AbstractWe used human Toll-like receptor 9 (hTLR9)-expressing HEK-Blue hTLR9 cells, which release secreted embryonic alkaline phosphatase (SEAP) upon response to CpG DNA, to evaluate the immunological properties of nucleic acid drug candidates. Our preliminary studies showed that phosphodiester CpG DNA hardly induced any SEAP secretion in HEK-Blue hTLR9 cells. In the current study, therefore, we developed HEK-Blue hTLR9 cells transduced with human macrophage scavenger receptor-1 (hMSR1), a cell-surface DNA receptor, and determined whether HEK-Blue hTLR9/hMSR1 cells respond to phosphorothioate (PS) CpG DNA and phosphodiester (PO) CpG DNA. We selected PS CpG2006, a single-stranded PO CpG DNA (ssCpG), and a tetrapod-like structured DNA (tetrapodna) containing ssCpG (tetraCpG) as model TLR9 ligands. Alexa Fluor 488-labeled ligands were used for flow cytometry. Unlike the mock-transfected HEK-Blue hTLR9 cells, the HEK-Blue hTLR9/hMSR1 cells efficiently took up all three CpG DNAs. SEAP release was almost proportional to the uptake. Treatment of HEK-Blue hTLR9/hMSR1 cells with an anti-hMSR1 antibody significantly reduced the uptake of ssCpG and tetraCpG. Collectively, reconstruction of TLR9-mediated responses to CpG DNA in HEK-Blue hTLR9 cells can be used to evaluate the toxicity of nucleic acid drug candidates with diverse physicochemical properties.