PROCESSING OF C5A BY HUMAN POLYMORPHONUCLEAR LEUKOCYTES

Uptake of human C5a by neutrophils was monitored in vitro using both I -125-labeled and unlabeled C5a. The Ligand was internalized by the cel ls in a dose-dependent manner and maximal binding/uptake was observed after 5 min of incubation. Neutrophils were incubated with labeled C5a and the cytosol and supernatant were analyzed by sodium dodecyl sulfa te-polyacrylamide gel electrophoresis and autoradiography, C5a degrada tion products were primarily observed in the supernatant, whereas most df the protein remained intact in the cytosol even after 60 min of in cubation. Cytosol from neutrophils incubated for 20 min with unlabeled C5a was examined by radioimmunoassay and found to contain antigenical ly intact C5a and retained the ability to induce a neutrophil (shape c hange) response. The functional activity of C5a recovered from the cyt osol was inhibited by antibodies to either C5a or the C5a receptor (CD 88). This data supports our hypothesis that although C5a is internaliz ed it remains antigenically intact and functionally active inside the cell and is primarily degraded extracellularly.