The increased susceptibility of neonates to infection may be due to defects in CB cellular immunity and cytokine production (Cairo, Blood 82:2269, 1993; Cairo, et al, Pediatr Res 30:362, 1991; and 31:574, 1992). These alterations may additionally account for the low incidence of GvHD post related and unrelated CBT (Wagner, et al, Blood 82:86a, 1993, Sweetman, et al, Blood 86: 388a, 1995). IL-15 is a novel cytokine that shares many biological properties with IL-12 in regulating many of the biologic functions of NK and T cells. In this study, we investigated the IL-15 gene expression in C and A MNC in order to examine the role of IL-15 in neonatal immunity. MNC were isolated from C and A PB with Ficoll-hypaque and stimulated with LPS (10 μg/ml, 6-48 hrs). Northern blots containing poly A+ RNA were hybridized with an antisense probe made by transcription of a human cDNA (kindly provided by Immunex) to detect IL-15 mRNA expression. Unstimulated C and A MNC did not constitutively express significant levels of IL-15 mRNA. Upon stimulation of cells with LPS (10μg/ml, 12 hrs), IL-15 mRNA expression was significantly induced in both C and A MNC. However, IL-15 mRNA expression in C MNC was only 25 ± 2.0% of that in A MNC. We next determined the relative rates of transcription of the IL-15 gene by nuclear run-on assays. The nuclei from unstimulated and LPS-stimulated cells were isolated and incubated with 32P-UTP. Newly elongated 32P-labeled transcripts were hybridized to slot blots of target DNA (0.5 kb IL-15 cDNA). The basal and stimulated levels of transcription were the same in C and A MNC. These preliminary data suggest that C MNC express less IL-15 mRNA compared to A in response to stimulation and this discrepancy is not secondary to the transcriptional regulation. Reduced expression of IL-15 mRNA from activated CB MNC may contribute to the immaturity of CB cellular immunity and to a decreased incidence of GvHD following both related and unrelated CB stem cell transplantation.