Fish soluble Toll-like receptor (TLR)5 amplifies human TLR5 response via physical binding to flagellin

Fish has a soluble form of TLR5 ortholog (TLR5S), which does not exist in mammals. We identified TLR5S from rainbow trout and named rtTLR5S, which was about 38% homologous to the extracellular domains of human (hu) and mouse TLR5. Adjuvancy of rtTLR5S to flagellin response by human TLR5 (huTLR5) was tested in this study. A chimera constructed of rtTLR5S and the intracellular TIR of huTLR5 expressed on HeLa cells signaled the presence of various species of bacterial flagellin resulting in NK-κB activation. huTLR5S, when co-expressed with rtTLR5S in HeLa cells, augmented response to flagellin resulting in robust huTLR5-mediated NF-κB activation. Physical binding of flagellin to rtTLR5S was detected under the conditions where huTLR5 induced rtTLR5S-amplified NF-κB activation. Signal amplification by rtTLR5S was specific to huTLR5: no other huTLRs tested were responded to rtTLR5S. These results suggest that the soluble TLR5 serves as an adjuvant augmenting flagellin-TLR5-mediated NF-κB activation even in human.