Down-regulation of surface membrane CD4 (smCD4) in phorbol ester stimulated T-cells resulted from internalization. Internalization (T 80506-E/asset/equation/feb2001457939080506e-math-si1.gif?v=1&s=dd0f4d2e8007bcbf06e4b55c0ecf67821513bfd4)
= 15 min at 50 ng 80506-E/asset/equation/feb2001457939080506e-math-si2.gif?v=1&s=50493053a87d71593464239036da51bd275c8a3f)
) 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.
