A functional role for soluble HLA-G antigens in immune modulation mediated by mesenchymal stromal cells from different sources. Evidence of discordant reactivity for various immunological markers

Biologic and clinical interest in mesenchymal stromal cells (MSC) has risen over the last years, mainly due to their immunosuppressive properties. Soluble factors produced by bone marrow (BM) MSCs have been suggested to play a fundamental role in mediating immune modulation. HLA-G antigens are MHC class Ib molecules characterized by a limited polymorphism and a splicing mechanism that regulates the production of membrane bound and soluble isoforms. IL-10 cytokine is one of the main up-modulators of soluble HLA-G antigen (sHLA-G) and increased IL-10 levels were reported as associated to MSC immune modulation. In our study we investigated , the possible role of sHLA-G molecules in the inhibition of the peripheral blood mononuclear cells (PBMCs) response to phytohemagglutinin (PHA) mediated by MSCs from different sources. We also investigated the basis of hMSC immunomodulant variability either in relation to the MSCs subset composition or to the different culture conditions, as well as culture passages. The results showed a significant correlation between the presence of increased levels of sHLA-G in the MSC/PBMC/PHA culture supernatants and lymphoproliferative inhibition. Neutralizing experiments performed with monoclonal antibodies directed against HLA-G and IL-10 molecules confirmed the inhibitory ability of sHLA-G antigens. Furthermore, exogenous IL-10 induced sHLA-G molecule secretion by MSCs alone in a polymorphic way, while a longitudinal analysis confirmed the loss of MSC inhibitory functions in relation to in vitro MSC aging. Our results further showed that hMSC from amniotic membrane, chorion and BM from patients with hematological malignancies (HM), showing a significant decrease in CD90 (Thy-1) surface molecule expression, elicited a lymphoproliferative allogeneic response in phytohemagglutinin (PHA)/PBMC cultures without any increase in sHLA-G and IL-10 levels. Overall the results obtained propose a functional role for sHLA-G molecules in inhibiting the PBMC response mediated by MSCs and strengthen the relationship between MSC activation, sHLA-G production and CD90 expression, suggesting the CD90 molecule as a novel predictive marker for hMSC inhibitory ability. These data may be of value in optimising MSC usage in transplantation and regenerative medicine settings.