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.

The germinal center (GC) is an important site for the generation and selection of B cells bearing high-affinity antibodies, yet GC cell migration and interaction dynamics have not been directly observed. Using two-photon microscopy of mouse lymph nodes, we revealed that GC B cells are highly motile and extend long cell processes. They transited between GC dark and light zones and divided in both regions, although these B cells resided for only several hours in the light zone where antigen is displayed. GC B cells formed few stable contacts with GC T cells despite frequent encounters, and T cells were seen to carry dead B cell blebs. On the basis of these observations, we propose a model in which competition for T cell help plays a more dominant role in the selection of GC B cells than previously appreciated.

The Drosophila transient receptor potential protein (TRP) and its mammalian homologues are thought to be Ca(2+)-permeable cation channels activated by G protein (G(q/11))-coupled receptors and are regarded as an interesting molecular model for the Ca(2+) entry mechanisms associated with stimulated phosphoinositide turnover and store depletion. However, there is little unequivocal evidence linking mammalian TRPs with particular native functions. In this study, we have found that heterologous expression of murine TRP6 in HEK293 cells reproduces almost exactly the essential biophysical and pharmacological properties of alpha(1)-adrenoceptor-activated nonselective cation channels (alpha(1)-AR-NSCC) previously identified in rabbit portal vein smooth muscle. Such properties include activation by diacylglycerol; S-shaped current-voltage relationship; high divalent cation permeability; unitary conductance of 25 to 30 pS and augmentation by flufenamate and Ca(2+); and blockade by Cd(2+), La(3+), Gd(3+), SK&F96365, and amiloride. Reverse transcriptase-polymerase chain reaction and confocal laser scanning microscopy using TRP6-specific primers and antisera revealed that the level of TRP6 mRNA expression was remarkably high in both murine and rabbit portal vein smooth muscles as compared with other TRP subtypes, and the immunoreactivity to TRP6 protein was localized near the sarcolemmal region of single rabbit portal vein myocytes. Furthermore, treatment of primary cultured portal vein myocytes with TRP6 antisense oligonucleotides resulted in marked inhibition of TRP6 protein immunoreactivity as well as selective suppression of alpha(1)-adrenoceptor-activated, store depletion-independent cation current and Ba(2+) influx. These results strongly indicate that TRP6 is the essential component of the alpha(1)-AR-NSCC, which may serve as a store depletion-independent Ca(2+) entry pathway during increased sympathetic activity.