Applied nutritional investigationExpression of retinoic acid receptors in intestinal mucosa and the effect of vitamin A on mucosal immunity
Introduction
Many recent investigations have indicated that vitamin A (VA) deficiency (VAD) in children is still a serious public health issue in low-income countries [1]. Children with VAD have an increased incidence of respiratory tract and enteric infections, such as measles, pneumonia, diarrhea, etc. [2], but supplementation with VA has been shown to prevent these diseases and aid treatment [3]. Because VA is one of the most important microelements for childhood development, the positive effect of VA on mucosal immunity against infections has been widely accepted. Although VA has been shown to increase the production of secretory immunoglobulin (Ig) A in mucosa [4] and modulate the function of T and B lymphocytes [5], the mechanism for VA promotion of mucosal immunity remains unclear.
Retinoic acid receptors can be divided into RARs (α, β, γ) and RXRs (α, β, γ), whose distributions are tissue specific [6]. Retinoic acid (RA) exerts its effects on cellular growth, differentiation, and function exclusively by these nuclear receptors [7]. Our laboratory previously investigated the expression of these receptors in lymph nodes and thymus [8], [9] and found that RAR-α plays a key role in the modulation of T and B lymphocytes by RA. In the present study, the expression of these retinoic acid receptors was examined for the first time in the intestinal mucosa of normal rats and rats with VAD to assess which receptor's expression is influenced most by VAD. Furthermore, in in vitro culture we used Ro 41-5253, which is a specific antagonist of RAR-α, to substantiate the role of RAR-α in RA modulation of intestinal mucosal immunity.
Dendritic cells (DCs), which are the most powerful antigen-presenting cells, are critical players in intestinal immune defense. Immature DCs reside dispersedly in the entire intestinal mucosa with prominent localization in the subepithelial dome of Peyer's patches (PPs), which are enriched in terminal ileum. There, DCs guard the sites of pathogen entry into the host, capture invading micro-organisms, and then migrate to interfollicular traffic areas of the PPs, gradually maturing during migration. Mature DCs initiate a primary immune response through expression of major histocompatibility complex and costimulatory molecules. Moreover, the interaction of DCs with T cells is important in directing T-helper type 0 (Th0) cell differentiation and in determining the resulting cytokine production profile and function [10]. The production of interleukin (IL)-12 (secreted mainly by DCs) and interferon-γ (IFN-γ; a Th1 cytokine) mediate Th1 differentiation, whereas the production of IL-4, IL-5, and IL-6 (Th2 cytokines) favors Th2 differentiation. In addition, IL-10 had been shown to skew T-cell responses toward T regulatory (Treg) cells that produce high levels of IL-10 and inhibit antigen-specific T-cell responses [11].
Previous studies from our laboratory [12] and other laboratories [13] have demonstrated that DCs are also the target cells of VA. Determining the role of VA in the differentiation, maturation, and function of intestinal DCs is therefore of great interest, because it could help to clarify the mechanism by which VA modulates intestinal mucosal immunity from the primary step of immune response. Rats with VAD were established as a model for in vivo studies, and PPs from the rats were cultured with all-trans RA (at-RA) and/or Ro 41-5253 in vitro to study the impact of VAD and VA supplementation on DCs and local cytokine production and on the retinoic acid receptors involved.
Section snippets
Rat model of VAD
The methods used for establishing the rat model of VAD have been described elsewhere [14]. Maternal Sprague-Dawley rats (Animal Center of Shanghai Medical Collage, Fudan University) that had been pregnant for 10 d were fed a VA-deficient (TD.86143) diet (Harlan Teklad, Madison WI, USA) until weaning. Then the male weanling offspring were fed the same diet until the state of VAD was reached, and they were then sacrificed. All animals appeared to be healthy except for VAD-related characteristics.
Results
The gene expression of RAR-α, β, γ and RXR-α, β, γ in the mucosa of terminal ileum in normal rats and rats with VAD was measured by RT fluorescence quantitative PCR. The results showed that the RAR-α mRNA in VAD rats was significantly lower than in control rats (n = 10, P = 0.029). Few significant differences were seen in the rest of the receptors between VAD and control rats (Fig. 1).
To investigate the role of RAR-α in the effects of at-RA on intestinal mucosa, the expression of RAR-α mRNA was
Discussion
As one of the most important microelements during childhood, the positive effect of VA in reinforcing the anti-infective immunity of the digestive tract has been widely accepted. Many studies have shown that a lack of VA can cause abnormalities in epithelial cells, decreased cell-mediated immunity [18], diminished Th2-dependent antibody responses [19], altered polarization of Th1/Th2 cytokines, etc. It is now appreciated that DCs play a primary role in oral tolerance and defense against mucosal
Acknowledgments
The authors thank F. Hoffmann-La Roche for providing us with Ro 41-5253.
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