XPR1 prevents T-cell hyperactivation through interaction with KIDINS220 for endocytic regulation of CD28 costimulation
This study demonstrates that XPR1 restricts CD28 costimulation and prevents T-cell hyperactivation by interacting with KIDINS220 to regulate endocytic membrane rearrangement and CD28 surface clustering.
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 This is an AI-generated explanation of a preprint that has not been peer-reviewed.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
The immune system relies on a delicate balance to protect the body. When a T cell, a type of white blood cell, encounters a threat, it needs two signals to fully activate and multiply. The first signal comes from recognizing a specific invader, while the second, known as costimulation, acts like a confirmation that the threat is real and requires a full-scale response. This second signal is delivered by a receptor on the T cell surface called CD28. If this costimulation is too weak, the T cell remains inactive and fails to fight infection. If it is too strong, however, the T cell can become hyperactive, leading to a dangerous overreaction that damages healthy tissue. Finding the molecular switches that keep this process in check is crucial for understanding how to treat autoimmune diseases or improve cancer therapies.
Researchers at the University Medical Center Hamburg-Eppendorf have identified a specific protein that acts as a brake on this process, preventing T cells from becoming overzealous. This protein, named XPR1, was previously known for its role in moving phosphate and calcium in and out of cells, but its function in the immune system remained a mystery. By studying mice engineered to lack the gene for XPR1 specifically in their T cells, the scientists discovered that without this protein, the cells lose their ability to regulate the CD28 signal. Instead of stopping at a safe level of activation, these T cells become hyperactive, producing excessive amounts of inflammatory chemicals and multiplying uncontrollably.
The study reveals that XPR1 does not work alone. It physically binds to another protein called KIDINS220, which is known to help manage how cells move materials in and out. In a healthy T cell, this partnership helps pull the CD28 receptors off the cell surface after they have done their job, effectively turning down the volume on the activation signal. When XPR1 is missing, this recycling process fails. The CD28 receptors remain stuck on the surface, clustering together and sending a continuous, amplified signal that drives the cell into a state of hyperactivation. The researchers confirmed this by using special peptides to block the connection between XPR1 and KIDINS220 in normal cells; doing so caused the same hyperactive behavior seen in the mice without XPR1, proving that the interaction between these two proteins is the key to keeping the immune response in check.
Importantly, the team ruled out the idea that this overactivation was caused by a buildup of phosphate, which is the primary job XPR1 performs in other parts of the body. They measured the phosphate levels in the T cells and found them to be normal, indicating that the immune defect stems specifically from the loss of the XPR1-KIDINS220 partnership rather than a general failure of phosphate transport. Furthermore, the hyperactivation was strictly dependent on the CD28 signal. When the researchers stimulated the T cells using methods that bypassed CD28, the cells behaved normally, showing that XPR1's role is specific to regulating this particular costimulation pathway.
This discovery suggests that XPR1 acts as a critical rheostat, or volume control, for T cell activation. In the absence of XPR1, the T cells develop poorly in the thymus, the organ where they mature, and the few that do reach the bloodstream are prone to excessive inflammation. The researchers also found that this mechanism works similarly in human T cells, as blocking the interaction between XPR1 and KIDINS220 in human cells led to increased production of inflammatory signals. These findings point to XPR1 as a potential new target for immunomodulation, offering a way to fine-tune the immune system's response in conditions where it is either too weak or too aggressive.
Drowning in papers in your field?
Get daily digests of the most novel papers matching your research keywords — with technical summaries, in your language.