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Toll-Like Receptors (TLR)

Toll-like receptors (TLR)

For medical students2 min readUpdated 2026-10-10

Toll-like receptors (TLRs) are critical components of the innate immune system that act as primary sensors for invading microorganisms. They ensure rapid recognition of foreign agents and trigger protective defense responses by linking immune cells to pathogens.

Primary targetConserved microbial structures (PAMPs)

Gram-negative bacteriaRecognized predominantly by TLR4 receptors

Intracellular signalActivation of transcription factor NF-$\kappa$B

TLR4 mutationsIncrease the risk of severe bronchiolitis

Localization and Molecular Targets

Toll-like receptors are strategically positioned on cells that are the first to encounter potential infectious agents. They are particularly abundant on patrolling leukocytes. In addition, these receptors are widely expressed in barrier tissues: the gastrointestinal epithelium, vascular endothelium, skin keratinocytes, and microglial cells.

The primary function of TLRs is to detect pathogen-associated molecular patterns (PAMPs). These are evolutionarily conserved microbial structures that are vital for pathogen survival but completely absent in the human body. Key types of PAMPs include:

Lipopolysaccharides (LPS), which form the basis of the outer membrane in Gram-negative bacteria.
Lipoteichoic acids, characteristic of Gram-positive microorganisms.
Peptidoglycans of the bacterial cell wall.
Specific nucleic acids: bacterial DNA and viral RNA.

Signal Transduction and the Role of CD14 Glycoprotein

Precise and efficient recognition of Gram-negative bacteria requires the participation of the specific CD14 glycoprotein. Cells expressing CD14 (monocytes, tissue macrophages, dendritic cells, and activated granulocytes) ensure exceptionally high affinity in the complex formed between bacterial LPS and the Toll-like receptor.

Binding of TLRs to PAMPs initiates a powerful intracellular response. With the mandatory involvement of adapter proteins, a signaling cascade is triggered, leading to the following events:

Activation of the critical nuclear transcription factor NF-$\kappa$B.
The activated factor translocates to the nucleus and initiates the expression of genes encoding pro-inflammatory cytokines.

Downstream effects of this cascade drive a full-scale immune response. Phagocytic cells are activated, T and B lymphocytes are stimulated, and the synthesis of protective immunoglobulins (Igs) is significantly enhanced.

Receptor Specificity to Pathogens

Innate immunity differentiates threats due to strict receptor specificity. Depending on the invading infectious agent, various TLR types are expressed on the surface of immune cells, allowing the selection of the most appropriate defense strategy.

Gram-positive bacteria: Recognized predominantly by TLR2, which responds to their surface structures.
Gram-negative bacteria: Identified via TLR4, tuned to detect lipopolysaccharides.
Viral infections: Trigger the expression of a broad spectrum of receptors, including TLR1, TLR2, TLR3, TLR7, TLR8, and TLR9, which respond to viral components.

TLR System Pathology and Immunodeficiencies

Because Toll-like receptors are a foundational element of innate immunity, any defects in their function inevitably lead to increased host susceptibility to infectious diseases.

Two main causes of such impairments exist:

Direct mutations in genes encoding the TLR receptors themselves.
Congenital defects in intracellular signaling pathways transmitting signals from the receptor to the nucleus.

In clinical practice, mutations in the TLR4 gene—specifically the Asp299Gly and Thr399Ile genetic variants—are of particular significance. The pathogenesis of these mutations involves an insufficient, attenuated response to bacterial lipopolysaccharide. Consequently, patients face a significantly higher frequency of infections caused by Gram-negative microbes, as well as an elevated risk of severe bronchiolitis.

Mnemonic

To remember bacterial specificity, use this rule: Gram(-) bacteria have a complex wall with LPS — they require a receptor with a higher number (TLR4). Gram(+) bacteria are structured differently — TLR2 is sufficient for them.

Frequently asked questions

What is the molecular structure of a Toll-like receptor (which domains are included)?

The molecular structure of a Toll-like receptor includes extracellular, transmembrane, and cytoplasmic domains.

LRR domain: Forms the extracellular portion; its ligand-binding region is horseshoe-shaped (the outer part is formed by $\alpha$-helices, and the inner part by $\beta$-sheets).
Transmembrane segment: Located between the LRR and TIR domains, responsible for membrane localization and anchoring.
TIR domain (Toll/IL-1 receptor and resistance domain): Represents the cytoplasmic (C-terminal) portion, consisting of a central $\beta$-sheet and five $\alpha$-helices, serving to interact with adapter molecules.

Which specific adapter proteins participate in intracellular signal transduction from TLRs?

Signal transduction from Toll-like receptors involves adapter proteins that interact with the intracellular TIR domain:

MyD88: Acts as a bridge between the active TLR dimer and the initial signaling kinase in the MyD88-dependent pathway (with TIRAP participation); this pathway is used by all TLRs except TLR3, while TLR4 uses both signaling pathways.
TIRAP: Participates in the MyD88-dependent pathway alongside MyD88.
TRIF: Participates in the TRIF-dependent pathway; utilized by TLR3, and noted alongside MyD88 for TLR4.
TRAM: Listed among the adapter proteins interacting with the TIR domain.
Mal: Listed as an adapter protein in the MyD88-dependent pathway using TLR4 as an example.

Which specific pro-inflammatory cytokines are synthesized following NF-$\kappa$B activation via TLRs?

Activation of the transcription factor NF-$\kappa$B via Toll-like receptor signaling pathways triggers the expression of pro-inflammatory cytokine genes.

Among the specific cytokines synthesized as a result of this process, the provided sources explicitly name only one:

Tumor necrosis factor (TNF).

Other specific pro-inflammatory cytokines produced directly after NF-$\kappa$B activation via TLRs are not detailed in the text.

What are PAMPs and how do they relate to TLRs?

PAMPs are pathogen-associated molecular patterns, i.e., typical conserved microbial structures (such as LPS or bacterial DNA). Toll-like receptors physically bind to them to recognize infection and initiate an immune response.

What is the function of the CD14 glycoprotein?

It is necessary to create high affinity for the complex between lipopolysaccharide (LPS) and the receptor. CD14 is expressed on monocytes, macrophages, activated granulocytes, and dendritic cells.

What is the outcome of a TLR4 gene mutation?

Mutations such as Asp299Gly cause a weak immune response to LPS. Consequently, the patient suffers more frequently from infections caused by Gram-negative flora and carries a high risk of developing severe bronchiolitis.

Go deeper

Structure and types of pathogen-associated molecular patterns (PAMPs)
Role of adapter proteins in TLR signal transduction
Mechanisms of nuclear transcription factor NF-$\kappa$B activation
Features of CD14 expression on various leukocyte subsets
Pathogenesis of severe bronchiolitis in innate immunity defects