Lymphomas in (CD27;)CD137L^−/− mice are of GC B-cell origin. (A) Immunohistochemistry of spleen sections of mice diagnosed with early-stage and late-stage FL by the use of antibody to B220 to detect B cells and antibody to CD3 to detect T cells (serial sections). (B) Histochemistry with PNA was performed to define GC B cells. Shown are spleen sections of mice diagnosed with different stages of FL development: (i) healthy aged WT control, (ii) preneoplastic lesions, (iii) early-stage FL, and (iv) late-stage FL. Original magnification (panels A-B) ×5. Specific staining is shown in brown and hematoxylin counterstaining in blue.