Can Someone With MS Please Comment on This?

Here is a question that has come in from someone with MS.  If you have experience with this and can comment, please do so!

“I have secondary progressive MS, walk with a wheeled walker, and continue to work although I feel I am losing these abilities rapidly. I have tried many other treatments for my MS (4 different disease modifying therapies, most recently Tecfidera) but none have altered my MS. My doctor is recommending a trial of Tysabri. I am JC virus antibody negative and trying to decide if I should try this medication. Is there anyone reading this with my level of disability or disease type (secondary progressive) or both who can comment on their experience with Tysabri? Studies have not been completed for this stage of the disease and I am looking to gain insight from those with progressive MS who have tried Tysabri or are still on Tysabri."