Involvement of graft-derived interleukin-15 in islet allograft rejection in mice.

The possibility that islets play a role in graft rejection during islet transplantation for type-1 diabetes patients holds promise for ex vivo islet manipulation and for specific anti-rejection therapy. Interleukin (IL)-15 is a T cell growth factor and chemoattractant that is expressed by non-T cells. Intragraft expression of IL-15 is elevated during acute rejection in patients and in mice, and systemic blockade of IL-15 in mice prolongs allograft survival. However, the source of IL-15 in these conditions is undetermined. Since epithelial cell-derived IL-15 promotes lymphocyte proliferation in culture, we sought to determine whether islet-derived IL-15 promotes rejection in mice. We designed antisense oligodeoxyribonucleotide molecules that target mouse IL-15. Uptake of FITC-labeled antisense molecules and efficacy of IL-15 inhibition in IFNgamma-stimulated islets were evaluated. Islets exhibited typical cytoplasmatic distribution of antisense molecules and produced IL-15 levels that were comparable to non-stimulated cells. Antisense-treated islet allografts, that were transplanted across multiple minor-histocompatibility-antigen mismatched strains of mice, were accepted at a higher rate than control-antisense treated islets or untreated islets (88.9% vs. 37.5% and 20%, respectively). Our results suggest that islet-derived IL-15 may be involved in acute islet allograft rejection.

CD40 ligation enhances IL-15 production by tubular epithelial cells.

Interleukin-15 (IL-15) is a potent T-cell growth factor and activator. Acute rejection of kidney allografts strongly correlated with elevated IL-15 mRNA in the graft. A role in the rejection process is also suggested for the interaction between CD40 ligand (CD154) expressed on membranes of activated T cells and its receptor (CD40). The effect of CD40 ligation on IL-15 production in human tubular epithelial cells (TEC) was investigated. TEC were co-cultured with a cell line genetically engineered to express CD154. CD154-expressing cells (CD40L cells) bind to TEC. Addition of the CD40L cells to the TEC culture resulted in elevated IL-15 levels. This enhanced production was not observed with control cells, when anti-CD154 antibody was added, and when direct contact between CD40L-cells and TEC was prevented with the use of a Trans-well system. CD40 activation resulted in a twofold increase of IL-15 mRNA transcripts as measured by reverse transcription-PCR and a concordant elevation in IL-15 protein production as measured by specific enzyme-linked immunosorbent assay. The intensity of activation by CD154 was linearly dependent on cell number, reaching plateau when the effector/target-ratio was 1:1. The increase of IL-15 levels was similar to that produced by stimulation with interferon-gamma (IFN-gamma). Combination of IFN-gamma and activation with CD154 resulted in an additive effect. To conclude, activated T cells may enhance IL-15 expression in two ways: by secreting IFN-gamma and by cell to cell contact using CD154. Each signal alone induces IL-15 in similar magnitudes, and both signals are additive. Because IL-15 is a major T-cell activator, this interaction may contribute to graft rejection.