A protein of apparent molecular weight 55000, designated protein 3, becomes newly detectable on the eosinophil surface as a specific consequence of interaction with antigen-antibody complexes immobilized in agar layers. The effect of various agents upon this interaction has been determined by monitoring the appearance of this protein by lactoperoxidase-catalysed iodination. Other parameters that have been measured include: the attachment of eosinophils to the agar layers and their subsequent degranulation, as measured by the release of granule peroxidase, and the degree of spreading of the eosinophils, as assessed by electron microscopy. Attachment of eosinophils to antibody-coated layers is inhibited by heat-aggregated immunoglobulin G (IgG), suggesting that this attachment is mediated via eosinophil Fc receptors. In addition, agents, such as the eosinophil chemotactic factor Ala-Gly-Ser-Glu, that enhance the expression of Fc receptors also enhance the appearance of protein 3, while agents, such as hydrocortisone, that inhibit the expression of Fc receptors reduce its appearance. It is concluded that the appearance of protein 3 parallels the expression of Fc receptors. Attempts to block the Fc region of the bound antibody with staphylococcal protein A were not successful. These experiments indicated that the Fc region of bound IgG has different binding sites for protein A and for the Fc receptor. The correlation between the appearance of protein 3 and subsequent degranulation of the eosinophils was confirmed by the use of agents, such as cytochalasin D and levamisole, that enhance both the appearance of protein 3 and degranulation. Conversely, hydrocortisone reduces the appearance of protein 3 and inhibits degranulation. Protein 3 does not appear when eosinophils adhere to agar layers coated with concanavalin A instead of antibody and the eosinophils do not degranulate. Addition of the calcium ionophore A23187, while causing the release of granule peroxidase, does not elicit the appearance of protein 3. These observations provided additional evidence that the appearance of protein 3 is a specific consequence of the interaction of eosinophils with antibody-coated surfaces. The fact that protein 3 appears at the eosinophil surface as a direct consequence of the interaction with antibody suggests that this protein is closely associated with the eosinophil Fc receptor. The enhancement of the appearance of protein 3 in the presence of cytochalasin D indicates that the movement and reorientation of both this protein and the Fc receptor are constrained by association with cytoplasmic microfilaments.

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