The use of human mesenchymal stem cells (hMSCs) in cell-based therapies has dramatically increased in recent years, as researchers have exploited the ability of these cells to migrate to sites of tissue injury, inflammation and tumors. Once at these sites, hMSCs facilitate the repair of damaged tissue through immunomodulation and angiogenesis or promote tumor progression via the same mechanisms. Our group established that hMSCs respond to “danger” signals—by-products of damaged, infected or inflamed tissues—via activation of Toll-like receptors (TLRs). However, little is known regarding downstream signaling mediated by TLRs in hMSC. The focus of this dissertation was to determine the downstream signaling pathways activated by TLR3; specifically the role suppressor of cytokine signaling (SOCS) play in hMSC function. We hypothesized that, when stimulated, TLR3 indirectly triggers a JAK/STAT signaling cascade resulting in the activation of SOCS1 and SOCS3, which downregulate the expression of both CXCR4 and CXCR7 and inhibit the migration of hMSCs.

We show here that TLR3-activated hMSCs secreted increased amounts of interferon β (IFNβ), which stimulated the JAK2/STAT1 pathway in an autocrine manner. At the same time, IFNβ increased expression of SOCS1 and SOCS3 suggesting each play a role in negatively regulating TLR3 and JAK/STAT signaling. In support of this notion, TLR3-mediated signaling was disrupted by SOCS overexpression in hMSCs. An overexpression of either SOCS disrupted TLR3-mediated NF-κB p65 signaling, while SOCS3 overexpression inhibited interferon regulatory factor 1 (IRF1) expression, and SOCS1 overexpression reduced STAT1 activation. Furthermore, our study was the first to demonstrate that when TLR3 was activated, expression of CXCR4 and CXCR7 were downregulated due to receptor internalization and exocytosis. This internalization was independent of CXCR4/CXCR7 ligand binding and receptor phosphorylation. SOCS3 overexpression increased CXCR4 exocytosis and inhibited internalization of both CXCR4 and CXCR7 following TLR3 stimulation. By contrast, SOCS1 overexpression had no effect on receptor exocytosis and inhibited CXCR7 internalization, but not CXCR4. These results collectively demonstrate that SOCS1 and SOCS3 play a distinct role in modulating TLR3, JAK/STAT, and CXCR4/CXCR7 signaling in hMSCs and shed further light on the way hMSCs respond to danger signals.