Arthritis is a chronic condition affecting more than 1 in 5 adults in the US. Under the umbrella of arthritic conditions, rheumatoid arthritis is a chronic autoimmune inflammatory condition thought to impact approximately 1% of the world’s population. It is characterized by chronic pain and progressive joint damage and can severely hinder patient quality of life.
Often, the targets of rheumatoid arthritis include the hands, wrists and knees, where inflammation of the synovial lining of these joints leads to long-term tissue damage. The causes of rheumatoid arthritis are not fully understood, but in a recent article published in Immunity, Friščić et al. investigate the cellular and molecular mechanisms underlying one aspect of rheumatoid arthritis: recurrent site-specific inflammation.
Also referred to as disease flares, these repeated periods of inflammation at the same location in the body are characteristic of multiple arthritis subtypes, including rheumatoid and gouty arthritis. Based on this pattern, tissues that have been inflamed in the past are more likely to become inflamed again in the future, a phenomenon referred to as “inflammatory tissue priming.”
The mechanisms responsible for this inflammatory tissue priming are not fully understood, but could be attributed to immunological memory, a phenomenon that enables the immune system to more readily fight off recurrent infections. Though previously considered only a feature of adaptive immune cells, in recent years, it has been uncovered that the innate immune system too has a form of memory. “Trained immunity” is the term given to the process by which innate immune cells, after being exposed to a stimulus, can undergo reprogramming so as to be better prepared to defend against repeated exposures to that same stimulus. However, it has been thought that this trained immunity is a systemic response to stimuli, and the known paradigms of trained immunity do not fully explain the site-specific phenomena seen in arthritis.
To investigate the mechanisms underlying this disease process, Friščić et al. focused on synovial fibroblasts, a stromal cell type found in the synovial sublining of joints. Typically, synovial fibroblasts assemble around the joint space in a thin layer that is dotted with tissue-resident macrophages. They play a key role in generating components of the synovial fluid and in mediating immune responses to pathogens. In arthritis, synovial fibroblasts are known to proliferate, thickening this synovial lining and ultimately damaging nearby cartilage. Given their key role in the development of arthritis, the authors hypothesized that these synovial fibroblasts could be contributing to the development and maintenance of tissue priming.
To begin, the authors manipulated rodent models of arthritis to confirm the presence of this site-specific immune response in a controlled experimental environment. They found that repeated injections with an arthritogenic serum lead to a more sustained inflammatory response than a single injection. Moreover, when they repeatedly injected one of a rat’s paws with an inflammatory agent called Zymosan, they observed prolonged arthritis in that paw. However, injecting one paw for the first time after repeated injections into the opposite paw did not lead to the same effect. These findings supported the hypothesis that joint-resident cells could be playing a key role in facilitating site-specific tissue inflammation.
Moving on to in vitro models, the authors developed 3D synovial organ cultures that enabled them to specifically manipulate synovial fibroblasts and observe the effects. Through exposure to Zymosan, they either fully or partially primed rodent synovial fibroblasts prior to introducing them into the culture system. What they found were gradations in an inflammatory stimulus-induced reorganization of the synovial lining dependent on the degree of tissue priming. Unprimed synovial fibroblasts formed a standard one-to-two cell layer, much like the synovial lining in a normal healthy joint. Partially primed synovial fibroblasts formed a thicker lining, comparable to that observed in arthritis, and fully primed synovial fibroblasts did not form a lining at all. Instead they assembled into a “densely interconnected cellular network” within the organ culture matrix.
Aside from the structural changes induced by repeated exposure to inflammation, both partially and fully primed synovial organoids also released the cytokine IL-6, an inflammatory molecule known to be associated with arthritis. In order to better understand the potential biochemical cues being produced by these primed cells, the authors performed bulk RNA-sequencing and ATAC-sequencing to examine both the transcriptional profile and chromatin accessibility landscape of synovial fibroblasts. In primed fibroblasts, they found upregulation of a host of key genes, including more inflammatory molecules, pathogenic fibroblast markers, enzymes implicated in cartilage and bone destruction, and also components of the complement system. The complement system is a defense mechanism consisting of a cascade of activated proteins that can both directly attack pathogens and—as suggested by their name—complement the efforts of other immune cells to neutralize threats to resident tissue. Through further experiments, the authors concluded that the complement system—especially the C3 protein—plays a key role in producing the pathogenic synovial fibroblast phenotype found at sites of recurrent inflammation.
Also upregulated were specific metabolic pathways. It turns out that these primed synovial fibroblasts had higher metabolic activity (consumed more glucose) than normal cells, and this higher metabolic activity was characterized by a shift toward aerobic glycolysis. The authors postulated that these changes in metabolism could be closely linked to the cells’ ability to mediate tissue priming.
Though the majority of their work was done in rodents, the authors also analyzed single cell RNA sequencing data from the synovial tissue of human RA patients and found many of the same molecular patterns, including expression of key proteins in the complement system. These findings suggest that many of the mechanisms elucidated from their study of rodent fibroblasts are generally translatable to human disease.
The results of this study have important implications in terms of potential therapies for arthritis. Many current treatments focus on immunosuppression, but the authors suggest that there may be value in specific interventions aimed at interrupting the sequence of events taking place in these synovial fibroblasts. From disrupting changes in activated synovial fibroblast metabolism (for example, through glycolysis inhibitors), to directly targeting components of the complement system, there may be ways to reverse or prevent the process of tissue priming that leads to recurrent disease flares and long-term tissue damage in arthritis patients. In coming years, it will be critical to build on this understanding of tissue priming in order to design more targeted and effective therapies for arthritis and other chronic inflammatory conditions.