Siemasko K, Eisfelder B J, Stebbins C, Kabak S, Sant A J, Song W, Clark M R
Section of Rheumatology, Department of Medicine, Committee on Immunology, University of Chicago, IL 60637, USA.
J Immunol. 1999 Jun 1;162(11):6518-25.
The B cell Ag receptor (BCR) is a multimeric complex, containing Ig alpha and Ig beta, capable of internalizing and delivering specific Ags to specialized late endosomes, where they are processed into peptides for loading onto MHC class II molecules. By this mechanism, the presentation of receptor-selected epitopes to T cells is enhanced by several orders of magnitude. Previously, it has been reported that, under some circumstances, either Ig alpha or Ig beta can facilitate the presentation of Ags. However, we now demonstrate that if these Ags are at low concentrations and temporally restricted, both Ig alpha and Ig beta are required. When compared with the BCR, chimeric complexes containing either chain alone were internalized but failed to access the MHC class II-enriched compartment (MIIC) or induce the aggregation and fusion of its constituent vesicles. Furthermore, Ig alpha/Ig beta complexes in which the immunoreceptor tyrosine-based activation motif tyrosines of Ig alpha were mutated were also incapable of accessing the MIIC or of facilitating the presentation of Ag. These data indicate that both Ig alpha and Ig beta contribute signaling, and possibly other functions, to the BCR that are necessary and sufficient to reconstitute the trafficking and Ag-processing enhancing capacities of the intact receptor complex.
B细胞抗原受体（BCR）是一种多聚体复合物，包含Igα和Igβ，能够内化特定抗原并将其递送至特殊的晚期内体，在那里抗原被加工成肽段以加载到MHC II类分子上。通过这种机制，受体选择的表位向T细胞的呈递增强了几个数量级。此前有报道称，在某些情况下，Igα或Igβ均可促进抗原呈递。然而，我们现在证明，如果这些抗原浓度较低且存在时间限制，则Igα和Igβ均不可或缺。与BCR相比，仅含其中一条链的嵌合复合物虽能内化，但无法进入富含MHC II类分子的区室（MIIC），也不能诱导其组成囊泡的聚集和融合。此外，Igα中基于免疫受体酪氨酸的激活基序酪氨酸发生突变的Igα/Igβ复合物同样无法进入MIIC或促进抗原呈递。这些数据表明，Igα和Igβ均为BCR提供信号传导以及可能的其他功能，这些功能对于重建完整受体复合物的 trafficking 和抗原加工增强能力而言是必要且充分的。
