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Toxoplasma gondii profilin does not stimulate an innate immune response through bovine or human TLR5

Toxoplasma gondii is responsible for one of the most prevalent infections in people. T. gondii profilin (TgPr) is a protein
integral to parasite movement and cellular invasion. Murine TLRs has been described to bind TgPr. Furthermore, more
recently, human TLR5 has been described to recognise recombinant TgPr, as well as bacterial flagellin. In addition to
infections in humans, T. gondii infects farm animals, but little information is available about its innate recognition. We
aimed to investigate whether, similarly to their human orthologue, bovine and porcine TLR5 could also be stimulated by
TgPr by using a combination of reporter cell lines expressing full length TLR5 from each species as well as primary cells.
Although human and bovine TLR5-transfected cells responded to flagellin, no response was detected upon stimulation
with profilin. Furthermore, TgPr failed to elicit IL-6 secretion in human peripheral blood mononuclear cells and CD14
monocytes. In contrast, exposure of RAW cells, known to express TLR11, to TgPr slightly increased the IL-6 response.
Our data cast doubts on the possibility that profilin is a specific ligand for human TLR5 and bovine TLR5. This leaves the
immunogenic properties of this potential target antigen (Ag) uncharacterised outside of the murine system.

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