Fate of intercellular MHC–peptide–T‐cell receptor complexes during T‐cell activation

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Abstract
Antigen‐specific T‐cell activation requires the formation of a transient cell‐cell conjugate between a T cell and an appropriate antigen presenting cell (APC). Focal aggregation of T‐cell receptor (TCR) molecules at the T‐cell–APC membrane interface accompanies formation of multiple non‐covalent intercellular bridges consisting of TCRs on the T cell and cognate MHC–peptide complexes on the APC. Enhanced adhesiveness and T‐cell activation follow the T‐cell signalling that results from crosslinking of T‐cell receptors (TCR). Models of AT‐cell activation propose that the APC and activated T cell separate following a decline in the enhanced adhesiveness. The rate of intercellular TCR‐(MHC‐peptide) complexes formed during T‐cell activation is unknown. Based on the reported CD4‐positive T‐cell internalization of the peptide moiety of preformed cognate MHC II–peptide complexes, it is proposed here that translocation of the peptide moiety leads to destabilization and decomposition of intercellular trimolecular TCR–(MHC–peptide) complexes in the T‐cell–APC interface. This decomposition accompanies or results in the decline in enhanced adhesiveness leading to separation of the APC and activated T cell.