Antibodies to Toll-Like Receptor (TLR) Family - FocusOn 004

The innate or natural immune system recognizes a wide spectrum of pathogens without a need for prior exposure. The main cells responsible for innate immunity, monocytes/macrophages and neutrophils, phagocytose microbial pathogens and trigger the innate, inflammatory and specific immune responses. Members of the Toll-like receptor (TLR) family, first discovered in drosophila, have been shown to be pattern recognition receptors, each member recognizing and responding to different microbial components to limit/eradicate invading microbes. Binding of pathogen-associated molecular patterns (PAMP) to TLRs induces the production of reactive oxygen and nitrogen intermediates, initiation of the pro-inflammatory cytokine network and upregulation of costimulatory molecules linking the rapid innate response to the adaptive immunity.

Fig. 1: CD281/TLR1 antibody Cat.-No. AP07365PU-N staining of formalin-fixed paraffin-embedded human small intestine

Fig.2: CD282/TLR2 antibody Cat.-No. AP19009PU-N staining of paraffin-embedded Ca922 xenograft at 1/100 dilution

The TLR family has significant homology in its cytoplasmic domain to the IL-1 receptor type I. Studies so far show that signaling pathways via TLRs originate from the conserved Toll/IL-1 receptor (TIR) domain. The TIR domain-containing MyD88 acts as a common adapter that induces secretion of inflammatory cytokines such as IL-1, IL-6, IL-8, IL-12 and TNF-alpha, and induction of co-stimulatory molecules. A MyD88-independent pathway also has been reported to induce type I Interferons (IFNs) in the TLR4 and TLR3 signaling pathway. Another TIR domain-containing adapter, TIRAP/Mal has recently been shown to mediate the MyD88-dependent activation in the TLR4 and TLR2 signaling pathway. Thus, individual TLRs may utilize different signaling systems that characterize their specific activities. In addition to induction of the cytokine network, MyD88 binds FADD and triggers apoptosis through the caspase cascade. Hence, activation of the apoptosis pathway via TLRs appears to contribute to the repertoire of defense mechanisms utilized by the innate immune response.

Fig.3: Western blot: CD281/TLR1 antibody Cat.-No. AP09159PU-N staining of mouse spleen tissue whole cell lysate. This IgG fraction of TLR1 antibody shows detection of a predominant band at ~90 kDa corresponding to TLR1 (arrowhead). The predicted MW of TLR1 is 90 kDa. TLR1 was detected using 1.0 µg/ml (Lane A, 2.0 µg/ml (Lane B) and 4.0 µg/ml (Lane C) concentrations of primary AB

Fig.4: Western blot: CD288/TLR8 antibody Cat.-No. AP09342PU-N staining of Daudi whole cell lysate. The membrane was probed with the primary AB diluted to 1/2000 (Lane A), 1/1000 (Lane B) and 1/500 (Lane C)

Fig. 5: Western blot analysis with TLR9 antibody Cat.-No. SM7105P in human PBMC (Lane A), human intestine (Lane B), mouse intestine (Lane C) and rat intestine tissue lysates (Lane D) using the AB at a dilution of 3 µg/ml. Lane E shows antibody tested at 5 µg/ml in human MCF7 cell lysate

The ligands for TLR2, TLR3, TLR4, TLR7 and TLR9 have been reported. TLR2, in association with TLR1 and/or TLR6, recognizes and signals bacterial lipoproteins (BLP) and peptidoglycans from Gram-positive bacteria. Recently, it was discovered that TLR3 recognizes dsRNA. LPS from Gram-negative bacteria signals through TLR4. The immune response modifiers, imiquimod and resiquimod, have been reported as agonists for TLR7. Cellular response to the CpG motif of bacterial DNA is mediated by TLR9. The cDNA for human TLR10 codes for a protein containing 811 amino acids. Human TLR10 is closely related to human TLR1 and TLR6. TLR10 is expressed in tissues and cells of the immune system. TLR10, originally identified from a spleen cDNA library, is on chromosome 4p14 and has been found to be expressed in lung and in B-lymphocytes. TLR10 is thought to be a potential asthma candidate gene because early life innate immune responses to ubiquitous inhaled allergens and PAMPs may influence asthma susceptibility.

In addition to the innate immune response, evidence implicates the involvement of the TLR family in a spectrum of systemic disorders following bacterial infections including sepsis, cardiac ischemia, peridontitis, and cerebral palsy. The rapid pace of research in the TLR family will elucidate their functions and potential therapeutic interventions.

Key references

 

Akira, S. et al. (2001) Toll-like receptors: critical proteins linking innate and acquired immunity.
Nat Immunol. 8:675-80.

 

Hopkins PA, Sriskandan S.: Mammalian Toll-like receptors: to immunity and beyond.
Clin Exp Immunol. Jun 2005;140(3):395-407.

Further readings

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