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 (CD 154) 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 Transwell 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.