Mammalian Toll-like receptors (TLRs) 3, 7, 8 and 9 initiate immune responses to infection by recognizing microbial nucleic acids1, 2; however, these responses come at the cost of potential autoimmunity due to inappropriate recognition of self nucleic acid3. The localization of TLR9 and TLR7 to intracellular compartments appears to play a role in facilitating responses to viral nucleic acids while maintaining tolerance to self nucleic acid, yet the cell biology regulating the trafficking and localization of these receptors remains poorly understood4-6. Here, we define the route by which TLR9 and TLR7 exit the endoplasmic reticulum (ER) and traffic to endolysosomes. Surprisingly, the ectodomains of TLR9 and TLR7 are cleaved in the endolysosome, such that no full-length protein is detectable in the compartment where ligand is recognized. Remarkably, though both the full-length and cleaved forms of TLR9 are capable of binding ligand, only the processed form recruits MyD88 upon activation, arguing that this truncated receptor, rather than the full-length form, is functional. Furthermore, conditions that prevent receptor proteolysis, including forced TLR9 surface localization, render the receptor non-functional. We propose that ectodomain cleavage represents a strategy to restrict receptor activation to endolysosomal compartments and prevent TLRs from responding to self nucleic acid.

Despite the paradigm that the innate immune system uses nucleic acid-specific receptors to detect viruses due to a lack of other conserved features, a number of viruses are recognized by TLR2 and TLR4. The relevance of this recognition for antiviral immunity remains largely unexplained. Here we report that TLR2 activation by viruses leads to production of type I interferon (IFN). TLR2-dependent induction of type I IFN only occurs in response to viral ligands, indicating that TLR2 is capable of discriminating between pathogen classes. We demonstrate that this specialized response is mediated by Ly6Chigh inflammatory monocytes. Thus, the innate immune system can detect certain non-nucleic acid features of viruses and links this recognition to specific antiviral gene induction.

Here, we identify the viral oncogenes of the DNA tumor viruses, including E7 from human papillomavirus (HPV) and E1A from adenovirus, as potent and specific inhibitors of the cGAS-STING pathway. We show that the LXCXE motif of these oncoproteins, which is essential for blockade of the Retinoblastoma tumor suppressor, is also important for cGAS-STING pathway antagonism. We find that E1A and E7 bind to STING, and that silencing of these oncogenes in human tumor cells restores cGAS-STING pathway signaling. Our findings reveal a host-virus conflict that may have shaped the evolution of viral oncogenes, with implications for the origins of the DNA viruses that cause cancer in humans.

Summary Toll-like receptors (TLRs) contribute to host resistance to microbial pathogens and drive the evolution of virulence mechanisms. We have examined the relationship between host resistance and pathogen virulence using mice with a functional allele of the Nramp-1 gene and lacking combinations of TLRs. Mice deficient in both TLR2 and TLR4 were highly susceptible to the intracellular bacterial pathogen Salmonella typhimurium, consistent with reduced innate immune function. However, mice lacking additional TLRs involved in S. typhimurium recognition were less susceptible to infection. In these TLR-deficient cells, bacteria failed to upregulate Salmonella pathogenicity island 2 (SPI-2) genes and did not form a replicative compartment. We demonstrate that TLR signaling enhances the rate of acidification of the Salmonella containing phagosome, and inhibition of this acidification prevents SPI-2 induction. Our results indicate that S. typhimurium requires cues from the innate immune system to regulate virulence genes necessary for intracellular survival, growth, and systemic infection.

Quantitative PCR technology may have a major role in the near future as an adjunctive diagnostic tool in both epidemiological and clinical studies in periodontology. However, culture techniques still hold some inherent capabilities, which makes this diagnostic tool the current reference standard in periodontal microbiology.