Cytokine activation sensitizes human monocytic and endothelial cells to the cytotoxic effect of an anti-CD40 immunotoxin

G28-5 sFv-PE40 is a single-chain immunotoxin that is cytotoxic toward malignant B cells expressing CD40. Human monocytes, which also express cell surface CD40, were found to be insensitive to the immunotoxin. Activation of the monocytic cell line THP-1, or freshly isolated peripheral blood monocytes with IFN-gamma, but not IL-3, GM-CSF, IL-6, or TNF-alpha, greatly sensitized the cells toward G28-5 sFv-PE40, lowering the EC(50) value from >10 mu g/ml to 80 ng/ml. This sensitization could not be explained simply by the two- to threefold increase in cell surface CD40 expression induced by IFN-gamma since TNF-alpha or the combination of TNF-alpha and IL-6 gave similar increases in CD40 expression but did not sensitize the cells to the immunotoxin. Internalization of G28-5 sFv-PE40 after IFN-gamma activation was also increased threefold, reflective of the increase in CD40 expression. IFN-gamma-treated but not -untreated THP-1 cells produced IL-6 and TNF-alpha following incubation with G28-5 sFv-PE40, indicating an association between CD40 signaling, which induces cytokine production, and sensitivity to the immunotoxin. HUVECs also express CD40 but were found to be insensitive to the anti-CD40 immunotoxin. A combination of IFN-gamma and TNF-alpha, but neither cytokine alone, sensitized the endothelial cells to G28-5 sFv-PE40. These data show that activation with cytokines can sensitize monocytes and endothelial cells to an immunotoxin targeted to CD40, most likely by altering the trafficking and/or processing of the immunotoxin after receptor-mediated internalization.