Lawsonia intracellularis flagellin protein LfliC stimulates NF-κB and MAPK signaling pathways independently of TLR5 interaction

Lawsonia intracellularis, a Gram-negative obligate intracellular bacterium and etiologic agent of porcine proliferative enteropathy, was observed to have a long, single, and unipolar flagellum. Bacterial flagellar filament comprises thousands of copies of the protein flagellin (FliC), and has been reported to be recognized by Toll-like receptor (TLR5) to activate the NF-κB and MAPK signaling pathways, thereby inducing the expression of proinflammatory genes. Recently, two L. intracellularis flagellin proteins, LfliC and LFliC, were reported to be involved in bacterial-host interaction and immune response. Here, to further explore the role of LfliC in proinflammatory response, we purified LfliC, and found that its exposure could activate NF-κB signaling pathway in both HEK293T and IPI-FX cells, as well as activate MAPK p38 and ERK1/2 in HEK293T cells but not in IPI-FX cells. However, our yeast two-hybrid and co-immunoprecipitation assay results revealed that LfliC has no interaction with the porcine TLR5 ECD domain though it harbors the conserved D1-like motif required for the interaction. Moreover, LfliC was identified as a substrate of the virulence-associated type III secretion system (T3SS) by using the heterologous Y. enterocolitica system. Transient expression of LfliC also activated the NF-κB and MAPK signaling pathway in HEK293T cells. Collectively, our results suggest that both the exposure and expression of L. intracellularis LfliC can induce the NF-κB and MAPK signaling pathway in mammalian cells. Our findings may provide important implications and resources for the development of diagnostic tools or vaccines and dissection of the pathogenesis of L. intracellularis.