The cause of death for this patient is only speculation.

The cause of death could have been PML. While the Tysabri was removed from the system, the immune response may have been delayed/affected by prior methotrexate treatment. I don't know how long this drug may affect immune response.

The cause of death could have been IRIS. Inflamation from the immune response could have worsened the syptoms and resulted in death. Maybe the PLEX should be less aggressive.

The cause of death could have been complications due to the patients rheumatological condition.

The cause of death could have been due to some other hospital or outside acquired infection.

From Biogen's 8-k

" Patient had also received prior therapy with methotrexate for a rheumatological condition. "