Toll-Like Receptors

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Toll-like receptors (TLRs) expression was initially detected in innate immune cells and epithelial cells where they mediate immune responses upon infection. In recent years, it has been observed that TLR expression is not restricted to the innate immune system. TLR was detected in B cells as well as in T cells and their subpopulations. Recent studies identified TLR expression in γδ T cells possibly playing an important role in early immune responses of γδ T cells against different pathogens.

Brief Introduction to TLR

TLRs were the first mammalian pattern-recognition receptors (PRR) to be cloned. They are the most well-characterized PRRs and subjects of considerable investigation. TLRs include 10 functional subtypes in humans (TLR1 10), of which TLR1, 2, 4, 5, 6, and 10 are expressed on the cell membrane, and TLR3, 7, 8, and 9 are located on the endosomal membrane. The differential subcellular localizations of these homologous TLRs largely correlate with the types of molecular patterns they recognize. Several studies have indicated that UNC93B1, a 12-pass transmembrane (TM) chaperone protein, mediates trafficking of TLR3, 5, 7, and 9 from the endoplasmic reticulum to the endolysosome membrane through direct interaction with TLRs or by recruiting an adaptor protein complex 2 (AP2). The innate immune system, governed by TLRs, performs a defensive role that contributes to the survival of the host by promptly neutralizing external or internal threats.

Important Relationship Between γδ T Cells and TLR

γδ T cells and toll-like receptors (TLR) serve as an important link between the innate and adaptive immune responses.

  • TLRs complement the cytotoxic potential of γδ T cells against tumor cells

γδ T cells can lyse different types of tumors and tumor-derived cell lines. In addition to TCR, γδ T cells use additional stimulatory co-receptors or ligands including TLRs to execute effector functions. The enhanced capability of γδ T cells to lyse tumor cells was attributed to increased expression of CD54 and downregulation of MHC class 1 on tumor cells. However, TLR7 surrogate ligand-induced downregulation of MHC class 1 molecule on tumor cells resulting in a reduced affinity for inhibitory receptor NKG2A on γδ T cells. Manipulation of TLR signaling by using TLR8 agonists reversed the suppressive potential of γδ Tregs found elevated in breast cancer. Polysaccharide K (PSK) known for its anti-tumor and immuno-modulatory function can also activate TLR2, leading to increased secretion of IFN-γ by γδ T cells on stimulation. The cell-cell contact between γδ T cells and DC was required for optimal activation of γδ T cells.

  • Co-stimulatory effects of TLR ligands on γδ T cells

The results with human γδ T cells demonstrate that TLR ligands on their own (except for TLR5 ligand flagellin) are not sufficient to exert a striking effect on γδ T cells. A co-stimulatory effect is induced in freshly isolated human γδ T cells after combined TCR- and TLR1/ 2/6, -3, or -5 ligand stimulation. However, in γδ T cell lines and clones, the addition of exogenous IL-2 (necessary for the expansion of γδ T cells) might overcome the co-stimulatory effects of TLR ligands. Besides, TLR8 ligands abolish the suppressive function of tumor-infiltrating Vd1 γδ T cells, and several other TLR ligands induce an indirect effect via DC or tumor cells on γδ T cell effector functions.

A summary of the direct co-stimulatory effects of TLR ligands on human γδ T cells. Fig.1 A summary of the direct co-stimulatory effects of TLR ligands on human γδ T cells. (Wesch, 2011)

  • TLRs and γδ T cells in diseases

Studies have demonstrated the protective role of γδ T cells in infection and inflammation. Activated γδ T cells through TLR3 and TLR4 ligands rescue the repressed maturation of virus-infected DCs and mount a potent antiviral response. Malarial infection in MyD88 deficient mice resultes in impairment in CD27-IL-17A-producing γδ T cell without affecting the IFN-γ producing γδ T cells. This study specifies the role of TLR in promoting the proliferation of proinflammatory γδ T cells. Another study showes that IL17 producing γδ T cells express TLR1 and TLR2 and expand in response to their ligands and mount an adequate response against heat-killed M. tuberculosis or C. Albicans infection.

Improving γδ T cell functions by TLRs in combinatorial therapy. Fig.2 Improving γδ T cell functions by TLRs in combinatorial therapy. (Dar, 2014)

Prospective Application

Due to their small number in the peripheral blood, human T cells are often expanded for several days, and thereby γδ T cell lines are established before functional studies are performed. However, the response to TLR ligand co-stimulation of expanded γδ T cell lines differs from the response of freshly isolated γδ T cells. The co-simulation of IFN-c production in TCR/TLR3 ligand stimulated freshly isolated γδ T cells compared to TCR stimulation alone is much higher than in similar experiments with γδ T cell lines. An explanation for this discrepancy is given by the enhanced IFN-c production of γδ T cell lines after TCR stimulation compared to the production of TCR-stimulated freshly isolated γδ T cells. Therefore, an additional stimulus via TLR-ligand stimulation is more effective in freshly isolated γδ T cells than in γδ T cell lines.

  • Application significance in vaccines

TLR ligands can modulate the effector functions of γδ T cells. The combined TCR-TLR ligand stimulation of γδ T cells (directly or indirectly) could be a strategy to optimize Th1-mediated immune responses as an adjuvant in vaccines against viruses or bacteria or could help to improve the therapeutic potential of cancer vaccines.

Recent reports have shown how important the involvement of TLR signaling in γδ T cell functions and their implications in harnessing γδ T cells for cancer immunotherapy are. As an industry-leading CRO company, Creative Biolabs provides attentive γδ T cells toll-like receptors services including γδ T cells toll-like receptors repertoires analysis, γδ T cells toll-like receptors transcriptome analysis, γδ T cells toll-like receptors generation, and γδ T cells toll-like receptors ligand reaction analysis.

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  1. Wesch, D.; et al. Modulation of gammadelta T cell responses by TLR ligands. Cell Mol Life Sci. 2011, 68(14): 2357-70.
  2. Dar, A. A.; et al. Insights into the relationship between toll-like receptors and gamma delta T cell responses. Front Immunol. 2014, 5: 366.
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