No reliable culture system exists for B‐lineage acute lymphoblastic leukaemia (ALL). Recently we found that many different mature B‐cell malignancies proliferate upon stimulation via the CD40 antigen, and this led us to investigate whether a similar CD40 activation on ALL cells could also induce proliferation. First, we measured CD40 expression in 21 ALL cases; all were CD40+, although mostly weak. Next, we triggered the CD40 antigen by anti‐CD40 antibodies and by a CD40 ligand‐expressing cell line. In addition, we measured the influence of IL‐3, IL‐4 and IL‐7 with and without these stimuli. In 8/10 cases proliferation, measured by3H‐thymidine incorporation, could be induced after CD40 crosslinking, especially in the presence of IL‐3. Stimulation via the CD40 ligand was more successful than using crosslinked anti‐CD40 antibodies. IL‐4 inhibited the spontaneous proliferation found in three cases, but stimulated proliferation after CD40 crosslinking. IL‐7 did not contribute to proliferation. Morphology, immunophenotyping and surface marker analysis, combined with DNA flow cytometry confirmed that the proliferation found could be ascribed to the ALL cells. In conclusion, B‐lineage ALL cases are CD40+, and many can be cultured us
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