Monoclonal Antibodies to a Purified Human Transferrin Receptor

BALB/c mice were injected with an affinity‐chromatography‐purified placental transferrin receptor preparation. Spleen cells were fused with NS‐1 myeloma cells. Sixteen hybrids producing monoclonal antibodies specific for the transferrin receptor and two hybrids specific for transferrin were identified by radioimmunoassay (RIA). Five hybrids were selected for cloning on the basis of antibody specificity and affinity. None of the antibodies inhibited the binding of transferrin to K562 cells. The binding of antibody ID9 to K562 cells was partially inhibited by transferrin or a polyclonal goat anti‐transferrin receptor antiserum. Of the five antibodies, two (IIB6 and IIB2) reacted only with the purified receptor and solubilized cells and not with whole cells. The other three antibodies, when tested with normal human cells and leukaemia and tumour cell lines, showed identical reaction patterns. The antibodies precipitated a glycoprotein from K562 cells with an apparent molecular weight of 94,000, estimated from sodium dodecyl sulphate‐polyacrylamide gel electrophoretograms run under reducing conditions, and a molecular weight of 188,000 when run under unreduced conditions. All antibodies have a high affinity with K_a values ranging from 1.44 ± 10⁹ to 3.56 ± 10¹⁰ (1/mol). The antigen precipitated by all five antibodies showed identical peptide maps after partial proteolytic digestion.