Cross-talk among Toll-like receptors and their ligands

Abstract

Toll-like receptors (TLRs) 4, 5, 7 and 9 belong to a family of proteins that recognize mainly conserved microbial motifs. Though each TLR has a highly specific ability to recognize a particular microbial pattern, recent papers suggest that some ligands are able to affect the expression of different TLRs. In this paper, we have investigated TLR4, 5, 7 and 9 expression, both at mRNA and protein level, following treatment of different intestinal epithelial cell lines with LPS, flagellin, loxiribine, CpG-oligodeoxynucleotide and peptidoglycan, to assess if the different TLR ligands may modulate the expression of the respective TLR and of the unrelated ones. Our results show that a cross-talk exists between TLRs and various ligands, indicating a cross-regulation among these pattern recognition receptors. In particular, TLR4 was generally down-regulated by treatment with ligands other than LPS, while flagellin and unrelated microbial-associated molecular patterns exerted a general stimulatory activity as regards TLR5 expression. Concerning TLR7 and 9, we have observed a more variable behaviour of the various cell lines with the different ligands. Together, our results demonstrate that the expression of TLRs in intestinal cells is highly dynamic and tightly regulated in response to encountered microbial stimuli.

Effect of natural commensal-origin DNA on toll-like receptor 9 (TLR9) signaling cascade, chemokine IL-8 expression, and barrier integritiy of polarized intestinal epithelial cells

“…Palazzo et alreported that pretreatment of IEC with peptidoglycan (PGN) and then challenge with a specific CpG sequence inhibited TLR9 expression. 45 As our results indicated that TLR9 silencing using TLR9 siRNA abolished the inhibitory effects of natural commensal-origin DNA on TNF-a-induced IL-8 production, we evaluated the effect of LGG DNA on the TLR9-associated intracellular signaling pathway, which leads to the inhibition of TNF-a-induced IL-8 secretion. Our study showed that apical stimulation of TLR9 with LGG DNA did not elicit NF-jB activation but rather attenuated TNFa-induced NF-jB activation by reducing the degradation of IjBa and p38 phosphorylation, 2 important downstream signaling proteins involved in NF-jB pathway and subsequent translocation of NF-jB into the nucleus, where it regulates various genes like IL-8.…”

“…The purity of the TLR9 agonists may play a role, as a previous study reported that pretreatment of IECs with peptidoglycan (PGN) and then challenge with a specific CpG sequence inhibited TLR9 expression. 45 Assessment of Barrier Integrity of Monolayers and Transmonolayer Movement of Natural Commensal-Origin DNA Because tight-junction integrity of polarized cell monolayers was a prerequisite for studying the recognition of natural commensal-origin DNA by apical/basolateral TLR9, we investigated if natural commensal-origin DNA affects epithelial cell integrity and TNF-a-induced epithelial resistance dysfunction and if natural commensal-origin DNA itself can translocate from the apical to the basolateral compartment. For this, 2 indices of epithelial barrier function were employed.…”

Early Life Activation of Toll-Like Receptor 4 Reprograms Neural Anti-Inflammatory Pathways

“…We found that postnatal LPS permanently enhances the expression of its receptor, TLR4, in immune-competent organs such as the liver and spleen. As TLR4 mRNA and protein expression appear to be regulated in tandem (Armstrong et al, 2004;Palazzo et al, 2008), it is possible that such increased expression of TLR4 participates in enhanced HPA axis responsiveness.…”

Alternative TLRs are stimulated by bacterial ligand to induce TLR2-unresponsive colon cell response

“…The mechanisms that drive the positive and negative TLR feedback loops are in view of maintenance of the intestinal homeostasis at any time point. Though each TLR has a specific ability to recognize a particular PAMP, Pallazo et al have shown altered TLR expression following treatment of various intestinal epithelial cell lines with several PAMPs having known TLR specificity [35]. In this study, we show porin-induced switching of TLRs, the mechanism of which remains unclear but indicate unique downstream signaling path do exist.…”