Comparison of inhibitory effect of galactose analogs on the binding and cytotoxicity of an anti‐globotriaosylceramide monoclonal antibody coupled or not coupled to pokeweed antiviral protein

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Abstract
The results described here provided an example of a human IgM monoclonal antibody against a tumor‐associated glycolipid and of the unusual properties of its corresponding immunotoxin (IT). The monoclonal antibody referred to as 38‐13 has been previously described and reacted with the globotriaosylceramide [Gb3: Gal(α1→4)‐Gal(β1→4)‐Glc(β1→1)ceramide] specifically expressed on surface membrane of Burkitt's lymphoma (BL) cells. An immunotoxin (38‐13 IT) combined with the pokeweed antiviral protein (PAP) toxin via S‐S bridges showed paradoxically a lower cytotoxic effect in BL Ramos cells than in non‐BL cells such as leukemic mouse L1210 cells, while these cells appeared not to be involved by flow cytometric analysis and complement‐dependent cytotoxicity. Consequently, the inhibitory effect of selective galactose analogs on the binding and the cytotoxicity of 38‐13 antibody conjugated or not to PAP toxin was compared on BL and non‐BL cells. Only the galactose blocked in α configuration provided a fine inhibition of 38‐13 binding on BL Ramos cells and both α and β‐galactose allowed us to establish a clear distinction between the pathway entry of 38‐13 IT in BL and non‐BL cells; (a) in close correlation with the 38‐13 binding specificity the 38‐13 IT cytotoxic effect in Ramos BL cells could also be prevented by α‐Gal only, suggesting that this toxic action is probably mediated through the IT binding to Gb3 antigenic sites. (b) In contrast, on apparently irrelevant L1210 cells, 38‐13 IT showed a cytotoxic effect which was inhibited preferentially by lactose (Gal in β configuration). It was discussed that IT binding alone to either antigenic sites which are inhibited by the hapten α‐Gal, or nonspecific sites which can compete with the hapten β‐Gal is unable to induce efficient killing of cells. But cooperation of both bindings might give an attractive explanation of IT cytotoxic effect. It was concluded that the unexpected activity of 38‐13 IT in non‐BL cells probably could be mediated through an active macromolecular transport process which could implicate a β‐galactoside‐binding protein (lectin).