Growth Inhibition in IL-7 Receptor Positive Childhood Burkitt’s Lymphoma Cells is not due to Induction of Apoptosis

Abstract

Interleukin-7 (IL-7) is a growth factor for normal precursor B and mature T-cells. We have previously shown that the IL-7 receptor was expressed to varying extent in all cells from a panel of thirteen Burkitt’s lymphoma (BL) cell lines derived from nine children as detected by reverse transcriptase polymerase chain reaction (RT-PCR). The expression in two of 13 BL cells were highly positive, and also detectable by Northern blotting. We therefore investigated the functional significance of this receptor expression. Proliferation of BL cells in the presence and absence of IL-7 was measured by 3H-thymidine incorporation, and by mitochondrial dehydrogenase activity using the MTS color reagent (Celltiter 96AQ, Promega). Apoptosis was measured on cytospins of BL cells by in-situ incorporation of digoxigenin-labeled dUTP into DNA strand breaks using Terminal deoxynucleotidyl Transferase (TdT). Incorporated Dig-dUTP was subsequently detected using an alkaline phosphatase labeled Dig-specific antibody. Apoptosis was also estimated by forward/sides-catter FACS analysis. All proliferation assays were done in serum-free medium. IL-7 inhibited 3H-thymidine incorporation by > 50% only in the three BL cell lines with the highest IL-7 receptor expression. MTS metabolism was inhibited in the same cell lines, but only to a lesser extent of 10–30%. In-situ TdT assays showed no increase in the percentage of apoptotic cells, and the percentage of cell death was not increased on forward/ sidescatter FACS analysis after IL-7 treatment. In contrast to the situation in Burkitt’s lymphoma, control precursor-B-cell ALL lines showed no inhibition, but unchanged or enhanced proliferation. We conclude that IL-7 can be a growth inhibitory cytokine for a subset of BL cells and that induction of apoptosis is not the mechanism of growth inhibition in these cells.