Humoral immune responses are characterized by antigen-driven B-lymphocyte differentiation processes that occur in distinct phases. Each phase has unique cellular starting points, specialized regional microenvironments, and its own set of controls that ultimately determine the resulting cell fate.
Secondary lymphoid organs such as the spleen and lymph nodes are the sites providing the critical environment in order to generate high-affinity antibodies to T-cell-dependent antigens. The spleen can be grossly subdivided into red and white pulp, the former containing largely erythrocytes whereas the latter represents the lymphoid area. The histological structure of the white pulp shows three major compartments: 1) a cluff of lymphocytes lining the splenic arterioles, referred to as the periateriolar lymphocyte sheath (PALS), which contains mainly T cells, 2) spheroid-shaped primary B-cell follicles located in the periphery of the PALS, which consist of naive, recirculating B cells, and 3) the marginal zone, a layer containing reticular cells, macrophages, and resident B cells, that surrounds the PALS and follicular areas (Janeway and Travers, 1996). Accessory cells are also segregated within the T and B-cell areas of the white pulp. Interdigitating dendritic cells (IDCs) belong to the hematopoietic lineage and are present in the PALS, whereas clusters of follicular dendritic cells (FDCs), a cellular lineage of controversial origin, are characteristically located in the B-cell follicles (Liu et al., 1996; Steinman et al., 1997a,b).
Upon immunization with T-cell-dependent antigens, B cells become activated in the PALS outside the lymphoid follicle. For this activation, antigen-specific B cells bind native antigens and receive accessory signals provided by T-helper cells, which themselves were primed and activated by IDCs. Activated B cells either form foci of short-lived plasma cells secreting germline V-bearing IgM or switched IgG isotype or they migrate into the primary follicles. These follicles subsequently are transformed into a histologically defined structure termed the germinal center (GC; Kelsoe, 1995a,b). In these sites, affinity maturation, immunoglobulin (Ig) class switching, and memory B-cell generation occur (Kelsoe, 1995a). These steps serve to expand and diversify the repertoire of the early immune response to antigens and help to select high-affinity variants for the long-term maintenance of protective immunity.
GCs were shown to be derived from rapid clonal expansion of one or a few antigen-specific founder B cells replicating every 6 -7 hours, which leads to the classical polarized structure. The dark zone of the GC houses the rapidly dividing B-cell blasts, the centroblasts, that continually give rise to the nondividing centrocytes in the light zone. GC B cells are surrounded by a so-called mantle zone consiting of resting B cells (MacLennan and Gray, 1986;MacLennan et al., 1990).
For a long time, GCs were thought to be the only place for affinity maturation by somatic hypermutation. This dogma has been broken by analyzing mutant mice lacking GC formation in the spleen which were still able to manifest a high-affinity immune response after immunization with high doses of antigen (Matsumoto et al., 1996a;Franzoso et al., 1997). These data indicate that affinity maturation is not absolutely dependent on the presence of GCs.
Signals leading to the induction of secondary lymphoid structures and the formation of a fully functional GC are complex. They are derived from cognate physiological cell-cell contacts and in addition interactions of soluble ligands and cytokines with their specific receptors. These interactions trigger intracellular signaling cascades that often result in the activation or inhibition of transcription factors or cofactors. In the meantime, many of the components participating in the GC have been identified and characterized. In addition to in vitro studies using combinations of purified primary and transformed cell lines stimulated with cytokines, genetically altered mice as well as naturally occurring mutants have provided powerful tools for dissecting the physiology of the immune responses to T-cell-dependent antigens (Fig. 1).
T-CELL HELP IS ESSENTIAL FOR GC FORMATION
T-cell help is critical for B cells to induce affinity maturation and memory B-cell development. T-helper cells exhibit their function either by cognate cell-cell contact or by secreting cytokines that act as costimulatory signals. Prior to these processes, T-helper cells themselves have to be primed and activated within the T-cell area of the spleen. T-helper cell activation requires at least two distinct signals provided by IDCs: 1) an antigen-specific signal