T and B Lymphocytes in Germinal Centers
As detailed in chapter 1, the germinal center can be histologically divided into a dark zone of proliferating centroblasts and a light zone of nonproliferating centrocytes.1–3 The dark zone, in proximity to the PALS, is populated by proliferating centrobl
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Tand BLymphocytes in Germinal Centers R.A. Insel and M.H. Nahm
Germinal Center B Cells General Overview of Germinal Center Reactions
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s detailed in chapter 1, the germinal center can be histologically divided into a dark zone of proliferating centroblasts and a light zone of nonproliferating centrocytes. 1- 3 The dark zone, in proximity to the PALS, is populated by proliferating centroblasts that clonally expand with a cell cycle time as short as 6-8 hours.4 Up to 10 B cell clones populate a germinal center, but through selection there is an outgrowth of only 3-5 of these clones. The signals that activate migration from the PALS of an activated B cell to populate a primary follicle and form a germinal center reaction are incompletely elucidated and discussed further in chapter 1. Also, the mechanism(s) of the rapid clonal expansion of centroblasts in germinal centers is unknown, but discussed further below. These rapidly dividing centroblasts undergo a process of somatic hypermutation of the V regions of their immunoglobulin genes, detailed in chapter 4. Centroblasts develop into nonproliferating centrocytes that move apically into the basal light zone and then to the apical light zone where they are interspersed with a network of follicular dendritic cells (FDC) and CD4+ T cells. It is in the light zone that centrocytes undergo massive apoptosis, with collection of condensed chromatin fragments as tingible bodies in macrophages, or escape apoptosis by a process of antigenic selection to develop into memory B cells or differentiate to become plasma cells. Germinal center B cells are programmed to die by apoptosis unless they are rescued by a positive signal. One The Biology o/Germinal Centers in Lymphoid Tissue, edited by G. Jeanette Thorbecke and Vincent K. Tsiagbe. © 1998 Springer-Verlag and R.G. Landes Company.
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The Biology of Germinal Centers in Lymphoid Tissue
such signal comes from the interaction between germinal center B cells and FDC. The primary signal between the two cells in vivo is believed to be dependent on interaction with antigen held as immune complexes on the surface of the FDC. This interaction may facilitate a transfer of native Ag from the surface of the FDC as an immune complex-coated body (iccosomes) to the B cell, where it may be internalized, processed, and presented to T cells. The activated B cell can present processed Ag on its surface to the T cell and activate cytokine production from T cells. In vitro interactions of FDC with B cells leads to upregulation of MHC class II and, in the presence of Ag, upregulation of B7-2. (CD86) on the B cell. The germinal center response tapers by 3 weeks after immunization.
Precursors of Germinal Center B Cells Both IgD+ and IgD- B lymphocytes have been shown to give rise to germinal center formation. 5-7 Although sIgD expression on B lymphocytes is not obligatory, germinal center formation and memory B cell production may be enhanced by IgD+ B cells through interaction with T cells that express an IgD receptor. 8 Furthermore, the pres
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