Toll-Like Receptors 2 and 4 Mediate the Capacity of Human Bone Marrow-Derived Mesenchymal Stem Cells to Support the Proliferation and Differentiation of CD34⁺ Cells In a Non-Synergistic Way.

Abstract 3850

Human bone marrow-derived mesenchymal stem cells (BM-MSCs) are multipotent nonhematopoietic progenitor cells, which can differentiate into osteoblasts, adipocytes, chondrocytes and other tissues. The most important function of BM-MSCs is to support hematopoiesis. Toll-like receptors (TLRs) are a conserved family of receptors that can be activated by both pathogen components and mammalian endogenous molecules such as heat-shock proteins and extracellular matrix breakdown products. In the past a few year, several studies reported that TLRs are expressed in hematopoietic and non-hematopoietic to modulate their biological functions. We hypothesized that MSCs are equipped with TLRs that enable them to dynamically change hematopoiesis-related cytokines expression pattern and level by sensing correspondent agonists, thus efficiently supporting hematopoiesis.

In this study, BM-MSCs were analyzed for mRNA expression of TLR 1–9 by reverse transcription-polymerase chain reaction. TLR 1–6, but not TLR 7–9 were expressed by MSCs. The expression of TLR2 and TLR4 was also confirmed by flow cytometic assay. We further explored the role of TLR2 and TLR4 in mediating the capacity of MSC to support the proliferation and differentiation of CD34⁺ cells. The pre-stimulation with TLR2 agonists (Pam3Cys) or TLR4 agonists (LPS) enable MSCs to enhance CD34+ cells proliferation and promote CD34+ cells differentiation towards the myeloid lineage (CD33⁺, CD11b⁺), as well as granulocyte colony formation by those cells. The production of interleukin 8 (IL-8), IL-11, stem cell factor (SCF), granulocyte colony-stimulating factor (CSF), macrophage CSF and granulocyte-macrophage CSF were also increased by stimulated MSCs. Interestingly, although Pam3Cys and LPS displayed different inductive magnitudes, they have no synergistic effect on MSCs. We hypothesized there may be some antagonistic effect between TLR2 and TLR4 intracellular signal conductive pathway, or they can downregulate the expressive level of the TLRs on MSCs.

Together, our findings suggest that TLR2 and TLR4 signalings may indirectly regulate hematopoiesis by modulating MSCs' functions. The increased haemopoietic proliferation and myeloid lineage differentiation could be mediated, at least in part, by augmented hematopoiesis-related cytokine production.