Toll-Like Receptor Overview

Members of the Toll-like receptor (TLR) family are responsible for the recognition of pathogen-associated molecular patterns (PAMPs) expressed by a wide spectrum of infectious agents. To date, over thirteen TLRs have been reported in human and mouse. TLRs activate the NF-κB pathway, which regulates cytokine expression, through several adaptor molecules including MyD88, TIRAP/Mal and TRIF. Activation of the NF-kB pathway links innate and adaptive immune response by production of inflammatory cytokines such as IL-1, IL-6, IL-8, TNF alpha, IL-12, chemokines and induction of costimulatory molecules such as CD80, CD86, and CD40. 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.

Constitutive expression (albeit at very low levels with some family members) of human TLR1, 2, 4 and mouse TLR4/MD2 and TLR2 on the surface of myeloid lineage cells has been confirmed with specific mAbs. Activation-induced upregulation of TLR2 and 4 on other cell types such as endothelial cells, epithelial cells and others have also been reported in the literature. TLR3, TLR7, TLR8 and TLR9 are localized mainly in endosomal/lysosomal compartments. Human TLR3 is detected in human fibroblast cell lines and TLR9 in in vitro-derived DCs. Specific antibodies to other members of human and mouse TLRs are used to correlate and confirm the expression of these molecules at the protein and mRNA levels.

Evidence implicates the involvement of the TLR family in a spectrum of systemic disorders following bacterial infections including sepsis, cardiac ischemia, periodontitis and cerebral palsy. The rapid pace of research in the TLR family shall elucidate their functions and potential therapeutic interventions.

TLR4 is a pattern recognition receptor and signaling molecule, responding to bacterial lipoproteins, which has been implicated in innate immunity and inflammation. TLR4 physically associates with another molecule called MD-2, and together with CD14, this complex is responsible for LPS recognition and signaling. TLR4 is expressed by peripheral blood monocytes. Most TLR cell surface expression, especially TLR1 and TLR4, occurs at low levels on monocytes and at even lower levels on other cell types including granulocytes and immature dendritic cells (iDC). While the exact mechanism is not clearly defined at the present time, antibodies to TLR4 have been reported to inhibit the LPS-induced cytokine production by human PBMCs and mouse peritoneal macrophages.

2x10e6 ml human PBMCs were treated with 10 ng/ml LPS for 24 hours. Cells in the Anti-TLR4 treated sample were preincubated with 20 ug/ml HTA 125 for one hour prior to addition of LPS. TNF alpha secretion was measured using human TNF alpha Ready-Set-Go! ELISA set.