Research Article

Role of Fat-Soluble Vitamins A and D in the Pathogenesis of Influenza: A New Perspective

Figure 2

Retinoic acid synthesis pathway in dendritic cells and its effects on lymphocytes. Retinoic acid is produced from vitamin A (retinol) via a two-step enzymatic pathway that oxidizes retinol to retinaldehyde and then retinaldehyde to RA. Oxidation of retinol to retinaldehyde requires the activities of several alcohol dehydrogenises (ADH-1, -4, -5) and subsequently, retinaldehyde is oxidized to retinoic acid by retinal dehydrogenases (RALDH). RA produced by DCs acts on T and B lymphocytes and induces the mucosal homing receptors α4β7-integrin, and CCR9. RA in the presence of TGF-β promotes the conversion of naïve T cells into Foxp3+ regulatory T cells and at high concentration inhibits the differentiation of Th-17 cells. In addition, RA synergizes with IL-6 and IL-5 and promotes class-switching to IgA in B cells (source: [67]).
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