The depletion of B220⁺ B cells in spleen is due to the increased apoptosis and arrested cell cycle.

<p>(A) Splenic cells from mice 5 days after infection with V and Ts were stained with PE-conjugated anti-B220 mAb, annexin V and propidium iodide and analyzed by flow cytometer. Numbers indicate the percentages of early apoptosis cells (Annexin-V⁺) and late apoptosis cells (Annexin-V⁺PI⁺) within B220 gated cells populations. The number was estimated by collecting 10,000 events. (B) After challenging, mice (n = 3/group) were euthanized on 5 days p.i. and the histological paraffin sections of spleens were prepared. Apoptotic cells in histological spleen sections were identified using the TUNEL assay. Magnification, ×200. (C) Following infection, spleen cells were prepared and stained with PE-Cy5-anti-FITC and PI and analyzed by flow cytometer. The results were analyzed using MultiCycle AV software. Each graph is from an individual mouse, but is representative of three mice per group and three independent experiments.