REDUCTION OF ANTIGEN EXPRESSION FROM DNA VACCINES BY COADMINISTERED OLIGODEOXYNUCLEOTIDES

Bacterial DNA or synthetic oligodeoxynucleotides (ODN) containing unme thylated CpG dinucleotides within the context of certain flanking base s (CpG motifs) have potent stimulatory effects on the vertebrate immun e system. CpG ODN with a synthetic nuclease-resistant phosphorothioate backbone (S-ODN) can be used as an adjuvant to augment both humoral a nd cell-mediated immune responses against a protein antigen, It has al so been shown that the presence of CpG motifs in DNA vaccines may be r esponsible, at least in part, for their efficacy, Here we evaluate the possibility of using CpG ODN as an adjuvant with DNA vaccines to furt her improve their efficacy, We show that it is not possible to directl y mix S-ODN with plasmid DNA because this will result in an ODN dose-d ependent reduction in gene expression from the plasmid, possibly becau se of competitive interference at binding sites on the surface of targ et cells, Although ODN with a phosphorothioate-phosphodiester chimeric backbone (SDS-ODE;) do not adversely effect the level of gene express ion (except when certain sequences, such as a poly G, are present), th is is not useful, as SDS-ODN are apparently also not sufficiently nucl ease resistant to exert a strong CpG adjuvant effect, Neither is it po ssible to augment responses to DNA vaccines by administering the CPG S -ODN at a different time or site than the plasmid DNA, Thus, at least for the present, it appears necessary to clone CpG motifs into DNA vac cine vectors to take advantage of their adjuvant effect.