We report here that the Intrinsic affinities of the antigen-specific T-cell receptors (TCR) of two unrelated CD8+ T-cell clones for their respective peptide-major histocompatibility complex (MHC) ligands are higher than the values generally thought to prevail for TCR. The TCR of one clone (2C) binds an aflogeneic class I MHC protein (Ld) in association with an a-ketoglutarate dehydrogenase nonapeptide (QLSPFPFDL, termed QL9) with an intrinsic anity (intrinsic equilibrium association constant) of 1-2 x 107 M-1. The TCR of the other clone (4G3) binds a syngeneic class I MHC protein (Kb) in association with an ovalbumin octapeptide (SIINFEKL, termed pOV8) with an inins affinity of 1.5x 106 M-l. A compain of the two clones, combined with current views of T-cell repertoire selection in the thymus, leads us to propose that TCR afities are generally likely to be higher for algeneic MHC-peptide complexes than for syngeneic MHC-peptide complexes.Because the somatic hypermutation that underlies the generation of high-affinity antibodies is not evident in T cells, it has long been surmised that the affinities of antigen-specific T-cell receptors (TCR) for their natural ligands, now known to be peptide-major histocompatibility complex (MHC) complexes, are likely to be low and in the narrow range (104 to 105 M-1) exhibited by antibodies made early in immune responses, before the onset of somatic hypermutation. Previously, however, we showed (1) that one of the two murine T-cell clones studied here (clone 2C) has a relatively high intrinsic affinity (2 x 106 M-1) for its ligand: Ld, a class I MHC protein, in association with an octapeptide (LSPF-PFDL, termed p2Ca) that derives from murine a-ketoglutarate dehydrogenase (a-KGDH) (2, 3). This clone also recognizes Ld in association with several other peptides from a-KGDH. One of them, the nonapeptide QLSPFPFDL (termed QL9), proved to be exceptionally active in sensitizing Ld+ target cells (T2-Ld) for lysis by 2C cells. We show here that the affinity of the TCR on 2C cells for the complex formed by this peptide with Ld (QL9-Ld) is about 10 times greater than for p2Ca-Ld and, to our knowledge, is the highest affinity value found so far for any TCR.Since the 2C clone arose in a mouse of the H-2b haplotype, its high-affinity binding of the peptide-Ld complex is an allogeneic reaction (alloreaction). To compare it with a CD8+ cytotoxic T lymphocyte (CTL) clone that is also highly reactive cytolytically but recognizes a syngeneic MHCpeptide complex, we took advantage of clone 4G3. This clone, which also arose in an H-2b mouse, reacts specifically with the ovalbumin octapeptide pOV8 (SIINFEKL) in association with Kb. In this syngeneic reaction, the TCR affinity for its peptide-MHC complex was about 10-fold lower than in the alloreaction. Though the comparison involves only two clones, when the findings are considered in light of recent studies ofT-cell repertoire selection in the thymus (4-6), they lead us to propose that the affinity ofa TCR for peptide-MHC complexes will ge...

No abstract

CTL clone 2C recognizes the allogeneic class I MHC molecule Ld in association with peptides derived from α-ketoglutarate dehydrogenase (oxoglutarate dehydrogenase (OGDH)), a ubiquitous intracellular protein. One of these peptides, QLSPFPFDL (QL9), elicits more vigorous cytolytic responses than two previously identified naturally processed peptides with overlapping sequences, LSPFPFDL (p2Ca) and VAITRIEQLSPFPFDL (p2Cb), from OGDH. In this study, we show that QL9 forms a more stable complex with cell surface Ld than does p2Ca or p2Cb and is processed from the longer, naturally occurring peptide p2Cb by 20S proteosomes in vitro. The N-terminal cyclized pyroglutaminyl QL9 (pyroQL9), a form of QL9 to which it is converted at the low pH used for peptide isolation from tissue extracts, is even more active than QL9 in cytotoxicity assays with 2C CTL. Overall, the results indicate that along with the abundant natural peptides p2Ca and p2Cb, the QL9 and other OGDH peptides of various lengths, sharing a conserved C-terminal sequence, are also processed and presented with Ld as allogeneic ligands for T cells expressing 2C TCR. All these peptides, each available in a low amount, could act in concert at the cell surface, resulting in a high density of cognate ligands that accounts for the exceptionally potent cytolytic response by 2C CTL.