Human mesenchymal stem cells constitutively express the serine protease granzyme B. (P1097)

Abstract

Abstract Human mesenchymal stem cells (MSC) are known for their immunoregulatory potential and are allegedly involved in tolerance induction. Several immune cells have been shown to exhibit their regulatory effects via expression of the cytotoxic serine protease granzyme B including regulatory T cells and plasmacytoid dendritic cells. Since MSC were recently found to express the GrB inhibitor PI-9, which is primarily expressed by cytotoxic cells, we hypothesized that MSC may also express GrB. Using various methods including Western immunoblotting, ELISpot, FACS analysis we found that MSC freshly isolated from healthy donor bone marrow biopsies indeed express considerable amounts of GrB protein. ELISpot assays demonstrated that GrB was not only expressed but also secreted by MSC. Moreover, a colorimetric GrB activity assay confirmed that GrB was secreted in its enzymatically active form. Importantly, when testing the effect of various cytokines we found that G-CSF, IFNg and TPO were capable of further enhancing GrB expression in MSC. Future studies have to elucidate which aspects of the suppressive activity of MSC possibly depend on GrB and which culture conditions may support a suppressive phenotype of these cells including GrB expression. Our results may open novel approaches to the use of MSC as immunoregulatory cellular therapeutics in inflammatory conditions such as graft-versus-host disease.