A dominant T cell population bearing the cross-reactive idiotype of T cell antigen receptor (TcR) has been obtained using an anti-TcR monoclonal antibody (mAb) developed in syngeneic mice. Forty-four cytotoxic T cell (CTL) clones with reactivity to a mAb (N9-127) were selected out of 396 H-2Db-restricted CTL clones specific for FBL-3 tumor antigen from C57BL/6 mice. These CTL clones were divided into two groups according to the blocking pattern of cytotoxic activities with mAb N9-127. All eight CTL clones chosen from both groups expressed TcR with a specific combination of alpha and beta chains (V alpha 1J alpha 112-2/V beta 10D beta 2.1J beta 2.7), and the difference in the blocking susceptibility resided in a single amino acid substitution (Gly to Asp) in the D-J joint of beta chain. This provides direct evidence for the molecular basis of cross-reactive idiotypes of TcR recognized by mAb.
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