Innate immune receptors in solid organ transplantation

The discovery of Pattern Recognition Receptors (PRRs) followed by that of their role in the early detection of pathogens and the ignition of the innate immune response has been a formidable progress for immunological research in the past 15 years. This has massively fueled investigations aiming at developing better strategies to fight off infectious diseases and/or to prevent their occurrence. However, infected individuals are for most part outliers in a given population and therefore, the primary function of these receptors should be considered in pathogen-free conditions. Our current understanding indicates that an important physiological function of PRRs resides in their capacity to maintain epithelial homeostasis in response to colonizing commensals. In addition, endogenous host-derived ligands, expressed under stressed, albeit sterile, conditions (called DAMPs for Danger-Associated Molecular Patterns) are also able to trigger PRR signaling. Solid organ transplantation represents a unique situation where both contributions of PRRs signaling can be studied. Indeed, dysbiosis (either caused by antibiotherapy preceding organ transplantation or simply due to the microbiota differences between the transplanted organ and the recipient host) is a characteristic feature of this situation, which is also marked by a massive synthesis and liberation of DAMPs as a result of hypoxia/reperfusion injury. Therefore, in the transplanted organ, at least two compartments (epithelial and that composed of immune cells) participate in graft rejection/acceptance depending on the activation status of expressed PRRs.