Frankly, with your history, I would be very reluctant. I am JC+ and on Tysabri (for more than four years) and getting scared myself. But part of me figures I'm on it now, so I am damned either way (which may be right or wrong). Having said that, it has been amazingly good for me, MS-wise, but your circumstances are different to mine.

Just take care of what you put into your body - 'cos we just don't know what it will do. Know the risks, weigh up the possibilities.