ESCAPE MECHANISMS OF HUMAN LEUKEMIC-CELLS TO LONG-TERM IMMUNOTOXIN TREATMENT IN AN IN-VITRO EXPERIMENTAL-MODEL

In kinetic assays, an anti-CD5-ricin A chain (ST.1-RTA) immunoconjugat e (immunotoxins, IT) specifically inhibited up to 40% the protein synt hesis of Jurkat target cells within the first 40 hr. Longer exposures of leukemia cells to ST.1-RTA resulted in a progressively higher numbe r of target cells escaping IT treatment and becoming resistant to furt her treatment with ST.1-RTA even in the presence of the RTA-IT enhance r monensin. Resistant Jurkat cells proliferated at the same rate as co ntrol untreated cells, and were as sensitive as control cells to a tra nsferrin-RTA IT, indicating that the ST.1-RTA-resistant tumor-cell pop ulation did not become insensitive to the enzymatic activity of RTA. B inding studies revealed that the anti-CD5 IT treatment induced a trans ient modulation of CD5 antigens but not of the functionally related CD 3 antigens. The CD5 antigens were re-expressed at the cell surface fol lowing removal of the IT molecules from the culture medium with 1.1% o f the total CD5 Ag being re-expressed per hr. When our experimental da ta on the kinetics of cell intoxication by the IT were corrected for t he proliferative potential of the resistant and of the sensitive tumor -cell populations, it appeared that the effect of ST.1-RTA treatment o n Jurkat cells was only to delay cell growth for a limited time period (20 hr) without reducing effectively the tumor-cell burden. Our resul ts may have implications for the long-term treatment of target tumor c ells with IT. (C) 1995 Wiley-Liss, Inc.