Systematic Modulation of Immune Responses by CpG DNA

Abstract

In recent years, it has become clear that the innate immune system regulates essentially all immune responses. An extremely important mechanism that controls the induction of innate immune responses is the presence of pattern recognition receptors (PRRs) which bind microbial structures that are not present in host tissues. For example, CD14 appears to function as a PRR for the detection of endotoxins and other PRRs detect high mannose proteins that are commonly present in bacteria. Viral RNA molecules appear to form double stranded structures which are thought to bind a PRR and trigger the activation of the double-stranded RNA-dependent protein kinase (Kumar et al. 1997). Another nucleic acid that now appears to also bind PRRs and activate innate immunity is bacterial DNA (Tokunaga et al. 1984, Yamamoto et al. 1988, Messina et al. 1991). It is now clear that the immune stimulatory effects of bacterial DNA result from its content of unmethylated CpG (cytosine followed by a guanine and linked by a phosphodiester bond) dinucleotides in particular base contexts (Krieg et al. 1995 b). CpG dinucleotides are present at approximately the expected frequency in bacterial DNA and are unmethylated. In contrast, CpG dinucleotides are “suppressed” to about 1/4 of the expected frequency and are heavily methylated in vertebrate DNA (Bird 1987). The immune stimulatory effects of bacterial DNA are abolished by methylation with CpG methylase, confirming that this is the structural difference by which the immune system recognizes bacterial DNA as being foreign (Krieg et al. 1995 b). Furthermore, the immune stimulatory effects can be mimicked by synthetic oligodeoxynucleotides (ODN) bearing unmethylated CpG dinucleotides in particular base contexts, which differ between murine and human cells (Yi et al. 1998 a and b, Hartmann and Krieg 1999, Hartmann et al. 1999). These observations demonstrate that the vertebrate immune system has evolved a defense system for the detection of foreign DNA which is based on the specific ability to detect these CpG motifs. The purpose of this chapter is to review the current level of understanding of the immune effects and mechanisms of CpG motifs with particular attention to their significance for gene therapy. Since essentially all gene therapy techniques involve the introduction or production of CpG motifs in host cells, an understanding of CpG DNA will be important for the further advancement of this field. Recent studies have also suggested significant immunologic consequences from the intracellular introduction of double stranded RNA or DNA through a CpG-independent mechanism (Suzuki 1999), but this potential new immune effect of nucleic acids is not yet well enough understood for inclusion in the present chapter.