LIF is a pro-algesic mediator in human and mouse models of arthritis-related pain
This study identifies Leukemia Inhibitory Factor (LIF) as a key pro-algesic mediator in arthritis-related pain, demonstrating that it activates JAK/STAT signaling in sensory neurons to upregulate pain-relevant genes and induce pain-like behaviors in both human and mouse models.
Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). This is an AI-generated explanation of a preprint that has not been peer-reviewed. It is not medical advice. Do not make health decisions based on this content. Read full disclaimer
Chronic pain in rheumatoid arthritis is a stubborn and complex problem. While modern medicines have become very good at calming the swelling and inflammation inside a joint, they often fail to stop the pain itself. For many patients, the joint may look calm under a microscope or on an ultrasound, yet it still aches with a intensity that disrupts daily life. Scientists have long suspected that this disconnect happens because the pain is being driven by something other than just the visible inflammation. Instead of a simple reaction to swelling, the pain might be fueled by a hidden chemical signal that directly tells the nerves to become hypersensitive, turning up the volume on pain signals even when the joint appears quiet.
A team of researchers at King's College London has now identified a specific chemical messenger that acts as this hidden signal. They found that a protein called leukemia inhibitory factor, or LIF, is present in the fluid surrounding painful joints. When this protein touches the sensory nerves that carry pain signals, it changes the nerves' behavior, making them more sensitive to pressure and more likely to fire off pain signals. The researchers showed that this protein is far more powerful at altering nerve function than other well-known inflammatory chemicals. By blocking LIF in laboratory models, they were able to stop the nerves from becoming hypersensitive, suggesting that targeting this specific protein could offer a new way to treat pain in people whose joints are not visibly inflamed but still hurt.
The journey to this discovery began with a simple observation: patients with inflammatory arthritis often report severe pain even when their joints show little sign of active disease. The researchers suspected that the cells lining the joint, known as synovial fibroblasts, were releasing chemical signals that directly irritated the nerves. They focused on a group of signaling proteins that are known to activate a specific pathway inside cells, a pathway that acts like a switchboard for genetic instructions. Previous work had shown that three of these proteins—interleukin-6, interleukin-11, and leukemia inhibitory factor—were present in the joint fluid. The team wanted to know which one was the primary driver of pain.
To find the answer, the scientists grew human sensory neurons in a dish. These are the nerve cells responsible for feeling pain, created from stem cells to ensure they were healthy and human. They exposed these neurons to fluid taken from the knees of patients with painful arthritis. The neurons reacted by turning on a specific protein inside them, a sign that they were receiving a distress signal. When the team added a substance that blocked leukemia inhibitory factor, the distress signal vanished. In contrast, blocking the other two proteins had very little effect. This experiment revealed that leukemia inhibitory factor was the dominant force driving the nerve cells to react to the joint fluid.
The researchers then looked deeper to see what was happening inside the nerve cells. They analyzed the genetic instructions, or the transcriptome, of the neurons after they were exposed to leukemia inhibitory factor. They found that this single protein triggered a massive change in the cell's behavior. It turned on hundreds of genes that are known to make nerves hypersensitive. These genes included instructions for building channels that let pain signals pass through the nerve cell more easily, and receptors that make the cell more alert to danger. When the team compared this to the other two proteins, interleukin-6 and interleukin-11, they found that leukemia inhibitory factor caused a much larger and more profound shift in the nerve's genetic activity. It was the only one capable of fully reprogramming the nerve to be in a state of high alert.
To confirm these findings in a living system, the team turned to mice. They induced arthritis in the mice and measured the levels of the protein in the affected joints. They found that the protein levels were significantly higher in the painful, inflamed knees compared to the healthy ones. They then tested whether this protein could cause pain on its own. They injected a small amount of the protein directly into the knee of a healthy mouse that had no arthritis. The mouse did not show signs of spontaneous pain, such as limping, but it became much more sensitive to pressure. When the researchers pressed on the knee, the mouse reacted to a much lighter touch than before. This proved that the protein alone was enough to lower the threshold for pain, making the joint feel painful even without inflammation.
The study also explored how this protein affects the physical structure of the nerves. Using a specialized device that allows scientists to watch nerve fibers grow, they found that when the protein was present, the nerves grew longer and more vigorously after being cut. This suggests that the protein not only makes nerves more sensitive to pain but also encourages them to grow and change their connections, which could contribute to long-term pain conditions. The researchers confirmed that the same changes happened in human nerve cells and in mouse nerve cells, showing that this mechanism is consistent across species.
The findings point to a clear mechanism for why pain persists in rheumatoid arthritis even when inflammation is under control. The synovial fibroblasts in the joint release leukemia inhibitory factor, which travels to the sensory nerves and rewrites their genetic code. This makes the nerves hypersensitive to pressure and more likely to send pain signals. The study suggests that blocking this specific protein could be a new way to treat pain, particularly for patients who do not respond to current anti-inflammatory treatments. By targeting the root cause of the nerve sensitivity rather than just the swelling, doctors might be able to offer relief to those who are currently suffering from unexplained joint pain. The research provides a concrete target for future therapies, moving beyond the idea that pain is just a symptom of swelling and recognizing it as a distinct biological process driven by specific chemical signals.
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