Xenopus laevis macrophage-like cells produce XCL-1, an intelectin family serum lectin that recognizes bacteria

Xenopus laevis Ca²⁺ -dependent lectin-1 (XCL-1) is an intelectin family serum lectin that selectively recognizes carbohydrate chains on the bacterial cell surface. Immunofluorescence examination of control spleen tissues from normal X. laevis revealed cells producing XCL-1 (XCL-1⁺ cells) exclusively in red pulps. Intraperitoneal injection of Escherichia coli lipopolysaccharide (LPS) caused a marked increase in the number of XCL-1⁺ cells in red pulps on day 3, followed by a rapid decrease to near control levels by day 7. XCL-1⁺ cells were also detected in peripheral blood leukocytes (PBLs) and peritoneal exudate cells (PECs), and their numbers increased upon LPS injection until day 7. The XCL-1⁺ cells exhibited the morphological characteristics of macrophages, with a large oval or lobulated nucleus and abundant cytoplasm with vacuoles and dendritic projections. Western blot analyses revealed concurrent increases in XCL-1 levels in the spleen, PBLs, and PECs. When LPS-stimulated frogs were intraperitoneally injected with paraformaldehyde-fixed, green fluorescent protein-labeled E. coli cells (GFP-Eco), these were phagocytosed by XCL-1⁺ PECs. The purified XCL-1 protein agglutinated GFP-Eco in a Ca²⁺ -dependent manner, which was blocked effectively by xylose and partly by LPS and Staphylococcus aureus peptidoglycan, but not by sucrose. These results indicate that X. laevis macrophage-like cells produce XCL-1 and suggest that XCL-1 promotes the clearance of invaded bacteria by facilitating phagocytosis.