Interaction of antibody and cell surface localized antigen

The changes occurring in the expression of a membrane-associated antigen following binding of specific antibodies and the fate of the bound antibodies were investigated. Daudi cells, carrying membrane 7S IgM molecules, were coated with radio-labelled anti-IgM antibodies and incubated under culture conditions at 37degrees C. Within 10 h, more than 80% of the cellbound radioactivity was shed from the cells into the culture medium. Concomitantly, IgM molecules were lost, as the cells had a diminished ability to bind a new aliquot of antibody. This phenomenon occurred also when the antibody was used at a low concentration, i.e. did not saturate the cell-surface antigens. The precipitability of the antibody released from the cells with 10% TCA and 40% saturated ammonium sulphate was lower than that of native antibody, indicating some degradation. Also, shed antibody could rebind to fresh Daudi cells less efficiently than native antibody. The presence of immune complexes in the culture supernatants was indicated by the following results: (1) some of the shed radioactivity could bind to IgM-negative but Fc receptor-bearing cells; and (2) immune precipitates of sheep anti-IgM shed from 3H-leucine labelled Daudi cells and of rabbit anti-sheep IgG antibodies contained 3H activity.