Benham A M, Neefjes J J
Division Cellular Biochemistry, The Netherlands Cancer Institute, Amsterdam.
J Immunol. 1997 Dec 15;159(12):5896-904.
For an effective CD8+ cytotoxic T cell response to occur during infection, MHC class I molecules must be loaded with antigenic peptides in the endoplasmic reticulum. The cytosolic factor responsible for peptide generation is believed to be the proteasome, with the TAP heterodimer mediating peptide transport into the endoplasmic reticulum. However, the rate-determining step(s) in this intracellular pathway of Ag presentation is currently unresolved. The availability of a specific and irreversible proteasome inhibitor called lactacystin has enabled us to determine the amount of proteasomes required for the peptide loading of MHC class I molecules in four cell types. In the absence of the IFN-gamma-inducible proteasome subunits LMP2 and LMP7, the trypsin-like (but not the chymotrypsin-like) activity of the proteasome is directly related to MHC class I peptide loading. However, IFN-gamma stimulation or assimilation of catalytic LMP2 and LMP7 subunits into proteasomes causes both chymotrypsin- and trypsin-like activities of the proteasome to become limiting for the loading of class I molecules. Our data suggest that upon full IFN-gamma stimulation, peptide supply by the proteasome is the limiting step in the assembly of MHC class I polypeptides. This mechanism may enable the cell to prevent competition between novel Ags and the pool of endogenous proteins for binding to MHC class I molecules.
为了在感染期间产生有效的CD8+细胞毒性T细胞应答，MHC I类分子必须在内质网中加载抗原肽。负责肽生成的胞质因子被认为是蛋白酶体，TAP异二聚体介导肽向内质网的转运。然而，目前尚不清楚这种细胞内抗原呈递途径中的限速步骤。一种名为乳胞素的特异性不可逆蛋白酶体抑制剂的可用性，使我们能够确定四种细胞类型中MHC I类分子肽加载所需的蛋白酶体数量。在缺乏IFN-γ诱导的蛋白酶体亚基LMP2和LMP7的情况下，蛋白酶体的胰蛋白酶样（而非糜蛋白酶样）活性与MHC I类肽加载直接相关。然而，IFN-γ刺激或催化性LMP2和LMP7亚基融入蛋白酶体，会使蛋白酶体的糜蛋白酶样和胰蛋白酶样活性都成为I类分子加载的限制因素。我们的数据表明，在完全IFN-γ刺激下，蛋白酶体提供的肽是MHC I类多肽组装中的限速步骤。这种机制可能使细胞能够防止新抗原与内源性蛋白质库之间竞争与MHC I类分子结合。
