In vitro interaction between BM mesenchymal SCs and MCs. (A) Immunolocalization analysis shows direct interaction between tryptase⁺ MCs and CD40⁺ SCs within BM SMZL infiltrates. Immunofluorescence with Alexa-488 and Alexa-568 fluorochromes; original magnification ×400. Microphotograph is relative to 1 representative case with high CD40 stromal expression (score 3). (B) BMSCs and LAD2 MCs are driven toward upregulation of CD40 and CD40L, respectively, on 24-hour coculture. (C) IL-6 (left) and TNF (right) release measured by ELISA in the coculture medium of unstimulated or IgE-Ag-activated MCs and BMSCs after 12, 24, and 48 hours interaction. BMSC culturing in the presence of MC results in significant release of the 2 proinflammatory cytokines, even in the absence of IgE-Ag stimulation. (D) Intracellular flow cytometry after 24 hours of coculture between BMSC and unstimulated MC reveals that MCs are the only source of TNF, whereas both BMSCs and MCs contribute to IL-6 production. **P < .01; ***P < .001.