Toll-like receptor 4 (TLR4)-deficient murine macrophage cell line as an in vitro assay system to show TLR4-independent signaling of Bacteroides fragilis lipopolysaccharide

Bacterial lipopolysaccharides (LPS) activate cells of innate immunity, such as macrophages, by stimulating signaling through toll-like receptor 4 (TLR4). We and others have hypothesized that LPS derived from different bacterial species may function through TLR4-independent mechanisms. To test this hypothesis, we have generated using a nonviral transformation procedure a bone marrow-derived macrophage cell line called 10ScNCr/23 from mouse strain C57BL/10ScNCr. This mouse strain has a deletion of the TLR4 locus, causing the mouse strain to be nonresponsive to stimulation by LPS from Escherichia coli while responding normally to other bacterial substrates, such as lipoteichoic acid (LTA) from Staphylococcus aureus, which signal TLR4 independently. Stimulation with LTA induces five- and sixfold increases in 10ScNCr/23 cell line tumor necrosis factor alpha and macrophage inflammatory protein-2 (MIP-2) secretion, but no increases in either cytokine were found when cells were stimulated with E. coli LPS. Bacteroides fragilis-derived LPS, however, can effectively stimulate MIP-2 expression in the absence of functional TLR4 in the 10ScNCr/23 cell line. This gives rise to the notion that LPS from some bacterial species will utilize alternative receptors to stimulate the innate immune response.