PS is localized in the inner membrane leaflet of murine B cells and exposed on stimulation. (A) Lymphocytes from C57BL/10 mice were labeled with anti-CD4^APC (T-cell-specific) and anti-CD19^PERCP (B-cell-specific) antibodies and preincubated with AV^FITC. (Ai) Density plots of the rate of PS exposure (binding of AV) by T cells (CD4⁺; left panel) and B cells (CD19⁺; right panel) following stimulation with 5 μM calcimycin (arrows). (Aii) PS exposure by T- and B-cell populations following stimulation. Data are from the experiment shown in panel Ai, plotted as histograms to compare AV binding by cell populations gated at t = 0 seconds (black line) and t = 500 seconds (gray line) in T cells (left panel) and B cells (right panel). (B) Lymphocytes from C57BL/10 mice were labeled with anti-CD19^PERCP (B-cell-specific) antibodies, preincubated with AV^FITC and stimulated with calcimycin at the doses shown. Density plots illustrate the rate and degree of PS exposure (AV binding). (C) Spontaneous PS exposure and cell death in B-lymphocyte populations ex vivo. Lymphocytes from C57BL/10 mice were labeled with anti-CD4^CYCHROME and anti-CD19^APC antibodies, to distinguish T and B cells, and incubated with AV^FITC. (Ci) Histograms show PS exposure (AV binding) by T cells (CD4⁺) and B cells (CD19⁺)30 minutes (black lines) and 2 hours (gray lines) after animals were killed. (Cii) Line graph shows the proportion (± SD) of cells stained with anti-CD19 antibody (B cells) within “live cell” gates as determined by forward and side light scatter at times shown following the time the animals were killed.