Accessible 5'-end of CpG-containing phosphorothioate oligodeoxynucleotides is essential for immunostimulatory activity.

In our ongoing efforts to decipher the sequence and structural requirements in the flanking region of the CpG motif in phosphorothioate oligodeoxynucleotides (PS-oligos), we have examined the requirement of free 5'- and 3'-ends of PS-oligos on immune stimulation. Our model studies using 3'-3'-linked (containing two free 5'-ends) and 5'-5'-linked (containing two free 3'-ends) CpG-containing PS-oligos demonstrate that immunostimulatory activity is significantly reduced when the 5'-end of the PS-oligo is not accessible, rather than the 3'-end, suggesting that the 5'-end plays a critical role in immunostimulatory activity.