Critical role of an amino acid residue in a T cell determinant is due to its interaction with a neighboring non-critical residue

Evidence for immunodominance between closely related epitopes in the selection of T cell repertoire: hierarchy of T cell epitopes in a repeating sequence

In a protein antigen the number of epitopes that are presented by the MHC molecules to the T cells is generally limited. This phenomenon of immunodominance determines the T cell response to a given antigen. To understand the molecular basis of epitope selection we have analysed the hierarchy of T cell epitopes in a repeating synthetic polypeptide antigen Poly 18, Poly EYK(EYA)5 in the mice of H-2d haplotype. Because of its repeating nature, all the potential epitopes of Poly EYK(EYA)5 generated as a result of antigen processing will have extensive sequence overlap, therefore, providing an excellent system to investigate the molecular basis of T cell peptide epitope selection in vivo. We have synthesized a series of 12 and 15 amino acid peptides to mimic these epitopes. In H-2d mice Poly EYK(EYA)5 elicits T cell clones that optimally react with 15 amino acid peptides EYK(EYA)4 and/or (EYA)5. Similar results were obtained when related 12 amino acid peptides EYK(EYA)3 and/or (EYA)4 are used. (EYA)5 reactive T cell clones appear to be very heterogenous and much larger in number than EYK(EYA)4 reactive clones. (EYA)5 reactive clones could be elicited by at least three short Poly-18 derived epitopes (EYA)4, EYK(EYA)3 and (EYA)3EYK while EYK(EYA)4 reactive clones elicited only by the EYK(EYA)3 epitope. However, we observed the dominance of (EYA)5 reactive clones even when EYK(EYA)3 was used as an immunogen and this could be related to the degeneracy of their antigen specificity. Our earlier antigen competition studies suggest that (EYA)5 does not compete with EYK(EYA)4 epitope in binding to I-Ad. Therefore, there is no intramolecular competition between these epitopes to activate T cells. The epitope (EYA)3EYK appears to be subdominant since it can elicit Poly EYK(EYA)5 specific T cells upon immunization but does not appear to be part of Poly EYK(EYA)5 repertoire. Peptides such as (EYA)2EYKEYA or EYAEYK(EYA)2 with lysine substitution in the middle of the sequence were non immunogeneic. Similar results were obtained when the larger 15 amino acid peptides were used as antigen. Another level of epitope immunodominance is seen when substituted peptides of the two immunodominant epitopes are used. Some of these epitopes have potential to be part of the Poly 18 repertoire but they are greatly under represented when intact Poly 18 is used as antigen. The unusual hierarchy observed for immunodominance in these overlapping epitopes of EYK(EYA)5 sequence suggest a bias in the selection of T cell repertoire based upon the crossreactivity between potential epitopes generated as a result of antigen processing.

Non-additive effects of multiple amino acid substitutions on antigen-antibody recognition

Synthetic peptides have been used to mimic the main antigenic site of foot-and-mouth disease virus (FMDV) of serotype C and of several variant isolates. This region includes multiple continuous B cell epitopes. The effect of single amino acid replacements, individually or in combination, on antigen specificity has been evaluated using monoclonal antibodies. Quantitative enzyme immunodot assays have shown that both additive and non-additive effects of multiple replacements occur in continuous B cell epitopes, with regard to antibody recognition. Antigenically critical single replacements may be compensated by other, non-critical replacements. Thus, the role of a single amino acid on antibody recognition depends on the sequence context in the antigenic domain. The non-additive effects of multiple replacements may modulate the extent of antigenic diversification of highly variable RNA viruses, and keep viruses confined within antigenic groups by precluding linear antigenic divergence.

Studies on antigenic variability of C strains of foot-and-mouth disease virus by means of synthetic peptides and monoclonal antibodies

Peptides representing the sequence of the immunodominant loop of foot-and-mouth disease virus strain C-S8 (YTASARGDLAHLTTTHARHLP, residues 136-156 of VP1) and of several variant viruses have been prepared by solid phase methods. In addition, five peptides with single-residue replacements at Leu147 (Ile, Nle, Val, Ala, Gly) have been synthesized. Tosyl and dinitrophenyl protections for histidine have been compared, the latter being found to give better synthetic products. The peptides have been tested in an immunodot assay against a panel of monoclonal antibodies directed towards the VP1 loop. Immunochemical results are discussed on the basis of the mobility of the region reproduced by the peptides and the nature of the side chain of residue 147.

Immunoregulatory activity of the T-cell receptor alpha chain demonstrated by retroviral gene transfer

We have previously described an antigen-specific I-Ad-restricted T-cell hybridoma, A1.1, that constitutively releases an antigen-specific immunoregulatory activity into supernatants. Using retrovirally mediated gene transfer, we have found that transfer of the T-cell receptor alpha chain (TCR alpha) gene from A1.1 to a number of other T-cell hybridomas effectively transferred the ability to produce the activity. Gene transfer of the TCR beta chain (TCR beta), however, did not transfer this ability. The regulatory activity from cells expressing the A1.1 TCR alpha bound to and was eluted from an anti-TCR alpha monoclonal antibody and displayed fine antigenic specificity identical to that of supernatants from A1.1. The possibility that this activity represents a secreted form of the TCR alpha (as opposed to shed cell-surface TCR) was examined in BW1100 cells, lacking TCR alpha and TCR beta, which produced the antigen-specific activity after gene transfer of the A1.1 TCR alpha gene. The expression of the immunoregulatory activity in supernatants correlated with a direct antigen-binding activity as detected by ELISA, thus raising the possibility that antigen binding is relevant to the mechanism of action of the soluble TCR alpha. We discuss these observations and our earlier studies suggesting an immunoregulatory role for soluble TCR alpha.

Two mechanisms of antigenic diversification of foot-and-mouth disease virus

The amino acid replacements that underlay the diversification of the main antigenic site A (VP1 residues 138 to 150) of foot-and-mouth disease virus (FMDV) of serotype C have been identified. Sixteen new VP1 sequences of isolates from 1926 until 1989 belonging to subtypes C1, C2, C3, C4, C5, and unclassified are reported. The reactivities in enzyme-linked immunoelectrotransfer blot assays of capsid protein VP1 with a panel of neutralizing monoclonal antibodies that recognize sites A or C (the VP1 carboxy-terminus) have been correlated with the amino acid sequence at the relevant epitopes. The analyses involving the immunodominant site A reveal two mechanisms of antigenic change. One is a gradual increase in antigenic distance brought about by accumulation of amino acid replacements at two hypervariable segments within site A. A second mechanism consists of an abrupt antigenic change manifested by loss of many epitopes, caused by one replacement at a critical position (particularly Ala (145)----Val or His (146)----Gln). The identification of the amino acid substitutions responsible for such large antigenic changes provides new information for the design of synthetic anti-FMD vaccines. However, the screening of isolates from six decades suggests that the virus, even within the confines of a single serotype, has exploited a minimum of its potential for antigenic variation.