Increased intragraft IL-15 mRNA expression after liver transplantation

To study T-cell/macrophage interactions at the molecular level in clinical allograft rejection, we measured intragraft mRNA expression of the T-cell derived cytokine IL-2 and the macrophage derived chemokine IL-15, a novel cytokine associated with T-cell activation, in post-transplant liver biopsies (n = 33) and in non-transplanted control liver tissue by reverse transcriptase-polymerase chain reaction (RT-PCR). We analyzed biopsies without evidence of rejection (n = 12), with spontaneously resolving histological rejection (n = 10), or with histological rejection accompanied with clinical rejection (n = 11) defined by rising serum bilirubin and aspartate amino transaminase levels. IL-15 mRNA expression was present in the majority of post-transplant liver biopsies (91%, 30/33) and was significantly upregulated as compared with non-transplanted liver tissue (p = 0.005). However, the increased intragraft IL-15 mRNA level was not indicative for rejection. In contrast to intragraft IL-15 mRNA expression, IL-2 mRNA transcription was measured in the minority of the post-transplant liver biopsies (15%, 5/33) and not detectable in control specimens. In addition, IL-2 mRNA was almost specifically measured in rejection biopsies concurrent with graft dysfunction (36%, 4/11 versus 1/22 without clinical rejection; p = 0.03). No relation between intragraft IL-2 and IL-15 mRNA expression was found. The IL-15 mRNA expression levels were not higher in the IL-2 negative rejections compared with those in IL-2 positive rejections. To conclude, in contrast to IL-2, the function of IL-15 in T-cell mediated rejection remains unclear. The overall high IL-15 mRNA levels in sites of immune responses suggests that the macrophage-derived mediator IL-15 is involved in a constant flow of T-cells from the circulation into the graft.