Iron uptake by MOLT 3 cells from transferrin/monoclonal antitransferrin antibody complexes

The ability of seven monoclonal antibodies (MAbs) reactive with human transferrin (Tf) to inhibit iron uptake from Tf by cells of the human lymphoid line MOLT 3 was compared with the effect of these MAbs on Tf binding to MOLT 3 surface Tf receptors. MAbs HTF-01, HTF-06, HTF-07, HTF-11 and HTF-14 inhibited both iron uptake and Tf binding. Complexing Tf with MAbs HTF-04 or HTF-05 resulted in an increased association of Tf with the cells but iron uptake was diminished. Following the intracellular kinetics of Tf/HTF-04 complex has shown that the whole complex is endocytosed and Tf iron is retained inside the cell, but Tf release from both the cell surface and the intracellular compartment is slower when compared to Tf alone. Iron uptake inhibition was therefore attributed to lengthening of the Tf cell cycle after complexing of Tf with HTF-04 and it is suggested that the rate of cellular Tf turnover might, together with the number of functioning Tf receptors, determine the rate of iron uptake by cells.