John I can help to answer one of the two questions. The retina is
protected by a blood brain barrier. In fact, as you probably know the
eyes are considered to be a direct extention of the brain itself. The
optic neurons are covered by dura mater, as is the spinal column. The
retina also has two different blood supplies the arteries and veins
(branches of the ophthalmic artery-->central retinal artery) supply the
inner half of the 10 layers of the retina, while the uvea (also called
choroid) supplies the outer layers of the retina. These blood supplies
each have zonula occludens (tight junctions) between adjacent cells
composing the retinal vasculature.

As for the Fas ligand, I don't have a lot of additional information.