Human mesenchymal stem cells (MSCs) can differentiate into many types of somatic cells, especially those residing in connective tissues. These stem cells can be found in different parts of our bodies. Increased interest in their clinical use has been additionally promoted by the fact, that they can modulate human immune responses, particularly by inhibiting T cell activation. This means that by applying MSCs we could prevent or treat acute graft versus host disease (aGvHD) that can arise following allogeneic hematopoietic stem cells (HSCs) transplantation. In this study we set up a goal to establish and optimize a set of in vitro non-radioactive functional cell proliferation tests for evaluation of immunosuppressive activity of MSCs. First, we tested various parameters related to in vitro cultivation and stimulation of peripheral blood mononuclear cells (PBMCs) to determine optimal conditions for non-radioactive assessment of T-cell proliferation. We found that the use of 96 well (U-shaped) tissue culture plates, BioTargetTM-1 serum-free medium, as well as 72-hour incubation of PBMCs following their activation with phytohemagglutinin (PHA), and 6 days long incubation in case of their allogeneic stimulation induced in one- and two-way mixed lymphocyte cultures (MLCs), are optimal for performing our tests. The extents of T-cell proliferation were assessed by measuring absorbances after the addition of MTS or CCK-8 reagents to cell cultures, followed by additional incubation. According to results we decided to use the MTS reagent and to select mitogenic (PHA) stimulation of PBMC, as well as the two-way MLC, set-up by mixing PBMCs of two unrelated donors, as the two most appropriate test models. Further on we applied both of them to evaluate the extent of T-cell proliferation inhibition caused by the presence of different amounts of allogeneic bone-marrow-derived MSCs. In case of PHA-induced proliferation of T lymphocytes the immunosuppressive action of MMCs was the greatest at the PBMC : MMC ratio of 8 : 1, while in the two-way MLCs such effect was observed at the PBMC : MMC ratio of 4 : 1. This difference could be a consequence of the fact that allogeneic stimulation only activates a limited number of T cell clones, while the mitogenic lectin (PHA) T-cell activation is complete.
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