UCSF

The germinal center (GC) is an important site for the generation and selection of B cells bearing high-affinity antibodies during an immune response. The GC is organized into two main compartments termed dark and light zones. However, despite extensive anatomical definition of these zones, the mechanisms by which GC compartmentalization and selection occur have remained poorly defined.

First, we considered the possible role of chemokines as GC organizers. We show here that GC organization into dark and light zones was absent in mice deficient in the chemokine receptor CXCR4. GC B cells migrated toward the CXCR4 ligand SDF-1, which was more abundant in the dark zone than in the light zone. B cells in the dark zone expressed more CXCR4 than B cells in the light zone, and CXCR4-deficient B cells were excluded from the dark zone in the context of a wild-type GC. These findings suggest that CXCR4 promotes dark zone localization in response to SDF-1. In contrast, CXCR5 helped direct cells to the light zone, which is rich in the CXCR5 ligand CXCL13. Deficiency in CXCL13 was associated with aberrant positioning of the light zone. To gain insight into the GC cell migration and interaction dynamics during selection, we imaged GCs in intact mouse lymph nodes by two-photon microscopy. We observed that GC B cells were highly motile and exhibited a dendritic morphology with long cell processes. GC B cell motility was partially dependent on the chemokine CXCL13. GC B cells transited between dark and light zones and divided in both zones, yet the cells resided in the light zone for only a few hours. GC B cells formed few stable contacts with GC T cells despite frequent encounters, and GC T cells were seen to carry dead GC B cell blebs.

On the basis of these observations, we conclude that GC organization into dark and light zones is a chemokine-driven process. We also suggest a new model for selection in which competition for T cell help plays a more dominant role than previously appreciated.