How Do You See CG?

tion of immune cells by foreign DNA appears to require Alan Aderem*‡ and David A. Hume† uptake into the cell by receptor-mediated endocytosis * Institute for Systems Biology and endosome acidification, suggesting that the recogSeattle, Washington nition structure is either endosome-associated or cyto† Institute for Molecular Bioscience plasmic (Krieg and Wagner, 2000). Therapeutic applicaUniversity of Queensland Q4072 tions of the response to foreign DNA have been Australia expedited by the observation that short, CG-containing oligonucleotides (CpG-ODN) retain activity, even when stabilized with noncleavable phosphorothioate bonds The mammalian immune response to microbial patho(Krieg and Wagner, 2000). The remarkable specificity of gens relies on both innate and adaptive components the response is seen in the observation that reversal of (Hoffmann et al., 1999). The immediate, innate response the CG motif to GC in active CpG-ODN is sufficient is mediated largely by white blood cells such as neutroto abolish activity. Aside from the core dinucleotide, phils and macrophages, cells that phagocytose and kill variation in flanking bases alters the dose response the pathogens, and that concurrently coordinate addicurve for activation, for example a 59 C residue (a CCG tional host responses by synthesizing a wide range of core) reduces the efficacy (Sester, et al., 1999, Krieg inflammatory mediators and cytokines (Aderem and Unand Wagner, 2000). Such structure–function studies imderhill, 1999). In macrophages, the infectious agent is ply the existence of a recognition protein that can bind killed and degraded within the maturing phagosome, to either single-stranded or double-stranded DNA and and components of the pathogen are presented to T that it has exquisite specificity for the CG core in an cells, resulting in the activation of the adaptive immune optimal context. The nature of that recognition molecule response and the establishment of protective immunity has thus far eluded identification. Two recent papers (Aderem and Underhill, 1999). A major challenge to the (Chu et al., 2000; Hemmi et al., 2000) promise a rapid innate immune system is the discrimination of a large resolution of this issue, but the connection between their number of potential pathogens from self. This challenge conclusions is not immediately apparent. has been met by the evolution of a variety of receptors Toll-like Receptor 9 and Components that recognize conserved motifs on pathogens that are of Its Signaling Pathway Are Required not found in higher eukaryotes; these so called patternfor the Response to CpG-DNA recognition receptors recognize pathogen-associated The study of Akira and colleagues (Hemmi et al., 2000) molecular patterns (Janeway and Medzhitov, 1998). builds on parallels between the recognition of foreign Every organism, even bacteria, have mechanisms of DNA and other microbial products. The Toll-like recepinnate host defense to protect against invasion by potors, first identified in Drosophila, have been implicated tential pathogens. The major defense mechanism emas the receptors that enable the innate immune system ployed by bacteria relies on recognition of foreign DNA. to recognize classes of molecules that are common to Bacteria methylate specific sequences in their own gepathogenic microorganisms but are absent or underrepnome and produce restriction enzymes that cleave unresented in the host, the so-called pathogen associated methylated sequences in the DNA of an invading pathomolecular patterns (Aderem and Ulevitch, 2000). In gen such as a bacteriophage (Murray, 2000). Only quite mammals, TLR4 is required for the immune response to recently have we come to recognize that specialized bacterial lipopolysaccharide (LPS), the major cell wall immune cells in mammals can employ a very similar component of Gram-negative bacteria, while TLR2 is mechanism. The CpG dinucleotide in particular serequired for the response to several components of quence contexts (PuPuCGPyPy) is greatly reduced in Gram-positive organisms, including lipopeptides and peptidoglycan as well as yeast particles (Aderem and frequency in the mammalian genome, and where it ocUlevitch, 2000). Different Toll-like receptors can also curs, the cytosine commonly is methylated. DNA concombine to extend their repertoire of detection; for extaining unmethylated CG motifs is able to activate muample TLR2 can combine with TLR6 to detect peptidorine myeloid cells (macrophages and dendritic cells) and glycan, whereas TLR2 appears to combine with a yet B lymphocytes, a response that stimulates immune deunknown TLR to detect lipopeptide (Ozinsky et al., fenses that may be desirable in a therapeutic or immuno2000). prophylactic context (Sester et al., 1999; Wagner, 1999, The mechanism by which TLRs stimulate the producChu et al., 2000; Hemmi et al., 2000; Krieg and Wagner, tion of inflammatory molecules is being clarified (Figure 2000, and references therein). Among native foreign 1) (Janeway and Medzhitov, 1998; Aderem and Ulevitch, DNAs tested, bacterial DNA is the most active immuno2000). Ligation of a TLR promotes dimerization and remodulator, but mammalian cells also can recognize and sults in the recruitment of MyD88, an adaptor protein respond to yeast, insect, and nematode genomic DNA containing two domains: a C-terminal TIR domain that while ignoring high concentrations of their own DNA, interacts with the TIR domain of the receptor, and an even when it is demethylated (Sun et al., 1997). ActivaN-terminal death domain. This death domain undergoes homophilic interaction with the death domain of a serine/ threonine protein kinase known as IRAK; this leads to ‡ To whom correspondence should be addressed (e-mail: aderem@