Toll-Like Receptors in Stem/Progenitor Cells

One of the bridges that control the cross-talk between the innate and adaptive immune systems is toll-like receptors (TLRs). TLRs interact with molecules shared and maintained by the source pathogens, but also with endogenous molecules derived from injured tissues (damage/danger-associated molecular patterns - DAMPs). This is likely why some kinds of stem/progenitor cells (SCs) have been found to express TLRs. The role of TLRs in regulating basal motility, proliferation, processes of differentiation, self-renewal, and immunomodulation has been demonstrated in these cells. In this book chapter, we will discuss the many different functions assumed by the TLRs in SCs, pointing out that, depending on the context and the type of ligands they perceive, they may have different effects. In addition, the role of TLR in SC's response to specific tissue damage and in reparative processes will be addressed, as well as how the discovery of molecules mediating TLR signaling's differential function may be decisive for the development of new therapeutic strategies. Given the available studies on TLRs in SCs, the significance of TLRs in sensing an injury to stem/progenitor cells and evaluating their action and reparative activity, which depends on the circumstances, will be discussed here. It could also be possible that SCs used in therapy could theoretically be exposed to TLR ligands, which could modulate their in vivo therapeutic potential. In this context, we need to better understand the mechanisms of action of TLRs on SCs and learn how to regulate these receptors and their downstream pathways in a precise way in order to modulate SC proliferation, survival, migration, and differentiation in the pathological environment. In this way, cell therapy may be strengthened and made safer in the future.