Polarization of gene expression in LZ and DZ B cells. (A) Heat maps showing differential expression in LZ and DZ of selected genes in mice and humans. Colors indicate fold change (Log₁₀ base) between one zone and the opposite zone within the same sample (human) or pool (mouse). (B) Immunofluorescence of human (top) and wild-type mouse (middle) GCs showing the anatomic distribution of AID protein. AID is stained in both species using the same rat monoclonal antibody (mAID-2). Light zones are defined by CD23 staining (green) in humans and Ig (anti-IgG, heavy and light chains, which stains mostly immune complexes deposited on follicular dendritic cells) in mice. The GC perimeter is defined by counterstaining with DAPI in human and Bcl-6 in mouse (both in blue). Immunofluorescent staining of an Aicda^−/− lymph node is shown as a control for the specificity of the anti-AID antibody (bottom). Scale bars represent 100 μm. Image acquisition parameters are described in “Microscopy.” (C) Mouse LZ and DZ signatures overlaid on human gene expression data, plotted as expression scatter plots (left) or volcano plots (right). Human gene expression data are shown (gray) with genes contained in mouse LZ or DZ signatures highlighted in blue or red, respectively. Interspecies orthologs defined as genes bearing the same Official Gene Symbol. Volcano plots: dotted line represents 2-fold P < .05. (D) Human LZ and DZ signatures overlaid on mouse gene expression data. Details are as in panel C.