In kinetic assays, an anti-CD5-ricin A chain (ST.I-RTA) immunoconjugate (immunotoxins, IT) specifically inhibited up to 40% the protein synthesis of Jurkat target cells within the first 40 hr. Longer exposures of leukemia cells to ST.I-RTA resulted in a progressively higher number of target cells escaping IT treatment and becoming resistant to further treatment with ST.I-RTA even in the presence of the RTA-IT enhancer monensin. Resistant Jurkat cells proliferated at the same rate as control untreated cells, and were as sensitive as control cells to a transferrin-RTA IT, indicating that the ST.I-RTA-resistant tumor-cell population did not become insensitive to the enzymatic activity of RTA. Binding studies revealed that the anti-CD5 IT treatment induced a transient modulation of CD5 antigens but not of the functionally related CD3 antigens. The CD5 antigens were re-expressed at the cell surface following removal of the IT molecules from the culture medium with 1.1% of the total CD5 Ag being re-expressed per hr. When our experimental data on the kinetics of cell intoxication by the IT were corrected for the proliferative potential of the resistant and of the sensitive tumor-cell populations, it appeared that the effect of ST.I-RTA treatment on Jurkat cells was only to delay cell growth for a limited time period (20 hr) without reducing effectively the tumor-cell burden. Our results may have implications for the long-term treatment of target tumor cells with IT.
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