Identification and characterization of specific receptors for the LD78 cytokine

LD78 is a small secreted protein that has a sequence similar to a number of other polypeptides, including murine macrophage inflammatory protein lα (MIP-1α), interleukin 8 (IL-8), Act-2, monocyte chemoattractant protein l (MCP-1), and others. These polypeptides are members of a novel cytokine superfamily that is involved in the inflammatory response, wound healing, hematopoiesis, and tumorigenesis. Specific receptors for purified clonal LD78 protein were measured using four cell lines (HL-60, U937, Jurkat, and MJ). ¹²⁵I-labeled recombinant LD78 bound most efficiently to U937 cells. We therefore characterized the receptors as being on the surface of U937 cells. Binding reached an equilibrium after incubation for 60 min at 4°C. Scatchard analysis showed that there were two classes of binding sites on U937 cells, high affinity sites (K(d) = 5.3 x 10^-9 M) and low affinity sites (K(d) = 9.3 x 10^-8 M), with the average number of binding sites per cell being ~ 30,000 and ~ 90,000, respectively. These receptors for LD78 were distinct from the receptors for γ-IFN and for IL-8. SDS-PAGE analysis of chemically crosslinked ¹²⁵I-labeled LD78 receptor complexes identified a single band of 52 kDa. The ability to detect specific LD78 receptors should prove valuable in efforts to molecularly clone these receptors and to dissect the biological actions of LD78.