Contribution of toll-like receptors to mesenchymal stem cell differentiation and immunomodulation

Abstract

characteristics along with several stem cell features such as lineage dependent differentiation and self-renewal capacity. MSCs are known to induce immunomodulatory activity and homing capacity to damaged tissue sites. Such diverse capabilities of MSCs make them distinct from adult stem cells and can be harnessed in several therapeutic applications. Toll-like receptors (TLR) can recognize conserved microbial byproducts and are mainly expressed by innate immune system cells as well as epithelial or endothelial cells. Recent findings suggest that in vitro generated MSCs express some of these pathogen recognition receptors. In our view, to broaden the breath of the therapeutic potential, TLR mediated activation of MSCs and demonstrate its impact on differentiation and immunomodulatory activity is critical. First, bone marrow-derived MSCs were generated and characterized via their surface marker expression by FACS (CD90, CD106 and CD45) at protein level and their message transcripts by RT-PCR (CD11b, CD29, CD34, CD45, CD71, CD73, CD90 and CD166). The most abundant marker was found to be CD90 over several passages. Following determination of TLR expression profile by RT-PCR, contribution of TLR ligands addition (TLR2, TLR3, TLR7 and TLR9) to MSCs during adipogenic or osteogenic differentiation was studied. TLR3 was found to be the most abundant type over several passages. The adipogenic differentiation of rMSCs was found to be facilitated in the presence of TLR2 TLR3 and TLR7 ligands. Additionally, changes in the adipogenic and osteogenic markers (LPL, PPAR-g for adipogenesis, and ALP, OC-1, RUNX for osteogenesis) were analyzed by RT-PCR. While adipogenic markers upregulated osteogenic markers were downregulated in response to TLR ligand treatment. The final part of this study was performed with mouse mesenchymal stem cells. In order to define the immunostimulatory/immunosuppressive potential of mouse MSCs, immunomodulatory character of MSCs were examined in the presence or absence of mouse spleen cells. Our data suggested that when mMSCs are primed with TLRL, a pro-inflammatory cascade as evidenced by increased IL-6 and IFN-γ secretion is initiated either alone or in co-culture with splenocytes. In conclusion, TLR priming of MSCs augments their differentiation primarily into adipogenesis, and mainly these cells are immunostimulatory.