Dexamethasone and 2,3,7,8-tetrachlorodibenzo-p-dioxin can induce thymic atrophy by different mechanisms in mice

Interpreting Stress Responses during Routine Toxicity Studies

Stress often occurs during toxicity studies. The perception of sensory stimuli as stressful primarily results in catecholamine release and activation of the hypothalamic–pituitary–adrenal (HPA) axis to increase serum glucocorticoid concentrations. Downstream effects of these neuroendocrine signals may include decreased total body weights or body weight gain; food consumption and activity; altered organ weights (e.g., thymus, spleen, adrenal); lymphocyte depletion in thymus and spleen; altered circulating leukocyte counts (e.g., increased neutrophils with decreased lymphocytes and eosinophils); and altered reproductive functions. Typically, only some of these findings occur in a given study. Stress responses should be interpreted as secondary (indirect) rather than primary (direct) test article–related findings. Determining whether effects are the result of stress requires a weight-of-evidence approach. The evaluation and interpretation of routinely collected data (standard in-life, clinical pathology, and anatomic pathology endpoints) are appropriate and generally sufficient to assess whether or not changes are secondary to stress. The impact of possible stress-induced effects on data interpretation can partially be mitigated by toxicity study designs that use appropriate control groups (e.g., cohorts treated with vehicle and subjected to the same procedures as those dosed with test article), housing that minimizes isolation and offers environmental enrichment, and experimental procedures that minimize stress and sampling and analytical bias.

This article is a comprehensive overview of the biological aspects of the stress response, beginning with a Summary (Section 1) and an Introduction (Section 2) that describes the historical and conventional methods used to characterize acute and chronic stress responses. These sections are followed by reviews of the primary systems and parameters that regulate and/or are influenced by stress, with an emphasis on parameters evaluated in toxicity studies: In-life Procedures (Section 3), Nervous System (Section 4), Endocrine System (Section 5), Reproductive System (Section 6), Clinical Pathology (Section 7), and Immune System (Section 8). The paper concludes (Section 9) with a brief discussion on Minimizing Stress-Related Effects (9.1.), and a final section explaining why Parameters routinely measured are appropriate for assessing the role of stress in toxicology studies (9.2.).

Activation of the aryl hydrocarbon receptor reduces the number of precursor and effector T cells, but preserves thymic CD4+CD25+Foxp3+ regulatory T cells

Aryl hydrocarbon receptor (AhR) activation suppresses immune responses, including allergic sensitization, by increasing the percentage of regulatory (Treg) cells. Furthermore, AhR activation is known to affect thymic precursor T cells. However, the effect of AhR activation on intrathymic CD4+CD25+Foxp3+ Treg cells is unknown. Therefore, we investigated the effect of AhR activation on the percentage and number of CD4+CD25+Foxp3+ Treg cells during allergic sensitization in relevant immunological organs. C3H/HeOuJ mice were treated on day 0 with 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and subsequently sensitized to peanut. On day 8, mice were sacrificed and thymus, spleen and mesenteric lymph nodes (MLN) were isolated. TCDD treatment decreased the number of CD4-CD8-, CD4+CD8+, CD4+CD8- and CD4-CD8+ precursor T cells, but not the number of thymic CD4+CD25+Foxp3+ Treg cells. TCDD treatment increased the number of splenic CD4+CD25+Foxp3+ Treg cells and decreased Th1, Th2 and cytotoxic T cells in the spleen. This appeared to be independent of allergic sensitization. In MLN, TCDD treatment suppressed the increase of the number of CD4+CD25+Foxp3+ Treg cells, Th1, Th2 and cytotoxic T cells induced by peanut sensitization. Together, TCDD treatment preserves thymic CD4+CD25+Foxp3+ Treg cells and decreases peripheral T helper and cytotoxic T cells. This effect of TCDD may contribute to the increased influence of CD4+CD25+Foxp3+ Treg cells on immune mediated responses and to the understanding of how AhR activation modulates immune mediated diseases, including food allergy.

Ezrin is highly expressed in early thymocytes, but dispensable for T cell development in mice

BACKGROUND

Ezrin/radixin/moesin (ERM) proteins are highly homologous proteins that function to link cargo molecules to the actin cytoskeleton. Ezrin and moesin are both expressed in mature lymphocytes, where they play overlapping roles in cell signaling and polarity, but their role in lymphoid development has not been explored.

METHODOLOGY/PRINCIPAL FINDINGS

We characterized ERM protein expression in lymphoid tissues and analyzed the requirement for ezrin expression in lymphoid development. In wildtype mice, we found that most cells in the spleen and thymus express both ezrin and moesin, but little radixin. ERM protein expression in the thymus was differentially regulated, such that ezrin expression was highest in immature thymocytes and diminished during T cell development. In contrast, moesin expression was low in early thymocytes and upregulated during T cell development. Mice bearing a germline deletion of ezrin exhibited profound defects in the size and cellularity of the spleen and thymus, abnormal thymic architecture, diminished hematopoiesis, and increased proportions of granulocytic precursors. Further analysis using fetal liver chimeras and thymic transplants showed that ezrin expression is dispensable in hematopoietic and stromal lineages, and that most of the defects in lymphoid development in ezrin(-/-) mice likely arise as a consequence of nutritional stress.

CONCLUSIONS/SIGNIFICANCE

We conclude that despite high expression in lymphoid precursor cells, ezrin is dispensable for lymphoid development, most likely due to redundancy with moesin.

Ultrastructural effects of DDT, DDD, and DDE on neural cells of the chicken embryo model

Human exposure to environmental compounds with estrogenic activity and the potential effects on human health is the subject of ongoing scientific debates. Their potential effects raise concern regarding neurological development after prenatal exposure. Central to this debate is the pesticide 1,1,1-trichloro-2,2-bis(4-chlorophenyl)ethane (DDT). Although it has apparent low acute toxicity in mammals, DDT has a long residual persistence and laboratory research has indicated that it acts on the CNS by interfering with Na(+)/K(+) pump mechanism of the neuronal membranes, causing disruption in Ca(2+) homeostasis. Potentially this may lead to both apoptosis and necrosis. The present study investigates the effects of DDT and two of its metabolites DDD and DDE on the ultramorphology of neural cells, using a previously published chicken embryo model. Results indicate cellular swelling, budding, and increased membrane permeability for all three chemicals, accompanied by karyolysis in the DDE group (typical features of oncosis). These results support the finding of other researchers as well as the concerns of the WHO that DDT and its metabolites may cause neurotoxicity after prenatal exposure.

Evidence for Involvement of p59fyn in Fasting-Induced Thymic Involution

p59fyn, a member of the src-family protein tyrosine kinase, is expressed abundantly in thymus. We examined the possible involvement of p59fyn in thymic involution induced by a fasting stress in Fyn-/- mice. An acute 48 h fast resulted in severe atrophy of the thymus and a marked decrease of the total thymocyte number with depletion of the CD4+CD8+[double positive (DP)] population in Fyn+/+ (control) mice. A remarkable increase in terminal deoxynucleotidyl transferase-mediated dUTP-biotin nick end labelling-positive signals was detected in the fasted group of control mice. However, these findings were not observed in Fyn-/- mice. Interestingly, MRL/MPJ-lpr/lpr, a Fas-deficient model animal, also showed no significant decrease of DP cell numbers in the fasted group. p59fyn is known to interact with Fas signalling, and these findings suggest that p59fyn is involved in fasting-induced thymic involution, raising the possibility that Fas/p59fyn-mediated signalling may, at least partially, be associated with the phenomenon.

Effects of PCB 126 on primary immune organs and thymocyte apoptosis in chicken embryos

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) and polychlorinated biphenyl (PCB) 126 produce thymic atrophy and immunosuppression. This study explored the hypothesis that the thymic atrophy produced by developmental exposure to PCB 126 is associated with an increase in apoptotic thymocytes at the end of incubation in chicken embryos. Eggs were injected via the air cell with PCB 126 (0.05, 0.13, 0.32, 0.64, and 0.80 ng/g egg) on d 0 of incubation, and tissues were collected on d 20. Controls included noninjected and vehicle-injected (sunflower oil) eggs. Thymocytes were cultured for 6 h and analyzed by flow cytometry for decreased DNA content (propidium iodide staining) and cell size (forward scatter), which indicate apoptosis. PCB 126 induced dose-dependent mortality with an LD50 of 1.01 ng/g and lowest-observed-effect concentration (LOEC) of 0.32 ng/g. Teratogenic effects commonly associated with TCDD and planar PCBs, including cranial and foot deformities and subcutaneous edema, tended to increase with dose of PCB 126. PCB 126 reduced thymus mass by approximately 20% at 0.64 and 0.8 ng/g, the number of viable thymocytes by approximately 20-24% at and above 0.13 ng/g, and the number of bursal lymphoid cells by 57% at 0.64 ng/g. The percentage of apoptotic thymocytes increased with dose, reaching levels 2 times greater than controls at 0.8 ng/g. Electrophoresis of low-molecular-weight DNA from thymocytes of all doses demonstrated fragments in multiples of 180 bp. This DNA laddering is a hallmark of apoptosis. At all doses, thymocytes exhibited caspase-3 activation, another indicator of apoptosis. The results of this experiment supported the hypothesis that the thymic atrophy produced by developmental exposure to PCB 126 in chicken embryos is associated with an increase in apoptotic thymocytes on embryonic d 20.

In vivo tracking of T cell development, ablation, and engraftment in transgenic zebrafish

Transgenic zebrafish that express GFP under control of the T cell-specific tyrosine kinase (lck) promoter were used to analyze critical aspects of the immune system, including patterns of T cell development and T cell homing after transplant. GFP-labeled T cells could be ablated in larvae by either irradiation or dexamethasone added to the water, illustrating that T cells have evolutionarily conserved responses to chemical and radiation ablation. In transplant experiments, thymocytes from lck-GFP fish repopulated the thymus of irradiated wild-type fish only transiently, suggesting that the thymus contains only short-term thymic repopulating cells. By contrast, whole kidney marrow permanently reconstituted the T lymphoid compartment of irradiated wild-type fish, suggesting that long-term thymic repopulating cells reside in the kidney.

Dioxin-induced adseverin expression in the mouse thymus is strictly regulated and dependent on the aryl hydrocarbon receptor

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD), a ligand for the ubiquitous, intracellular aryl hydrocarbon receptor (AhR), up-regulates the actin-modulating protein adseverin in mouse lymphoid tissues, a response that may be correlated to the immunotoxicity of TCDD. Here, by using chimeric mice with TCDD-responsive (AhR(+/+)) hematopoietic cells and TCDD-unresponsive (AhR(minus sign/minus sign)) thymic stroma, or the reverse, we show that TCDD-induced expression of adseverin in thymus is dependent on AhR expression in hematopoietic cells but not in stroma. The use of fetal thymic organ cultures also indicates that TCDD-induced expression of adseverin is confined to the thymocytes. The thymic stroma showed no induction of adseverin expression after TCDD exposure, although TCDD clearly activated the AhR in these cells, as indicated by the induction of CYP1A1. Adseverin was not induced in the thymus of normal adult C57BL/6 mice exposed to beta-estradiol or dexamethasone, two other agents, which also cause thymic atrophy. This further supports that adseverin induction is a specific gene regulatory effect by TCDD on thymocytes.

Differential effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin, bis(tri-n-butyltin) oxide and cyclosporine on thymus histophysiology

Recent advances in the histophysiology of the normal thymus have revealed its complex architecture, showing distinct microenvironments at the light and electron microscopic level. The epithelium comprising the major component of the thymic stroma is not only involved in the positive selection of thymocytes, but also in their negative selection. Dendritic cells, however, are more efficient than epithelial cells in mediating negative selection. Thymocytes are dependent on the epithelium for normal development. Conversely, epithelial cells need the presence of thymocytes to maintain their integrity. The thymus rapidly responds to immunotoxic injury. Both the thymocytes and the nonlymphoid compartment of the organ can be targets of exposure. Disturbance of positive and negative thymocyte selection may have a major impact on the immunological function of the thymus. Suppression of peripheral T-cell-dependent immunity as a consequence of thymus toxicity is primarily seen after perinatal exposure when the thymus is most active. Autoimmunity may be another manifestation of chemically mediated thymus toxicity. Although the regenerative capacity of thymus structure is remarkable, it remains to be clarified whether this also applies to thymus function. In-depth mechanistic studies on chemical-induced dysfunction of the thymus have been conducted with the environmental contaminants 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and bis(tri-n-butyltin)oxide (TBTO) as well as the pharmaceutical immunosuppressant cyclosporine (CsA). Each of these compounds exerts a differential effect on the morphology of the thymus, depending on the cellular targets for toxicity. TCDD and TBTO exposure results in cortical lymphodepletion, albeit by different mechanisms. An important feature of TCDD-mediated thymus toxicity is the disruption of epithelial cells in the cortex. TBTO primarily induces cortical thymocyte cell death. In contrast CsA administration results in major alterations in the medulla, the cortex remaining largely intact. Medullary epithelial cells and dendritic cells are particularly sensitive to CsA. The differential effects of these three immunotoxicants suggest unique susceptibilities of the various cell types and regions that make up the thymus.

Alterations in splenocyte protein kinase C (PKC) activity by 2,3,7,8-tetrachlorodibenzo-p-dioxin in vivo

The effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) on growth factor-coupled activation of nuclear protein kinase C (nPKC) and on the subcellular distribution of PKC activity in rat splenocytes were investigated. Seven days after a single injection of TCDD (50 micrograms/kg body weight), cytosolic and particulate PKC activity was significantly higher in splenocytes from TCDD-treated rats or pair-fed control rats compared to ad libitum-fed animals. In a separate experiment, purified splenocyte nuclei from TCDD-treated animals and controls were used to study activation of nPKC by growth factors and other trophic agents. Growth factor-stimulated nPKC activation was attenuated in splenic nuclei from TCDD-treated rats compared to vehicle-treated controls. Evidence presented here suggests that the cellular mechanism of TCDD toxicity leading to immunosuppression in rodents may be mediated in part by uncoupling of growth factor receptors linked to PKC activation at the level of the nucleus. However, changes in total splenocyte PKC activity appear to be correlated with hypophagia since cytosolic and particulate PKC levels were elevated in TCDD-treated rats and their pair-fed partners.

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD)-induced reduction of adenosine deaminase activity in vivo and in vitro

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) is a potent immunosuppressant in several animal species. The purpose of this study was to determine if TCDD affected the activity of adenosine deaminase (ADA), a purine metabolizing enzyme that is vital to the proper functioning of the immune system. The effect of TCDD on ADA activity was studied in various tissues of male Balb/c mice (a TCDD-responsive strain) and DBA/2 mice (a less-responsive strain). Of the tissues examined after administration of TCDD in vivo (115 micrograms/kg, i.p.), ADA activity was found to be significantly reduced in thymic and splenic tissues of Balb/c mice at 24 hours postadministration. The enzyme activity in these affected tissues remained consistently low through 10 days postadministration. Such an effect of TCDD was both dose and time related in the thymic tissue of Balb/c mice. In contrast, no appreciable alterations in ADA activity were evident in any of the tissues of DBA/2 mice at any of the sampling intervals, indicating that such an effect of TCDD is likely to be mediated through the Ah receptor. This in vivo effect of TCDD on thymic ADA activity was also reproducible in situ where isolated whole thymuses were directly incubated with 10 nM TCDD. In this model, TCDD's effects on ADA activity were antagonized by known protein kinase or phosphorylation inhibitors such as quercetin, genistein, tyrphostin, and neomycin. These results indicate that the effect of TCDD on ADA activity in the thymus may be related to its property to elevate protein kinase activities in this tissue. ADA activity was also reduced in 3T3 cells that were treated with 10 nM TCDD in a low (1%) serum media. In contrast, 25 ng/mL epidermal growth factor (EGF) under such conditions consistently stimulated ADA activity. Interestingly, EGF at a similar concentration failed to elicit a stimulatory effect on ADA activity when cells were pretreated with TCDD. The property of TCDD to lower ADA activity under in vivo, in situ, as well as in vitro conditions appears to be largely related to its action to modulate protein phosphorylation activities.

Induction of apoptosis in mouse thymocytes by cyclosporin A: in vivo study

We investigated the in vivo effect of cyclosporin A (CsA) on mouse thymus and thymocytes. Administration of CsA (10 mg/kg of body weight) was found to induce a marked reduction in the size, weight and consistency of the thymus. These modifications were associated with thymic reticulo-epithelial cells (TREC) and thymocyte damage. Some of the damaged thymocytes displayed characteristic of cells undergoing apoptosis. Ultrastructural study of thymocytes and thymic tissue, as well as DNA electrophoresis of thymocytes, showed chromatin condensation, cellular shrinkage, and nuclear fragmentation in oligonucleosomal fragments. DNA labeling with propidium iodide (PI) of thymocytes from CsA treated mice cultured for 24 hrs showed an increased number of apoptotic nuclei. Furthermore, flow cytometric analysis using monoclonal antibodies (mAbs) specific for thymocyte subsets confirmed that CsA induces a large decrease in the relative number of mature single positive (SP) CD4+CD8- and CD8+CD4- thymocytes expressing high densities of CD3 and T cell receptor ab (TCR alpha beta) surface molecules, but also a decrease in the absolute number of the other thymocyte subsets. These results suggest that CsA causes macroscopic and ultrastructural modifications of the thymus, associated with an active process of cell death in mouse thymocytes in vivo. In line with these results we formulate a hypothesis concerning the stage of T-cell development at which CsA induces apoptosis.

Differential effects of dexamethasone on the thymus and spleen: alterations in programmed cell death, lymphocyte subsets and activation of T cells

The kinetics of DEX-induced changes in lymphocytes from the thymus and spleen of normal mice were examined in relation to cell numbers, programmed cell death (PCD), in vitro proliferative response to anti-CD3 antibodies or Con-A, and changes in lymphocyte subsets by flow cytometry. The above aspects were examined at 48, 24, 18, and 3 h after a single intraperitoneal injection of DEX. Profound reduction of thymocyte numbers was noticed particularly at 48 (74-84%) and 24 (43-55%) h after DEX administration. PCD of thymocytes was not detectable at 48 h, marginally detectable at 24 h, markedly evident at 18 h, and minimally detectable at 3 h pi of DEX. PCD in splenic lymphocytes of DEX-treated mice was not clearly evident at these time points. Thymocytes from mice exposed to DEX (48 or 24 h) proliferated vigorously when cultured in the presence of anti-CD3 or Con-A, thereby suggesting that the remaining thymocytes can transduce activation signals. In contrast, splenic lymphocytes from the same animals responded poorly to these stimulants. Flow cytometric studies revealed that there was a marked increase in number of thymocytes expressing CD3+ (4-6 fold) and alpha beta TCR+ (2-7 fold) surface molecules. On the other hand, no major changes in CD3+ or alpha beta TCR+ cells were noticed in the spleen of DEX-treated mice. Although the total numbers of cells expressing heat stable antigen, M1/69, were not markedly altered after DEX treatment, the fluorescent intensity profile was modified. There were no remarkable changes in CD45RB+ cells in these mice. CD44+ cells were not decreased in DEX-treated thymocytes or splenic lymphocytes. Our results suggest that DEX has differential effects on the thymus and the spleen.

Involution of rat thymus: characterization of cytoplasmic glucocorticoid receptors, evidence of glucocorticoid resistant dexamethasone receptor-positive cells

In rats, thymic relative weight increased after birth reaching maximum values between days 15-30 and then decreased markedly in a similar way in both sexes, while the organ's absolute weight continued to increase until days 80-90 and declined slowly with apparent sex differences from day 30 onward. Scatchard analysis revealed that the [3H] dexamethasone (Dexa) receptor sites concentration showed a pattern comparable to that found in relative thymic weight, with no change in the apparent KD. The reduction of lymphocytes mitotic activity resulting in reduction of immature thymocytes production must be accompanied by a fall of the number of glycocorticoid receptors in ageing thymuses. Despite the profound decrease in the glucocorticoid receptor sites levels, the thymus sensitivity to Dexa remained unchanged during development. Indeed, in prepuberal and adult rats, the steroid administration was followed 4 days after by a transient thymic weight loss of about 70-80% which was mainly linked to the reduction in the cortical area. In contrast, the density of [3H] Dexa binding sites was reduced unexpectedly by 25% only after steroid treatment. These findings provided evidence that Dexa receptor-positive population cells in thymus was formed in a large part by relatively glucocorticoid resistant cells.

Characterization of apoptosis in thymocytes isolated from dexamethasone-treated rats

The induction of apoptosis by glucocorticoids in isolated thymocytes has been studied extensively. However, it is not known whether or not the same changes occur after in vivo glucocorticoid treatment. In order to investigate this, we have studied the changes occurring in thymocytes isolated from rats, from 2-24 hr after a dose of dexamethasone (1 mg/kg), which caused 50% thymic atrophy. Thymocytes were separated into four fractions by isopycnic Percoll gradients. A loss of cells occurred within 2-8 hr, primarily in only one of the two major fractions of normal thymocytes. This loss of normal thymocytes coincided with the appearance of small dense cells with characteristic features of apoptosis including condensed chromatin, increased DNA fragmentation, internucleosomal DNA cleavage and a "hypodiploid" peak on flow cytometric analysis. Striking differences occurred in the cellular composition of the different Percoll fractions with time. Initially (up to 4 hr), the pattern of changes occurring in vivo resembled those found in vitro. However, at later times, the complex fate of apoptotic cells in vivo, such as phagocytosis, are not observed in the in vitro studies.

Programmed cell death: concept, mechanism and control

Programmed cell death or apoptosis occurs under physiological conditions as a result of physiological effectors. It is a relatively slower process and requires active participation of the cell in the suicidal mechanism. Apoptosis is controlled by precise intrinsic genetic programme and may be induced by almost all those stimuli causing necrosis. The role played by the intensity in determining the death process and the underlying mechanism is imperfectly understood. Morphologically apoptotic cells appear as small condensed body. The chromatin is dense and fragmented, packed into compact membrane-bound bodies together with randomly distributed cell organelles. The plasma membrane loses its characteristic architecture and shows extensive blebbing. It buds off projections so that the whole cell may split into several membrane-bound apoptotic bodies. Significant chemical changes take place in the plasma membrane. This helps in recognition of the apoptotic bodies by phagocytes. At this moment it is unclear if all cells can undergo apoptosis or it is a characteristic of only some tissues which are predisposed to apoptotic death being directly under the control of hormones or growth factors. Experimental studies aimed at comparison of induction of apoptosis in cells of different origin are warranted to elucidate this point. Biochemically a pre-commitment step for induction of death programmation through macromolecular synthesis is essential for most systems. The double-stranded linker DNA between nucleosomes is cleaved at regular inter-nucleosomal sites through the action of a Ca2+, Mg(2+)-sensitive neutral endonuclease. Zinc is a potent inhibitor of the enzyme. Calcium probably plays a key controlling role in activation of the enzyme since prevention of Ca2+ increase prevents endonuclease activation. It is becoming evident that signal transduction through appropriate receptors control the Ca2+ flux in the cells. Most apoptotic cells require synthesis of RNA and proteins. Delay or abrogation of apoptosis by inhibition of macromolecular synthesis is well known. The dying cells show high mRNA levels for several enzymes. Several degradative enzymes become active. Regulatory proteins maintain control over the apoptotic cascade. At the molecular level, search has been initiated for the mammalian equivalents of the cell death (ced) gene. Activation of several specific genes is indicated. Specific expression of cell death-associated gene products (e.g. TRPM-2/SGP-2) has been reported in several unrelated apoptotic cell systems. Sequential induction of c-fos, c-myc and 70 kDa heat shock protein is reported. Studies demonstrate that certain genes must remain in a transcriptionally active demethylated state during programmed cell death. Recent evidences clearly indicate that apoptosis may be positively or negatively modulated by certain genes.(ABSTRACT TRUNCATED AT 400 WORDS)