Beekman N J, van Veelen P A, van Hall T, Neisig A, Sijts A, Camps M, Kloetzel P M, Neefjes J J, Melief C J, Ossendorp F
Department of Immunohematology, Leiden University Medical Center, Leiden, The Netherlands.
J Immunol. 2000 Feb 15;164(4):1898-905. doi: 10.4049/jimmunol.164.4.1898.
CTL directed against the Moloney murine leukemia virus (MuLV) epitope SSWDFITV recognize Moloney MuLV-induced tumor cells, but do not recognize cells transformed by the closely related Friend MuLV. The potential Friend MuLV epitope has strong sequence homology with Moloney MuLV and only differs in one amino acid within the CTL epitope and one amino acid just outside the epitope. We now show that failure to recognize Friend MuLV-transformed tumor cells is based on a defect in proteasome-mediated processing of the Friend epitope which is due to a single amino acid substitution (N-->D) immediately flanking the C-terminal anchor residue of the epitope. Proteasome-mediated digestion analysis of a synthetic 26-mer peptide derived from the Friend sequence shows that cleavage takes place predominantly C-terminal of D, instead of V as is the case for the Moloney MuLV sequence. Therefore, the C terminus of the epitope is not properly generated. Epitope-containing peptide fragments extended with an additional C-terminal D are not efficiently translocated by TAP and do not show significant binding affinity to MHC class I-Kb molecules. Thus, a potential CTL epitope present in the Friend virus sequence is not properly processed and presented because of a natural flanking aspartic acid that obliterates the correct C-terminal cleavage site. This constitutes a novel way to subvert proteasome-mediated generation of proper antigenic peptide fragments.
针对莫洛尼鼠白血病病毒（MuLV）表位SSWDFITV的细胞毒性T淋巴细胞（CTL）可识别莫洛尼MuLV诱导的肿瘤细胞，但不能识别由密切相关的弗瑞德MuLV转化的细胞。潜在的弗瑞德MuLV表位与莫洛尼MuLV具有很强的序列同源性，仅在CTL表位内的一个氨基酸以及表位外的一个氨基酸上有所不同。我们现在表明，无法识别弗瑞德MuLV转化的肿瘤细胞是基于蛋白酶体介导的弗瑞德表位加工缺陷，这是由于表位C末端锚定残基紧邻的单个氨基酸取代（N→D）所致。对源自弗瑞德序列的合成26肽进行蛋白酶体介导的消化分析表明，切割主要发生在D的C末端，而不是莫洛尼MuLV序列中的V的C末端。因此，表位的C末端没有正确生成。额外带有C末端D的含表位肽片段不能被抗原加工相关转运体（TAP）有效转运，并且对MHC I类Kb分子没有显著的结合亲和力。因此，由于天然存在的侧翼天冬氨酸消除了正确的C末端切割位点，弗瑞德病毒序列中存在的潜在CTL表位没有被正确加工和呈递。这构成了一种颠覆蛋白酶体介导的合适抗原肽片段生成的新方式。
