We investigated how one calcitriol dose plus vitamin D3 reverses experimental autoimmune encephalomyelitis (EAE), a multiple sclerosis model. This protocol rapidly increased CD4+ T cell Ikzf2 transcripts, Helios protein, and CD4+Helios+FoxP3+ 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 CD4+Helios+FoxP3+Tregulatory 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.
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We extend our gratitude to Professors Norlin Benevenga, Timothy Garrow and Brian Yandell for discussions on one carbon metabolism, BHMT1 enzymology, and data analysis, respectively. We thank Dr. Brigitte Huber for reviewing the manuscript before submission. We are especially grateful to Thomas Wolfe and Patricia Powers for establishing the “Multiple Sclerosis Research Fund in Biochemistry” at the University of Wisconsin Foundation. We also thank Dr. Brian Fox, without whose support and encouragement this manuscript would not have been completed. We are grateful to Ms. Lauren Barta and Ms. Corinne Praska for assistance with the research. We acknowledge the University of Wisconsin Carbone Cancer Center, United States Support Grant P30 CA014520 for supporting the Flow Cytometry Laboratory where we collected and analyzed data for this research. We received research support from the University of Wisconsin Graduate School Research Committee, United States research grant # 13008 , a HATCH McIntyre Stennis Award, United States ( MSN119798 , PRJ18KV ), a Pilot Award from the Center for Autoimmune Disease Research at the University of Minnesota, United STates (UMF0020624 to J.A.S.), and a generous bridge funding award from the University of Wisconsin Department of Biochemistry, United States.