CD5+ B cells express higher levels of Fas ligand (FasL) than other splenic populations. Freshly isolated splenocytes from type II collagen/complete Freund's adjuvant (cII/CFA)-immunized, cII T-cell receptor transgenic mice were stained with anti-CD19, anti-CD5, and anti-FasL antibodies as described in Materials and methods. (a) Cells were initially gated as small nongranular lymphocytes by forward scatter and side scatter criteria (not shown) and then categorized by staining with anti-CD19 and anti-CD5 as T cells (CD5hi/CD19-), CD5+ B cells (CD5+/CD19+), B-2 cells (CD5-/CD19+), or double-negatives. (b) Comparison of FasL expression on CD5+ B cells from four mice, with representative disease severity scores shown in parentheses. Numbers in the box are mean fluorescence intensity (MFI) values of FasL staining on CD5+ B cells from the four mice. (c) Histograms and MFI values compared for lymphocyte subsets of two representative mice. (d) Average MFI values of FasL expression on gated splenic lymphocytes from mice with severe (n = 6), moderate (n = 6), and mild or no arthritis (n = 6).