Histiocytes in the T cell–rich zones express PD-L1. Representative follicular lymphoma paraffin sections stained for PD-L1 (5H1) (A,C) and CD3 (B,D). Panels A and C show PD-L1-staining cells predominantly in the T cell–rich zones between the neoplastic follicles. Inset of panel C is a high-power photomicrograph that shows the dendritic process of the 5H1 staining histiocytes. Panels B and D stained for CD3 shown staining predominantly between the neoplastic follicles in the same distribution as the PD-L1-staining cells. Panels E and F show the interfollicular zone stained for PD-L1 (brown) and PD-1 (red). F indicates follicle; IF, interfollicular zone. Hematoxylin counterstain; original magnification ×50 (A-B), ×200 (C-D), ×400 (inset in panel C); ×250 (E), and ×400 (F).