Microbial Nucleic Acid Sensing in Oral and Systemic Diseases

Abstract

One challenge in studying chronic infectious and inflammatory disorders is understanding how host pattern recognition receptors (PRRs), specifically toll-like receptors (TLRs), sense and respond to pathogen- or damage-associated molecular patterns, their communication with each other and different components of the immune system, and their role in propagating inflammatory stages of disease. The discovery of innate immune activation through nucleic acid recognition by intracellular PRRs such as endosomal TLRs (TLR3, TLR7, TLR8, and TLR9) and cytoplasmic proteins (absent in melanoma 2 and DNA-dependent activator of interferon regulatory factor) opened a new paradigm: Nucleic acid sensing is now implicated in multiple immune and inflammatory conditions (e.g., atherosclerosis, cancer), viral (e.g., human papillomavirus, herpes virus) and bacterial (e.g., Helicobacter pylori, pneumonia) diseases, and autoimmune disorders (e.g., systemic lupus erythematosus, rheumatoid arthritis). Clinical investigations reveal the overexpression of specific nucleic acid sensors in diseased tissues. In vivo animal models show enhanced disease progression associated with receptor activation. The involvement of nucleic acid sensors in various systemic conditions is further supported by studies reporting receptor knockout mice being either protected from or prone to disease. TLR9-mediated inflammation is also implicated in periodontal diseases. Considering that persistent inflammation in the oral cavity is associated with systemic diseases and that oral microbial DNA is isolated at distal sites, nucleic acid sensing may potentially be a link between oral and systemic diseases. In this review, we discuss recent advances in how intracellular PRRs respond to microbial nucleic acids and emerging views on the role of nucleic acid sensors in various systemic diseases. We also highlight new information on the role of intracellular PRRs in the pathogenesis of oral diseases including periodontitis and oral cavity cancer, which might offer future possibilities for disease prevention and therapy.

Keywords: AIM2; DAI; infection; inflammation; periodontal disease; toll-like receptor.

Figure 1.

Cellular trafficking pathways of the nucleic acid sensing toll-like receptors (TLRs). The chaperone protein UNC93B associates with TLR3, TLR7, and TLR9 and facilitates their exit from the endoplasmic reticulum (ER) to the Golgi apparatus via coat protein complex II vesicles. With the exception of TLR3, TLR7 and TLR9 require the additional folding chaperone proteins gp96 and PRAT4 for exiting the ER. TLRs are sorted into endosomes through the Golgi apparatus. Prior to endosomal localization, TLR3 and TLR9 translocate to the cell membrane, where they remain functionally inactive. TLR7 and TLR9 shuttling into endosomes require adaptor protein complex 4 and 2, respectively. After the arrival to the endosome, the TLRs are cleaved by cathepsin and asparagine endopeptidase (AEP) to yield an active, functional receptor.

Figure 2.

Signaling pathways of nucleic acid sensors. Toll-like receptor 3 (TLR3) recognizes dsRNA to initiate a signaling cascade with the recruitment of toll/interleukin-1 receptor domain-containing adaptor protein inducing interferon β (TRIF), signaling through TANK-binding kinase (TBK), resulting in the nuclear translocation of interferon regulatory factor 3 (IRF3) to induce type I interferon production. Additionally, TRIF can signal via tumor necrosis factor receptor–associated factor 6 (TRAF6), the transforming growth factor β–activated kinase 1 (TAK1)/TAK-binding protein (TAB) complex, and the IκB kinase (IKK) complex to activate nuclear factor κ light-chain enhancer of activated B cells (NF-κB) for the induction of proinflammatory cytokines. The recognition of viral ssRNA and CpG DNA activates TLR7 and TLR9, respectively, to recruit myeloid differentiation primary response gene 88 (MYD88). MYD88 recruitment triggers a signaling cascade via interleukin-1 receptor–associated kinase 4 (IRAK4) to activate activator protein 1 (AP-1), NF-κB, and IRF5 for the transcription of proinflammatory cytokines and type I interferons. Concurrently, MYD88 signals via TRAF6 and TRAF3 for the IRF7-mediated transcription of type I interferons. Absent in melanoma 2 (AIM2) recognizes dsDNA for the recruitment of caspase-1 via apoptosis-associated speck-like protein containing a CARD (ASC) to activate interleukin 1β (IL-1β) and IL-18. dsDNA recognition by DNA-dependent activator of interferon regulatory factor (DAI) triggers TBK1, signaling through the IKKε complex for the IRF3-mediated transcription of type I interferons. Additionally, DAI-mediated dsDNA recognition activates receptor-interacting protein 1 (RIP1)/RIP3 signaling through the IKK complex to activate NF-κB for the transcription of proinflammatory cytokines.

Figure 3.

Microbial nucleic acid sensors in periodontitis. (A) Toll-like receptor 8 (TLR8), TLR9, and DNA-dependent activator of interferon regulatory factor (DAI) gene expression were significantly elevated in periodontitis lesions compared to healthy sites, and TLR9 expression was the highest of all the other innate sensors. (B) Immunohistochemistry analyses revealed increased TLR9 and DAI expression in periodontitis lesions compared to healthy tissues. Adapted from Sahingur et al. (2013). (C) Following the induction of periodontitis using oral gavage with Porphyromonas gingivalis, bacteria-infected TLR9^–/– mice did not exhibit significant alveolar bone loss compared to sham-infected TLR9^–/– mice. There was significant bone loss in P. gingivalis–infected wild-type (WT) mice compared to sham-infected WT mice and P. gingivalis–infected TLR9^–/– mice. (D) Micro–computed tomography images of P. gingivalis–infected TLR9^–/– and WT mice versus sham-infected controls. Adapted from Kim et al. (2015). *P < 0.05, **P < 0.01.

Figure 4.

Diseases related to nucleic acid sensors. Nucleic acid sensing has been implicated in the pathogenesis of several diseases including infectious and inflammatory conditions, autoimmune disorders, and cancer. *Systemic disorders associated with periodontal disease and highlighted in this review.

Figure 5.

Potential areas of investigation in relation to nucleic acid sensing within the oral cavity.