Gevaert Toll-Like Receptor 8 (TLR8)

Toll-Like Receptor 8 (TLR8)

An Unusual Receptor in Innate Immunity

Introduction

The immune system constantly monitors the body for invading microorganisms. One of its fundamental abilities is the capacity to distinguish between self and non-self molecules, allowing it to eliminate pathogens while maintaining tolerance toward host tissues.

This recognition is mainly performed by a group of receptors known as Pattern Recognition Receptors (PRRs). These receptors detect conserved molecular structures found in microbes, called Pathogen-Associated Molecular Patterns (PAMPs).

Among the different PRRs identified so far, Toll-Like Receptors (TLRs) are the most extensively studied. They play a crucial role in coordinating both innate and adaptive immune responses by recognizing a wide variety of microbial components and activating intracellular signaling pathways that trigger inflammation and immune defense mechanisms.

Among the TLR family, Toll-Like Receptor 8 (TLR8) stands out because of its unique regulatory properties and its involvement in immune regulation and disease.

Toll-Like Receptors

Discovery and Biological Role

The role of Toll receptors in immunity was first discovered in the fruit fly Drosophila melanogaster. Initially, the Toll receptor was identified as a transmembrane protein required for establishing dorsoventral polarity during embryonic development.

Later studies demonstrated that mutations in the Toll signaling pathway led to increased susceptibility to fungal infections due to defective production of antimicrobial peptides. These findings highlighted the essential role of Toll receptors in immune defense.

Subsequently, researchers identified homologous receptors in mammals. One of the first discovered was TLR4, which shares structural and functional similarities with the Toll receptor in Drosophila.

Structural Characteristics of TLRs

Toll-Like Receptors are type I transmembrane proteins composed of three main structural domains:

1. Extracellular Domain

This domain contains Leucine-Rich Repeats (LRR) responsible for recognizing microbial molecules.

2. Transmembrane Domain

This region anchors the receptor within the cell membrane.

3. Intracellular Domain

The intracellular region contains a TIR (Toll/Interleukin-1 receptor) domain, which transmits signals to downstream signaling molecules after ligand recognition.

Activation of TLRs leads to intracellular signaling cascades that stimulate the production of cytokines and chemokines, molecules that regulate immune responses and inflammation.

TLR Family Members and Localization

Currently, 10 functional TLRs have been identified in humans and 12 in mice. TLR1 to TLR9 are conserved between both species, although some differences exist in gene functionality and expression.

TLRs can be categorized according to their cellular localization:

Cell Surface TLRs

These receptors detect structural components of microorganisms.

Examples include:

  • TLR1
  • TLR2
  • TLR4
  • TLR5
  • TLR6

They mainly recognize bacterial membrane components, such as lipopolysaccharides or lipoproteins.

Intracellular TLRs

Other TLRs are located inside the cell within endosomes and lysosomes.

These include:

  • TLR3
  • TLR7
  • TLR8
  • TLR9

Intracellular TLRs are specialized in detecting microbial nucleic acids, particularly viral RNA or bacterial DNA.

Recognition of Endogenous Molecules

Although TLRs primarily detect microbial molecules, increasing evidence shows that they can also recognize endogenous molecules released during tissue damage or cell death.

These endogenous ligands can activate TLR signaling pathways even in the absence of infection, leading to sterile inflammation.

Such mechanisms are involved in several pathological conditions, including:

  • autoimmune diseases
  • cancer development
  • chronic inflammatory disorders
  • tissue injury responses

Endogenous TLR ligands may include:

  • proteins and peptides
  • nucleic acids
  • proteoglycans
  • phospholipids

Intracellular Toll-Like Receptors and Nucleic Acid Recognition

Intracellular TLRs specialize in detecting viral and bacterial nucleic acids.

TLR3

TLR3 recognizes double-stranded RNA (dsRNA) produced during viral replication.

TLR7 and TLR8

TLR7 and TLR8 are closely related receptors encoded by genes located on the X chromosome. Both receptors recognize single-stranded RNA (ssRNA) derived from RNA viruses.

Examples of viruses detected by these receptors include:

  • Influenza virus
  • Vesicular stomatitis virus
  • Human immunodeficiency virus (HIV)

TLR9

TLR9 recognizes unmethylated CpG DNA motifs, which are abundant in bacterial and viral genomes but rare in mammalian DNA.

Regulation of Intracellular TLR Activity

Recognition of nucleic acids poses a challenge for immune cells because both host and microbial nucleic acids share similar structures. To prevent unwanted immune activation against self-molecules, intracellular TLRs are tightly regulated.

Key regulatory mechanisms include:

Endosomal Localization

TLR3, TLR7, TLR8, and TLR9 are active only within endolysosomal compartments, reducing the risk of recognizing host nucleic acids.

Endoplasmic Reticulum Transport

These receptors are initially stored in the endoplasmic reticulum and transported to endosomes by the membrane protein UNC93B1.

Proteolytic Activation

Once inside endosomes, several resident proteases cleave the receptors, generating the active form capable of initiating immune signaling.

The Unique Role of TLR8 in Immune Regulation

Despite its importance, TLR8 remains one of the least understood members of the TLR family.

Historically, mouse TLR8 was considered non-functional, which limited experimental studies. However, recent research has demonstrated that TLR8 plays an important role in regulating immune responses, particularly in controlling the activity of TLR7.

Studies in mice lacking TLR8 have shown that its absence leads to:

  • increased activation of dendritic cells
  • excessive TLR7 signaling
  • enhanced NF-κB activation
  • development of autoimmune symptoms

These findings suggest that TLR8 acts as a regulatory checkpoint, preventing excessive immune activation driven by TLR7.

TLR8 and Autoimmune Diseases

In experimental mouse models, deficiency of TLR8 leads to several autoimmune features resembling systemic lupus erythematosus (SLE).

Observed effects include:

  • splenomegaly
  • reduced marginal zone B cells
  • increased autoantibody production
  • immune complex deposition in kidneys

Interestingly, mice lacking both TLR7 and TLR8 do not develop these abnormalities, highlighting the central role of TLR7 in disease development and the regulatory function of TLR8.

TLR7–TLR8 Interaction Mechanisms

Although the exact mechanism remains unclear, several hypotheses explain how TLR8 regulates TLR7 signaling.

One possibility is the formation of heterodimers between TLR7 and TLR8. These complexes may be unable to recognize RNA ligands or initiate signaling, thereby inhibiting TLR7 activity.

Another potential regulatory mechanism involves competition for the UNC93B1 transport protein, which controls the trafficking of endosomal TLRs from the endoplasmic reticulum to endosomes.

TLR8 Function in Humans

In humans, TLR8 is expressed in several immune cell types, including:

  • monocytes
  • myeloid dendritic cells
  • B lymphocytes

Activation of human TLR8 triggers strong inflammatory responses and stimulates the production of cytokines such as:

  • TNF-α
  • IL-12
  • MIP-1α

Compared with TLR7 activation, TLR8 stimulation generally results in stronger pro-inflammatory cytokine production, whereas TLR7 preferentially induces type I interferons.

Therapeutic Potential of TLR7 and TLR8 Agonists

Synthetic molecules that activate TLR7 and TLR8 have significant clinical applications. Among the most studied compounds are imidazoquinoline derivatives, such as:

  • Imiquimod
  • Resiquimod

These compounds act as immune response modifiers and possess strong antiviral and antitumor properties.

For example, imiquimod is widely used in dermatology to treat conditions such as:

  • basal cell carcinoma
  • genital warts
  • actinic keratosis
  • Kaposi’s sarcoma

These agents stimulate immune cells and activate the transcription factor NF-κB, resulting in enhanced cytokine production and improved immune responses against tumors and infections.

TLR8 in Infectious Diseases

TLR8 also plays a role in viral infections. In the case of HIV-1, viral RNA can activate TLR8 in dendritic cells, triggering NF-κB signaling and promoting transcription of the integrated viral genome.

This discovery suggests that TLR8 may represent a potential therapeutic target for controlling viral replication and disease progression.

TLR8 as a Vaccine Adjuvant Target

There is growing interest in using TLR agonists as vaccine adjuvants. TLR8 agonists are particularly promising because they strongly stimulate antigen-presenting cells and enhance immune responses.

Interestingly, TLR8 agonists are also effective in activating immune responses in newborns, a population that typically shows weak responses to many vaccine adjuvants.

This property makes TLR8 a promising candidate for the development of next-generation vaccines.

Conclusion

Toll-Like Receptor 8 is an important component of the innate immune system with unique regulatory functions. Beyond its role in recognizing viral RNA, TLR8 participates in the fine regulation of immune signaling pathways, particularly through its interaction with TLR7.

Recent discoveries have highlighted the involvement of TLR8 in autoimmune diseases, viral infections, and immune modulation. Furthermore, synthetic TLR8 agonists are being explored for their potential applications in cancer therapy, vaccine development, and treatment of infectious diseases.

Continued research into TLR8 signaling and regulation will be essential for developing new therapeutic strategies targeting immune responses.