Surface expression of TRAIL receptors in erythroid cultures. Surface TRAIL receptor (TRAIL-R1, TRAIL-R2, TRAIL-R3, and TRAIL-R4) expression was evaluated by flow cytometry in CD34⁺ cells freshly purified (A) and in CD34⁺ cells cultured in the presence of EPO + IL-3 + SCF (B-C). In panels B-C, erythroid differentiation was monitored at 6 and 12 days of liquid culture by analysis of surface GPA expression. In panels A-B, shaded histograms represent cells stained with MoAbs specific for the indicated surface antigens (CD34, glycophorin A, TRAIL receptors), whereas unshaded histograms represent the background fluorescence obtained from the staining of the same cultures with isotype-matched control MoAbs. In panel C, surface TRAIL-R1 and TRAIL-R2 expression was analyzed in combination with surface glycophorin A at 6 and 12 days of culture. Horizontal axis indicates the relative surface glycophorin A expression detected by Cy-chrome fluorescence intensity. Vertical axis indicates the TRAIL-R1 or TRAIL-R2 expression detected by indirect PE fluorescence intensity. Representative negative controls, constituted by cells stained with irrelevant (Irr.) isotype-matched MoAbs, are shown in the top panels. A representative of 5 (A-B) and 3 (C) separate experiments is shown.