CD40 BM stromal expression correlates with the number of CD40L-expressing infiltrating MCs. (A) Analysis of the number of infiltrating CD4⁺ T helper cells, CD68⁺ macrophages, DC-Sign⁺ myeloid dendritic cells, and tryptase⁺ MCs in SMZL BM infiltrates, subdivided according to high or low stromal CD40 expression. The degree of CD40 expression on SCs significantly correlated with the number of infiltrating MCs, but not with that of other infiltrating cells. (B) Representative immunohistochemical stainings for CD40L on SMZL BM infiltrates revealing that CD40L (brown signal) is mainly expressed by infiltrating cells with large monocytoid morphology suggestive of MCs (insets) and by scattered small cells with lymphoid morphology. Microphotograph is relative to 1 representative case of the 66 evaluated. Immunohistochemistry, STREPTavidin–biotin–peroxidase complex method, 3,3′-diaminobenzidine (DAB) chromogen; original magnification X100, insets X400; (C) Double-marker immunofluorescence for CD40L (green signal) and tryptase (red signal) expression in SMZL BM infiltrates showing that CD40L-expressing cells mostly coexpress MC tryptase (yellow signal). Microphotographs are relative to 1 representative case with high (score 3) CD40 stromal expression. Immunofluorescence with Alexa-488 and Alexa-568 fluorochromes; original magnification upper panels, ×200; lower panels, ×400.