Evaluation of the Mechanism of Post-Traumatic Immunosuppression---The Role of TLR4 Trafficking and microRNA

Trauma is the leading cause of death, especially among people aged 15 through 44 years. On the other hand, trauma contributes to the major burden of disease in the world, maybe ahead of ischemic heart disease in 2030. Traumatic injury is a battle between pro-inflammatory and anti-inflammatory responses. As compensatory anti-inflammatory response syndrome predominates, trauma results in suppression of the immune system and increases the susceptibility to sepsis. Sepsis after trauma increases rates of organ failure and in-hospital mortality. Systemic responses to trauma are a complex interplay involving multiple levels and, however, there is currently no trauma cell model established for investigation of post-traumatic immuno -suppression. About 90% of multiply injured patients have long bone fractures and patients dying from sepsis after trauma frequently have orthopedic injuries. Therefore, orthopedic trauma is one of the major parts in the field of trauma research. Evidence has showed that localized interaction with the immune cell at the fracture site might be responsible for the overall systemic reaction and outcome. Monocytes are chosen to set up the model not only because of the central role in innate immunity but also of the ability to orchestrate adaptive immunity. Besides, more than any one of the other Toll-like receptor (TLR) family member, as the recognizer of damage-associated molecular patterns, TLR4 sits at the interface of microbial and sterile (such as trauma) inflammation. TLR4 is the receptor of endotoxin and TLR4 downregulation is central to endotoxin tolerance, indicating immunosuppression. Thus, TLR4 dowregulation on monocytes after bone component exposusre represents post-traumatic immunosuppression. Our preliminary data showed that bone components impaired TLR4 trafficking to the cell membrane by downregulating gp96 expression; meanwhile, upregulated the expression miR-146a, both indicated the fact of immunosuppression induced by bone components. This study will build up a trauma cell model and examine if bone components alter the direction of monocyte differentiation. Importance of specific microRNAs and RNA-binding proteins will be investigated with regard to the effect of bone component exposure on LTR4 expression. Then, through research focusing on gp96 will be performed on this trauma cell model. We consider miR-148/miR-152 the possible keyplayer modulating the expression of TLR4 and gp96, which will be examined step by step and confirmed by animal model though the results of post-fracture splenocyte function and organ injury in this project. We hope our results will provide information for trauma care and innovate the strategy for preventing post-traumatic sepsis.