Tysabri (natalizumab) works by blocking immune cells from crossing out of the bloodstream and into the brain and spinal cord, where they cause the damage seen in multiple sclerosis. It does this by attaching to a specific protein on the surface of immune cells, preventing them from latching onto blood vessel walls and squeezing through into nervous system tissue. The result is a powerful reduction in the inflammation that drives MS relapses and lesion formation.
How Tysabri Blocks Immune Cell Migration
In MS, certain white blood cells become misfired. They treat the brain’s own nerve coatings as threats, crossing from the bloodstream into the central nervous system through the blood-brain barrier, a tightly sealed layer of cells lining the brain’s blood vessels. To make that crossing, white blood cells use a surface protein called alpha-4 integrin as a kind of molecular handshake with a partner protein on blood vessel walls called VCAM-1. When the two connect, the immune cell sticks to the vessel wall and migrates through.
Tysabri is a lab-engineered antibody that binds directly to alpha-4 integrin on the immune cell’s surface. Interestingly, it doesn’t physically block the spot where VCAM-1 would normally attach. Instead, it binds nearby and changes the shape of the connection site, making it roughly 12 times harder for VCAM-1 to latch on. Researchers at Harvard confirmed this is an allosteric (indirect) mechanism: Tysabri distorts the integrin’s structure enough that the immune cell can no longer grip the blood vessel wall effectively. With that grip weakened, immune cells stay in the bloodstream and never reach the brain.
Effects Beyond the Brain
The same mechanism applies in the gut. In Crohn’s disease, immune cells use alpha-4 integrin to interact with a different partner protein called MadCAM-1 on the blood vessels lining the intestinal wall. By blocking alpha-4 integrin on these cells too, Tysabri reduces the flood of immune cells into the intestinal lining, calming the chronic inflammation that drives Crohn’s symptoms. This dual action in the brain and gut is why Tysabri is approved for both conditions, though its use in Crohn’s disease is generally reserved for patients who haven’t responded to other treatments.
How Effective It Is
Tysabri is one of the most effective MS therapies available. In the landmark AFFIRM trial, which tested it against placebo in relapsing-remitting MS, Tysabri reduced the annual relapse rate by 68%. Its impact on MRI-visible disease activity was even more striking: new or growing brain lesions dropped by 83%, and actively inflamed lesions (those lighting up on contrast-enhanced MRI) fell by 92%.
These effects show up fast. In a phase II study, differences in new inflammatory brain lesions between Tysabri-treated and placebo-treated patients were already visible after the first infusion, within one month. That rapid onset distinguishes Tysabri from many other MS therapies that take months to reach full effectiveness.
Dosing and Administration
The standard dose is 300 mg delivered as an intravenous infusion over about one hour, repeated every four weeks. Tysabri has a half-life of roughly 11 days, meaning it takes about two weeks for half the drug to clear from your system after each dose.
Some patients now receive infusions on an extended schedule, typically every six weeks instead of four. Multiple studies, including data from large registries in Italy and North America, have found that extending the interval to six weeks produces similar relapse rates and disability outcomes compared to the standard four-week schedule. The shift toward extended dosing is partly driven by evidence suggesting it may cut the risk of a serious side effect, PML, by about half. A large observational study from the TOUCH database found that patients averaging 35 to 43 days between infusions had roughly half the PML risk of those on standard dosing.
The PML Risk
The most significant concern with Tysabri is progressive multifocal leukoencephalopathy, or PML, a rare but potentially devastating brain infection caused by a common virus called JC virus. Most people carry JC virus harmlessly, but when Tysabri suppresses immune surveillance in the brain for an extended period, the virus can reactivate and damage brain tissue.
Not everyone on Tysabri faces the same level of risk. Three factors determine how concerned you and your doctor need to be: whether you carry JC virus antibodies (detectable through a blood test), how long you’ve been on the drug, and whether you’ve previously taken other immune-suppressing medications. Patients who test negative for JC virus antibodies have an extremely low risk. For those who test positive, the antibody level matters. A JC virus antibody index at or below 0.9 (or 0.45 for patients with prior immunosuppressant exposure) is associated with much lower risk.
To put the numbers in perspective: among JC virus-positive patients without prior immunosuppressant use and with a high antibody index (above 1.7), the cumulative risk of PML reaches roughly 11 per 1,000 patients after four years and about 30 per 1,000 after six years. For those with a low antibody index (0.6 or below and no prior immunosuppressant use), the cumulative risk remained at zero through four years of treatment in one Bayesian analysis, only reaching 0.3 per 1,000 at the five-year mark.
Monitoring During Treatment
Because of the PML risk, Tysabri is only available through a restricted program called TOUCH (Tysabri Outreach: Unified Commitment to Health). Before every infusion, your treatment center completes a standardized checklist. You’ll be asked whether you’ve experienced any new or worsening neurological symptoms over the past month, such as changes in thinking, vision, balance, or strength that have lasted several days. You’ll also be asked whether you’ve taken any other immune-suppressing medications recently and whether you have any condition that weakens your immune system.
If you answer yes to any of these screening questions, the infusion is held until your prescribing doctor reviews your situation. You’ll also need to read the patient medication guide before every infusion, not just the first one. JC virus antibody testing is typically repeated every six months to catch any new seroconversion, since about 2 to 3% of patients convert from negative to positive each year.