TOSO protein expression in lymph nodes from healthy donors, patients with follicular lymphoma, and patients with CLL. (A) Inguinal lymph node with small, regressive germinal center (*). Single TOSO-positive cells are seen in the germinal center as well as within the sinus. Cytomorphologically, these cells are assigned to plasmocytic cells. (B) Cervical lymph node with prominent germinal center (*), mantle zone (**), and small marginal zone (***). Single TOSO-positive plasmocytoid cells within the germinal center; ample TOSO positive cells in the marginal zone. (C) Lymph node infiltrated by a low-grade follicular lymphoma. Within the neoplastic follicle, no TOSO expression is visible. (D-F) Two examples of lymph node infiltration by CLL. (D) TOSO expression is seen as a distinct staining in almost all neoplastic B cells. (E) Distinct clusters of strongly positive cells are visible within the infiltrate. These clusters are not necessarily limited to proliferation centers within the infiltrates. (F) Magnification of panel E. (A-E) Magnification ×200; (F) magnification ×400. Immunohistochemistry was performed using 6B10 antibody on paraffin-embedded sections.