Germinal-center organization and cellular dynamics

Abstract

Germinal centers (GCs) are important sites of antibody affinity maturation. In the classical model, the GC dark zone contains large centroblasts that are rapidly proliferating and undergoing somatic hypermutation of their antibody variable-region genes. Centroblasts give rise to smaller nonproliferating centrocytes in the light zone that compete for binding antigen on follicular dendritic cells. Recently, the approach of real-time imaging of GCs by two-photon microscopy of intact lymph nodes has provided new insights into GC dynamics that both support and challenge fundamental aspects of this model. Here we review recent and older findings on cell migration, proliferation, and interaction dynamics in the GC and discuss a model in which dark- and light-zone cells are morphologically similar, proliferation occurs in both zones, and GC B cells compete for T cell help as well as antigen.

Figure 1.. Schematic representation of GC compartments and cellular dynamics

The cellular composition of the compartments is summarized on the left and possible functions of the compartments are described on the right. The diagram depicts the structure of an acute GC but chronic GCs, such as those typical of tonsils, may have additional levels of compartmentalization. GC B cells are medium sized blasts expressing low surface Ig and exhibiting a dendritic morphology, in both the dark and light zones. In humans, dark zone cells also express somewhat higher amounts of CD77 (Hardie et al., 1993) whereas CD44 may be higher on light zone cells (Feuillard et al., 1995). Additional cell types may sometimes be present including dendritic cells (Grouard et al., 1996), and CD4⁺CD3⁻ cells (Kim et al., 2003). GC B cells migrate extensively within their respective compartments and move between compartments, most likely after modulating CXCR4 protein levels. Blebs of apoptotic GC B cells can associate with T cells, possibly limiting the availability of T cell help. Antigen is displayed as immune complexes (ICs) on FDCs but also possibly on tingible body macrophages (TBMØs) and motile B cells. Soluble antigen may also be present. Note the greater cellular heterogeneity and thus lower GC B cell density in the light zone than in the dark zone. See text for further details.

Figure 2.. GC B cell movement within and between GC dark and light zones

The light zone FDCs are identified by in vivo deposited PE immune complexes (red). Colored lines show tracks of GC B cells that moved within the light or dark zones or that crossed between zones, as indicated, during a 30 minute imaging session. The gridlines are separated by 20µm. Cell tracks were manually classified as being in the light or dark zones if the entire track stayed within the PE⁺ or PE⁻ region, respectively, during the imaging session. Cell tracks were manually classified as traveling between zones if the tracks originated in one zone, crossed an approximately 20 µm border between the zones, then entered at least 10 µm into the other zone, and stayed in that zone for the duration of the analysis. Some tracks could not be definitively classified by these criteria, for example tracks that were too close to the border between the light and dark zones. From Allen et al., Science 315: 528 (2007).

Figure 3.. Two models of GC B cell selection within the GC

In the first model, discrimination between cells of different affinity for the foreign antigen occurs solely at the level of different strengths of BCR signaling. In the second model, sufficient BCR engagement is still necessary but further selection occurs due to competition for T cell help. B cells that have captured, processed, and presented more antigen as MHC-peptide complexes go on to receive T cell help at the expense of cells that have captured less antigen. See text for details.