Rosloniec E F, Beard K S, Freed J H
Department of Medicine, National Jewish Center for Immunology and Respiratory Medicine, Denver, CO 80206.
Mol Immunol. 1993 Apr;30(5):491-501. doi: 10.1016/0161-5890(93)90117-t.
The MHC class II molecules bind antigenic peptides and present them to T cells. Their ability to carry out these functions depends, in a critical way, on the detailed structure of the membrane-distal alpha 1 and beta 1 domains of these molecules. Using the I-Ak molecule and a series of hen egg lysozyme (HEL) peptide-specific, I-Ak-restricted T cell hybridomas as a model, we have examined the effect of altering essentially all of the polymorphic residues of the murine class II molecule on its ability to present Ag. Our results support the following conclusions: (1) both the location and the structural alteration introduced in a specific amino acid interchange are important in determining the effect the interchange will have on Ag presentation; and (2) changes in amino acids in the floor of the putative Ag binding cleft of the class II molecule can exert a major influence on the presentation of peptides to T cells. By carrying out direct binding experiments between the HEL(46-61) peptide and two mutant I-A molecules that fail to present HEL(46-61) to appropriate T cells, we were able to assess, in a quantitative fashion, the role played by peptide binding in the failure to present Ag. Our results suggest that, in the two cases studied, the failure to bind the HEL(46-61) peptide was not primarily responsible for the failure of the mutant class II molecule to present that peptide. Specifically, an A beta chain mutant that possesses d allelic residues at positions 65-67 in the second PMR of the Ak beta chain actually binds HEL(46-61) at wild type (I-Ak) levels. In contrast, an A alpha chain chimera in which b allelic residues are inserted in the third PMR of the Ak alpha chain, binds HEL(46-61) about three- to four-fold less well than wild type. While this decrease in binding affinity may be partially responsible for the inability of the latter chimeric molecule to present HEL(46-61), it can not be the total explanation because increasing the peptide concn even by an order of magnitude does not restore Ag presentation by APC expressing this chimeric molecule. These results are discussed in terms of the currently accepted model of the class II molecule.
MHC II类分子结合抗原肽并将其呈递给T细胞。它们执行这些功能的能力在很大程度上取决于这些分子膜远端α1和β1结构域的详细结构。以I-Ak分子和一系列鸡卵溶菌酶（HEL）肽特异性、I-Ak限制性T细胞杂交瘤为模型，我们研究了改变小鼠II类分子基本上所有多态性残基对其呈递抗原能力的影响。我们的结果支持以下结论：（1）特定氨基酸互换中引入的位置和结构改变对于确定该互换对抗原呈递的影响很重要；（2）II类分子假定抗原结合裂隙底部的氨基酸变化可对肽向T细胞的呈递产生重大影响。通过进行HEL(46-61)肽与两种不能将HEL(46-61)呈递给合适T细胞的突变I-A分子之间的直接结合实验，我们能够以定量方式评估肽结合在抗原呈递失败中所起的作用。我们的结果表明，在所研究的两种情况下，不能结合HEL(46-61)肽并非突变II类分子不能呈递该肽的主要原因。具体而言，在Akβ链第二个多态性区域（PMR）的65-67位具有d等位基因残基的Aβ链突变体实际上以野生型（I-Ak）水平结合HEL(46-61)。相反，在Akα链第三个PMR中插入b等位基因残基的Aα链嵌合体结合HEL(46-61)的能力比野生型低约三到四倍。虽然这种结合亲和力的降低可能部分导致了后一种嵌合分子不能呈递HEL(46-61)，但这不能完全解释，因为即使将肽浓度提高一个数量级也不能恢复表达这种嵌合分子的抗原呈递细胞的抗原呈递能力。根据目前被广泛接受的II类分子模型对这些结果进行了讨论。
