Inhibition of T Lymphocyte (TL) Proliferation by Mesenchymal Stem Cells (MSCs): Role of Cell Contact between MSCs and TL, Role of MSCs Dose Effect, and Role of Soluble Factors and Adhesion Molecules Expressed by MSCs.

Mesenchymal stem cells (MSCs) can inhibit T lymphocyte (TL) proliferation in mixed lymphocyte reaction (MLR) and are candidate to induce tolerance of allogeneic haematopoietic stem cell graft. Results of previous studies evaluating the role of cell contact between MSCs and TL are contradictory. Some soluble factors produced by MSCs have been shown to be involved in TL inhibition such as Transforming Growth Factor-b (TGF-b), Hepatocyte Growth Factor (HGF), and Indoleamine 2,3 dioxygenase (IDO).

In this study inhibition of TL proliferation in MLR was evaluated in cultures with MSCs-TL contact and in cultures without MSCs-TL contact (transwell), at different concentration of MSCs (ratio MSCs/TL: 0.3, 0.1, 0.03, 0.01). Expression of mRNA encoding cytokines and adhesion molecules produced by MSCs were analysed by semi-quantitative RT-PCR. Inhibitory cytokines studied were IDO, HLA-G, LIF, IL-10, TGF-b and HGF; adhesion molecules studied were VCAM and LFA3.

1. Results of MSCs dose effect. In MLR with MSCs-TL contact, inhibition of proliferative index was related to the dose of MSCs. The percentage of inhibition was 74%, 60%, 48%, and 28% at ratio 0.3, 0.1, 0.03 and 0.01 respectively (p<0.05). In MLR without MSCs-TL contact, no dose effect was observed: the percentage of inhibition was 48%, 46% and 46% at ratio 0.3, 0.1 and 0.03 respectively; no inhibition was observed at ratio 0.01.

2. Results of comparison between inhibition with cell contact and without cell contact. At ratio 0.3 the percentage of inhibition was 74% with cell contact and 48% without cell contact (p<0.05), at ration 0.1 the percentage of inhibition was 60% and 46% respectively (p<0.05). These results confirmed that TL inhibition is mediated by soluble factors but is increased when MSCs and TL are in contact.

3. Results of mRNA RT-PCR in MSCs showed an overexpression of IDO, HLA-G and LIF in cultures with cell contact and without cell contact. In cultures with cell contact an overexpression of IL10 and TGF-b was observed, but not in cultures without cell contact. Adhesion molecules VCAM and LFA3 were overexpressed in both types of cultures with and without cell contact. In conclusion although cell contact is not mandatory to inhibit T cell proliferation, inhibition is higher when TL and MSCs are in contact, concomitantly an overexpression of additional inhibitory molecules is observed.