Characterization of the interaction of a TCR alpha chain variable domain with MHC II I-A molecules

The ap TCR recognizes peptides bound to MHC molecules. In the present study , we analyzed the interaction of a soluble TCR a chain variable domain (V(a lpha)4.2-J(alpha)40; abbreviated to V(alpha)4.2) with the MHC class II mole cule I-AU. V-alpha,4.2 bound specifically to I-AU expressed on the surface of a transfected thymoma cell line. Modifications in the amino acid residue s located within the three complementarity-determining regions (CDRs) of th e V alpha, domain did not markedly affect this interaction. However, mutati on of glutamic acid to alanine at position 69 of the fourth hypervariable r egion (HV4 alpha) significantly increased the binding. Antibody inhibition studies suggested that the binding site was partly contributed by a region of the beta chain of I-A(u). Furthermore, the binding of V(alpha)4.2 to the MHC molecule was dependent on the nature of the peptide bound in the groov e. Soluble V(alpha)4.2 specifically inhibited the activation of TCR transfe ctants by I-Au-expressing cells pulsed with an N-terminal peptide of myelin basic protein. V(alpha)4.2 also bound to MHC class II-expressing spleen ce ll populations from mice of the H-2(U) and H-2(d) haplotypes, The binding o f V(alpha)4.2 to I-A molecules might explain the immunoregulatory effects r eported previously for TCR a chains. This V(alpha)4.2 interaction may also be relevant to models of antigen presentation involving the binding of inta ct proteins to MHC class II molecules followed by their processing to gener ate epitopes suitable for T cell recognition.