Resistance of myeloid leukaemia cell lines to ricin A-chain immunotoxins.
Nineteen monoclonal antibodies that recognize antigens on myeloid leukaemia cells were screened upon HL60, KG1, U937 and K562 cells for their ability to form effective ricin A-chain immunotoxins. The screening was performed using an indirect assay in which the cells were treated firstly with the test antibody and then with a Fab' immunotoxin directed against mouse immunoglobulin. Only two antibodies, MEM75 and 120-2A3, both directed against the transferrin receptor (TfR) were predicted to form immunotoxins that would inhibit protein synthesis by the cells by 50% at a concentration (IC50) of 10(-8) M or less. This prediction was subsequently confirmed using several of the antibodies directly conjugated to ricin A-chain. By contrast, the same immunotoxins were highly toxic to non-myeloid cells which shared the target antigens. A comparison was made between the rates of endocytosis and degradation by HL60 cells of an anti-TfR immunotoxin 120-2A3.dgA, that was effective at killing myeloid cells, and a CD33 immunotoxin, p67-7.dgA, that bound to myeloid cells but did not kill them. The difference in potency of the two immunotoxins on HL60 cells was not due to deficient uptake of p67-7.dgA but was probably due to the more rapid intracellular degradation of p67-7.dgA. Fast and effective degradation in lysosomes, if a general finding, could explain the poor susceptibility of myeloid cells to ricin A-chain immunotoxins.
Engert
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Drug Targeting Laboratory, Imperial Cancer Research Fund, London, U.K.
Brown
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Thorpe
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D000818:Animals; D000911:Antibodies, Monoclonal / Q000276:immunology; D015703:Antigens, CD / Q000276:immunology; D015214:Antigens, Differentiation, Myelomonocytic / Q000276:immunology; D002460:Cell Line / Q000187:drug effects; D002470:Cell Survival / Q000187:drug effects; D004351:Drug Resistance; D006801:Humans; D000922:Immunotoxins / Q000276:immunology / Q000302:isolation & purification; D007700:Kinetics; D007951:Leukemia, Myeloid / Q000276:immunology / Q000378:metabolism*; D051379:Mice; D012276:Ricin / Q000276:immunology; D063268:Sialic Acid Binding Ig-like Lectin 3
D000911:Antibodies, Monoclonal; D015703:Antigens, CD; D015214:Antigens, Differentiation, Myelomonocytic; C571163:CD33 protein, human; C571165:Cd33 protein, mouse; D000922:Immunotoxins; D063268:Sialic Acid Binding Ig-like Lectin 3; D012276:Ricin