TLR-independent induction of human monocyte IL-1 by phosphoglycolipids from thermophilic bacteria

The structures of phosphoglycolipids PGL1 and PGL2 from the thermophilic bacteria Meiothermus taiwanensis, Meiothermus ruber, Thermus thermophilus, and Thermus oshimai are determined recently (Yang et al. in J Lipid Res. 47:1823-1932, 2006). These bacteria belong to Gram-negative bacteria that do not contain lipopolysaccharide, but high amounts of phosphoglycolipids and glycoglycerolipids. Here we show that PGL1/PGL2 mixture (PGL1: PGL2 = 10:1 ~ 10:2) from M. taiwanensis and T. oshimai, but not T. thermophilus and M. ruber, up-regulate interleukin-1beta (IL-1beta) production in human THP-1 monocytes and blood-isolated primary monocytes. PGL2 was purified after phospholipase A2 hydrolysis of PGL1 in the PGL1/PGL2 mixture followed by column chromatography. PGL2 did not induce proIL-1 production, even, partially (35-40%) inhibited PGL1-mediated proIL-1 production, showing that PGL1 is the main inducer of proIL-1 production in PGL1/PGL2 mixture. The production of proIL-1 stimulated by phosphoglycolipids was strongly inhibited by specific PKC-alpha, MEK1/2, and JNK inhibitors, but not by p38-specific inhibitor. The intracellular calcium influx was involved in phosphoglycolipids-mediated proIL-1 production. Using blocking antibody and Toll-like receptor (TLR)-linked NF-kappaB luciferase assays, we found that the cellular receptor(s) for phosphoglycolipids on proIL-1 production was TLR-independent. Further, phosphoglycolipids isolated from T. thermophilus and M. ruber did not induce proIL-1 production, even though T. thermophilus possess more PGL1 than PGL2 (6:4). Specially, the fatty acid composition of phosphoglycolipids from both T. thermophilus and M. ruber consists of a low percentage of C15 (<10%) and a high percentage of C17 (>75%). It suggests, the C15 percentage of PGL may play a critical role in PGL-mediated proIL-1 induction.