An in vivo and in vitro study of selenium deficiency and infection in rats

Role of Selenoproteins in Bacterial Pathogenesis

The trace element selenium is an essential micronutrient that plays an important role in maintaining homeostasis of several tissues including the immune system of mammals. The vast majority of the biological functions of selenium are mediated via selenoproteins, proteins which incorporate the selenium-containing amino acid selenocysteine. Several bacterial infections of humans and animals are associated with decreased levels of selenium in the blood and an adjunct therapy with selenium often leads to favorable outcomes. Many pathogenic bacteria are also capable of synthesizing selenocysteine suggesting that selenoproteins may have a role in bacterial physiology. Interestingly, the composition of host microbiota is also regulated by dietary selenium levels. Therefore, bacterial pathogens, microbiome, and host immune cells may be competing for a limited supply of selenium. Elucidating how selenium, in particular selenoproteins, may regulate pathogen virulence, microbiome diversity, and host immune response during a bacterial infection is critical for clinical management of infectious diseases.

Dietary selenium supplementation and whole blood gene expression in healthy North American men

Selenium (Se) is a trace nutrient required in microgram amounts, with a recommended dietary allowance of 55 μg/day in humans. The nutritional functions of Se are performed by a group of 25 selenoproteins containing the unusual amino acid selenocysteine at their active sites. The selenoproteins with known activities are oxidation-reduction enzymes with roles in antioxidant protection, redox homeostasis and signaling, and thyroid hormone metabolism. Both deficiencies and excesses of Se are associated with impaired innate and adaptive immune responses. We supplemented 16 healthy men for 1 year with 300 μg Se/day as high-Se yeast or placebo yeast and measured whole blood gene expression with DNA microarrays before and after supplementation. Protein phosphorylation was the main biological process in common among the Se-responsive genes, which included a prominent cluster of protein kinases, suggesting that protein phosphorylation in leukocytes is sensitive to Se supplementation. We found highly ranked clusters of genes associated with RNA processing and protein transport, suggesting that dietary Se may regulate protein expression in leukocytes at both the posttranscriptional and posttranslational levels. The main functional pathway affected by Se supplementation was FAS apoptosis signaling, and expression of genes associated with T cell and natural killer cell cytotoxicity was increased. At the same time, the numbers of circulating natural killer and T cells expressing activation markers decreased. These changes are consistent with an anti-inflammatory effect of Se supplementation exerted through regulation of protein phosphorylation.

The effect of selenium supplementation on DTH skin responses in healthy North American men

The trace element selenium (Se) is essential for immune system development and function in animals. However, the exact functions of Se in the human immune system and the achievable health benefits from Se supplementation remain unclear. To test whether an increased intake of dietary Se affects immune function, we conducted a randomized, controlled trial of Se supplementation in healthy free-living men. Forty-two men were administered 300microg of Se a day as high-Se Baker's yeast, or low-Se yeast for 48 weeks. Serum immunoglobulins, differential complete blood counts and lymphocyte sub-populations were measured every 6 weeks. Tests of delayed-type hypersensitivity (DTH) skin responses to mumps, candida, trychophyton, tuberculin-purified protein, and tetanus were performed at baseline and at the end of 48 weeks of treatment. Supplementation increased blood Se concentration by 50%. Surprisingly, consumption of the low-Se yeast induced anergy in DTH skin responses and increased counts of natural killer (NK) cells and T lymphocytes expressing both subunits of the high affinity interleukin-2 receptor (IL2R). DTH skin responses and IL2R+ cells did not change in the high-Se group, suggesting Se supplementation blocked induction of DTH anergy. There were no differences between groups in quality of life indicators, number of days sick, other leukocyte phenotypes, serum immunoglobulins, or complement factors. These results suggest that Se plays a role in immunotolerization, a cell-mediated process involved in many aspects of immune function.

Interactions among infections, nutrients and xenobiotics

During recent years there have been several incidents in which symptoms of disease have been linked to consumption of food contaminated by chemical substances (e.g., 2,3,7,8-tetrachlorodibenzo-p-dioxin, TCDD). Furthermore, outbreaks of infections in food-producing animals have attracted major attention regarding the safety of consumers, e.g., Bovine Spongiform Encephalitis (BSE) and influenza in chicken. As shown for several xenobiotics in an increasing number of experimental studies, even low-dose xenobiotic exposure may impair immune function over time, as well as microorganism virulence, resulting in more severe infectious diseases and associated complications. Moreover, during ongoing infection, xenobiotic uptake and distribution are often changed resulting in increased toxic insult to the host. The interactions among infectious agents, nutrients, and xenobiotics have thus become a developing concern and new avenue of research in food toxicology as well as in food-borne diseases. From a health perspective, in the risk assessment of xenobiotics in our food and environment, synergistic effects among microorganisms, nutrients, and xenobiotics will have to be considered. Otherwise, such effects may gradually change the disease panorama in society.

The effect of subclinical selenium toxicosis on pregnant beef cattle

A field investigation conducted by the South Dakota Animal Disease Research and Diagnostic Laboratory suggested that subclinical selenium toxicosis in pregnant cows may have contributed to an outbreak of aborted/stillborn calves in a high-selenium region of South Dakota. This study was undertaken to evaluate the relationship between abortion and subclinical selenium toxicosis in the dam and to assess the effects of subclinical selenium toxicosis on the bovine immune system. Fifteen pregnant cows were fed diets containing 0.25 (control), 6.0, and 12.0 ppm selenium beginning at 80-110 days gestation. Although selenium toxicosis has been reported to cause abortion, this study failed to reproduce abortions. A single cow in the 12-ppm selenium treatment group gave birth to a weak calf, which subsequently died. This calf had myocardial lesions consistent with those described for selenium toxicosis and had hepatic selenium levels of 9.68 ppm (wet weight). Elevated dietary selenium resulted in the depression of several leukocyte function parameters in pregnant cows. A statistically significant depression in forced antibody response was identified in both selenium-supplemented groups. A significantly diminished mitogenic response to concanavalin A and pokeweed mitogen was also observed in the 12-ppm selenium group. Although a similar pattern of depression was also observed with phytohemagglutinin, differences were not significant. These findings indicate that even in the absence of clinical alkali disease, elevated selenium levels may adversely affect both pregnancy outcome and the bovine immune system.

Effects of selenium and vitamin E on the immune responses of domestic animals

The gross effects of selenium and vitamin E deficiency on animal production and the potential benefits of supplementation are recognised, and the influence of these micronutrients on the resistance of animals to a variety of infections has also been reported. As a result, deficiencies could compromise the immune system and result in a decline in production and performance before gross effects become apparent. Data obtained by laboratories studying different animal species indicate that the responses of domestic animals, particularly ruminants, differ from those of human beings and laboratory animals as well as differing among themselves. This paper provides a comprehensive review of the effects of selenium and vitamin E on the immune responses of domestic animals and discusses their effects with respect to the differences in the basal nutritional status of the animals concerned, the type of supplements used, the route and timing of their administration and the different agents which have been used to stimulate an immune response.

Effect of selenium and reducing agents on in vitro immunoglobulin M synthesis by bovine lymphocytes

A series of experiments was conducted to evaluate the effects of inorganic and organic forms of Se with or without reducing agents on in vitro IgM production by bovine lymphocytes. Peripheral mononuclear cells were isolated from nonlactating Jersey cows fed a diet with adequate Se. Cells were stimulated with pokeweed mitogen and, in addition, were cultured with various Se compounds at a concentration of 100 ng Se/ml. Mercaptoethanol (50 microM) and glutathione (1 mM) were included in cultures of cells stimulated by pokeweed mitogen with and without inorganic Se. Sodium selenite was less effective than selenomethionine and selenocystine in augmenting pokeweed mitogen-induced Ig synthesis. The addition of mercaptoethanol to pokeweed mitogen-stimulated control cultures enhanced in vitro IgM production, whereas the addition of glutathione had a negligible effect, but addition of either in combination with sodium selenite dramatically depressed IgM production. These results suggest that Se in inorganic or organic forms enhances B-cell function in vitro.

Advances in the role of minerals in immunobiology

Immunology and our understanding of its various cells, immunoglobulins and lymphokines are recent events that date from the work of Pasteur and Metchnikoff in the late nineteenth century. Experimental evidence has shown the importance of adequate dietary protein and vitamins. The present review examines past and recent experimental evidence for the role of minerals in the functioning of the immune system. Included is in vivo and in vitro information on the macrominerals; calcium and magnesium, the micro-(trace) minerals; iron, zinc, copper, and selenium as they affect various components of the immune system. The effects of gold as either gold-thiomaleate or gold-thioglucose on selenium is also reviewed.

Inhibition of hepatic deiodination of thyroxine is caused by selenium deficiency in rats

Selenium (Se) deficiency produced up to a 14-fold decrease in hepatic tri-iodothyronine (T3) production from thyroxine (T4) in vitro. The T3 production rate could not be restored by the addition of a variety of cofactors, nor by the addition of control homogenate. The impairment in hepatic T3 production observed in Se deficiency was reflected in the concentrations of thyroid hormones circulating in plasma, T4 being increased approx. 40% and T3 being decreased by 30%. However, the fall in plasma T3 concentrations was smaller than might be expected in view of the marked decreased in T3 production. Se deficiency had no measurable effect on plasma reverse-tri-iodothyronine concentrations. The data suggest that Se deficiency produces an inhibition of both 5- and 5'-deiodination, consistent with the widely held view that these reactions are catalysed by the same enzyme complex. The mechanism of inhibition appears not be mediated by changes in thiol levels, but a direct role of Se in the activity of the deiodinase complex cannot be excluded.