Topology of T cell receptor-peptide/class I MHC interaction defined by charge reversal complementation and functional analysis

The molecular interactions between the CD8 co-receptor dependent N15 and N26 T cell receptors (TCRs) and their common ligand, the vesicular stomatitis virus octapeptide (VSV8) bound to H-2Kb, were studied to define the docking orientation(s) of MHC class I restricted TCRs during immune recognition. Guided by the molecular surfaces of the crystallographically defined peptide/MHC and modeled TCRs, a series of mutations in exposed residues likely contacting the TCR ligand were analyzed for their ability to alter peptide-triggered IL-2 production in T cell transfectants. Critical residues which diminished antigen recognition by 1000 to 10,000-fold in molar terms were identified in both N15 Valpha (alphaE94A or alphaE94R, Y98A and K99) and Vbeta (betaR96A, betaW97A and betaD99A) CDR3 loops. Mutational analysis indicated that the Rp1 residue of VSV8 is critical for antigen recognition of N15 TCR, but R62 of H-2Kb is less critical. More importantly, the alphaE94R mutant could be fully complemented by a reciprocal charge reversal at Kb R62 (R62E). This result suggests a direct interaction between N15 TCR Valpha E94R and Kb R62E residues. As Rp1 of VSV8 is adjacent to R62 in the VSV8/Kb complex and essential for T cell activation, this orientation implies that the N15 Valpha CDR3 loop interacts with the N-terminal residues of VSV8 with the Valpha domain docking to the Kb alpha2 helix while the N15 Vbeta CDR3 loop interacts with the more C-terminal peptide residues and the Vbeta domain overlies the Kb alpha1 helix. An equivalent orientation is suggested for N26, a second VSV8/Kb specific TCR. Given that genetic analysis of two different class II MHC-restricted TCRs and two crystallographic studies of class I restricted TCRs offers a similar overall orientation of V domains relative to alpha-helices, these data raise the possibility of a common docking mode between TCRs and their ligands regardless of MHC restriction.