Identification of a human monocyte antigen (Mo3e) associated with cellular activation and lymphokine responsiveness

A murine monoclonal IgM antibody recognizes an antigen (Mo3e) found on the surface of human peripheral blood monocytes stimulated with phorbol 12‐myristate 13‐acetate (PMA), lipopolysaccharide and muramyl dipeptide and blocks the monocyte response to migration inhibitory factor (MIF). We utilized Western blot and immunoprecipitation analyses of whole cell lysates to investigate the biochemical nature of this monocyte antigen. Two distinct bands (75 kDa and 50 kDa) were detected by anti‐Mo3e after monocyte lysates were resolved by sodium dodecyl sulfate‐polyacrylamide gel electrophoresis (SDS‐PAGE) and transblotted onto nitrocellulose paper. Stimulation of monocytes with PMA resulted in a marked increase in the amount of the 50‐kDa species. Immunoprecipitation of Mo3e from lysates of surface‐iodinated cells demonstrated one broad band at 55–80 kDa which increased after PMA stimulation. The epitope identified by anti‐Mo3e was resistant to 2‐mercaptoethanol and heat treatment (100°C/5 min) and was sensitive to trypsin or papain treatment. Two‐dimensional SDS‐PAGE analysis revealed that the 75‐kDa species has an isoelectric point of 7.0 and the 50‐kDa species is more acidic with an isoelectric point of 5.3. These results indicate that anti‐Mo3e antibody defines unique monocyte proteins that may play a role as suggested by previous studies, in monocyte activation and responsiveness to MIF.