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-Chain Is an Active Process Requiring TCR-Mediated Signaling and c-Cbl1
* Department of Immunology, The Scripps Research Institute, La Jolla, CA 92037; and
Laboratory of Immunology, National Institute of Allergy and Infectious Diseases, National Institutes of Health, Rockville, MD 20852
Phenotypic allelic exclusion at the TCR
locus is developmentally regulated in thymocytes. Many immature thymocytes express two cell surface -chain species. Following positive selection, the vast majority of mature thymocytes and peripheral T cells display a single cell surface -chain. A posttranslational mechanism occurring at the same time as positive selection and TCR up-regulation leads to this phenotypic allelic exclusion. Different models have been proposed to explain the posttranslational regulation of the -chain allelic exclusion. In this study, we report that allelic exclusion is not regulated by competition between distinct -chains for a single 
-chain, as proposed by the dueling -chain model, nor by limiting CD3 
-chain in mature TCRhigh thymocytes. Our data instead favor the selective retention model where the positive selection signal through the TCR leads to phenotypic allelic exclusion by specifically maintaining cell surface expression of the selected -chain while the nonselected -chain is internalized. The use of inhibitors specific for Lck and/or other Src kinases indicates a role for these protein tyrosine kinases in the signaling events leading to the down-regulation of the nonselectable -chain. Loss of the ubiquitin ligase/TCR signaling adapter molecule c-Cbl, which is important in TCR down-modulation and is a negative regulator of T cell signaling, leads to increased dual -chain expression on the cell surface of double-positive thymocytes. Thus, not only is there an important role for TCR signaling in causing -chain allelic exclusion, but differential ubiquitination by c-Cbl may be an important factor in causing only the nonselected -chain to be down-modulated.
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