Multiple sclerosis (MS) is the most common cause of acquired disability in adults. It typically affects people between 20 and 50, but it can strike at any age. It affects two to three times as many women as men and is more common among people with northern European ancestry. Approximately 400,000 Americans and as many as 2.5 million people worldwide have MS (National Multiple Sclerosis Society). There is no cure for multiple sclerosis. Although interleukin 17 (IL17) mediates and vitamin D mitigates MS pathogenesis, the biochemical mechanisms by which they do so were relatively unknown until recently. A research team led by co-principal investigators Dr. Sylvia Christakos at New Jersey Medical School and Dr. Lawrence Steinman at Stanford University has provided new insight on how vitamin D interferes with the complex MS biochemical pathways (Joshi et al. 2011). As a result of their findings, novel therapies for multiple sclerosis may be on the horizon.
Interleukin 17 and vitamin D
IL17 is an inflammatory cytokine produced by several kinds of T cells, including a recently identified CD4+ T cell subset called Th17 cells. IL17 is implicated in numerous autoimmune diseases, including MS, which is modeled in mice as experimental autoimmune encephalomyelitis (EAE). In contrast, vitamin D, more commonly known for its role in maintaining calcium and phosphate homeostasis, down-regulates autoimmunity and offers some protection against both multiple sclerosis and experimental autoimmune encephalomyelitis. To determine the action mechanisms of both IL17 and vitamin D in the context of MS and EAE, the Christakos-Steinman team conducted a series of experiments using human and mouse cells (some of them genetically altered) and several mouse models of multiple sclerosis.
Experimental autoimmune encephalomyelitis
Laboratory mice are not susceptible to multiple sclerosis. However, a disorder similar to MS, experimental autoimmune encephalomyelitis, can be induced in mice via injections of myelin peptides, such as myelin basic protein, proteolipid protein, or myelin oligodendrocyte glycoprotein, suspended in complete Freund's adjuvant. Among the several EAE-susceptible mouse strains are C57BL/6J (B6J, 000664) and SJL/J (SJL, 000686).
Vitamin D mitigates EAE by down-regulating IL17A expression
To determine vitamin D's effects on multiple sclerosis and experimental autoimmune encephalomyelitis, the Christakos-Steinman team conducted several in vitro and in vivo experiments. They found that the active form of vitamin D – 1,25(OH)2D3 – down-regulates the expression of IL17A in cultures of CD4+ T cells isolated from healthy human donors and in cultures of various splenocyte and lymph node T cell subsets isolated from naïve "myelin oligodendrocyte glycoprotein (MOG) T cell receptor transgenic" (2D2) mice. The active form of the vitamin significantly reduces IL17A expression in cultures of splenocytes and draining lymph node cells isolated from EAE-induced SJL mice seven days after EAE is induced (before clinical symptoms appear).
Substantiating the findings of previous researchers, the Christakos-Steinman team found that administering the active form of vitamin D to experimental autoimmune encephalomyelitis-induced SJL mice on the day of EAE induction and every other day thereafter markedly mitigates disease progression. Administering the active form of vitamin D to SJL mice paralyzed by EAE or to B6J mice with ongoing EAE slows disease progression and reverses paralysis and EAE symptoms, especially if the active form of the vitamin is administered repeatedly during the course of the disease. Eighteen days after experimental autoimmune encephalomyelitis induction, IL17A expression from the spinal cord and brain CD4+ mononuclear cells of the diseased SJL and B6J mice are down-regulated, indicating that the improvement of EAE symptoms is associated with reduced IL17A expression. B6J mice inoculated with Th17 cells differentiated from CD4+ cells isolated from EAE-induced 2D2 mice and cultured with the active form of vitamin D develop a milder EAE, have fewer IL17A-expressing cells, and express less splenic and central nervous system IL17A than do B6J mice inoculated with Th17 cells cultured without the active form of vitamin D.
These findings indicated that the active form of vitamin D mitigates experimental autoimmune encephalomyelitis by down-regulating IL17A expression.
Vitamin D mitigates EAE by repressing IL17A transcription
To determine the biochemical pathways through which the active form of vitamin D down-regulates IL17A expression, the Christakos-Steinman team conducted a series of cell culture experiments involving several kinds of genetically engineered cells. They identified several molecules that play key roles in vitamin D-mediated IL17A down-regulation:
- The ability of the active form of vitamin D to inhibit human IL17A expression is considerably less if the vitamin D receptor's (VDR's) IL17A promoter binding domain is absent.
- The "nuclear factor for activated T" cells (NFAT), an essential regulator of T cell-mediated IL17A transcription, dose dependently mediates IL17A's down-regulation by the active form of vitamin D.
- A complex interaction between the retinoid X receptor (RXR), vitamin D receptor (VDR), and other co-regulatory proteins enhances vitamin D repression of IL17A transcription.
- At least in part, the active form of vitamin D down-regulates IL17A expression by recruiting histone deacetylase (HDAC) to the IL17A promoter.
- The active form of vitamin D decreases the EAE-induced recruitment of "runt-related transcription factor 1" (RUNX1) (an IL17A transcription regulator) to its binding sites on the IL17A promoter.
- The active form of vitamin D significantly increases the number of immunosuppressive Foxp3+ regulatory T (Treg) cells in the spleen, brain and spinal cord of EAE-induced B6J mice. The IL17-repression domain in the Foxp3 gene is highly conserved in mice and humans. FOXP3 inhibits RUNX1 and NFAT, which in turn results in vitamin D repression of IL17A transcription.
In summary, the Christakos-Steinman team was the first to identify the molecular mechanisms by which vitamin D down-regulates autoimmune phenotypes characteristic of multiple sclerosis and experimental autoimmune encephalomyelitis. They demonstrated that the active form of vitamin D represses IL17A transcription by blocking NFAT, recruiting HDAC, complexing with the VDR and sequestering RUNX1, and inducing FOXP3. Their work provided a much-needed framework for initiating clinical trials to test the efficacy of vitamin D or its analogs in fighting MS and other autoimmune diseases.