Phosphorothioate Backbone Modification Changes the Pattern of Responses to CpG

Bacterial DNA is immunostimulatory due to the presence of specific unmethylated CpG-containing sequences (1). The activity of bacterial DNA can be mimicked by oligonucletides (ODN) (2), and this has been critical in establishing the sequence requirements for activation. Both native phosphodiester oligonucleotides and phosphorothioate-modified oligonucleotides (PO-ODN and PS-ODN) of various sequences can activate macrophages, dendritic cells, and B lymphocytes. Although PO-ODN are the most relevant to the role of bacterial or viral DNA in the host response to infection, stabilized synthetic oligonucleotides have great potential in immunotherapy. Normal phosphodiester oligonucleotides are short-lived in vivo (3) and incapable of giving effective therapeutic immunostimulation. The most frequent means of stabilizing oligonucleotides is by phosphorothioate modification of the backbone, whereby one of the nonbridging oxygens of the phosphate group is converted to sulfur. Phosphorothioates are poor substrates for most cellular nucleases. CpG PS-ODN display many of the activities of bacterial DNA, but owing to evidence of phosphorothioate-specific activity, caution has been urged in their use as a model of bacterial DNA in infection (4,5). Although PS-ODN are now widely used in studies on the immunostimulatory effects of CpG-containing DNA, it is appropriate to ask whether they mimic all the actions of native phosphodiester DNA.