Members of the toll-like receptor (TLR) family bind to pathogen-associated molecular patterns and subsequently induce the activation of the innate and adaptive immune responses. We investigated here the expression and role of TLRs in mesenchymal stromal cells (MSC), which are bone marrow-derived mesenchymal progenitors with recently described antigen presenting cell (APC) properties [Stagg et al, Blood (March 2006), Romieu et al., JI (in press)]. We observed that human MSCs express basal levels of TLR3 and TLR4, and these responded to their respective ligands, e.g. polycytidylic acid (poly I:C) or lipopolysaccharide (LPS), by the production of IL-6, IL-8 and IL-1b but not of active IL-12 p70 dimer. Exposure of MSCs to TNF-a increased the expression of TLR2 and TLR4 while IFN-a strongly upregulated TLR3 expression. By contrast, IFN-g had only a marginal effect on the expression of any TLR. IFN-a and poly I:C, as well as IFN-g and poly I:C or LPS were observed to work in synergy for the up-regulation of expression of several cytokines or inflammatory mediators, such as IL-12 p35 (IL-12A), TNF-a, RANTES, TRAIL, IFN-b and NOS2A in MSC. In contrast, none of these treatments induced the upregulation of IL-12 p40 (IL-12B), which may explain the lower levels of IL-12 p70 produced in MSCs compared to primary macrophages. Since c-Rel/p50 NF-kB dimers as well as IFN-induced IRF-1 were shown to control IL-12B and/or IL12A promoters, respectively, we next investigated expression of these transcription factors in MSCs. Results showed that MSC expressed the ubiquitous RelA NF-kB subunit but displayed significantly lower expression of c-Rel compared to macrophages. IRF-1 was upregulated following treatments with IFN-g or, to a lesser extent, with IFN-a. Therefore, the lower levels of IL-12 p70 seen in MSCs compared to macrophages may be explained by the absence of c-Rel in MSCs, which hinders the upregulation of IL-12A and IL-12B after exposure to single TLR ligands. However, the induction of IRF-1 by IFN-g or IFN-a may explain the synergistic effect observed in the upregulation of IL-12A in MSC treated with IFN-g or IFN-a and TLR ligands. In conclusion, TLR expression in human MSCs plays a role in their immune activation, although the outcome of this activation is different from the result of macrophage activation by TLR.

Author notes

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