TLR2 was first shown to be involved in the recognition of LPS by studies on the overexpression of TLR2 in human embryonic kidney 293 cells. However, subsequent studies in mice lacking TLR2 or CHO cells genetically lacking functional TLR2 demonstrated that TLR2 is not involved in the recognition of LPS. Overexpression of TLR2 in 293 cells seems to confer a response to the TLR2 ligand contaminating the preparation of LPS used. Indeed, repurification of the LPS resulted in no response by 293 cells with overexpression of TLR2. However, TLR2 presumably recognizes atypical LPS from Leptospira interrogans or Porphyro-monas gingivalis, both of which differ from the typical LPS of Gram-negative bacteria in several biochemical and physical properties (Hirschfeld et al., 2001; Werts et al., 2001).

Several lines of evidence demonstrate that TLR2 recognizes components from a variety of microbial pathogens. These include lipoproteins from pathogens such as Gram-negative bacteria, Mycoplasma fermentans, Treponema pallidum, and Borrelia burgdorferi, peptidoglycan and lipoteichoic acid from Gram-positive bacteria, lipoarabinomannan from mycobacteria, glycosylphosphatidylinositol anchors from Trypanosoma cruzi, a phenol-soluble modulin from Staphylococcus epidermis, zymosan from fungi, and glycolipids from Treponema malto-philum. A prerequisite role for TLR2 in the recognition of peptidoglycan and lipoproteins has been shown in TLR2 knockout mice (Akira et al., 2001; Medzhitov, 2001). The mechanism by which TLR2 recognizes a wide variety of microbial components is now explained by the fact that TLR2 cooperates with other TLRs such as TLR1 and TLR6 to discriminate between the specific patterns.

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