CpG motif) and its receptor, Toll‐like receptor (TLR)‐9. Methods CpG‐ and non‐CpG lipoplex consisting of CpG‐ or non‐CpG plasmid DNA, respectively, and N‐[1‐(2,3‐dioleyloxy)propyl]‐N,N,N‐trimethylammonium chloride/cholesterol liposomes were intravenously injected into mice. IFN‐β and interleukin (IL)‐6 in the serum and organs were measured by the enzyme‐linked immunosorbent assay. The involvement of TLR9, phagocytic cells and the spleen in the responses was evaluated using TLR9−/−, clodronate liposome‐treated‐, and splenectomized mice, respectively. Accumulation of blood cells in the lung was evaluated histologically. Results CpG lipoplex induced a large increase in the levels of IFN‐β and IL‐6 in the serum, liver, spleen, lung and kidney, whereas non‐CpG lipoplex scarcely had any effect. Neither formulation led to significant cytokine production in TLR9−/− mice. Clodronate liposome‐treated mice showed a large reduction in both IFN‐β and IL‐6 levels. Splenectomized mice receiving CpG lipoplex also showed a significantly low production of IL‐6 but a similar level of IFN‐β production to that of unsplenectomized mice. A large number of monocytes were found in the capillary vessels around the pulmonary alveoli of mice receiving lipoplex. Conclusions These findings indicate that, in contrast to the production of IL‐6 from splenic macrophages, IFN‐β is produced from phagocytic cells other than splenic macrophages after the injection of CpG lipoplex through the TLR9‐dependent pathway. Copyright © 2009 John Wiley & Sons, Ltd.