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The results of research by scientists at Trinity College Dublin and St. Vincent’s University Hospital offer new insights into the cellular and genetic features of joint inflammation in rheumatoid arthritis (RA), which could point to new therapeutic approaches and the identification of predictive biomarkers in support of personalized medicine.
Headed by Ursula Fearon, PhD, professor of molecular rheumatology, School of Medicine, Trinity College Dublin, and Megan Hanlon, PhD, post-doctoral fellow in Molecular Rheumatology, School of Medicine, the team carried out in-depth RNA sequencing and metabolic analysis of macrophages residing in the synovium of RA patients, compared with “individuals-at-risk” (IAR) of RA, and healthy controls (HC).
The study findings demonstrated, for the first time, the presence of a dominant macrophage subtype (CD40-expressing CD206+CD163+) in the inflamed RA synovium, which was associated with disease-activity and treatment response, and which was also found early in disease development, before clinical symptoms were manifest.
The team suggests that identifying an early pathogenic macrophage cell/gene signature in the RA joint inflammatory environment represents a unique opportunity for early diagnosis and therapeutic intervention.
Fearon commented, “This is an important breakthrough in our understanding of what goes wrong at the initial stages of disease in RA, which also has an impact on patient’s progression and relapse. We have identified a dominant macrophage subtype/gene signature associated with driving the pro-inflammatory responses early in disease and therefore reprogramming of macrophages towards resolution of inflammation has the potential to be therapeutically targeted.”
Reporting on their findings in Science Advances “Loss of synovial tissue macrophage homeostasis precedes rheumatoid arthritis clinical onset” the researchers concluded, “We describe the presence of a CD40-high myeloid signature that becomes activated early in RA disease pathogenesis, before clinical signs and symptoms, and correlates with disease activity and response to treatment. The findings, they continued, represent “… a unique opportunity for early diagnosis and therapeutic intervention.”
Rheumatoid arthritis is a progressive autoimmune disease that affects one percent of the population, the authors wrote. The disorder is characterized by synovial inflammation, hyperplasia, and structural damage to cartilage and bone, which impact on mobility and quality of life. RA is in addition associated with substantial comorbidities including atherosclerosis, diabetes, cardiovascular disease, and obesity. “A significant proportion of patients are nonresponders to current therapeutic targets, and it is currently impossible to predict who will develop severe, erosive disease and who will respond to treatment,” the team continued.
This means that a trial-and-error approach to treatment prevails, leading to potential irreversible joint damage before the patient has received the correct treatment. “Therefore, better understanding of the disease at the site of inflammation will allow the development of new treatment strategies or predictive biomarkers,” they noted.
Although synovial tissue macrophages have been identified as the most common immune cell present in the normal synovium, the function of macrophages in the healthy synovial tissue isn’t well understood,” the investigators noted. Macrophages are “pivotal players” in joint destruction, they continued, and recent research has uncovered diverse macrophage phenotypes in heath and disease. “This has further emphasized the need to explore and characterize the phenotype and ontogeny of macrophage populations in RA.”
For their reported study the team used techniques including multiparameter flow cytometry, bulk RNA-seq and scRNA-seq, and noninvasive fluorescent lifetime imaging microscopy (FLIM) metabolic imaging and functional analysis to better identify the spectrum of macrophage activation states within the synovium of patients with RA, as well as individuals at risk of RA, and healthy controls.
Their analyses identified a CD40-expressing CD206+CD163+ macrophage subtype in the inflamed RA synovium, and showed that these were the dominant macrophages in patients with active RA. The team found that these cells are resident in the joint which, in health play a protective role, but in disease—for unknown reasons—become pro-inflammatory, release cytokines that induce inflammation, and also have the ability to activate the invasive fibroblast cell type which leads to cartilage and bone destruction.
“Here, we observe a marked increase in the diversity of macrophage subsets residing in RA compared to healthy synovial tissue biopsies with a dominance of CD206+CD163+ macrophages observed in both,” they noted. “This subset in healthy synovium lacks expression of the inflammatory marker CD40, which was expressed on most RA CD206+CD163+ macrophages.”
The CD40-expressing CD206+CD163+ macrophages were also found to be transcriptionally distinct with “unique tissue-resident gene signatures and metabolic capacities, and can activate healthy synovial fibroblast responses,” the investigators wrote.
Importantly the study findings showed that this macrophage subtype is present and becomes activated in individuals at risk of developing RA, and so prior to clinical signs and symptoms. The analyses in addition indicated that the pro-inflammatory status of the macrophages is maintained by specific signaling and metabolic pathways within the joint, targeting of which may induce resolution of inflammation.
In parallel, the team’s studies showed that protective barrier macrophages (CX3CR1+) were depleted in established RA, exhibiting a switch in the dominance of joint macrophages from protective macrophages to pro-inflammatory macrophages.
The identification of the dominant CD40-expressing CD206+CD163+ cells suggests that targeting CD40 signaling could represent a new strategy for patients who currently don’t respond to treatment. Hanlon, who is now based in Harvard University, said “The presence of these macrophages in individuals at risk of developing RA highlights the possibility of an early cellular biomarker of disease onset, resulting in early treatment intervention.”
The authors further concluded, “Uncovering the early molecular patterns and cues that transform this immunoregulatory macrophage population into a dysfunctional inflammatory activation state may provide opportunities to reinstate joint homeostasis in patients with RA.”