Expression of alpha-fetoprotein receptors by human T-lymphocytes during blastic transformation

Alpha-fetoprotein (AFP) and transferrin (Tf) are actively internalized by many growing cells during ontogenic and neoplastic development, including human malignant T- and B-lymphoblastoid cells. Their internalization is, on the contrary, greatly diminished or absent in mature, non-proliferating elements. In the present work, peripheral blood mononuclear cells (PBMCs) and T-lymphocytes, harvested from normal human donors, were induced to blastic transformation with phytohemagglutinin (PHA) and their ability to uptake AFP and Tf was measured and compared with Tf uptake in the same conditions. The capacity of the cells to internalize both proteins was quantified by fluorescence activated cell sorter (FACS) using fluoresceinated derivatives of these proteins. The results obtained show a significant uptake of AFP by T-lymphocytes upon PHA stimulation. The values of AFP incorporation were similar for all the cells studied (PBMCs, T-cells and T4, T8 cell subsets). The time course of AFP uptake paralleled, under the same conditions, the uptake of Tf and the expression of IL2 receptors. AFP uptake increased rapidly from the zero time (resting T-cells) and reached a maximum around 72 hr after PHA activation. Scatchard analysis of kinetic data at 4 degrees C revealed for Hu-AFP one single group of specific binding sites in PHA activated T-lymphocytes with a dissociation constant of 3.03 x 10(-7) M and around 88,000 sites/cell. There results strongly suggest the transitory expression of AFP receptors in T-lymphocytes during blastic transformation.