Surface membrane CD4 turnover in phorbol ester stimulated T‐lymphocytes

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Abstract
Down‐regulation of surface membrane CD4 (smCD4) in phorbol ester stimulated T‐cells resulted from internalization. Internalization (T = 15 min at 50 ng ) was followed by degradation of CD4‐bound antibodies. Degradation in unstimulated T‐cells was comparatively insignificant. Release of degradation products was PMA dose‐dependent and could be inhibited by methylamine. Uptake and degradation continued after maximal down‐regulation of surface membrane CD4, and methylamine did not inhibit reappearance of smCD4 antigens. Metabolic labelling of T‐cells further showed that ongoing synthesis rather than recycling contributed to an accelerated smCD4 turnover in activated cells.