HSS Researchers Identify Hidden Mechanism Driving Joint Tissue Growth in Rheumatoid Arthritis

Researchers at Hospital for Special Surgery (HSS) have identified a specialized population of immune cells that play a central role in driving abnormal tissue growth in rheumatoid arthritis (RA), offering new insight into disease progression and potential treatment strategies.

The study, published in Science Translational Medicine, reveals that a subset of immune cells known as SPP1ʰⁱ macrophages work in tandem with structural cells and protein scaffolds to promote excessive tissue expansion in the joints of patients with RA.

RA is a chronic autoimmune disease that causes inflammation of the synovium-the tissue lining the joints-leading to pain, swelling, and eventual damage to cartilage and bone. While current therapies largely focus on controlling inflammation, the mechanisms that drive the abnormal growth of synovial tissue-and its invasive portion, known as pannus, which erodes cartilage and bone-have remained poorly understood.

A New Model of Tissue Growth in RA

Using advanced spatial transcriptomics to analyze human tissue samples, the HSS research team discovered that SPP1ʰⁱmacrophages cluster within fibrin-rich niches in the synovium. Fibrin, a protein typically involved in blood clotting and wound healing, appears to act as a temporary scaffold that supports new tissue formation.

Within these niches, the macrophages were found to:

• break down fibrin scaffolds through enzymatic activity and cellular uptake;
• stimulate nearby fibroblasts, key structural cells, to proliferate; and
• promote tissue remodeling and expansion independent of traditional inflammatory pathways.

"This work suggests that rheumatoid arthritis is not only driven by inflammation, but also by dysregulated tissue repair processes," said Laura Donlin, PhD, senior author and scientist at the HSS Research Institute. "We are seeing a coordinated interaction between immune cells, structural cells, and the extracellular matrix that fuels abnormal growth."

Beyond Fibrosis: A Distinct Pathway

Interestingly, while SPP1ʰⁱmacrophages resemble cells involved in fibrotic diseases such as lung or liver fibrosis, the RA tissue environment lacks the dense collagen buildup characteristic of fibrosis. Instead, the researchers describe this process as "pro-generative" tissue remodeling, more closely resembling wound healing gone awry.

Therapeutic Implications

The findings also suggest that targeting SPP1ʰⁱmacrophages and their signaling pathways could represent a new therapeutic approach. The study highlights the role of IL-6 signaling in sustaining these cells, offering further insight into why IL-6-targeting therapies, already used in RA, may be particularly effective in certain patient populations.

"Current treatments focus on suppressing inflammation, but our findings point to additional pathways that drive disease progression," said Dr. Donlin. "Targeting these tissue remodeling processes could open the door to more precise and effective therapies."

Broader Impact

The study's findings may extend beyond RA. Similar immune cell populations and fibrin-based remodeling processes have been implicated in other conditions, including interstitial lung disease, lupus, cancer, and traumatic injury, suggesting a broader role for this pathway in human disease.