Carrageenan-induced innate immune response is modified by enzymes that hydrolyze distinct galactosidic bonds

The common food additive carrageenan (CGN) predictably induces intestinal inflammation in animal models. Mechanisms of CGN-induced nuclear factor κB and interleukin-8 (IL-8) stimulation include an immune-mediated pathway involving toll-like receptor 4 (TLR4) and B-cell lymphoma/leukemia 10 (BCL10) and a reactive oxygen species (ROS)-mediated pathway. To determine how the structure of CGN contributes to its initiation of inflammation through these two distinct mechanisms, we treated CGNs with galactosidases and carrageenases (CGNases) and determined the impact on IL-8 secretion and BCL10 production. Hydrolysis of CGN by the enzyme α-1→(3,6)-galactosidase significantly reduced increases in IL-8 and BCL10, but other galactosidases tested, including α-11→6-galactosidase, α-1→4-galactosidase and α-1→3,6-galactosidase, had no effect. In contrast, specific κ-CGNases or κ-CGNases, which hydrolyze β-1,4-galactosidic bonds, produced increases in IL-8 and BCL10 attributable to increased exposure of the immunogenic α-1→3-galactosidic epitope of CGN to TLR4. These results were consistent with induction of innate immune response by an interaction of TLR4 with the unusual α-d-Gal-(1→3)-d-Gal epitope present in CGN. Activation of the ROS-mediated pathway was unaffected by treatment of κ-CGN with either κ-CGNase (3 mg/L), α-1→(3,6)-galactosidase (20 mU/ml) or these enzymes in combination, indicating that changes in IL-8 production were attributable to the effects of induction of inflammation on the TLR4-BCL10-mediated innate immune pathway. These findings provide new information about the specificity of carbohydrate-protein interaction between CGN and TLR4 and may help to devise treatments that modify the immune reactivity induced by carbohydrate antigens.