λ-Carrageenan initiates inflammation via activation of heterodimers TLR2/6 and TLR4/6

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Abstract λ-Carrageenan (λCGN) is frequently used to model local inflammation, however, the mechanism by which λCGN functions has not been fully examined. Swine whole blood was stimulated with λCGN and biomarkers were evaluated. Monocyte Chemotactic Protein-1(MCP-1) and Interleukin-8 protein levels were increased, while Serum Amyloid A (SAA) and Tumor Necrosis Factor-α (TNFα) protein levels were unchanged. Gene expression of MCP-1, Phosphatidylinositol 3-Kinase, and Alveolar Macrophage Chemotactic–II were increased; gene expression of SAA, Interleukin-6, and TNFα were unchanged; and gene expression of Thrombospondin-2 Precursor were decreased. Assessment of which TLRs (Toll-Like Receptors) were involved in the λCGN activation pathway, HEK-293 cells transfected with one plasmid containing either TLR2 or TLR4, along with a second plasmid to release SEAP (secreted embryonic alkaline phosphatase) upon NFκB activation were stimulated with λCGN. TLR2 and TLR4 both showed increased SEAP release after λCGN stimulation. The lack of TNFα production after λCGN stimulation suggested that it was not working through a classic TLR4 homodimer. To determine if a common mechanism could explain these results, RNA silencing studies were initiated. TLR6 was silenced in SEAP HEK-293 cells containing a plasmid for either TLR2 or TLR4. In both cell lines where TLR6 was silenced, λCGN was unable to induce a SEAP response, while these cells maintained responsiveness to a TLR1/2 ligand and a TLR 4 ligand, respectively. These results suggest that λCGN initiates inflammation via TLR2/6 and TLR4/6 heterodimers.