1,25-Dihydroxyvitamin D increases the methionine cycle, CD4 T cell DNA methylation and HeliosFoxp3 T regulatory cells to reverse autoimmune neurodegenerative disease.

We investigated how one calcitriol dose plus vitamin D reverses experimental autoimmune encephalomyelitis (EAE), a multiple sclerosis model. This protocol rapidly increased CD4 T cell Ikzf2 transcripts, Helios protein, and CD4HeliosFoxP3 T regulatory cells. It also rapidly increased CD4 T cell Bhmt1 transcripts, betaine:homocysteine methyltransferase-1 (BHMT1) enzyme activity, and global DNA methylation. BHMT1 transmethylates homocysteine to replenish methionine. Targeting the Vdr gene in T cells decreased Ikzf2 and Bhmt1 gene expression, reduced DNA methylation, and elevated systemic homocysteine in mice with EAE. We hypothesize that calcitriol drives a transition from encephalitogenic CD4 T cell to Treg cell dominance by upregulating Ikzf2 and Bhmt1, recycling homocysteine to methionine, reducing homocysteine toxicity, maintaining DNA methylation, and stabilizing CD4HeliosFoxP3Tregulatory cells. Conserved vitamin D-responsive element (VDRE)-type sequences in the Bhmt1 and Ikzf2 promoters, the universal need for methionine in epigenetic regulation, and betaine's protective effects in MTHFR-deficiency suggest similar regulatory mechanisms exist in humans.