They found, by
using HEK293 cells transfected with both TLR2 and CD14, that TLR2 is recruited within lipid rafts following LTA stimulation, that LTA is internalized in a lipid-raft-dependent manner and that TLR2 is co-localized with LTA in the Golgi apparatus.15 However, they concluded that LTA internalization is not dependent on TLR2, because LTA internalization occurs even in HEK293 cells transfected with only CD14.15 This is in good agreement with our finding that FSL-1 is internalized into PMφs from TLR2−/− mice (Fig. 7c,e). However, their findings that LTA Proteases inhibitor is internalized into a cell in a lipid-raft-dependent manner and is co-localized with TLR2 in the cytosol15 are in contrast to our findings that FSL-1 is internalized in a clathrin-dependent manner (Figs. 3,4) and FSL-1 is not co-localized with TLR2 in the cytosol (Fig. 7a). This discrepancy may be because of the difference in cell types and ligands used. Triantafilou et al. used non-phagocytic HEK293 transfectants with LTA, whereas we used professional phagocytes, RAW264.7 cells. In addition, several
lines of evidence have indicated that LTA is not a TLR2 ligand.34–36 They have described that contaminants in the LTA preparation, but not LTA itself, are responsible PF-562271 for TLR2-mediated activation of innate immune cells. For these reasons there can be no doubt about the difference in uptake mechanisms between LTA and FSL-1. More recently, Triantafilou et al.37 have also reported that TLR2 is co-localized with TLR6 and CD36 in the Golgi apparatus after stimulation with FSL-1 in HEK293 cells transfected with CD14, TLR2, TLR1, TLR6 and CD36, although they did not investigate whether FSL-1 is co-localized with TLR2 in the cytosol.37 Taken together, these results suggest that TLR2 ligands are internalized into cells irrespective
of the presence of TLR2 after recognition by TLR2. There was great interest as to what kind of receptors other than TLR2 are involved in the FSL-1 uptake. We speculated that CD14 or CD36 may mediate the Atorvastatin uptake, because they function as co-receptors of TLR2 to recognize lipopeptide.32,33 CD36 is a glycosylated transmembrane protein that is expressed in various cell types and tissues including monocytes/macrophages.38 Especially for innate immune responses, Hoebe et al.32 showed that CD36 is involved in the recognition of TLR2/6 ligands. CD36 is also known as a class B scavenger receptor, and it has been reported that the C-terminal cytoplasmic domain of CD36 is required for bacterial internalization.39 Therefore, it is reasonable that CD36 is responsible for FSL-1 uptake, although Mairhofer et al.40 showed that most of the CD36 is in the lipid-raft fraction. CD14 is found in a soluble form in serum or as a glycosylphosphatidylinositol-anchored protein on the cell membrane, and is one of the essential accessory proteins for lipopolysaccharide recognition.41 It is also known that CD14 functions as a co-receptor of TLR2 for the recognition of a triacylated lipopeptide.