CpG motifs in pDNA-sequences increase anti-PEG IgM production induced by PEG-coated pDNA-lipoplexes

Gene therapy is largely dependent on the development of efficient delivery vehicles. To prolong their circulating time, PEGylation of the surface of a delivery vehicle is frequently applied. However, we have reported previously that anti-PEG IgM produced by intravenous injection of PEG-coated liposome is responsible for enhanced clearance of second dose PEG-coated liposomes, which is known as the "accelerated blood clearance (ABC) phenomenon." A similar phenomenon has been observed with PEG-coated pDNA-lipoplexes (PDCLs) upon their repeated injection. But the effect of the sequence of pDNA in PDCLs on inducing the ABC phenomenon has not been thoroughly investigated. Here, we focus on CpG motifs in pDNA, which are known to have a potent immune-stimulatory activity. PDCLs with non-CpG pDNA (PNDCL) diminished the anti-PEG IgM response, resulting in significant accumulation of a second dose in tumor tissue, comparable to that of a single injection, but not in enhanced accumulation in liver. In addition, PDCL induced proliferation of IgM(+) splenic cells including B cells. These results suggest that the CpG motif is a major cause of the induction of the ABC phenomenon when PDCLs are repeatedly injected. Immunogenicity is a relevant point of concern for non-viral delivery systems. Our results indicate that the use of non-CpG pDNA may allow meaningful repeated dosing of pDNA formulations without the induction of a strong immune reaction and thus may have important implications for therapeutic use of liposomal formulations of nucleic acids.