Stages of GC maturation and associated tissue niches. (A) Schematic representation of the stages of germinal center (GC) maturation. (B - E) Immunofluorescence staining of popliteal lymph node (B) and spleen (C-E) from C57BL/6 mice that received an adoptive transfer of OVA-specific T cells and antigen-specific B cells enriched by immunomagnetic purification (StemCell Technologies). T cells specific for OVA were obtained from spleens of OT-II mice while either B1-8+ Jκ-/- or MD4 mice served as donors for anti-NP or anti-HEL specific B cells respectively. Recipients were immunized after transfer with 50 μg of either NP-OVA/CFA in the footpad (lymph node, panels B), NP-OVA/alum injected i.p. (spleen, panels C, E), or HEL-OVA/alum (spleen, panel D). (B) Popliteal lymph node histology time-course demonstrating GC maturation after immunization. Stages of maturation represented include the unimmunized follicle (d0/naïve), maturation of the FDC network and light zone expansion (day 4), centroblast increase, zonal segregation and follicle swelling (day 5), and a mature GC (day 10). Sections were stained to highlight CD4+ T cells (blue), IgD+ non-responding B cells within the follicular mantel (green), and CD35+ FDC network (red). (C) Histology showing a maturing GC (day 6) stained to highlight IgD+ naïve B cells (grayscale), PNA+ GC cells (green), and the distribution of sonic hedgehog (SHH, red) in relation to the FDC network stained with CD35 (blue). (D) Representative histology of splenic GCsdemonstrating initial differentiative events shortly after the arrival of Ag-specific B cells to the follicle interior(day 3). Sections were stained to highlight GFP+ Ag-specific B cells (green), CD35 FDCs (blue), Bcl6 (red) toidentify GC committed B cells, and IgMa to assess the level of cytosolic immunoglobulin. Arrows point to Bcells within the FDC network that are cIg hi (consistent with ASC effector lineage), Bcl6hi (GC lineage), andBcl6neg cIg lo (of an undefined alternative fate). (E) Histology showing GFP+ Ag-specific B cell interactionwith dendritic cells (DC) at the T-B border (day 6). Section were stained for CD4+ T cells (blue), B220+ Bcells (red), GFP+ Ag-specific B cells (green) and CD11c+ DC cells (grayscale). Higher magnification imagesof the T-B border demonstrate the ability of GC B cells, DCs and T cells at the T zone border to directly engage each other.