Microheterogeneity in the recognition of a HLA-DR2-restricted T cell epitope from a meningococcal outer membrane protein

The trimolecular interaction of T cell receptor (TcR), antigen and major histocompatibility complex (MHC) class II was analyzed using a panel of HLA-DR2-restricted T cell clones recognizing the 49-61 region of a meningococcal class I outer membrane protein (OMP). The clones, all CD3⁺CD4⁺CD8^?TcRct/β⁺, were selected by restimulation with the synthetic peptide OMP(49-61), which contains an immunodominant T helper determinant. Using a series of peptides that were sequentially truncated from the N or C terminus, four different epitope fine-specificity patterns were identified. Furthermore, each clone was found to exhibit a distinct recognition pattern for a panel of 20 single-residue substitution analogues of the minimal epitope OMP(50-58). Most substitutions that were not tolerated in the nonamer were allowed when the analogues were prepared departing from the native peptide OMP(49-61). Obviously, the residues outside the minimal epitope contribute to stabilization of the trimolecular complex. These findings suggest that defining the minimal size of T cell determinants may be of limited value. By performing proliferation competition assays putative MHC and TcR contact residues were identified in the peptide. Most likely, He 51 and Phe 54 act as MHC-anchoring residues, whereas Asp 53 represents a critical TcR contact residue for all of the clones. MHC anchoring may be provided by other residues as well, since He 51 and Phe 54 can be substituted by conservative residues as OMP(50-58) and OMP(49-61) analogues] and with Ala as OMP(49-61) analogues only]. Some evidence was found for interaction of particular side chains at other positions with TcR molecules, but this contribution was not equally important for all clones. Apparently, the clonotypic TcR can see a single epitope in different ways in the context of the same MHC restriction element. Since most clones use different V_α and V_β genes (which encompass the putative MHC-binding regions first and second complementarity-determining regions, CDR1 and CDR2) different modes of interaction with the HLA-DR2 molecule indeed are likely to occur.