Ae Kyung Yi

The role of CpG dinucleotides in DNA vaccines

DNA vaccines can induce potent humoral and cellular immune responses without any additional adjuvant. Recent studies indicate that unmethylated CpG dinucleotides within DNA vaccines are immune stimulatory and exert an essential endogenous adjuvant activity. These CpG motifs can be added deliberately to DNA or conventional protein vaccines to enhance the Th1 immune response.

Mechanism of action of CpG DNA

In recent years, the position of the innate immune system in regulating nearly all immune responses has become well established. A central tenet in understanding the function of the innate immune system is the concept that it is triggered by pattern recognition receptors (PRRs), which bind microbial structures that are not present in host tissues. Examples of PRRs that have become well accepted are the lipopolysaccharide (LPS) receptors, such as CD14, and mannose-binding protein. Likewise, complement proteins have a primitive ability to bind and be activated by microbial structures. Viral RNAs are thought to adopt certain structural conformations that are bound by PRRs and lead to the activation of the double-stranded-RNA-dependent protein kinase PKR (Kumar et al. 1997). This mechanism is thought to be responsible for immune activation by the polynucleotide (rI, rC) (Talmadge et al. 1985; Wiltrout et al. 1985). Evidence that bacterial DNA (bDNA) may also possess some structural feature that activates innate immune defenses was first provided by Tokunaga et al. who reported its Surprising antitumor activity and ability to activate natural killer (NK) cells (Tokunaga et al. 1984; Yamamoto et al. 1988). Further studies by these investigators led to the proposal that this immune-stimulatory activity of bDNA resided in certain self-complimentary, palindromic sequences that contained CpG dinucleotides (Kuramoto et al. 1992). Interestingly, methylation of the CpGs was initially thought to have no influence on the immune stimulatory activities of DNA (Kuramoto et al. 1992).

Rescue of B cells from apoptosis by immune stimulatory CpG DNA.

ConclusionsCpG DNA is extremely effective at protecting primary B cells or B cell lines against apoptosis induced by multiple different stimuli. These protective effects are mediated through a chloroquine-sensitive pathway and are associated with increased activity of NF-κB and increased expression of the anti-apoptotic factor, BCLXL. This anti-apoptotic property of CpG DNA is yet another manifestation of its important role in regulating lymphocyte homeostasis. It seems likely that the anti-apoptotic effects of CpG DNA contribute to its remarkable efficacy as a vaccine adjuvant.

Activation of B cells by CpG Motifs in Bacterial DNA

Although the immune system possesses exquisitely antigen-specific receptors on T cells and B cells, the generation of such adaptive immune responses relies on the assistance of the innate immune defenses, such as dendritic cells, which must be activated in order to trigger optimal immune responses. These cells of the innate immune system lack such highly specific antigen receptors, instead relying on a set of “pattern recognition receptors” (PRRs) which have a general ability to detect certain molecular structures that are common to many pathogen occasions, but are not present in self tissues (1,2). One such PRR is represented by unmethylated CpG dinucleotides in particular base contexts, which are prevalent in bacterial and many viral DNAs, but are heavily suppressed and methylated in vertebrate genomes (1–5). Thus, the immune system appears to use the presence of this molecular structure as a “danger signal” that indicates the presence of infection and activates appropriate defense pathways. B cells are one of a very small subset of immune cells that express TLR-9, the putative CpG receptor, giving them a key role in the initiation of CpG-induced immune responses. The purpose of this review is to examine the B-cell effects of CpG DNA, and consider how these may affect the activation of innate and acquired immunity.

Immune Effects of Bacterial DNA and Their Possible Role in the Pathogenesis of Lupus

Since the discovery of anti-DNA antibodies in lupus approximately four decades ago, there have been many investigations into how these antibodies arise. Extensive studies demonstrated that DNA was a poor antigen and that it did not readily induce immune responses. On the other hand, DNA is a polyanion and, like other polyanions, such as dextran sulfate, can have a broad range of biologic effects. In recent years it has become clear that fairly minor structural differences in the DNA of vertebrates and bacteria can confer dramatic immune-activating properties, including the ability to induce production of anti-DNA antibodies. This chapter reviews the history of the immune effects of nucleic acids, the current state of this field, and its possible implications for our understanding of lupus pathogenesis.