Bacterial lipopolysaccharides stimulate production of XCL1, a calcium-dependent lipopolysaccharide-binding serum lectin, in Xenopus laevis.

Xenopus laevis serum lectin XCL1 is a newly identified molecule of the XCGL (or X-lectin) family, a unique group of Ca(2+)-dependent lectins that have a fibrinogen-like domain. The XCL1 protein was purified from lipopolysaccharide (LPS)-stimulated frog sera by sequential affinity chromatography on heparin-acrylic beads and galactose-Sepharose. XCL1 comprises multiple oligomeric proteins consisting of 37-kDa subunit polypeptides, as revealed by sodium dodecyl sulfate-polyacrylamide electrophoresis (SDS-PAGE) and Western blot analyses using the monoclonal antibody (mAb) produced against the recombinant XCL1 polypeptide. In the presence of Ca(2+), the protein bound to Escherichia coli, Staphylococcus aureus, LPS and galactose and the bound XCL1 was competitively eluted using ribose and xylose, and the elution was as efficient as that using EDTA, whereas elution using hexoses, GalNAc or GlcNAc was less effective. In reverse transcription-polymerase chain reaction (RT-PCR) and Western blot analyses, XCL1 expression was ubiquitously detected in frog tissues, with relatively high levels in hematopoietic tissues including the spleen, liver and kidney. Intraperitoneal injection of E. coli, S. aureus or 100-300μg S-type LPS from various bacteria induced several-fold increases in serum XCL1 concentrations on day 3, and the elevated levels retained up to day 12. It also caused a remarkable increase of the splenic XCL1 expression on day 3, followed by a rapid decline to nearly nonstimulated control levels by day 7. The R-type LPS with shortened polysaccharide chains was less effective in inducing the serum XCL1 response, indicating that the sugar chains of LPS were important, if not essential, for the stimulation of XCL1 production. These results suggest that XCL1 is a pathogen recognition molecule involved in antimicrobial innate immunity in Xenopus.