You wouldn't happen to know exactly what Tysabri targets, would you?
If you mean the action of Tysabri...it keeps T-cells from crossing the blood brain barrier. This helps the MS patient by limiting exacerbations, as t-cells are what cause destruction of the myelin.
But since the PML cases cropped up, it was wondered if the action of blocking t-cells from crossing the blood brain barrier was what caused the JC virus to go unchecked and the MS patients to come down with PML.
PML is usually only seen in severely immunesuppressed patients, such as AIDS patient.
The PML was found only in the MS patients who had taken Avonex and Tysabri, but since Avonex (an interferon and immune modulator) has been on the market for about 10 years without incidence of PML, the question arose as to whether it was the Tysabri that led to the PML.
Of course, steroids (immune suppresants) were thrown in the mix once the MS patients in the trial started having worsening symptoms, because the worsening symptoms were thought to be an MS exacerbation.
One MS trial patient died, the other, last I heard, had been moved from a hospital to a rehab and was making a slow recovery from the effects of the PML.
So the question that remain to be answered if Tysabri is reintroduced is: Why did PML develop when Tysabri was given to the MS patients? Did it have something to do with the combination of the Avonex and the Tysabri?
(There was a third patient who came down with PML and died, this patient was a Crohns patient, and was not on an interferon. However, this patient had been on other immunosuppressant drugs, in addition to the Tysabri.)