CD137L is expressed on B cells that are in close contact with FDC. CLSM analysis of CD137L expression on B cells and/or FDCs in GC of immunized mice. Cryostat sections of spleens from (A) WT and (B) CD137L^−/− mice used for the microarray experiments were stained with mAbs to detect CD137L (red), FDC (FDC-M2³⁶ ; green) and B cells (B220; blue). In the WT sample, B cells that are adjacent to FDC express CD137L, as indicated by the pink color. In the CD137L^−/− sample, this signal is virtually absent, indicating specificity of CD137L detection. Original magnification ×40.