Rheumatoid arthritis has long been framed as an immune-system disease. New research from the University of Oxford's Kennedy Institute of Rheumatology suggests some finger joints may be primed for the condition long before a baby is born.
The study, published in Nature Immunology, comes from a team that mapped human finger joints as they formed during fetal development. They focused on two joints that behave very differently in patients.
One is the proximal interphalangeal (PIP) joint, the middle knuckle of each finger, which rheumatoid arthritis attacks over and over. The other is the distal interphalangeal (DIP) joint, closest to the fingertip, which the disease almost always spares.
Even before those joints were fully formed, the vulnerable ones already looked different. PIP joints held larger volumes of synovial tissue, the soft lining inside a joint, and were enriched for a type of cell called PI16-positive fibroblasts.
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The cells that respond too loudly
These fibroblasts cluster near blood vessels and where tendons attach to bone. When the Oxford team exposed them to inflammatory signals, they switched on inflammatory pathways and shut down the housekeeping functions that keep tissue orderly.
PI16-positive fibroblasts were more abundant in PIP joints than in DIP joints. That gap in wiring, laid down in the womb, may be enough to tilt one joint toward decades-later inflammation and leave its neighbor alone.
What it could mean for treatment
Standard treatment for rheumatoid arthritis aims at the immune system. Drugs like methotrexate and biologic agents that block specific inflammatory signals slow joint damage but do not target the tissue's built-in vulnerability.
The Oxford findings do not change today's treatment. But they point at a different target for tomorrow.
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If PI16-positive fibroblasts are the local cells that turn a joint into a flare, drugs that calm them or restore their normal role could one day complement immune-based therapies.
Rheumatoid arthritis was the second most common arthritis subtype among U.S. adults with the condition, accounting for about 15.8 percent of diagnosed cases in a recent CDC analysis of national survey data from 2017 to March 2020.
Its earliest signs are often mistaken for something else. Common early warning signs of rheumatoid arthritis include prolonged morning stiffness, symmetrical swelling in the small joints of the hands and feet, and unexplained fatigue.
For people already living with RA, the new work may also help explain why treatment response varies so widely. Two patients on identical drug regimens may have joints with different underlying cellular landscapes, set before either was born.
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Anyone with persistent joint stiffness or swelling lasting more than six weeks should see a doctor. The Oxford team plans to trace PI16-positive fibroblasts across other joints and other inflammatory conditions to see whether the same principle applies beyond the fingers.
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This article is made and published by Mie Hermansen, who may have used AI in the preparation.