Nuclear AID is detected inside and outside the GC. (A) AID⁺ cells (green) express membrane CD20 (red; orange color when coexpressed) and are surrounded by CD3⁺ cells (blue) outside the GC. Several nuclear AID⁺ extrafollicular blasts (right, arrow) and small centroblasts (left) are shown. The areas marked by white rectangles are magnified below; the color split and phase contrast image of the same field were added. Original magnification ×40. (B) AID⁺ GC cells are strongly cytoplasmic cREL⁺ (red) in the GC (left field, orange color when coexpressed). Nuclei are counterstained with DAPI. The areas marked by white rectangles (GC on the left; extrafollicular on the right) are magnified below; the color split and nuclear AID⁺ cells are marked by a white arrow. Note the total absence of nuclear cREL in the nAID⁺ cells, while nuclear cREL⁺ nAID^– cells (yellow arrowhead) are shown nearby. Original magnification ×40. (C) Confocal microscopy of AID (green) and Ki-67 (red) costained tonsil shows nuclear coexpression (yellow; arrows) in small centroblasts (left) and extrafollicular blasts (right). (D) Prior staining for CD20 (brown) prevents cytoplasmic AID staining, except in 2 AID⁺ CD20 ± extrafollicular blasts (red arrows). Nuclear AID (purple) can be shown in extrafollicular cells (black arrows) and in centroblasts (blue arrows). On the right, nAID⁺ centroblasts (left column) and large extrafollicular cells (right column) are shown, enlarged. Original magnification ×40.(E) As a control, no nuclear staining is shown in CD20 (brown) and CD79a (purple) double-stained tonsil. Original magnification ×40. Scale bar: 1 μm at × 40, 4 μm at ×10, and 10 μmat ×4.