title: Blocked and non-blocked ricin immunotoxins against the CD4 antigen exhibit higher cytotoxic potency than a ricin A chain immunotoxin potentiated with ricin B chain or with a ricin B chain immunotoxin.
abstract: An immunotoxin consisting of ricin A chain linked to the monoclonal antibody M-T151, recognising the CD4 antigen, was weakly toxic to the human T-lymphoblastoid cell line CEM in tissue culture. The incorporation of [3H]leucine by CEM cells was inhibited by 50% at an M-T151--ricin-A-chain concentration (IC50) of 4.6 nM compared with an IC50 of 1.0 pM for ricin. In contrast, immunotoxins made by linking intact ricin to M-T151 in such a way that the galactose-binding sites of the B chain subunit were either blocked sterically by the antibody component or were left unblocked, were both powerfully cytotoxic with IC50 values of 20-30 pM. The addition of ricin B chain to CEM cells treated with M-T151--ricin-A-chain enhanced cytotoxicity by only eight-fold indicating that isolated B chain potentiated the action of the A chain less effectively than it did as an integral component of an intact ricin immunotoxin. Ricin B chain linked to goat anti-(mouse immunoglobulin) also potentiated weakly. Lactose completely inhibited the ability of isolated ricin B chain to potentiate the cytotoxicity of M-T151--ricin-A-chain and partially (3- to 4-fold) inhibited the cytotoxicity of the blocked and non-blocked ricin immunotoxins. Thus, in this system, the galactose-binding sites of the B chain contributed to cell killing regardless of whether isolated B chain was associated with the A chain immunotoxin or was present in blocked or non-blocked form as part of an intact ricin immunotoxin. The findings suggest that the blocked ricin immunotoxin may become unblocked after binding to the target antigen to re-expose the cryptic galactose-binding sites. However, the unblocking cannot be complete because the maximal inhibition of [3H]leucine incorporation by the blocked immunotoxin was only 80% compared with greater than 99% inhibition by the non-blocked immunotoxin.
authors.affiliation: Drug Targeting Laboratory, Institute of Cancer Research, Sutton, England.
authors.forename: E J
authors.initials: EJ
authors.lastname: Wawrzynczak
authors.forename: G J
authors.initials: GJ
authors.lastname: Watson
authors.forename: A J
authors.initials: AJ
authors.lastname: Cumber
authors.forename: R V
authors.initials: RV
authors.lastname: Henry
authors.forename: G D
authors.initials: GD
authors.lastname: Parnell
authors.forename: E P
authors.initials: EP
authors.lastname: Rieber
authors.forename: P E
authors.initials: PE
authors.lastname: Thorpe
mesh_terms: D000818:Animals; D000911:Antibodies, Monoclonal / Q000276:immunology; D001402:B-Lymphocytes / Q000473:pathology; D001665:Binding Sites; D001667:Binding, Competitive; D015704:CD4 Antigens / Q000276:immunology*; D002470:Cell Survival / Q000187:drug effects; D002478:Cells, Cultured; D005690:Galactose / Q000378:metabolism; D006041:Goats; D006801:Humans; D000922:Immunotoxins / Q000276:immunology / Q000378:metabolism / Q000494:pharmacology*; D007785:Lactose / Q000494:pharmacology; D051379:Mice; D009369:Neoplasms / Q000276:immunology / Q000378:metabolism / Q000473:pathology; D010455:Peptides / Q000378:metabolism / Q000494:pharmacology / Q000633:toxicity; D012276:Ricin / Q000378:metabolism / Q000494:pharmacology* / Q000633:toxicity; D013601:T-Lymphocytes / Q000166:cytology / Q000187:drug effects / Q000276:immunology / Q000378:metabolism; D014407:Tumor Cells, Cultured
chemical_list: D000911:Antibodies, Monoclonal; D015704:CD4 Antigens; D000922:Immunotoxins; D010455:Peptides; D012276:Ricin; D007785:Lactose; D005690:Galactose