IL-15 and IL-15 receptor expression by pre-B ALL cells and correlation with CNS infiltration. (A) RT-PCR analysis of IL-15 isoform and IL-15 receptor expression by cell lines and primary patient samples. Expected sizes in brackets; LSP-IL-15 (201 bp), SSP-IL-15 (320 bp), IL-15Rα (402 bp), IL-15/2Rβ (403 bp), IL-15/2Rγ (447 bp), GAPDH (housekeeping control) (115 bp). Representative examples are shown; individual results are listed in supplemental Table 1. (B) SYBR green relative quantification of IL-15 mRNA expression in primary patient samples using SD-1 as the calibrator (expression in patient samples reported as fold change relative to the level of expression in SD-1 cells, arbitrarily set at 1). (C) Histologic analysis of murine brains and spinal cord from xenografts. SD-1, Sup-B15, and REH hematoxylin and eosin–stained sections showing dark purple leukemic cells infiltrating the meninges (thick arrows) (top panel, left to right). All original magnification ×10; black scale bars represent 100 µm. Immunohistochemistry for human CD45 confirms the human origin of these infiltrating cells (bottom panel; isotype control shown in small inset panel). Original magnification ×40; black scale bars represent 100 µm. Section through spinal cord from SD-1 xenograft (bottom right); stars mark the sites of leukemic infiltration in the meningeal covering of the spinal cord. Original magnification ×20; black scale bars represent 100 µm. (D) Time to hind-limb paralysis (HLP) of cell-line xenografts (4-8 mice per cell line). (E-H) TaqMan qPCR analysis of IL-15 and all 3 components of the IL-15 receptor complex in xenografted cell lines. Three independent cultures were analyzed per cell line, and all results are expressed relative to the level in Sup-B15 cells, arbitrarily set at 1.0. All data are mean ± standard error of the mean and were analyzed by Student t tests comparing SD-1 cells to each of the other cell lines. ***P < .001, **P < .01, *P < .05, n.s = not significant. RQ, relative quantification.