Weber D A, Buck L B, Delohery T M, Agostino N, Pernis B
Department of Microbiology, College of Physicians and Surgeons, Columbia University, New York, NY 10032.
J Mol Cell Immunol. 1990;4(5):255-66; discussion 266-8.
The antibody response to protein antigens requires specific cooperation between B and T cells. In order to deliver the helper signal, T cells must recognize, in the context of Class II MHC, processed antigen on the membrane of B cells. Processed antigen is in the form of peptides bound in a given site of the Class II MHC molecule; in order to address the question of where, in the B cell, the complex of Class II MHC and processed antigen is formed, we studied the subcellular localization of these two molecules. Since the formation of this complex is the crucial step in antigen processing and presentation, the answer to this question is central to the whole problem of the physiology of antigen handling by B cells. To collect information pertinent to the question, we have compared, in B cells, the intracellular traffic of Class II MHC and of monovalent and divalent anti-immunoglobulin antibodies used as protein ligands of the membrane immunoglobulins. We have done so by two-color immunofluorescence microscopy, and we have detected extensive confluence of Class II MHC molecules with the immunoglobulin ligand, both mono- and bi-valent, in the endosomes of LPS-activated murine B cells. Whereas the ligand clearly reaches the endosomes by internalization from the cell membrane, the Class II MHC molecules could reach the same location either by endocytosis from the membrane or through targeting to the endosomes of newly synthesized Class II MHC molecules. We have collected quantitative evidence for endocytosis of Class II MHC by following, with the fluorescence activated cell sorter, the quenching of the fluorescence of fluoresceinated Fab' anti Class II MHC in LPS-activated murine B cells; this quenching indicates the entry of the label into an acidic intracellular compartment. Together with the results of others, obtained with different methods, our observations support the concept that, at least in mature activated B cells, Class II MHC molecules reach the organelles where they meet processed protein antigens, mainly through the endocytic route. Since activated B cells endocytose their membrane Class II MHC, and not their membrane Class I, our results contribute to the understanding of how B cells present antigens, that have bound to their membrane immunoglobulins, to Class II-restricted helper T cells and not to Class I-restricted cytolytic T cells.
对蛋白质抗原的抗体应答需要B细胞和T细胞之间的特异性协作。为了传递辅助信号，T细胞必须在II类主要组织相容性复合体（MHC）的背景下，识别B细胞膜上经过处理的抗原。经过处理的抗原以结合在II类MHC分子特定部位的肽的形式存在；为了解决在B细胞中II类MHC与经过处理的抗原的复合物在何处形成的问题，我们研究了这两种分子的亚细胞定位。由于这种复合物的形成是抗原处理和呈递的关键步骤，这个问题的答案对于B细胞处理抗原的生理学整个问题至关重要。为了收集与该问题相关的信息，我们在B细胞中比较了II类MHC以及用作膜免疫球蛋白蛋白配体的单价和二价抗免疫球蛋白抗体的细胞内运输情况。我们通过双色免疫荧光显微镜做到了这一点，并且我们在脂多糖激活的小鼠B细胞的内体中检测到II类MHC分子与单价和二价免疫球蛋白配体广泛融合。虽然配体显然是通过从细胞膜内化到达内体的，但II类MHC分子可以通过从膜内吞或通过靶向新合成的II类MHC分子的内体到达相同位置。我们通过用荧光激活细胞分选仪跟踪脂多糖激活的小鼠B细胞中荧光素化的抗II类MHC Fab'的荧光淬灭，收集了II类MHC内吞的定量证据；这种淬灭表明标记物进入酸性细胞内区室。连同其他人用不同方法获得的结果，我们的观察结果支持这样的概念，即至少在成熟的活化B细胞中，II类MHC分子主要通过内吞途径到达它们与经过处理的蛋白质抗原相遇的细胞器。由于活化的B细胞内吞其膜II类MHC，而不是其膜I类MHC，我们的结果有助于理解B细胞如何将与其膜免疫球蛋白结合的抗原呈递给II类限制性辅助性T细胞，而不是呈递给I类限制性细胞毒性T细胞。
