Mechanisms of growth inhibition by anti-transferrin receptor monoclonal antibodies.

In previous studies, an immunoglobulin A, anti-transferrin receptor antibody (42/6) inhibited growth of a variety of normal and malignant human hemopoietic cells. To determine its mechanism of growth inhibition, we compared effects of 42/6 and B3/25, an immunoglobulin G anti-transferrin receptor antibody which does not inhibit lymphocyte growth, on transferrin (TF) binding and uptake. As in previous studies, affinity constants of TF and anti-TF receptor antibodies for human TF receptors at 4 degrees C were similar, but the number of calculated binding sites was higher for the antibodies. Antibody B3/25 did not inhibit TF binding at either 4 degrees C or 37 degrees C. At 4 degrees C, antibody 42/6 inhibited TF binding to normal, mitogen-stimulated mononuclear cells. However, TF did not inhibit 42/6 binding, suggesting 42/6 inhibited TF binding by noncompetitive, possibly steric, mechanisms. When cells were simultaneously exposed to labeled TF and unlabeled anti-TF receptor antibodies at 37 degrees C, 42/6 inhibited TF binding only slightly. Initial uptake of antibodies and TF at 37 degrees C was rapid, but when mononuclear cells or HL60 cells were cultured with either 42/6 or B3/25 for 2 days, TF binding and immunoreactive TF receptor sites decreased. However, TF bound to cells cultured with B3/25 continued to enter the cell, whereas cells cultured with 42/6 would no longer take up bound TF. Studies using HL60 cells grown with soluble iron in lieu of TF showed that changes in TF binding sites and TF uptake were not secondary to growth inhibition. We conclude that incubation with both inhibitory (42/6) and noninhibitory (B3/25) anti-TF receptor antibodies results in decreased TF binding sites. However, exposure to 42/6 also inhibits TF uptake and causes growth inhibition by iron deprivation. Monoclonal antibodies to receptors transporting critical nutrient molecules, such as iron, may inhibit cell growth by blocking ligand access to the cell's interior.