ABSTRACT
BACKGROUND: Familial Mediterranean fever (FMF) is a prototypical autoinflammatory syndrome associated with phagocytic cell activation. Pyrin mutations are the genetic basis of this disease, and its expression has been shown in monocytes, granulocytes, dendritic cells, and synovial fibroblasts. Pyrin functions as a cytosolic pattern recognition receptor and forms a distinct pyrin inflammasome. The phagocyte-specific protein S100A12 is predominantly expressed in granulocytes and belongs to the group of damage associated molecular patterns (DAMP). S100A12 can be detected at massively elevated levels in the serum of FMF patients, even in clinically inactive disease. Whether this is crucial for FMF pathogenesis is as yet unknown, and we therefore investigated the mechanisms of S100A12 release from granulocytes of FMF patients presenting clinically inactive. RESULTS: We demonstrate that FMF neutrophils from patients in clinical inactive disease possess an intrinsic activity leading to cell death even in exogenously unstimulated neutrophils. Cell death resembles NETosis and is dependent on ROS and pore forming protein gasdermin D (GSDMD), as inhibitors for both are capable of completely block cell death and S100A12 release. When pyrin-activator TcdA (Clostridium difficile toxin A) is used to stimulate, neutrophilic cell death and S100A12 release are significantly enhanced in neutrophils from FMF patients compared to neutrophils from HC. CONCLUSIONS: We are able to demonstrate that activation threshold of neutrophils from inactive FMF patients is decreased, most likely by pre-activated pyrin. FMF neutrophils present with intrinsically higher ROS production, when cultured ex vivo. This higher baseline ROS activity leads to increased GSDMD cleavage and subsequent release of, e.g., S100A12, and to increased cell death with features of NETosis and pyroptosis. We show for the first time that cell death pathways in neutrophils of inactive FMF patients are easily triggered and lead to ROS- and GSDMD-dependent activation mechanisms and possibly pathology. This could be therapeutically addressed by blocking ROS or GSDMD cleavage to decrease inflammatory outbreaks when becoming highly active.
ABSTRACT
Translational research aims at closely linking basic research and clinical observations so that important mechanistic insights identified in one field should trigger progress in the other. Particularly in the field of pediatric rheumatology this approach has significantly improved the understanding and therapy of several diseases in recent years. One focus of our research in this respect is on the structure, release mechanisms and function of damage associated molecular patterns (DAMP), particularly S100 proteins. Due to their huge potential as inflammation biomarkers for more specific diagnostics these proteins are of particular clinical interest. Overactivated cells of the innate immune system play a crucial role in the development of rheumatic diseases. Innate mechanisms, such as the generation of neutrophil extracellular traps (NETosis) were linked to the pathogenesis of inflammatory diseases, such as systemic lupus erythematosus and rheumatoid arthritis. Furthermore, it became increasingly more evident that various excessive sterile inflammatory mechanisms and reactions significantly contribute to an activation of adaptive immune responses and thus to the development of autoimmunity. Studying such potentially DAMP-dependent pathways at the interface between innate and adaptive immunity can provide a better understanding of autoinflammatory conditions in pediatric rheumatology and to identify novel targets for optimization of therapy.
