We studied cytotoxic T lymphocyte (CTL) clones expressing cytoplasmic domain-deleted CD3delta and CD3gamma chains. These cells retained efficient antigen-specific cytolysis. Because the cytoplasmic domains of native CD3delta and CD3gamma chains contain a dileucine-based and a tyrosine-based motif thought to be important for receptor endocytosis, we compared TCR-CD3 down-modulation on the CTL clones expressing or not these domains. We found that antigen-induced TCR-CD3 down-modulation was not dependent on either the CD3delta or CD3gamma cytoplasmic domains. This contrasts with phorbol ester- and anti-CD3 mAb (soluble or plastic-coated)-induced TCR-CD3 down-modulation, that are respectively dependent on CD3gamma and on either CD3delta or CD3gamma cytoplasmic domains, suggesting that differences may exist between the mechanisms of TCR-CD3 down-modulation in response to the three stimuli. TCR-CD3 down-modulation in response to antigen was demonstrated by confocal microscopy to be associated with TCRbeta chain internalization, whether CD3delta and CD3gamma were native or truncated. Inhibition by the protein tyrosine kinase inhibitor PP1 of TCR-CD3 down-modulation in response to antigen was also similar whether CD3delta and CD3gamma cytoplasmic domains were present or not. These properties of receptor down-modulation are discussed with respect to the requirements for TCR engagement on antigen-presenting cells.
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