Engaged and Bystander T Cell Receptors Are Down-modulated by Different Endocytotic Pathways

T cell antigen receptor (TCR) engagement by stimulatory antibodies or its major histocompatibility complex-antigen ligand results in its down-modulation from the cell surface, a phenomenon that is thought to play a role in T cell desensitization. However, TCR engagement results in the down-modulation not only of the engaged receptors but also of non-engaged bystander TCRs. We have investigated the mechanisms that mediate the down-modulation of engaged and bystander receptors and show that co-modulation of the bystander TCRs requires protein-tyrosine kinase activity and is mediated by clathrin-coated pits. In contrast, the down-modulation of engaged TCRs is independent of protein-tyrosine kinases and clathrin pits, suggesting that this process is mediated by an alternate mechanism. Indeed, down-modulation of engaged TCRs appears to depend upon lipid rafts, because cholesterol depletion with methyl-beta-cyclodextrin completely blocks this process. Thus, two independent pathways of internalization are involved in TCR down-modulation and act differentially on directly engaged and bystander receptors. Finally, we propose that although both mechanisms coexist, the predominance of one or the other mechanisms will depend on the dose of ligand.