Immunostimulatory activity of CpG oligonucleotides containing non-ionic methylphosphonate linkages

Bacterial DNA and synthetic oligodeoxynucleotides containing unmethylated CpG-motifs in a particular sequence context activate vertebrate immune cells. We examined the significance of negatively charged internucleoside linkages in the flanking sequences 5' and 3' to the CpG-motif on immunostimulatory activity. Cell proliferation and secretion of IL-12 and IL-6 in mouse spleen cell cultures, and spleen weights of mice increased significantly when a non-ionic linkage was placed at least four or more internucleoside linkages away from the CpG-motif in the 5'-flanking sequence. When the non-ionic linkage was placed closer than three internucleoside linkages in the 5'-flanking sequence to the CpG-motif, immunostimulatory activity was suppressed compared with that observed with the unmodified parent oligo. In general, the placement of non-ionic linkage in the 3'-flanking sequence to the CpG-motif either did not affect or slightly increased immunostimulatory activity compared with the parent oligo. These results have significance in understanding CpG oligonucleotide-receptor interactions and the development of potent immunomodulatory agents.