I forgot to add a couple of links in my earlier post. The history of Tysabri is here http://www.webmd.com/multiple-sclerosis/news/20060605/fda-lets-ms-drug-tysabri-return and the TGA document is here http://www.msra.org.au/news/documents/Tysabri-July07.pdf
On thinking about it a bit more I guess the major problem with having no T-cells in the brain would be if there's a rogue virus around as there'd be no defence mechanism in place to knock it out. I also wonder what effect the extra protein on the T-cell would have when it's elsewhere in the body.
The human body is such an amazingly complex organism that I question the validity of using pharmaceuticals that target specific chemical processes in isolation. I wonder if we know enough of the big picture to completely understand the ramifications of a particular action. That's why I'd rather not experiment on myself and why I choose to stay chemical free in the management of my MS.