Vitamin D as an immunomodulator
In addition to the classical role in the homeostasis of calcium and phosphorus, Vitamin D shows a regulatory effect on a number of different cells, especially its anti-proliferative and pro-differential biological function. Through its own receptor in the immune cells, Vitamin D increases the phagocytic activity of macrophages and natural killer (NK) cells. Also, by binding to the regulatory sequences of antimicrobial peptides genes, Vitamin D increases the microbicidal activity of phagocytes.
Inhibition of differentiation and maturation of antigen-presenting dendritic cells, as well as direct influence on their contact with T lymphocytes, it significantly influences the type of immune response. Dendritic cells under the influence of Vitamin D induce a suppressor T cells, which can inhibit Th1 cell response and are critical in the regulation of immune tolerance. Vitamin D inhibits proliferation of Th1 and Th17 cells, as well their cytokine production, and suppresses the differentiation and maturation of B lymphocytes. Due to all these functions, Vitamin D has shown beneficial effects in the prevention and modification of a number of autoimmune diseases.
Vitamin D and immunomodulation: Is it time to change the reference values?
Redefining Vitamin D reference values seems to be either a critical or urgent issue. There is not so far, a univocal consensus on the reference values of Vitamin D status, mainly because of such reference value rates on the basis of the parathyroid hormone (PTH) plasma response. Commercial kits for the determination of Vitamin D propose reference values which greatly differ, contributing to increasing such uncertainty on the definition of hypovitaminosis. Although the development of new serum range has been proposed, literature suggestions for an optimal range are often discrepant, not to mention that Vitamin D deficiency (Vitamin D < 32 ng/mL) is still determined on the basis of PTH response.
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Vitamin D immunomodulatory effect
Vitamin D can be designated as a selective immune system regulator and modulator of growth, differentiation and function of immune cells. It affects both types of immune response, by emphasizing the innate and suppressing the acquired immunity, contributing to immune tolerance. Vitamin D directly regulates the expression of antimicrobial peptides and improves the function of macrophages in defense against mycobacteria. Also, Vitamin D shows a useful role in the prevention and treatment of various autoimmune diseases and the prevention of transplant organ rejection. Vitamin D receptor agonists, particularly hypocalcemic analogues, are eligible candidates for the prevention and treatment of infections and some autoimmune disorders.
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