Gene Expression Alterations in Immune System Pathways following Exposure to Immunosuppressive Chemicals

Predicting disease onset in clinically healthy people

Virtually all human disease is induced by oxidative stress. Oxidative stress, which is caused by toxic environmental exposure, the presence of disease, lifestyle choices, stress, chronic inflammation or combinations of these, is responsible for most disease. Oxidative stress from all sources is additive and it is the total oxidative stress from all sources that induces the onset of most disease. Oxidative stress leads to lipid peroxidation, which in turn produces Malondialdehyde. Serum malondialdehyde level is an additive parameter resulting from all sources of oxidative stress and, therefore, is a reliable indicator of total oxidative stress which can be used to predict the onset of disease in clinically asymptomatic individuals and to suggest the need for treatment that can prevent much human disease.

Psychobiological processes: a gene-environment transactional hypothesis

More and more concern over gene-environment interplay has been emerging recently. However studies covering this field of interest mainly focus on environmental toxicants. In this hypothesis I briefly review some outcomes of this area of interest related to social and physical environment, and try and elucidate a psychobiological pathway that could explain some of the mechanisms underlying this interplay. A gene-environment transactional approach related to neurobiological processes is proposed in this perspective.

The embryonic stem cell test combined with toxicogenomics as an alternative testing model for the assessment of developmental toxicity

One of the most studied in vitro alternative testing methods for identification of developmental toxicity is the embryonic stem cell test (EST). Although the EST has been formally validated, the applicability domain as well as the predictability of the model needs further study to allow successful implementation of the EST as an alternative testing method in regulatory toxicity testing. Genomics technologies have already provided a proof of principle of their value in identification of toxicants such as carcinogenic compounds. Also within the EST, gene expression profiling has shown its value in the identification of developmental toxicity and in the evaluation of factors critical for risk assessment, such as dose and time responses. It is expected that the implementation of genomics into the EST will provide a more detailed end point evaluation as compared to the classical morphological scoring of differentiation cultures. Therefore, genomics may contribute to improvement of the EST, both in terms of definition of its applicability domain as well as its predictive capacity. In the present review, we present the progress that has been made with regard to the prediction of developmental toxicity using the EST combined with transcriptomics. Furthermore, we discuss the developments of additional aspects required for further optimization of the EST, including kinetics, the use of human embryonic stem cells (ESC) and computational toxicology. Finally, the current and future use of the EST model for prediction of developmental toxicity in testing strategies and in regulatory toxicity evaluations is discussed.

Gene expression alterations in immune system pathways in the thymus after exposure to immunosuppressive chemicals

BACKGROUND

Dysregulation of positive and negative selection, antigen presentation, or apoptosis in the thymus can lead to immunosuppression or autoimmunity. Diethylstilbestrol (DES), dexamethasone (DEX), cyclophosphamide (CPS), and 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) are immunosuppressive chemicals that induce similar immunotoxic effects in the thymus, however, the mechanism of toxicity is purported to be different for each compound.

OBJECTIVES

We hypothesized that genomic analysis of thymus after chemical-induced atrophy would yield transcriptional profiles that suggest pathways of toxicity associated with reduced function.

METHODS

Female B6C3F1 mice were exposed to these immunosuppressive agents and changes in gene expression and immune cell subpopulations were evaluated.

RESULTS

All four chemicals induced thymic atrophy and changes in both the relative proportion and absolute number of CD3(+), CD4(+)/CD8(-), CD4(-)/CD8(+), and CD4(+)/CD8(+) thymocytes. The most significant impact of exposure to DEX, DES, and CPS was modulation of gene expression in the T-cell receptor (TCR) complex and TCR and CD28 signaling pathways; this could represent a common mechanism of action and play a pivotal role in lineage commitment and development of T cells. Up-regulation of genes associated with the antigen presentation and dendritic cell maturation pathways was the most distinctive effect of TCDD exposure. These elements, which were also up-regulated by DEX and DES, contribute to positive and negative selection.

CONCLUSIONS

Genomic analysis revealed gene expression changes in several pathways that are commonly associated with xenobiotic-induced immune system perturbations, particularly those that contribute to the development and maturation of thymic T cells.

In vitro testing for direct immunotoxicity: state of the art

Immunotoxicity is defined as the toxicological effects of xenobiotics including pharmaceuticals on the functioning of the immune system and can be induced in either direct or indirect ways. Direct immunotoxicity is caused by the effects of chemicals on the immune system, leading to immunosuppression and subsequently to reduced resistance to infectious diseases or certain forms of nongenotoxic carcinogenicity.In vitro testing has several advantages over in vivo testing, such as detailed mechanistic understanding, species extrapolation (parallelogram approach), and reduction, refinement, and replacement of animal experiments. In vitro testing for direct immunotoxicity can be done in a two-tiered approach, the first tier measuring myelotoxicity. If this type of toxicity is apparent, the compound can be designated immunotoxic. If not, the compound is tested for lymphotoxicity (second tier). Several in vitro assays for lymphotoxicity exist, each comprising specific functions of the immune system (cytokine production, cell proliferation, cytotoxic T-cell activity, natural killer cell activity, antibody production, and dendritic cell maturation). A brief description of each assay is provided. Only one assay, the human whole blood cytokine release assay, has undergone formal prevalidation, while another one, the lymphocyte proliferation assay, is progressing towards that phase.Progress in in vitro testing for direct immunotoxicity includes prevalidation of existing assays and selection of the assay (or combination of assays) that performs best. To avoid inter-species extrapolation, assays should preferably use human cells. Furthermore, the use of whole blood has the advantage of comprising multiple cell types in their natural proportion and environment. The so-called "omics" techniques provide additional mechanistic understanding and hold promise for the characterization of classes of compounds and prediction of specific toxic effects. Technical innovations such as high-content screening and high-throughput analysis will greatly expand the opportunities for in vitro testing.

Does cyclophosphamide play a protective role against neuronal loss in chronic T. cruzi infection?

In this study, we verified the possible role of cyclophosphamide (CY) in protecting or not against neuronal losses in young and aged male Calomys callosus chronically infected with the MORC-1 strain of Trypanosoma cruzi through numerical quantification of neurons from the myenteric plexus of the colon and quantification of nitric-oxide concentration (NO) during the acute and chronic phase of infection. For this purpose, groups of young C. callosus were infected with the MORC-1 strain of T. cruzi. A group of infected animals received i.p. 0.2 mg/ml genuxal dissolved in distilled water treatment with CY. NO concentration in aged animals displayed reduced levels when compared to those found in young animals. No significant alterations in the number of neurons were observed in young animals, but for aged ones, a protective role of CY in reducing neuron loss was noted, in addition to enhancing the neuronal volume, area, and perimeter. These results suggest that CY administration, depending on the dose and time span, can act as a protective agent against neuronal losses.

In vitro immunotoxicity of bis(tri-n-butyltin)oxide (TBTO) studied by toxicogenomics

The biocide and environmental pollutant bis(tri-n-butyltin)oxide (TBTO) causes thymus atrophy in rodents. Whether the depletion of thymic lymphocytes by tributyltin compounds may be the result of inhibition of cell proliferation or induction of apoptosis is subject of debate. We examined gene expression profiles in primary rat thymocytes exposed to TBTO in vitro at dose levels of 0, 0.1, 0.3, 0.5, and 1.0microM. By measuring cell viability and apoptosis, exposure conditions were selected that would provide information on changes in gene expression preceding or accompanying functional effects of TBTO. Several processes related to TBTO-induced toxicity were detected at the transcriptome level. Effects on lipid metabolisms appeared to be the first indication of disruption of cellular function. Many transcriptional effects of TBTO at higher dose levels were related to apoptotic processes, which corresponded to present or subsequent thymocyte apoptosis observed phenotypically. The gene expression profile was, however, not unambiguous since expression of apoptosis-related genes was both increased and decreased. Stimulation of glucocorticoid receptor signaling appeared to be a relevant underlying mechanism of action. These findings suggest that TBTO exerts its toxic effects on the thymus primarily by affecting apoptotic processes, but the possibility is discussed that this may in fact represent an early effect that precedes inhibition of cell proliferation. At the highest dose level tested, TBTO additionally repressed mitochondrial function and immune cell activation. Our in vitro toxicogenomics approach thus identified several cellular and molecular targets of TBTO that may mediate the toxicity towards thymocytes and thereby its immunosuppressive effects.