Mesenchymal stem cells (MSC) have recently been used successfully in humans to control severe graft-versus-host disease. However, the mechanisms involved in their immunomodulatory effects remain a matter of debate. Here, we show that MSC are unable to activate allogeneic T cells even in the presence of T-cell growth factors. We then found that MSC inhibit T-cell proliferation triggered either by allogeneic, mitogenic or antigen-specific stimuli. Interestingly, MSC inhibit T-cell proliferation by inducing apoptosis of activated T cells, but have no effect on resting T cells. Furthermore, we show that this apoptosis could be related to the conversion of tryptophan into kynurenine by indoleamine 2,3-dioxygenase expressed by MSC in the presence of IFNc. Moreover, we show that the inhibitory effect of MSC is neither abrogated nor modified during expansion in culture or after irradiation. Together, these results bring new insight to the mechanisms of immunosuppression induced by MSC and might help to develop their clinical use controlling immune-related adverse effects in humans.

VP22, a structural protein from herpes simplex virus type I, exhibits the unique property of intercellular trafficking. This protein is exported from primary expressing cells and subsequently imported into neighbouring cells. This property is conserved when VP22 is genetically fused to a protein, making it a promising tool to enhance the delivery of a gene product. We chose to study the intercellular transport and biological effect of a fusion protein between the putative tumour suppressor gene p27 Kip1 and VP22. We show that in vitro, P27VP22 is able to spread as efficiently as VP22. Functionality of the P27VP22 protein was demonstrated by its ability to inhibit cyclin/CDK2 complexes activity. In proliferation and clonogenicity assays, transfection with the P27VP22 plasmid resulted in a stronger cell growth inhibition when compared to transfection with the p27 Kip1 vector. In vivo, sub cutaneous tumours established in nude mice were injected with naked DNA encoding P27 or P27VP22. Our results show that P27VP22 can spread in vivo and that injections of the P27VP22 plasmid resulted in a significantly greater antitumour activity than injections of the P27 plasmid. This study confirms the usefulness of VP22-mediated delivery and suggests that P27VP22 may have applications in cancer gene therapy.