XmAb5574 caused reversible involution of germinal centers in the spleen. Spleen tissue was collected 15 and 85 days (D15 and D85) after administration of either vehicle (A,B) or 10 mg/kg XmAb5574 (C-F). Tissue sections were stained either for CD20 (B-cell immunostain) or with hematoxylin and eosin (H&E) for tissue morphology as indicated. The CD20 immunostained area (brown-stained cells) was decreased by XmAb5574 (C) in comparison to vehicle control (A), and this was associated with germinal center involution (B,D). CD20 immunostaining (E) and germinal center morphology (F) had recovered to baseline levels at day 85. For all panels, sections were examined using an Olympus BH-2 microscope equipped with a 60×/0.17 NA objective. Images were captured with a Leica DC300 camera using Image Capture Leica IM50 software and processed with Adobe Photoshop version 6.0.