Differential TLR recognition of leptospiral lipid A and lipopolysaccharide in murine and human cells

Leptospira interrogans is a spirochete that is responsible for leptospirosis, a zoonotic disease. This bacterium possesses an unusual LPS that has been shown to use TLR2 instead of TLR4 for signaling in human cells. The structure of its lipid A was recently deciphered. Although its overall hexa-acylated disaccharide backbone is a classical feature of all lipid A forms, the lipid A of L. interrogans is peculiar. In this article, the functional characterization of this lipid A was studied in comparison to whole parental leptospiral LPS in terms of cell activation and use of TLR in murine and human cells. Lipid A from L. interrogans did not coagulate the Limulus hemolymph. Although leptospiral lipid A activated strongly murine RAW cells, it did not activate human monocytic cells. Results obtained from stimulation of peritoneal-elicited macrophages from genetically deficient mice for TLR2 or TLR4 clearly showed that lipid A stimulated the cells through TLR4 recognition, whereas highly purified leptospiral LPS utilized TLR2 as well as TLR4. In vitro experiments with transfected human HEK293 cells confirmed that activation by lipid A occurred only through murine TLR4-MD2 but not through human TLR4-MD2, nor murine or human TLR2. Similar studies with parental leptospiral LPS showed that TLR2/TLR1 were the predominant receptors in human cells, whereas TLR2 but also TLR4 contributed to activation in murine cells. Altogether these results highlight important differences between human and mouse specificity in terms of TLR4-MD2 recognition that may have important consequences for leptospiral LPS sensing and subsequent susceptibility to leptospirosis.