Porcine TLR8 and TLR7 are both activated by a selective TLR7 ligand, imiquimod

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Toll-like receptors (TLRs) are a family of highly conserved germline-encoded pattern-recognition receptors (PRR), which are utilized by the innate immune system to recognize microbial components, known as pathogen-associated molecular patterns (PAMP). We cloned and characterized porcine TLR7 and TLR8 genes from pig lymph node tissue. Sequence analysis showed that the aa sequence identities of porcine TLR7 with human, mouse and bovine TLR7 are 85, 78 and 90%, respectively, whereas porcine TLR8 aa sequence identities with human, mouse and bovine TLR8 are 73, 69 and 79%, respectively. Both porcine TLR7 and TLR8 proteins were expressed in cell lines and were N-glycosylated. The stimulatory activity of TLR7 and TLR8 ligands to porcine and human TLR7 and TLR8 in transiently transfected Cos-7 and 293T cells were analyzed using a NF-κB reporter assay. Two imidazoquinoline molecules, imiquimod and gardiquimod, markedly activated both porcine TLR7 and TLR8 whereas only human TLR7, but not TLR8, was activated by the ligands. Therefore, receptor specificity for porcine TLR8 is clearly species specific. We further showed that porcine TLR7 and TLR8 are located intracellularly and are mainly within the endoplasmic reticulum. Moreover, activation of transfected cells and porcine PBMC by TLR7 ligands was inhibited by bafilomycin A1 indicating the requirement of endosomal/lysosomal acidification for activation of the receptors.

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