Differential effects of TLR3 and TLR4 activation on MSC-mediated immune regulation

Mesenchymal stromal cells (MSCs) have evolved as an invaluable therapeutic cell type due to their broad therapeutic properties. Bone marrow-derived MSCs are currently being applied in numerous clinical trials, and the initial results have been encouraging. However, heterogeneous responsiveness amongst patients is also being experienced; therefore, the efficacy of MSCs in vivo is still debatable. Host microenvironment plays an essential role in determining the fate of MSCs in vivo. Recent studies have indicated the role of toll-like receptors (TLR) in modulating the biological properties of MSCs. TLRs are expressed by MSCs, and activation of TLR3 and TLR4 can alter the functionality of MSCs. While MSCs can suppress the effector and memory T cell function by promoting regulatory T cells, the effect of TLR activation on MSC-mediated immune cell induction is still not well understood. This study was performed to understand the TLR licensing of MSCs and its impact on MSC-mediated immunomodulation. We found that TLR3 mediated activation of MSCs (TLR3-MSCs) increased the expression of G-CSF & IL-10 while TLR4-mediated activation of MSCs led to an increase in CXCL-1, CXCL-10, and CXCL-12. To study the immunological aspect, an in vitro co-culture model was established-to imitate the brief in vivo interaction of MSCs and immune cells. We found that TLR3-MSCs led to increase in CD4 and CD8 naive T (T_NAI) cells and vice versa for effector (T_EFF) and memory T (T_MEM) cells, while TLR4-MSCs did not show any effect.

Moreover, only TLR3-MSCs led to a non-significant increase in the regulatory T cells (T_REGS) and Double negative regulatory cells. No change in B cell profile was evident while TLR3-MSCs depicted an increasing trend in regulatory B cells which was not statistically significant. TLR3 MSCs also inhibited the T cell proliferation in our setup. Our data indicate that TLR3 priming may regulate the function of MSCs through immunomodulation.

Understanding the role of TLRs and other microenvironmental factors causing subdued responses of MSCs in vivo would allow the uninhibited use of MSCs for many diseased conditions.