Molecular requirements for T cell recognition by a major histocompatibility complex class II-restricted T cell receptor: The involvement of the fourth hypervariable loop of the V alpha domain

The role of two central residues (K68, E69) of the fourth hypervariable loo p of the Vol domain (HV4 alpha) in antigen recognition by an MHC class II-r estricted T cell receptor (TCR) has been analyzed. The TCR recognizes the N H2-terminal peptide of myelin basic protein (Ac1-11, acetylated at NH2 term inus) associated with the class II MHC molecule I-A(u). Lysine 68 (K68) and glutamic acid 69 (E69) of HV4 alpha have been mutated both individually an d simultaneously to alanine (K68A, E69A). The responsiveness of transfectan ts bearing wild-type and mutated TCRs to Ac1-11-I-A(u) complexes has been a nalyzed in the presence and absence of expression of the coreceptor CD4. Th e data demonstrate that in die absence of CD4 expression, K68 plays a centr al role in antigen responsiveness. In contrast, the effect of mutating E69 to alanine is less marked. CD4 coexpression can partially compensate for th e loss of activity of the K68A mutant transfectants, resulting in responses that, relative to those of the wild-type transfectants, are highly sensiti ve to anti-CD4 antibody blockade. The observations support models of T cell activation in which both the affinity of the TCR for cognate ligand and th e involvement of coreceptors determine the outcome of the T cell-antigen-pr esenting cell interaction.