Digging a bit deeper into refs 8 and 26 in my post above.

(8) UV radiation suppresses experimental autoimmune encephalomyelitis independent of vitamin D production
by Bryan R Becklund , Kyle S Severson, Souriya V Vang, Hector F DeLuca

From the abstract:

Our results indicate that continuous treatment with UVR dramatically suppresses clinical signs of [the experimental autoimmune encephalomyelitis] EAE. Interestingly, disease suppression occurs with only a modest, transient increase in serum 25(OH)D(3) levels. Further analysis demonstrated that the levels of 25(OH)D(3) obtained upon UVR treatment were insufficient to suppress EAE independent of UVR treatment. These results suggest that UVR is likely suppressing disease independent of vitamin D production, and that vitamin D supplementation alone may not replace the ability of sunlight to reduce MS susceptibility.

(26) Suppression of experimental autoimmune encephalomyelitis by ultraviolet light is not mediated by isomerization of urocanic acid
by Amy A. Irving, Steven J. Marling, Lori A. Plum, and Hector F. DeLuca

From the Discussion:

It is clear that ultraviolet B exposure efficiently prevents EAE, though we find no evidence to support [urocanic acid] UCA isomerization as the mediator of this suppression. This is not the first study to discount UCA as a mediator of UV immunosuppression, or that UCA administration does not mimic the effects of UV exposure.