ENZYME-LINKED IMMUNOABSORBENT ASSAY DETECTION OF A SOLUBLE FORM OF UROKINASE PLASMINOGEN-ACTIVATOR RECEPTOR IN-VIVO

The receptor for urokinase plasminogen activator (uPA-R, CD87) is a gl ycosylphosphatidylinositol (GPI)-anchored 50 to 65 kD glycoprotein tha t, by regulating membrane-associated plasmin activity, may facilitate- the invasion of inflammatory and malignant cells. Certain other GPI-an chored glycoproteins are shed from the cell membrane and exist as solu ble products in vitro and in vivo. To determine if uPA-R undergoes a s imilar phenomenon, we have developed a sensitive enzyme-linked immunoa bsorbent assay (ELISA) (using a rabbit antiserum as both capture and d etection reagents) to measure the quantity of soluble uPA-R (suPA-R) i n tissue culture supernatants and biologic fluids. Using this ELISA, w e have detected suPA-R in the culture supernatants of U-937 cells and human monocytes stimulated in vitro by certain soluble inflammatory me diators (Sitrin et al, Blood 84:1268, 1994; Mizukami et al., Clin Res 42:115A, 1994). To determine if suPA-R exists in vivo, we have screene d the plasma of 20 normal volunteers (mean +/- SD, 3 +/- 3 ng/mL; medi an, 2 ng/mL; range, 1 to 11 ng/mL [serum values slightly higher]); the plasma of 13 ICU patients with clinical sepsis syndrome (mean +/- SD, 30 +/- 11 ng/mL; median, 11 ng/mL; range, 4 to 221 ng/mL); and the ex travascular fluids (pleural, pericardial, and peritoneal) of 84 indivi duals with presumed inflammatory or malignant conditions (mean +/- SD, 21 +/- 39 ng/mL; median, 10 ng/mL; range, 2 to 253 ng/mL). Among the latter specimens, most were inflammatory exudates (only six were malig nant by positive cytology) with the highest quantities of suPA-R assoc iated with neutrophilic exudates. The solubility of suPA-R contained w ithin these fluids was confirmed by reanalysis after ultracentrifugati on to remove particulate material. When tested in a uPA ligand capture ELISA, representative specimens of extravascular body fluids and seps is plasma contained suPA-R capable of binding uPA ligand (generally re presenting a small fraction of the immunoreactive material). We conclu de from these data that suPA-R is immunologically detectable in vitro and in vivo with high concentrations of receptor found under condition s of inflammatory stimulation. The possibility of suPA-R's biologic ac tivity is suggested by its partial retention of ligand binding capacit y. (C) 1995 by The American Society of Hematology.