Bacteroides fragilis signals through Toll-like receptor (TLR) 2 and not through TLR4.

Although it is desirable to identify the interactions between endotoxin/LPS and the innate immune mechanism, it is often not possible to isolate these interactions from other cell wall-related structures of protein or polysaccharide origin. There is no universally accepted method to extract different LPSs from different bacteria, and their natural state will be influenced by their interactions with the associated molecules in the bacterial outer membrane. It is now believed that Toll-like receptor (TLR) 4 is the main signal transducer of classical LPS (i.e. Escherichia coli LPS), while TLR2 is used by certain non-classical LPSs. There are contradictory reports as to whether Bacteroides fragilis LPS, a non-classical LPS, signals primarily through TLR2 or TLR4. This study was designed to address this problem. Different non-purified and purified B. fragilis LPSs extracted by different methods together with different heat-killed, whole-cell populations of B. fragilis were used to elucidate the TLR specificity. All of these B. fragilis preparations showed a significant signalling specificity for TLR2 but not for TLR4. This indicates that changing the extraction methods, with or without applying a repurification procedure, and varying the cell populations do not alter the TLR specificity of B. fragilis LPS.