Mechanisms of inhibition of mononuclear cell activation by the iron-chelating agent desferrioxamine.

Iron-withholding by the chelating agent desferrioxamine abrogates the proliferative response of human peripheral blood mononuclear cells (PBMC) to phytohaemagglutinin (PHA). The present study investigated whether desferrioxamine operates late in the activation process or, as recently suggested, at an early stage, by inhibiting the appearance of the interleukin-2 (IL-2) receptor. Human PBMC were stimulated with PHA (10 micrograms/ml) and [3H]thymidine ([3H]TdR) incorporation determined after 66 hr of culture. Greater than 90% inhibition was achieved by concentrations of desferrioxamine as low as 5 mumol/l present throughout culture, while IL-2 receptor expression (anti-Tac), analysed by FACS, was maintained at up to 75% of control levels. 300 mumol/l desferrioxamine present throughout culture abrogated [3H]TdR incorporation and additionally suppressed IL-2 receptor to 10-15% of control levels. In contrast, the same high dose of desferrioxamine when added for 2 hr to cells previously cultured for 66 hr produced 80% inhibition of [3H]TdR incorporation but failed to inhibit expression of the IL-2 receptor. Desferrioxamine rapidly achieved equilibrium across the cell membrane (within 60 min) and chelated 59Fe delivered to activated cells by the transferrin endocytic cycle. These results indicate that desferrioxamine can inhibit T-cell activation either early or late in the process by chelating iron and independently of an effect on the IL-2 receptor. In support of a dual effect of the drug is the finding that at 50 mumol/l, desferrioxamine-enhanced expression of the transferrin receptor occurred, an adaptive response made to intracellular iron depletion, while IL-2 receptor expression was inhibited.