p2Ca

The well-defined 2C T cell was used to investigate alloreactive degeneracy. A panel of class I molecules that are known ligands for the 2C TCR were sensitized with three known peptide ligands, p2Ca (LSPFPFDL), dEV-8 (EQYKFYSV), and SIYR-8 (SIYRYYGL). The peptide p2Ca was originally identified as the allopeptide seen in the Ld class I molecule by 2C T cells, 2C recognizes the dEV-8 peptide as the ligand in the Kbm3 class I molecule, and SIYR-8 was recently identified as a peptide ligand for 2C in the context of the Kb class I molecule. Strong recognition of all three Ag-presenting molecules occurred in the context of their respective allopeptides, but 2C recognized all three peptides to a measurable extent in the context of Kb. Molecular modeling of these Kb/peptide complexes revealed a high degree of similarity between dEV-8 and SIYR-8, but very little conformational similarity of either of these peptides with p2Ca. Furthermore, the structural changes in the mutant Kbm3 binding site resulted in generalized changes in the conformation of each of five bound peptides compared with those of the same peptides bound to Kb. The finding that degenerate recognition occurs on Kb, the restriction element responsible for selecting 2C T cells, suggests a unique relationship between a TCR and the Ag-presenting molecule that mediates its positive selection.

The self-restricted T cell repertoire exhibits a high frequency of alloreactivity. Because these alloreactive T cells are derived from the pool of cells selected on several different self MHC alleles, it is unknown how development of the alloantigenic repertoire is influenced by homology between a self MHC allele and an alloantigen. To address this, we used the 2C transgenic TCR that is selected by K(b), is alloreactive for L(d), and cross-reacts with L(q). L(q) is highly homologous to L(d) and binds several of the same peptide ligands, including p2Ca, the peptide recognized by 2C. We find that L(d)/p2Ca is a high avidity agonist ligand, whereas L(q)/p2Ca is a low avidity agonist ligand for 2C T cells. When mice transgenic for the 2C TCR are bred to L(q)-expressing mice, 2C(+) T cells develop; however, they express lower levels of either the 2C TCR or CD8 and require a higher L(d)/p2Ca ligand density to be activated than 2C(+) T cells selected by K(b). Furthermore, the 2C T cells selected in the presence of L(q) fail to detect L(q)/p2Ca complexes even at high ligand density. Thus, despite possessing the identical TCR, there is a functional avidity difference between 2C(+) T cells selected in the presence of L(q) vs K(b). These data provide evidence that homology between the selecting ligand and an alloantigen can influence the avidity of the T cell repertoire for the alloantigen, and suggest that thymic selection can fine tune T cell avidity independent of intrinsic TCR affinity.

Cytotoxic T-lymphocytes (CTL) typically recognize foreign peptides bound to class I products of the major histocompatibility complex. A function of CTL is to identify and eliminate tumor cells that bear inappropriately expressed peptide/class I complexes (i.e., mutated self-peptides or self-peptides that are expressed at abnormally high levels). The processes that result in tolerance to self-antigens can undermine the effectiveness of this system by deleting or inactivating T-cells that might potentially be reactive with tumor-associated antigens. To up-regulate the response to tumor antigens it will be useful to develop methods whereby CTL responses to specific self-peptides can be elicited without damage to normal tissue. In this report a CTL response was generated in BALB/c mice against the ubiquitous self-peptide p2Ca (LSPFPFDL), which binds to Ld and is derived from the mitochondrial enzyme alpha-ketoglutarate dehydrogenase. CTL derived in vitro recognize specifically the p2Ca/Ld complex and use V beta 8 regions predominantly. The cultured cells lysed target cells with lower levels of p2Ca than the levels used for induction. This result suggests that it may be possible to use peptides at high concentrations to elicit CTL react with endogenous levels of a peptide/class I complex. The in vivo potential of the response was demonstrated by the observation that BALB/c mice, coinjected with a syngeneic BALB/c myeloma and exogenous p2Ca, are able to reject the tumor. The p2Ca/Ld system may thus provide a model for evaluating the parameters for effective immunotherapy with tumor-associated peptides.