Scientists have identified a group of immune cells that may be sabotaging multiple sclerosis (MS) treatments by hiding in the brain and spinal cord, where current therapies cannot reach them. This discovery could explain why some patients with MS continue to experience tissue damage even when their disease appears well controlled, and it opens the door to entirely new treatment approaches.

What Are Tissue Resident Memory T Cells and Why Do They Matter?

Your immune system relies on T cells, a type of white blood cell that patrols your body looking for signs of infection or disease. In MS, something goes wrong with this system. Instead of protecting you, some T cells mistakenly attack the central nervous system, which includes your brain and spinal cord, causing inflammation and tissue damage.

Over the past several decades, doctors have developed therapies that block these rogue T cells from entering the central nervous system. These treatments have been a major breakthrough, dramatically reducing relapses, or periods of severe symptoms, in many MS patients. However, researchers at the Benaroya Research Institute have discovered a critical problem with this approach.

"Unlike patrolling T cells, which circulate throughout the body, tissue resident memory T cells remain in the brain," explained Estelle Bettelli, PhD. "Because these cells don't circulate through the body, current disease-modifying therapies may not effectively target them."

Estelle Bettelli, PhD, Benaroya Research Institute

Tissue resident memory T cells, or TRMs, are fundamentally different from the patrolling T cells that current MS drugs target. Once TRMs settle into the brain and spinal cord, they stay there permanently, creating a persistent source of inflammation that existing therapies cannot reach. This may explain why some MS patients continue to experience disease progression and tissue damage even when their condition appears stable.

How Are Researchers Planning to Target These Hidden Cells?

Dr. Bettelli and her team have received grants from the National Institutes of Health (NIH) to investigate what makes tissue resident memory T cells unique and whether targeting them could lead to new MS treatments. Their research focuses on three key areas:

• Unique Molecules and Receptors: Researchers are identifying what molecules or receptors distinguish TRMs from other immune cells, which could provide targets for new drugs.
• Energy Source Dependencies: If TRMs rely on a specific energy source, scientists may be able to slow them down or limit the damage they cause without harming healthy brain tissue.
• Localization Mechanisms: Understanding how these cells stay in the central nervous system instead of circulating throughout the body could allow researchers to make them leave the brain.

The challenge is significant. The brain and spinal cord are vital organs, so any new therapy must be able to target TRMs without harming other important cells. Dr. Bettelli emphasized this delicate balance in her research approach.

"We're especially looking for unique characteristics that would enable us to target TRMs without harming other significant cells," stated Dr. Bettelli. "This is particularly important because the brain and spinal cord are vital organs."

Estelle Bettelli, PhD, Benaroya Research Institute

What Role Do B Cells Play in MS Progression?

The story becomes even more complex when you consider that tissue resident memory T cells may not work alone. Many studies show that B cells, another type of immune cell, and T cells may work together to drive inflammation and disease progression in MS. Dr. Bettelli is investigating whether TRMs and B cells collaborate to cause ongoing damage.

In a study funded by the National Multiple Sclerosis Society (NMSS), researchers are using a state-of-the-art disease model to examine if and how TRMs and B cells interact. This research could have major implications for treatment, because if TRMs depend on B cells for certain functions, then therapies that already target B cells might be able to curb their activity.

"We do not know whether TRMs rely on B cells for certain functions," noted Dr. Bettelli. "It is important to determine whether therapies that target B cells could curb their activity."

Estelle Bettelli, PhD, Benaroya Research Institute

Why MS Requires Multiple Treatment Approaches

MS is a complex condition involving many different types of immune cells working in concert to damage the nervous system. Because of this complexity, it may take a combination of medicines and different approaches tailored to individual patients to reduce the long-term impact of the disease. The discovery of tissue resident memory T cells suggests that the next generation of MS treatments will need to address multiple targets simultaneously.

Dr. Bettelli acknowledged that this research takes time because scientists must first build the models that allow them to study these complex disease processes. However, with ongoing support from the NIH and NMSS, researchers are moving closer to better therapies for MS patients who currently have limited options.

"This work takes time because we first have to build the models that allow us to study these complex disease processes," explained Dr. Bettelli. "We're very grateful for the ongoing support from the NIH and NMSS that enables us to keep asking important questions and moves us closer to better therapies for MS."

Estelle Bettelli, PhD, Benaroya Research Institute

For the millions of people living with MS, these discoveries offer hope that future treatments will be more effective at stopping disease progression and preventing the long-term disability that currently affects many patients, even those whose symptoms appear well controlled.