A soluble form of FcγRIII is present in human serum and other body fluids and is elevated at sites of inflammation

Abstract

We have developed a highly sensitive and specific sandwich enzyme-linked immunosorbent assay (ELISA) to measure the concentration of FcγRIII in serum and other body fluids. This ELISA is based on the use of monoclonal antibody (MoAb) (3G8) to FcγRIII and a rabbit antiserum against FcγRIII. The lower limit of detection of this ELISA was 1.5 nmol/L. The concentration of soluble FcγRIII in normal serum ranged from 7.3 to 75.9 nmol/L. Soluble FcγRIII was also present in other normal biologic fluids such as saliva, urine, and seminal fluid, but at much lower concentrations than that found in serum. Rabbit anti-FcγRIII immunoblotted polypeptides immunoprecipitated with MoAb 3G8. FcγRIII immunoprecipitated from a neutrophil lysate migrated from 40 to 76 Kd, whereas FcγRIII immunoprecipitated from serum from the same donor migrated from 40 to 66 Kd. The soluble form of FcγRIII apparently was bound to serum IgG, because immunoprecipitation of soluble FcγRIII by MoAb 3G8 coprecipitated polypeptides that were identified by goat antihuman IgG. Incubation of neutrophils in vitro at 4°C and 37°C showed that FcγRIII was released after 30 minutes of incubation at 37°C. To determine whether there was a correlation between the concentration of soluble FcγRIII in biologic fluids and inflammatory diseases, we measured the concentration of FcγRIII in the bronchoalveolar lavage fluid from patients with adult respiratory distress syndrome (ARDS) and in the synovial fluid from patients with various forms of arthritis. In ARDS, we found concentrations of soluble FcγRIII that were five to seven times higher than that found in the bronchoalveolar lavage fluids from healthy adults. The concentration of soluble FcγRIII in the synovial fluid from patients with rheumatoid arthritis ranged from 10 nmol/L to 28 μmol/L. These results suggest that activated neutrophils, such as those at sites of inflammation, may release FcγRIII. © 1992 by The American Society of Hematology.