Another sources of cfDNA are injuries and mitochondrial DNA, which is released during systemic inflammatory response syndrome 16, 17. Extracellular unprotected cell-free (cf)DNA and NETs 14 are processed by extracellular waste-management DNases before endocytosis 15. However, even in the absence of microbes, the destruction of host cells, including necrotic cell death and neutrophil extracellular traps (NETs), causes the release of nucleic acids, which can trigger sterile inflammation mediated by TLR9 (ref. In addition to the compartmentalization, limited activation by the autologous nucleic acids 9 is accomplished by their selectivity against methylated-CpG motifs, the requirement for the proteolytic cleavage of the TLR9 ectodomain 10, and by the acidic pH of the activating compartments 11, 12. Nucleic acids are a common constituent of all microbes 8, yet their activation of TLRs is contained within acidic cellular compartments with proteolytic and nuclease activity, which restricts activation by the circulating endogenous nucleic acids. A-class ODNs, which comprise oligomerizing segments, convert to B-class ODNs upon degradation via endosomal deoxyribonuclease (DNase) II 7. By contrast, mouse (m)TLR9 is activated by ODNs comprising a single CpG positioned 4–6 nt from the 5′-end 6. The minimal B-class ODNs that activate human (h)TLR9 comprise two CpG motifs separated by 6–10 nt, with the first CpG motif being proceeded by the 5′-thymidine 5. Minimal sequence motifs for B-class ODNs have been defined previously for human and mouse TLR9. Twelve nucleotides (nt) long B-class ODN and TLR9 were crystalized in a symmetric TLR9-CpG complex with a 2:2 stoichiometry 4, identifying the residues involved in single-stranded DNA (ssDNA) recognition, which were distributed over several LRR segments of the TLR9 ectodomain. Several types of CpG-containing ODNs, with distinct sequences and secondary structures, are potent activators of TLR9, inducing B-cell-proliferation, plasmacytoid dendritic cell (pDC) maturation, and the secretion of interleukin (IL)-6, IL-10, IL-12 and type I interferons 2, 3. Synthetic oligodeoxyribonucleotides (ODN) with non-methylated-deoxycytidylyl-deoxyguanosine dinucleotide (CpG) motifs recapitulate receptor activation by natural substrates.

The subgroup of TLRs (TLR3, TLR7, TLR8, TLR9 and TLR13) that reside in the endosomal compartments bind nucleic acids 1. Toll-like receptors (TLRs), as pattern recognition receptors, are critical for the recognition of the prototypical molecules characteristic of microbes, as well as autologous danger signals.

48 strand dna activation