T Cell Receptor Engagement Leads to Phosphorylation of Clathrin Heavy Chain during Receptor Internalization

T cell receptor (TCR) internalization by clathrin-coated vesicles after encounter with antigen has been implicated in the regulation of T cell responses. We demonstrate that TCR internalization after receptor engagement and TCR signaling involves inducible phosphorylation of clathrin heavy chain (CHC) in both CD4⁺ and CD8⁺ human T cells. Studies with mutant Jurkat T cells implicate the Src family kinase Lck as the responsible enzyme and its activity in this process is influenced by the functional integrity of the downstream signaling molecule ZAP-70. CHC phosphorylation positively correlates with ligand-induced TCR internalization in both CD4⁺ and CD8⁺ T cells, and CHC phosphorylation as a result of basal Lck activity is also implicated in constitutive TCR endocytosis by CD4⁺ T cells. Remarkably, irreversible CHC phosphorylation in the presence of pervanadate reduced both constitutive and ligand-induced TCR internalization in CD4⁺ T cells, and immunofluorescence studies revealed that this inhibition affected the early stages of TCR endocytosis from the plasma membrane. Thus, we propose that CHC phosphorylation and dephosphorylation are involved in TCR internalization and that this is a regulatory mechanism linking TCR signaling to endocytosis.