Multiple sclerosis and environmental factors: the role of vitamin D, parasites, and Epstein-Barr virus infection.

Pathogenic mechanisms underlying multiple sclerosis development have yet to be clearly identified, but considerable evidence indicates that autoimmunity plays an important role in the etiology of the disease. It is generally accepted that autoimmune diseases like MS arise from complex interactions between genetic susceptibility and environmental factors. Although environmental factors unequivocally influencing MS development have yet to be established, accumulating evidence singles out several candidates, including sunlight-UV exposure or vitamin D deficiency, viral infections, hygiene, and cigarette smoking. Vitamin D deficiency has been associated with different autoimmune diseases. Several investigations indicate 125 (OH)2 vitamin D plays a critical role in shaping T-cell response and inducing T cells with immunosuppressive properties. Likewise, helminth infections represent another potential environmental factor exerting immunomodulatory properties. Both epidemiological and experimental data provide evidence to support autoimmune down-regulation secondary to parasite infections in patients with MS, through regulatory T- and B-cell action, with effects extending beyond simple response to an infectious agent. Finally, different epidemiological studies have demonstrated that Epstein-Barr virus infection confers added risk of developing MS. Proposed mechanisms responsible for this association include activation and expansion of self-reactive T and B cells, lower threshold for self-tolerance breakdown, and enhanced autoreactive B-cell survival, all to be discussed in this review. Understanding environmental factors influencing propensity to MS will lead to new and more effective approaches to prevent and treat the disease.