Immune stimulation mediated by autoantigen binding sites within small nuclear RNAs involves Toll-like receptors 7 and 8.

Systemic lupus erythematosus (SLE) is an autoimmune disease characterized by the production of autoantibodies to certain cellular macromolecules, such as the small nuclear ribonucleoprotein particles (snRNPs), which had been considered to be passive targets of the autoimmune response. SLE is also characterized by the increased expression of type I interferon (IFN), which appears to be associated with the development and severity of disease. Here, we show that specific, highly conserved RNA sequences within snRNPs can stimulate Toll-like receptors (TLRs) 7 and 8 as well as activate innate immune cells, such as plasmacytoid dendritic cells (pDCs), which respond by secreting high levels of type I IFN. SLE patient sera containing autoantibodies to snRNPs form immune complexes that are taken up through the Fc receptor gammaRII and efficiently stimulate pDCs to secrete type I IFNs. These results demonstrate that a prototype autoantigen, the snRNP, can directly stimulate innate immunity and suggest that autoantibodies against snRNP may initiate SLE by stimulating TLR7/8.

Sequences derived from self-RNA containing certain natural modifications act as suppressors of RNA-mediated inflammatory immune responses.

The ability of the host to distinguish between self and foreign nucleic acids is one of the critical factors contributing to the recognition of pathogens by Toll-like receptors (TLRs). Under certain circumstances, eukaryotic self-RNA may reach TLR-containing compartments allowing for self-recognition. Specific modifications were previously demonstrated to suppress immune activation when placed at several positions in an immune stimulatory RNA or silencing RNA (siRNA). However, we show that even a simple natural modification such as a single 2'-O-methylation at different nucleotide positions throughout a sequence derived from a self-RNA strongly interferes with TLR-mediated effects. Such a single modification can even have an inhibitory effect in vitro and in vivo when placed in a different than the immune stimulatory RNA strand acting as suppressive RNA. Several safeguard mechanisms appear to have evolved to avoid cellular TLR-mediated activation by self-RNAs that may under other circumstances result in inflammatory or autoimmune responses. This knowledge can be used to include as few as a single 2'-O-methyl modification at a specific position in a siRNA sense or anti-sense strand to avoid TLR immune effects.

Characterization of conserved viral leader RNA sequences that stimulate innate immunity through TLRs.

Viruses of the order Mononegavirales encompass life-threatening pathogens with single-stranded segmented or nonsegmented negative-strand RNA genomes. The RNA genomes are characterized by highly conserved sequences at the extreme untranslated 3' and 5' termini that are most important for virus infection and viral RNA synthetic processes. The 3' terminal genome regions of negative-strand viruses such as vesicular stomatitis virus, Sendai virus, or influenza virus contain a high number of conserved U and G nucleotides, and synthetic oligoribonucleotides encoding such sequences stimulate sequence-dependent cytokine responses via TLR7 and TLR8. Immune cells responding to such sequences include NK cells, NK/T cells, plasmacytoid, and myeloid dendritic cells, as well as monocytes and B cells. Strong Th1 and pro-inflammatory cytokine responses are also induced upon in vivo application of oligoribonucleotides. It appears possible that the presence of highly conserved untranslated terminal regions in the viral genome fulfilling fundamental functions for the viral replication may enable the host to induce directed innate immune defense mechanisms, by allowing pathogen detection through essential RNA regions that the virus cannot readily mutate.

Modulating responsiveness of human TLR7 and 8 to small molecule ligands with T-rich phosphorothiate oligodeoxynucleotides.

Toll-like receptors (TLR) 7 and 8 are closely related members of the TLR family of pathogen-associated molecular pattern recognition receptors and have an important function in activation of innate immune responses upon viral infection. TLR7 can be activated selectively by the guanosine analogue loxoribine, whereas the imidazoquinoline derivative Resiquimod (R-848) activates both TLR7 and TLR8. We demonstrate that co-incubation of R-848 with thymidine homopolymer oligodeoxynucleotides (ODN) significantly increased activity of R-848 on TLR8-expressing HEK 293 cells, but abolished TLR7-mediated signaling. Similarly, the combination of loxoribine and thymidine ODN redirected the stimulatory effect of loxoribine away from TLR7, and toward TLR8. This alteration in ligand specificity was demonstrated both in TLR-transfected HEK cells, and also in human PBMC, with a corresponding change in cytokine production away from IFN-alpha secretion by TLR7-expressing plasmacytoid DC and toward IL-12, TNF-alpha and IFN-gamma secretion by TLR8-expressing monocytes and NK cells. These results demonstrate an unexpected plasticity in the ligand specificities of TLR7 and TLR8, and suggest a novel sequence-selective interaction between these receptors and synthetic phosphorothioate ODN.

Characterization of three CpG oligodeoxynucleotide classes with distinct immunostimulatory activities.

Oligodeoxynucleotides (ODN) with unmethylated deoxycytidyl-deoxyguanosine (CpG) dinucleotides (CpG ODN) mimic the immunostimulatory activity of bacterial DNA and are recognized by the Toll-like receptor 9 (TLR9). CpG ODN of the B-Class stimulate strong B cell and NK cell activation and cytokine production. The highest degrees of NK stimulation as well as IFN-alpha secretion by plasmacytoid DC were found to occur only with A-Class ODN. A third class of CpG ODN combines the immune effects of A- and B-Class CpG ODN. C-Class ODN strongly stimulate B cell or NK cell activation and IFN-alpha production. In contrast to the A-Class, the C-Class is wholly phosphorothioate, has no poly-G stretches, but has palindromic sequences combined with stimulatory CpG motifs. All classes stimulate TLR9-dependent signaling, but with strikingly different dose-response relationships that are quite in contrast to those observed for IFN-alpha. Effects similar to those on human cells were observed on mouse splenocytes. In contrast, splenocytes from TLR9-deficient mice did not show any response to the three CpG ODN classes. In vivo studies demonstrate that C-Class ODN are very potent Th1 adjuvants. C-Class ODN may represent new therapeutic drugs that combine the effects of A- and B-Class ODN for broad applications in infectious disease or cancer therapy.