The optic nerve inflammation, demyelination, and axonal loss observed after RSA59 infection, but not after RSMHV2 infection, demonstrate that the MHV-A59 spike protein is required for the induction of optic neuritis. This suggests that the spike protein may mediate retrograde axonal transport of the virus, allowing it to travel from brain regions containing RGC axonal projections along the optic nerve. Viral antigen was consistently detected within thalamic neurons in RSA59-infected mouse brains (Figs. 5C, D) , as well as in some RSMHV2-infected brains (Fig. 5B) . RSA59 viral antigen was also detected in the superior colliculi in some animals (data not shown). Light diffuse staining detected in optic nerves from RSA59-infected mice (Fig. 5G ), but not RSMHV2-infected mice (Fig. 5F ), suggests that only the RSA59 viral antigen is transported to the optic nerves. To further investigate this, the optic nerves were isolated 3 or 6 days pi and frozen sections were examined for the presence of EGFP. Viral antigenY positive EGFP signal was observed in optic nerves from RSA59-infected mice (Figs. 5I, J) but not RSMHV2-infected mice (Fig. 5H) . The punctate EGFP signal in optic nerve sections observed by fluorescent microscopy in RSA59-infected mice (Fig. 5J ) further suggests axonal transport through the optic nerve and was seen as early as 3 days pi.