CpG oligodeoxynucleotide

The "p" refers to the phosphodiester link between consecutive nucleotides, although some ODN have a modified phosphorothioate (PS) backbone instead.

[6] Since then, synthetic CpG ODN have been the focus of intense research due to the Type I pro-inflammatory response they elicit and their successful use as vaccine adjuvants.

PS modification protects the ODN from being degraded by nucleases such as DNase in the body and poly G tail enhances cellular uptake.

[7] The poly G tails form intermolecular tetrads that result in high molecular weight aggregates.

This led to the creation of five unofficial classes or categories of CpG ODN based on their sequence, secondary structures, and effect on human peripheral blood mononuclear cells (PBMCs).

[11] B ODN have been studied extensively as therapeutic agents because of their ability to induce a strong humoral immune response, making them ideal as a vaccine adjuvant.

The different classes of ODN elicit different responses in pDC and B cells. Class A strongly stimulates pDC and the production of IFNα. Class B strongly stimulates B cells and antibody production. Class C moderately stimulates both cell types. Class P elicits a response similar to Class A while Class S competitively inhibits the response.
The different classes of ODN elicit different responses in pDC and B cells.