PRIME Cells: Fibroblast-like Cells in the Blood
To further confirm the potential source of this genetic pattern, Dr. Orange’s group performed fluorescence-activated flow
sorting—a type of flow cytometry—on a group of blood samples. These were collected from an additional 19 patients with rheumatoid arthritis, irrespective of disease activity, as well as 18 healthy controls.
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Explore This IssueDecember 2020
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The team identified the cells based on the same membrane protein markers that Zhang et al. had used to sort synovial fibroblast cells, sorting for cells that did not express CD45 (indicating cells not of hematopoietic origin), did not express CD31 (indicating non-endothelial cells) and did express podoplanin (PDPN; a marker associated with fibroblasts). Cells expressing these specific characteristics of synovial fibroblasts were more common in the blood of patients with rheumatoid arthritis than they were in controls.
The team then performed RNA sequencing on these cells and found that they too were expressing large amounts of genes from the A3 gene cluster, including many extracellular matrix genes, as well as characteristic synovial fibroblast genes and proteins.
Dr. Orange explains, “Because it was so strange to think of a fibroblast in the blood, we didn’t want to call them that. So we called them PRIME cells, standing for preinflammatory mesenchymal, meaning they are not hematopoietic but mesenchymal in origin (because they are CD45 negative).”
Interpretation & Disease Model
Other data from humans demonstrate that some types of synovial fibroblasts play a key role in the pathogenesis of rheumatoid arthritis, furthering inflammation and causing joint degradation.2 Inflammatory sublining fibroblasts have previously been found next to the blood vessels in inflamed synovium from patients with rheumatoid arthritis, where they secrete proinflammatory cytokines.4 PRIME cells also appear to be phenotypically similar to a fibroblast-like cell that exacerbated inflammatory arthritis after passive cell transfer in a mouse model.5
Under the model proposed by Dr. Orange et al., B cells activate these PRIME cells just prior to flares, as indicated by upregulation of group A2 gene clusters and later the A3 gene clusters. “Since PRIME cells overlap with these cells that we see in rheumatoid arthritis synovium, we now hypothesize that they left the blood and trafficked to the synovium,” explains Dr. Orange. This is inferred from the drop in expression of A3 gene clusters in blood samples noted during the flare itself. In the synovium these PRIME cells—or perhaps their descendant cells—may promote joint inflammation.
Dr. Orange adds, “We’re not sure at this point whether they are a precursor to a sublining synovial fibroblast. They did express some mesenchymal stem cell-like genes, so that’s one possibility. Or it could be that the synovial fibroblasts somehow get out of the tissue and go into the blood. It’s not clear if they are upstream or downstream, but they are very closely related.”
The study authors suggest that B cell activation may have triggered these PRIME cells, based on what we know of the timing of gene expression and B cells’ general roles as potential immune activators. But currently the causation is not completely clear. Dr. Orange says, “It could be that there is some upstream activator that activates B cells and also activates PRIME cells, but the PRIME cells just take longer to get going.”
This work, suggesting one pathophysiological pathway for B cell activation, streams into an existing body of research showing that B cells play a key role in driving autoimmune responses, working as antigen-presenting cells and producing cytokines and antibodies.2 “There are lot of reasons to think that B cells play some role in arthritis,” notes Dr. Orange. “Some of the genetic risk factors for rheumatoid arthritis are expressed by B cells; patients have autoantibodies which are made by B cells; and B cell depletion therapy is approved for rheumatoid arthritis.”