INFLUENCE OF THE NH2-TERMINAL AMINO-ACID OF THE T-CELL RECEPTOR-ALPHACHAIN ON MAJOR HISTOCOMPATIBILITY COMPLEX (MHC) CLASS-II PLUS PEPTIDERECOGNITION

The alpha/beta T cell receptor (TCR) recognizes peptide fragments boun d in the groove of major histocompatibility complex (MHC) molecules. W e modified the TCR alpha chain from a mouse T cell hybridoma and teste d its ability to reconstitute TCR expression and function in an alpha chain-deficient valiant of the hybridoma. The modified alpha chain dif fered from wild type only in its leader peptide and mature NH2-termina l amino acid. Reconstituted cell surface TCR complexes reacted normall y with anti-TCR and anti-CD3 antibodies. Although cross-linking of thi s TCR with an antibody to the TCR idiotype elicited vigorous T cell hy bridoma activation, stimulation with its natural MHC + peptide ligand did not. We demonstrated that this phenotype could be reproduced simpl y by substituting the glutamic acid (E) at the mature NH2 terminus of the wild type TCR alpha chain with aspartic acid (D). The substitution also dramatically reduced the affinity of soluble alpha/beta-TCR hete rodimers for soluble MHC + peptide molecules in a cell-free system, su ggesting that it did not exert its effect simply by disrupting TCR int eractions with accessory molecules on the hybridoma. These results dem onstrate for the first time that amino acids which are not in the cano nical TCR complementarity determining regions can be critical in deter mining how the TCR engages MHC + peptide.