Novak Z, Fraga E, Singh B
Department of Immunology, University of Alberta, Edmonton, Canada.
Mol Immunol. 1992 Dec;29(12):1467-76. doi: 10.1016/0161-5890(92)90220-r.
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.
在蛋白质抗原中，主要组织相容性复合体（MHC）分子呈递给T细胞的表位数量通常是有限的。这种免疫显性现象决定了T细胞对特定抗原的反应。为了理解表位选择的分子基础，我们分析了H-2d单倍型小鼠中重复合成多肽抗原Poly 18、Poly EYK(EYA)5的T细胞表位层次结构。由于其重复性质，抗原加工产生的Poly EYK(EYA)5的所有潜在表位将具有广泛的序列重叠，因此，提供了一个极好的系统来研究体内T细胞肽表位选择的分子基础。我们合成了一系列12和15个氨基酸的肽来模拟这些表位。在H-2d小鼠中，Poly EYK(EYA)5引发的T细胞克隆与15个氨基酸的肽EYK(EYA)4和/或(EYA)5反应最佳。当使用相关的12个氨基酸的肽EYK(EYA)3和/或(EYA)4时，也得到了类似的结果。(EYA)5反应性T细胞克隆似乎非常异质，数量比EYK(EYA)4反应性克隆多得多。(EYA)5反应性克隆可由至少三个短的Poly-18衍生表位(EYA)4、EYK(EYA)3和(EYA)3EYK引发，而EYK(EYA)4反应性克隆仅由EYK(EYA)3表位引发。然而，即使将EYK(EYA)3用作免疫原，我们也观察到(EYA)5反应性克隆占优势，这可能与其抗原特异性的简并性有关。我们早期的抗原竞争研究表明，(EYA)5在与I-Ad结合时不与EYK(EYA)4表位竞争。因此，这些表位之间不存在激活T细胞的分子内竞争。表位(EYA)3EYK似乎是次显性的，因为它在免疫时可引发Poly EYK(EYA)5特异性T细胞，但似乎不是Poly EYK(EYA)5库的一部分。序列中间有赖氨酸取代的肽，如(EYA)2EYKEYA或EYAEYK(EYA)2是非免疫原性的。当使用较大的15个氨基酸的肽作为抗原时，也得到了类似的结果。当使用两个免疫显性表位的取代肽时，会出现另一层次的表位免疫显性。其中一些表位有可能成为Poly 18库的一部分，但当完整的Poly 18用作抗原时，它们的代表性大大不足。在EYK(EYA)5序列的这些重叠表位中观察到的不寻常的免疫显性层次结构表明，基于抗原加工产生的潜在表位之间的交叉反应性，T细胞库的选择存在偏差。
