The mechanism of Fc-mediated interaction of eosinophils with immobilized immune complexes. II. Identification of two membrane proteins, modified by the interaction.

Human peripheral blood eosinophils attach to and flatten down onto antibody-coated surfaces and subsequently degranulate. An antibody-coated surface was prepared by treating a layer of agar, containing tetanus toxoid antigen and eosinophil chemotactic factor (ECF), with human anti-tetanus immunoglobin. Changes in eosinophil surface proteins during attachment to the antibody-coated agar layer were detected by lactoperoxidase catalysed iodination. Purified eosinophils were pre-treated with unlabelled iodide, lactoperoxidase and hydrogen peroxide to block pre-existing accessible tyrosine residues on the cell surface. They were then allowed to interact with the agar layer, and subsequently treated with lactoperoxidase and 125I-labelled iodide to label newly accessible surface proteins. Separation of the radioactive proteins by sodium dodecyl sulphate/polyacrylamide gel electrophoresis revealed that, while incubation of the cells in suspension restored the major proteins to the cell surface, interaction with the antibody-coated agar layer caused the appearance of additional proteins of apparent molecular weight 55K, 30K, 28K and 18K. The 55K, 28K and 18K proteins were greatly reduced when antibody was absent, but the 55K protein was distinguishable from immunoglobulin G (IgG) heavy chain, since it could be detected in low amounts even in the absence of antibody. It was found in purified plasma membranes and it could be separated from IgG heavy chain by iso-electric focusing. The possibility is discussed that this protein is either linked to the receptor for the Fc portion of IgG, or that it is itself the receptor. The 18K protein required both antibody and ECF for maximum expression, but was seen in limited amounts with ECF alone. Possibly it is concerned with an ECF-mediated recognition of IgG. Unlike the 55K protein, it binds concanavalin A. Plasma membranes were prepared from eosinophils by lysis in borate, followed by purification on a glass-bead column. Both the 55K and the 18K proteins were found to be major components of the eosinophil membrane.