Secondary structures in CpG oligonucleotides affect immunostimulatory activity.

Area of Science:

• Immunology
• Molecular Biology
• Oligonucleotide Therapeutics

Background:

• Oligodeoxynucleotides (ODNs) containing CpG dinucleotides are potent immune activators in vertebrates.
• Previous research indicated that the 5-end of CpG ODNs must be accessible for receptor binding and immune stimulation.
• Blocking the 5-end, such as via a 5'-5' linkage, abrogates immune activity.

Purpose of the Study:

• To investigate the impact of terminal secondary structures (hairpin loops, terminal dimers) on the immune activity of CpG oligonucleotides.
• To determine if secondary structures at the 5-end versus the 3-end have differential effects on immune stimulation.
• To gain insights into the mechanism of CpG receptor recognition and its implications for DNA vaccine design.

Main Methods:

• Synthesis of CpG oligonucleotides with varying terminal secondary structures (5' and 3' ends).
• Assessment of immune-stimulating activity of modified CpG oligonucleotides in vitro or in vivo models.
• Comparative analysis of activity reduction caused by 5'-terminal versus 3'-terminal secondary structures.

Main Results:

• Secondary structures at the 5-end of CpG oligonucleotides significantly inhibit immune activity.
• Secondary structures at the 3-end have a less pronounced effect on immune activity compared to the 5-end.
• An open 5-end appears necessary for the receptor to recognize and initiate immune stimulation.

Conclusions:

• The accessibility of the 5-end of CpG oligonucleotides is critical for their immune-activating function.
• The findings suggest that the immune receptor interacts with the DNA sequence starting from the 5-end.
• Optimizing CpG motif placement in DNA vaccines, considering end accessibility, may enhance Th1-type immune responses.