CpG oligonucleotides (CpG ODNs or CpG) are short single-stranded DNA oligonucleotides containing a cytosine guanine (CpG) motif within their sequence. The “p” in this motif refers to the phosphodiester group linking the two nucleotides.
Oligonucleotides containing unmethylated CpG motifs act as immunostimulants for the innate immune system. Immune-stimulating oligonucleotides (ISOs) activate or stimulate the innate immune system via their interaction with pattern recognition receptors, encode immunostimulatory proteins or peptides, or silence specific genes to block negative regulators of the immune system. Several specific nucleic acids and oligonucleotides can act as immunostimulants.
During viral detection, cellular antiviral defenses sense foreign nucleic acids among abundant self-nucleic acids. This mechanism is known as “immune sensing.” However, an effective antiviral defense requires a balanced process of sensing foreign nucleic acids and ignoring self-nucleic acids.
This balance is accomplished by a multilevel system combining the immune sensing of pathogen-specific nucleic acid structures with specific labeling of self-nucleic acids and nuclease-mediated degradation.
Sensing nucleic acids released from pathogens and damaged or malignant cells is the primary mechanism by which innate immune cells recognize “foreign” substances and activate signaling pathways to initiate their antimicrobial and proinflammatory functions.
TLR9 recognizes unmethylated CpG regions and, when stimulated, activates B cells and human plasmacytoid dendritic cells. The activation results in a potent T helper-1 (Th1)-type immune response and an antitumor response in mouse tumor models and patients. Mammals mainly express TLR9 in subsets of Dendritic Cells and B cells. TLR9 receptors recognize different CpG motifs. Optimal sequences are GTCGTT and GACGTT for human TLR9.
Unmethylated CpG DNA containing CpG-dinucleotides is more common in bacterial genomes than in vertebrate genomes. Methylation at the CG sites generally inhibits the activity of CpG dinucleotides. The CpG motif stimulates immune cells via the toll-like receptor 9 signaling pathway.
Krieg et al. (1995) iteratively determined that the immunostimulatory activity of DNA sequences is restricted to a stretch of 12 to 20 base pairs containing CpG dinucleotides with selective flanking bases with the motif 5′-Pu-Pu-CpG-Pyr-Pyr-3′ as being biologically active.
The diverse functions of nucleic acids and oligonucleotides are critical components in vaccine development and cancer immunotherapy. For example, vaccination of the mucous membranes, the inner lining tissue cells of the nose, mouth, lungs, and stomach can initiate enhanced systemic and mucosal humoral and cellular immune responses, resulting in protection against pathogens, even at a different mucosal site. Also, combining CpG ODNs with radiation and chemotherapy can be effective. For example, treating tumors with CpG ODNs combined with radiation and docetaxel enhances the response and improves the cure rate of murine tumors.
Unfortunately, the instability, poor pharmacokinetics profile, non-specific biodistribution, and difficulty in accessing intracellular targets of CpG oligonucleotides make it challenging to develop CpG oligonucleotide-based therapeutics.
CpG 1018 is an adjuvant used in Heplisav-B vaccine made up of CpG motifs. When CpG 1018 is included in vaccines, it increases the body’s immune response.
Reference
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