IL-7 and Thymic Stromal Lymphopoietin (TSLP) Stimulate Proliferation of All Cells and Reverse mTOR Inhibitor-Induced Growth Inhibition, Suggesting a Role for IL-7Rα Signaling in All.

Signal transduction inhibitors have shown promise and proof of principle as effective biologic agents to treat leukemia. We have shown activity of mTOR inhibitors (MTI) in models of both human and murine precursor B acute lymphoblastic leukemia (pre-B ALL). MTI control disease in transgenic and xenograft models of pre-B ALL in part by inducing apoptosis. Furthermore, we have shown that IL-7, a cytokine that is an important regulator of progenitor B cell development and proliferation, acts to reverse the inhibitory effects of MTI on human and murine pre-B ALL cells. These observations have led us to examine the effect of TSLP, another cytokine active in B cell development, on ALL cells and their response to MTI treatment. An effect of TSLP on ALL cells has not been previously described. TSLP is closely related to IL-7 and provides important survival signals to developing B cells. The receptor for TSLP consists of a TSLPR chain and an IL-7Rα chain, while the IL-7 receptor contains an IL-7Rα chain and a γ_c chain. Thus, signaling via IL-7Rα chain, but not the γ_c chain, is shared by TSLP and IL-7 mediated signaling. Here, we show that TSLP, like IL-7, modulated growth of pre-B ALL cell lines in vitro in a dose-dependent manner. Co-treatment with TSLP rescued the growth inhibitory effect induced by MTI, but to a lesser extent as compared to IL-7. At concentrations of 3-30 ng/ml, TSLP almost completely reversed the inhibitory effects of low dose MTI (1 ng/ml) from <50% to >85% of the baseline. In the presence of high does MTI (100 ng/ml), TSLP only partially rescued ALL cell lines from <20% to approximately 50% of the baseline. As seen with IL-7, TSLP also prevented apoptosis in MTI-treated ALL cell lines. We also examined the effect of TSLP on MTI-treated primary lymphoblasts from a patient with pre-B ALL. Viability in response to MTI and TSLP was similar to those found in the ALL cell lines. The MTI rapamycin caused a 3-fold increase rate of cell death as compared to untreated cells, while TSLP rescued these primary lymphoblasts from this effect. As indicated by Annexin-V- and 7-AAD- positive staining on flow cytometric analysis, 80% of primary lymphoblasts treated with 1 ng/ml rapamycin alone were dead, while only 30% were dead when treated with TSLP alone; 25% of cells were dead when co-treated with 1 ng/ml rapamycin and TSLP. In addition, the total number of cells among the groups at the time of analysis revealed a 24-fold increase between rapamycin-treated cells and the other conditions: no treatment: 1.2 X 10⁶ cells; rapamycin-treatment: 5 X 10⁴ cells; TSLP-treatment: 1.9 X 10⁶ cells; and rapamycin + TSLP co-treatment: 1.1 X 10⁶ cells. These data suggest that 1) both Il-7 and TSLP modulate proliferation of ALL cells; 2) the IL-7Rα chain, and not the γ_c chain, is required to transduce the signal responsible for rescuing cells from the inhibitory and apoptotic effects of MTI in ALL cells; and 3) mTOR and IL-7Rα signaling pathway intermediates are therapeutic targets for ALL.