Toll-like receptors (TLRs) generate an innate immune signaling response upon recognizing broadly conserved microbial extracellular structures. Viral RNA is recognized by!TLR3, TLR7 and TLR8, and microbial DNA is recognized by TLR9. Until recently the prevailing paradigm was that TLR9 recognized unmethylated CpG DNA motifs, which are abundant in bacteria but relatively scarce in mammalian DNA. However, recent studies including our own preliminary data suggest that TLR9 binds natural DNA ligands independently of their sequence and methylation state. We propose a comprehensive analysis of the structural properties that allow TLR9 to recognize microbial DNA including sequence, length, duplex content, methylation state, backbone chemistry (phosphodiester versus phosphorothioate), curvature and higher order structure (such as junctions). We show in preliminary studies that DNA curvature-inducing proteins!HMGB1 and histones H2A and H2B significantly enhance binding to the C-terminal cleavage fragment of TLR9, suggesting that TLR9 preferentially recognizes curved DNA backbones. To determine the extent to which DNA curvature alone is responsible for the binding enhancement, we propose to measure the TLR9 binding affinity of DNA minicircles containing 75 to 120 base pairs. Since nucleosomes can induce TLR-dependent auto immunogenic signaling, we propose an in vitro biophysical analysis of whole nucleosomes as TLR9 ligands! These in vitro studies will be validated in vivo by measuring TLR9-dependent signaling responses in cells stimulated with various DNA or protein-DNA ligands including minicircles, nucleosomes, junctions and methylated DNA ligands. The TLR7/8/9 ectodomains must be proteolytically cleaved in order to produce receptors that are capable of signaling. In the first study with cleaved TLR9, we show in our preliminary data that both the N- and C-terminal TLR9 ectodomain fragments participate in ligand binding and receptor dimerization. We therefore hypothesize that the two fragments remain associated after proteolytic cleavage in the endosome, and that cleavage may be necessary for TLR9 to undergo the ligand-induced conformational change that activates the receptor. To test this hypothesis, we will explore the physical and functional relationships between the two TLR9 ectodomain fragments, and elucidate the physical basis of proteolytic activation using biophysical approaches. The lack of structural information for TLR7/8/9 limits our understanding of nucleic acid recognition by these receptors. We propose to use electron cryomicroscopy and X-ray crystallography as complementary approaches to gain insight into the structural basis of TLR9-DNA recognition. By providing a molecular-level understanding of the recognition of microbial DNA by TLR9, this project will provide the necessary tools to create more potent vaccine adjuvants, and a new class of anti-inflammatory therapeutics with a wide range of applications including in particular systemic lupus erythematosus, asthma, septic shock syndrome and organ transplant rejection.

Public Health Relevance

Innate immune receptors in specialized cellular compartments recognize the genetic material of invading pathogens. This recognition event triggers an inflammatory response that normally serves to fight infection, but in some cases can trigger the autoimmune disease lupus. By providing a molecular-level understanding of pathogen recognition, this project will provide some of the necessary tools to create more potent vaccine adjuvants, and a new class of anti-inflammatory therapeutics with a wide range of applications including in particular lupus, asthma, septic shock syndrome and organ transplant rejection.!

National Institute of Health (NIH)
National Institute of General Medical Sciences (NIGMS)
Research Project (R01)
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Special Emphasis Panel (ZRG1-IMM-M (08))
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Dunsmore, Sarah
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Yale University
Schools of Medicine
New Haven
United States
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Wong, Emily V; Cao, Wenxiang; Vörös, Judit et al. (2016) P(I) Release Limits the Intrinsic and RNA-Stimulated ATPase Cycles of DEAD-Box Protein 5 (Dbp5). J Mol Biol 428:492-508
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Modis, Yorgo (2015) An Inheritable variant of the innate immune receptor melanoma differentiation-associated gene 5 promotes clearance of hepatitis C virus. Hepatology 61:418-20
Reikine, Stephanie; Nguyen, Jennifer B; Modis, Yorgo (2014) Pattern Recognition and Signaling Mechanisms of RIG-I and MDA5. Front Immunol 5:342
Wang, Jimin; Li, Yue; Modis, Yorgo (2014) Exploiting subtle structural differences in heavy-atom derivatives for experimental phasing. Acta Crystallogr D Biol Crystallogr 70:1873-83
Wang, Jimin; Li, Yue; Modis, Yorgo (2014) Structural models of the membrane anchors of envelope glycoproteins E1 and E2 from pestiviruses. Virology 454-455:93-101
Li, Yue; Modis, Yorgo (2014) A novel membrane fusion protein family in Flaviviridae? Trends Microbiol 22:176-82

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