Géraldine Grouard

Generation of memory B cells and plasma cells in vitro

After germinal center B cells undergo somatic mutation and antigen selection, they become either memory B cells or plasma cells, but the signal requirements that control entry into either pathway have been unclear. When purified human germinal center cells were cultured with interleukin-2, interleukin-10, and cells expressing CD40 ligand, cells with characteristics of memory B cells were generated. Removal of CD40 ligand from the system resulted in terminal differentiation of germinal center B cells into cells with the characteristics of plasma cells. These results indicate that CD40 ligand directs the differentiation of germinal center B cells toward memory B cells rather than toward plasma cells.

Follicular Dendritic Cells and Germinal Centers

Follicular dendritic cells (FDCs) are stromal cells unique to primary and secondary lymphoid follicles. Recirculating resting B cells migrate through the FDC networks, whereas antigen-activated B cells undergo clonal expansion within the FDC networks in a T cell-dependent fashion, thereby generating germinal centers. Here, B cells undergo somatic mutation, positive and negative selection, isotype switching and differentiation into high-affinity plasma cells and memory B cells. Since the discovery of FDCs by electron microscopy as long-term antigen-retaining cells 30 years ago isolation of FDCs and generation of FDC-like cells lines and of FDC-specific monoclonal antibodies have been achieved. FDCs express all three types of complement receptors as well as Ig-Fc receptors, through which antigen-antibody immune complexes are retained. However, the mechanism that prevents FDCs from internalizing the antigens and retaining them in native form for long periods of time remains obscure. Substantial evidence derived from cultures in vitro indicates that FDCs contribute directly to the survival and activation of peripheral B cells. The adhesion between FDCs and B cells is mediated by ICAM-1 (CD54)-LFA-1(CD11a) and VCAM-VLA-4. T cells may interact with FDCs in a CD40/CD40-ligand-dependent fashion. Whether FDCs originate from hematopoietic progenitors or from stromal elements is still a controversy. New evidence suggests the presence of two types of dendritic cells within human germinal centers: (i) the classic FDCs that express DRC-1, KiM4, and 7D6 antigens represent stromal cells; and (ii) the newly identified CD3-CD4-CD11c- germinal center dendritic cells (GCDC) represent hematopoietic cells that may be analogous to the antigen-transporting cells described in mice. Finally, FDCs appear to be involved in the growth of follicular lymphomas and in the pathogenesis of HIV infection.

Regulation of Human B Cell Activation by Follicular Dendritic Cell and T Cell Signals

Follicular dendritic cells (FDC) represent a unique cell type (Nossal et al. 1968; Szakal and Hanna 1968) within the B cell follicles of the secondary lymphoid tissues. During T cell-dependent humoral immune responses, B cells undergo proliferation, somatic mutation, isotype switching, positive selection, and differentiation into memory B cells and plasma cells within the FDC networks (Berek 1992; Kosco and Gray 1992; Kroese et al. 1987; Liu et al. 1992; Maclennan 1994; Nossal 1992). The close physical contact between FDC and B cells that occurs when B cells undergo dramatic phenotypic and genetic changes suggests that FDC may be directly involved in all these events. Based on the histological observations depicted in Fig. 1, follicular B cell response can be divided into three distinct stages, in which different B cell events occur: 1. Exponential growth of germinal center precursor cells within the primary follicles. Germinal center reaction is initiated by the rapid proliferation of about Open image in new window three precursor cells Kroese et al. 1987; Liu et al. 1991; Jacob end Kelsoe 1992), giving rise to a germinal center containing about 1.4 x 104 cells after 4 days of immunization (Liu at al. 1991. 2.A A fully developed germinal center has e dark zone and a light zone. Centroblasts in the dark zone proliferate and mutate (Berek et al. 1991; Jacob et al. 1991; Koppers et al. 1993; McHeyzer-Williams et al. 1993). They give rise to non-proliferating cemrocytes in the light zone. The high-affinity centrocytes ore positively selected (Weiss et al. 1992) by antigens deposited on FIDC end differentiates 1o either memory B cells (MCHevzer-Wiumms et al. 199. 1; Schittek and Rajewsky 1992; Weiss and Rajewsky 1990) or plasma cells (Kosco et al. 1989. The low-affinity centrooytes which have failed to be selected die by apoptosio(Liu et al. 1989). 3. After 3-4 weeks of immunization, fully developed germinal 6entena are replaced by small follicular foci which consist of a few antigen-specific B blasts associated with immune complex networks (Liu et al. 18B1). These foci may represent the chronic stimulation of memory B cells, which are fundamental for the rnain1ainenoe of serum imrnunoglobulin (lg) level (Tew et al. 1980) and long-lived memory B cell clones (Gray and Smarvall 1988).

Human Follicular Dendritic Cells Promote Both Proliferation and Differentiation of CD40 Activated B Cells

Follicular dendritic cells (FDCs) are unique stromal cells within the germinal centers, where intensive B cell proliferation was identified. The main feature of these cells is their capacity to trap antigenantibody immune complexes for long periods of time (1). FDCs have been shown to have a stimulatory effect on B cell proliferation in both human and mouse systems by many groups (2, 3, 4, 5). In contrast, Freedman et al (6) have shown that FDCs inhibit SAC dependent B cell proliferations.