Preincubation with TNF- increases CD34⁺blood cell alloantigen-presenting function. Freshly isolated CD34⁺ blood cells were either stained with CD40 MoAb or incubated for 24 hours with medium alone, TNF-, or GM-CSF, then washed and evaluated both for CD40 expression by flow cytometry (A) and, after irradiation, for their capacity of stimulating allogeneic MNC in primary MLC with 5 × 10⁴ responders at different stimulator/responder ratios (B). Results are the mean cpm ± SEM of triplicate cultures. At 1:2 stimulator/responder ratio, the differences of the alloresponse to CD34⁺ blood cells preincubated with TNF- versus GM-CSF and medium, are both significant (P = .01).