The role of chemical-induced stress responses in immunosuppression: a review of quantitative associations and cause-effect relationships between chemical-induced stress responses and immunosuppression

Multi-biomarker approach to evaluate the toxicity of chlorpyrifos (active ingredient and a commercial formulation) on different stages of Biomphalaria straminea

Biomphalaria straminea is a freshwater gastropod native to South America and used in toxicological assessments. Our aim was to estimate 48 h-LC50 and sub-chronic effects after the exposure to low concentrations of chlorpyrifos as commercial formulation (CF) and active ingredient (AI) on B. straminea adult, embryos and juveniles. Concentrations between 1 and 5000 μg L-1 were chosen for acute exposures and 0.1 and 1 μg L-1 for the sub-chronic one. After 14 days biochemical parameters, viability and sub-populations of hemocytes, reproductive parameters, embryotoxicity and offspring' survival were studied. Egg masses laid between day 12 and 14 were separated to continue the exposure and the embryos were examined daily. Offspring' survival and morphological changes were registered for 14 days after hatching. 48 h-LC50, NOEC and LOEC were similar between CF and AI, however the CF caused more sub-lethal effects. CF but not the AI decreased carboxylesterases, catalase and the proportion of hyalinocytes with respect to the total hemocytes, and increased superoxide dismutase and the % of granulocytes with pseudopods. Also CF caused embryotoxicity probably due to the increase of embryos' membrane permeability. Acetylcholinesterase, superoxide dismutase, hemocytes sub-populations, the time and rate of hatching and juveniles' survival were the most sensitive biomarkers. We emphasize the importance of the assessment of a battery of biomarkers as a useful tool for toxicity studies including reproduction parameters and immunological responses. Also, we highlight the relevance of incorporating the evaluation of formulations in order to not underestimate the effects of pesticides on the environment.

Reproduction and caste ratios under stress in trematode colonies with a division of labour

Trematodes form clonal colonies in their first intermediate host. Individuals are, depending on species, rediae or sporocysts (which asexually reproduce) and cercariae (which develop within rediae or sporocysts and infect the next host). Some species use a division of labour within colonies, with 2 distinct redial morphs: small rediae (non-reproducing) and large rediae (individuals which produce cercariae). The theory of optimal caste ratio predicts that the ratio of caste members (small to large rediae) responds to environmental variability. This was tested in Philophthalmus sp. colonies exposed to host starvation and competition with the trematode, Maritrema novaezealandensis. Philophthalmus sp. infected snails, with and without M. novaezealandensis, were subjected to food treatments. Reproductive output, number of rediae, and the ratio of small to large rediae were compared among treatments. Philophthalmus sp. colonies responded to host starvation and competition; reproductive output was higher in well-fed snails of both infection types compared with snails in lower food treatments and well-fed, single infected snails compared with well-fed double infected snails. Furthermore, the caste ratio in Philophthalmus sp. colonies was altered in response to competition. This is the first study showing caste ratio responses to environmental pressures in trematodes with a division of labour.

Maternal and early life stress effects on immune function: relevance to immunotoxicology

Stress is triggered by a variety of unexpected environmental stimuli, such as aggressive behavior, fear, forced physical activity, sudden environmental changes, social isolation or pathological conditions. Stressful experiences during very early life (particularly, maternal stress during fetal ontogeny) can permanently alter the responsiveness of the nervous system, an effect called programming or imprinting. Programming affects the hypothalamic-pituitary-adrenocortical (HPA) axis, brain neurotransmitter systems, sympathetic nervous system (SNS), and the cognitive abilities of the offspring, which can alter neural regulation of immune function. Prenatal or early life stress may contribute to the maladaptive immune responses to stress that occur later in life. This review focuses on the effect of maternal and early life stress on immune function in the offspring across life span. It highlights potential mechanisms by which prenatal stress impacts immune functions over life span. The literature discussed in this review suggests that psychosocial stress during pre- and early postnatal life may increase the vulnerability of infants to the effects of immunotoxicants or immune-mediated diseases, with long-term consequences. Neural-immune interactions may provide an indirect route through which immunotoxicants affect the developing immune system. A developmental approach to understanding how immunotoxicants interact with maternal and early life stress-induced changes in immunity is needed, because as the body changes physiologically across life span so do the effects of stress and immunotoxicants. In early and late life, the immune system is more vulnerable to the effects of stress. Stress can mimic the effects of aging and exacerbate age-related changes in immune function. This is important because immune dysregulation in the elderly is more frequently and seriously associated with clinical impairment and death. Aging, exposure to teratogens, and psychological stress interact to increase vulnerability and put the elderly at the greatest risk for disease.

Hormetic Dose-Response Relationships in Immunology: Occurrence, Quantitative Features of the Dose Response, Mechanistic Foundations, and Clinical Implications

This article provides an assessment of the occurrence of immune-system-related hormetic-like biphasic dose-response relationships. Such dose-response relationships are extensive, with over 90 different immune response-related endpoints reported, induced by over 70 endogenous agonists, over 100 drugs, and over 40 environmental contaminants. Such hormetic responses were reported in over 30 animal models, over a dozen mammalian and human cell lines. These findings demonstrate that immune-system-related hormetic-like biphasic dose-response relationships are common and highly generalizable according to model, endpoint, and chemical class. The quantitative features of the dose response are generally consistent with previously published examples of hormetic dose responses for other biological endpoints. These findings were generally recognized and explicitly discussed by the original authors, often with consideration given to possible mechanistic foundations as well as numerous clinical implications. Despite the recognition by individual authors of the hormetic nature of these observed responses, the overall widespread nature of immune-related hormetic responses has been only little appreciated, with a general lack of insight into the highly generalizable nature of this phenomenon as well as the complex regulatory networks affecting biological switching mechanisms that result in the hormetic responses.

Enhancement of developmental toxicity effects of chemicals by gestational stress. A review

Risk assessment of developmental toxicants is almost exclusively based on single chemicals studied in animals under controlled experimental conditions, as to reduce stress. Although humans may be exposed simultaneously to numerous hazards, little is known about the interaction of prenatal chemical exposures with other factors, such as maternal stress, itself a modifier of fetal development. Gestational stress has been hypothesized to enhance the developmental toxicity of chemicals. This review identified 36 animal studies investigating if maternal stress may enhance the effects of prenatal chemical exposure, and evaluated the presented hypothesis. Studies of a broad range of chemicals and developmental endpoints support the notion, that maternal stress is able to enhance the effects of developmental toxicants, although stress mitigated chemically induced effects in a few cases. Maternal stress most often enhanced chemical developmental toxicity at dose levels associated with severe maternal toxicity or where test agents were already above threshold for effect. Thus, LOAEL(chemical) was generally similar to LOAEL(chemical+stress), although not necessarily for the same endpoint. It should be noted that the database contained a limited number of studies, and only a single high dose level was applied in most studies, rendering establishment of NOAELs for combined exposures impossible. Furthermore, for some compounds, the margin between human exposure levels and the LOAEL(chemical+stress) seems small. Future studies are recommended to investigate compounds, for which maternal stress was already proven as an enhancer, at lower dose levels. Interactive response seems to depend on stressor severity and timing of chemical exposure relative to maternal stress which should be further scrutinized.

Characterization of thymic atrophy and the mechanism of thymocyte depletion after in vivo exposure to a mixture of herbicides

3,4-Dichloropropionanilide (propanil) and 2,4-dichlorophenoxyacetic acid (2,4-D) are two commonly used herbicides that are marketed as a chemical mixture. It was hypothesized that the interaction between these two herbicides, when administered as a mixture, would result in a greater effect on the immune system than the individual components of the mixture. The present study demonstrates in a murine model that a mixture of propanil and 2,4-D, when compared to single herbicide exposures, exacerbates decreases in thymocyte populations 2 d postexposure and inhibits the repopulation of T-cells in the thymus 7 d postexposure. Exposure to 150 mg herbicide/kg body weight of propanil or 2,4-D alone had no effect on thymus weight. In contrast, decreases in the ratio of thymus weight to body weight (TW:BW) occurred 2 d after treatment with the mixture of 150 mg propanil/kg body weight + 150 mg 2,4-D/kg body weight (150/150). Thymic atrophy was associated with a decrease in the double-positive thymocyte population (CD4+CD8+) and correlated with sera corticosterone levels from 600 to 1000 pg/ml. Therefore, the hypothesis was tested that glucocorticoids, induced after exposure to herbicides, were responsible for the thymic atrophy and depletion of thymocytes. However, similar levels of corticosterone were induced after exposure to 50, 100, or 150 mg propanil/kg body weight, and 50/50 or 100/100 mixture treatments, doses that did not produce thymic atrophy or cell loss. In addition, RU 486, a glucocorticoid receptor blocker, only partially abrogated the thymic atrophy in mice exposed to the 150/150 mixture of herbicides. These results suggest that glucocorticoids are only partially responsible for herbicide-induced thymic atrophy. This study demonstrates that the effects of exposure to a mixture of chemicals cannot always be predicted based on single exposure data and emphasizes the importance of mixture-based studies.

Effects of prenatal exposure to chronic mild stress and toluene in rats

The aim of the present study was to elucidate whether prenatal chronic stress, in combination with exposure to a developmental neurotoxicant, would increase effects in the offspring compared with the effects of either exposure alone. Development and neurobehavioral effects were investigated in female offspring of pregnant rats (Mol:WIST) exposed to chronic mild stress (CMS) during gestational days (GD) 9-20, or 1500 ppm toluene, 6 h/day during gestational days 7-20, or a combination of the two. Prenatal CMS was associated with decreased thymic weight and increased auditory startle response. The corticosterone response to restraint seemed modified by prenatal exposure to toluene. Lactational body weight was decreased in offsprings subjected to CMS, primarily due to effects in the combined exposure group. Cognitive function was investigated in the Morris water maze, and some indications of improved function due to CMS were observed. In the present experimental setting, there was no indication of the two exposures potentiating each other with respect to adverse effects on the nervous system. However, the effects of prenatal CMS indicate that stress during fetal life may interfere with the development of the thymus and increase the reactivity (startle reflex) of the offspring.

Regulatory immunotoxicology: does the published evidence support mandatory nonclinical immune function screening in drug development?

Recent immunotoxicity guidance documents from the EU CHMP and the US FDA apply significantly different weightings to immune function testing; whereas the former mandates (as a starting point) incorporation of immune function tests (IFTs) to screen for immunotoxic potential in sub-chronic rodent toxicity studies, the more cautious 'for cause' FDA approach recommends the use of IFTs only when warranted by evidence obtained from conventional nonclinical and/or clinical studies. Conclusions from detailed evaluations of several key drugs, including salmeterol and some opioids, challenge the notion that data on these examples support the need for IFTs to detect unintended immunosuppression. Given the virtual absence of convincing pharmaceutical examples and the rarity of unintended immunosuppression, routine immune function testing of all new pharmaceuticals is not considered justified. Resources currently being employed in this manner in an attempt to detect a seemingly rare phenomenon would appear to be better applied to the development of reliable predictive assays for drug hypersensitivity, which is known to cause significant patient morbidity. Any moves towards a globally harmonised guideline that recommends the use of concern-based IFTs, need ideally to be accompanied by the establishment of appropriate historical control reference intervals and interpretation criteria to support a reliable weight-of-evidence approach to data evaluation.

Neurologic and immunologic effects of exposure to corticosterone, chlorpyrifos, and multiple doses of tri-ortho-tolyl phosphate over a 28-day period in rats

An animal (rat) model of chronic stress (corticosterone in the drinking water) was used to study the interaction of stress and the organophosphorus (OP) neurotoxicants chlorpyrifos (60 mg/kg subcutaneously in a single dose) and tri-ortho-tolyl phosphate (TOTP, at 75, 150, or 300 mg/kg given 7 times orally in a 2-wk period). Adult male Long-Evans rats were provided with corticosterone in drinking water (400 microg/ml, w/v) for a total of 28 d, which led to significantly decreased weight and decreased cellularity of the thymus and spleen. Seven days after initiation of corticosterone treatment, half of the rats were given chlorpyrifos, and an additional 7 d later the 2-wk, 7-dose treatment of TOTP was initiated. During the 28-d test period, behavior of rats was evaluated using a functional observational battery (FOB), motor activity, and passive avoidance. Reductions in body weight, grip strength, and ambulatory movements occurred as a result of corticosterone treatment. Decreased body weight and grip strength were also elicited by TOTP, and the interactions of corticosterone and TOTP enhanced the effects on body weight and grip strength. Blood cholinesterase levels were obtained during the 28-d study period and found useful for monitoring OP exposure. At the end of the 28-d testing period, rats were sacrificed and activities of cholinesterase, neurotoxic esterase (neuropathy target esterase), and/or carboxylesterase were evaluated in blood, liver, and/or brain regions (basal forebrain, caudate putamen, cerebral cortex, hippocampus). All these esterases in brain were inhibited in a dose-related manner by TOTP, with some enhancement in rats drinking corticosterone-containing water. In addition, choline acetyltransferase, glial acidic fibrillary protein (GFAP), glutathione peroxidase, and superoxide dismutase were evaluated in one or more of the brain regions already identified. Choline acetyltransferase, glutathione peroxidase, and superoxide dismutase activities were unaffected by treatments. However, GFAP was elevated above control levels in the cerebral cortex of rats by all treatments (corticosterone, chlorpyrifos, TOTP). Neuropathological examination revealed early stages of dose-related increased distal myelinated fiber axonal degeneration seen in the medullary fasciculus gracilis at only the highest dose of TOTP (300 mg/kg).

Extended histopathology in immunotoxicity testing: interlaboratory validation studies

There has been considerable interest in the use of expanded histopathology as a primary screen for immunotoxicity assessment. To determine the utility of a semiquantitative histopathology approach for examining specific structural and architectural changes in lymphoid tissues, a validation effort was initiated. This study addresses the interlaboratory reproducibility of extended histopathology, using tissues from studies of ten test chemicals and both negative and positive controls from the National Toxicology Program's immunotoxicology testing program. We examined the consistency between experienced toxicologic pathologists, who had varied expertise in immunohistopathology in identifying lesions in immune tissues, and in the sensitivity of the individual and combined histopathological endpoints to detect chemical effects and dose response. Factor analysis was used to estimate the association of each pathologist with a so-called "common factor" and analysis-of-variance methods were used to evaluate biases. Agreement between pathologists was highest in the thymus, in particular, when evaluating cortical cellularity of the thymus; good in spleen follicular cellularity and in spleen and lymph node-germinal center development; and poorest in spleen red-pulp changes. In addition, the ability to identify histopathological change in lymphoid tissues was dependent upon the experience/training that the individual pathologist possessed in examining lymphoid tissue and the apparent severity of the specific lesion.

Potentiation of trichothecene-induced leukocyte cytotoxicity and apoptosis by TNF-alpha and Fas activation

Trichothecene mycotoxins cause immunosuppression by inducing apoptosis in lymphoid tissue. Trichothecene-induced leukocyte apoptosis can be augmented by bacterial lipopolysaccharide (LPS) but the mechanisms involved in this potentiating effect are not completely understood. The objective of this study was to test the hypothesis that the trichothecene deoxynivalenol (DON, vomitoxin) can interact with LPS directly and other mediators or agonists associated with immune/inflammatory responses to induce apoptosis in primary murine leukocyte cultures. Primary leukocyte suspensions were prepared from murine thymus (TH), spleen (SP), bone marrow (BM) and Peyer's patches (PP) and then cultured with DON in the absence or presence of LPS, prostaglandin E2 (PGE2), anti-immunoglobulin (as antigen mimic), dexamethasone, Fas ligand, or TNF-alpha. Cytotoxicity and apoptosis were evaluated by MTT assay and morphologic assays, respectively. DON was found to inhibit LPS-induced proliferation and dexamethasone-induced apoptosis in SP cultures. In contrast, potentiation of DON-induced apoptosis and cytotoxicity was observed in BM cultures treated with anti-Fas and in TH cultures treated with TNF-alpha. When potentiation of DON-induced apoptosis by TNF-alpha was assessed using pharmacological inhibitors, generation of ROS, intracellular Ca2+, p38/SAPK, and caspase-3 activation were found to play roles. Taken together, these data demonstrate that LPS and its downstream mediators can interact with trichothecenes to modulate proliferative, cytotoxic and apoptotic outcomes in leukocytes in a tissue-specific manner.

Stress and the immune system

Stressors can positively or adversely affect immune and inflammatory responses. However, the current understanding of these effects at the cellular and molecular levels is not sufficient to allow prediction of the effects of a particular stressor on a particular immune or inflammatory function. Three complementary conceptual frameworks are presented that may prove useful in developing such an understanding. In addition, specific examples of the action of particular stress mediators on particular immune or inflammatory end points are discussed, and the relationship of these observations to the conceptual frameworks is indicated. Several of the effects discussed are relevant clinically, and the prospects for pharmacological intervention to prevent adverse effects of stressors on the immune system are discussed. Finally, some of the factors that can (sometimes unexpectedly) influence the outcome of stress-immunology studies and some of the pitfalls that continue to make this area of research controversial in some circles are discussed.

Toluene depresses plasma corticosterone in pregnant rats

Combined exposure to stressors and chemicals may result in synergistic effects. The effects of prenatal exposure to the organic solvent toluene resemble those observed in offspring of gestationally stressed dams, a possible common mechanism being transfer of stress-/toluene-induced increments of corticosteroids from the maternal to the foetal compartment. Pregnant rats were subjected to either 1500 ppm toluene 6 hr/day and/or a schedule of "Chronic mild stress" during the last two weeks of gestation. Exposure to toluene was associated with reduced birth weight and lower maternal weight gain, the latter being enhanced by maternal stress. A depressant effect of toluene on maternal corticosterone was observed, hence the study does not provide immediate evidence that transfer of elevated levels of corticosterone from the maternal to the foetal compartment mediates the effects of prenatal exposure to toluene.

Nonlinear determinism in the immune system. In vivo influence of electromagnetic fields on different functions of murine lymphocyte subpopulations

Animal studies of the effects of low-frequency electromagnetic fields (EMFs) on the immune system appear inconsistent, and recent evidence indicates that inconspicuous experimental problems are not responsible. We hypothesized that the inconsistencies resulted from use of linear methods and models to study inherently nonlinear input-output relationships. Using a novel analytical method, we found that exposure of mice to 5 G, 60 Hz, for 1-105 days in 6 independent experiments consistently affected a broad panel of immune variables when and only when the reaction of the immune system was modeled to allow the possibility of nonlinearity in the relationship between the field and the immune variables. It was possible to mimic the pattern observed in the immune data by sampling from a known chaotic system, suggesting the possibility that the observed pattern was the result of intrinsic nonlinear regulatory mechanisms in the immune system. Overall, the results suggested that lymphoid sub-populations were vulnerable to the physiological consequences of EMF transduction, that it may never be possible to predict specific changes in particular immune-system variables, and that the underlying behavior of the immune system (that which occurs in the absence of specific inputs) may be governed by laws that manifest extreme sensitivity to prior states.

Gender-based profiles of developmental immunotoxicity to lead in the rat: assessment in juveniles and adults

Gender-based differences in immunotoxicity induced by the heavy metal lead (Pb) have been observed both in the juvenile chicken and the adult rat following low-level exposure during embryonic development. To better define the gender-based differences, as related to dose following in utero exposure to Pb, potential differential sensitivities were examined after exposure of F344 rats to low concentrations of Pb (0, 50, 100, or 250 ppm Pb) ad libitum throughout gestation. Immune assessment was performed in juveniles (5 wk old) and young adults (13 wk old). At the highest (250 ppm) Pb concentration examined, the delayed-type hypersensitivity (DTH) response was depressed in females relative to gender-matched controls at both ages; relative spleen weights and relative neutrophil numbers were increased while relative and absolute monocyte numbers and relative basophil numbers were decreased at 13 but not 5 wk of age. In contrast, 250 ppm Pb-treated males did not differ in these endpoints. With in utero exposure to 100 ppm Pb, 13-wk-old females again had decreased relative and absolute monocyte numbers and increased relative neutrophil numbers, although the DTH response was unchanged. Males (with 100 ppm Pb) had increased relative neutrophil numbers, decreased relative lymphocytes, and transiently increased nitrite production seen at 5, but not 13, wk of age. After gestational exposure to 50 ppm Pb, minimal immunotoxic effects were observed in either males or females at either developmental age assessed. These results suggest that differential gender-based immunotoxicity profiles exist after gestational Pb exposure depending on the concentration of Pb administered to the dam. In utero exposure of dams to 250 ppm Pb results in more profound immunotoxicity in females than males. Males arenot more sensitive to lower concentrations of Pb than females. Since the 50 ppm exposure produced minimal changes, these data may provide information to establish a no-observed-adverse-effect level (NOAEL) for in utero exposure to Pb. Additionally, while most effects were evident at both juvenile and adult ages, some changes were not fully evident until measured in the adult. Most changes were persistent with only one exception (male nitrite levels at 100 ppm).

Quantitative aspects of stress-induced immunomodulation

Recent studies indicate that neuroendocrine-immune interactions can cause sufficient immunosuppression to adversely affect human health, but quantitative relationships between stress-related hormones or neurotransmitters and immune function have not been well documented. The mechanisms of stress-induced immunomodulation cannot be fully understood solely by identifying the hormones, neurotransmitters, and cytokines involved. Quantitative relationships and interactions must also be understood. Depending on the nature and duration of the stressor and the immunological parameter under investigation, stress responses can enhance, have no effect, or suppress immunological parameters. These quantitative relationships have implications with regard to safety assessment of drugs and chemicals and with regard to potential development of pharmacological interventions to ameliorate some of the immunosuppressive effects of stress. This review describes selected studies that relate the quantity and duration of exposure to stress-related neuroendocrine mediators to modulation of the immune system. These studies provide a useful starting point, but they also illustrate how much work remains to achieve a fully integrated qualitative and quantitative understanding of stress-induced immunomodulation.

Role of corticosterone in ethyl carbamate-induced immunosuppression in female BALB/c mice

We have recently demonstrated that the antibody response to the T-cell-dependent antigen, sheep red blood cells (SRBCs), was suppressed by ethyl carbamate in female BALB/c mice. At the same doses, ethyl carbamate decreased in the numbers of splenic macrophages, B cells, total T cells, CD4(+) T cells and CD8(+) T cells. In addition, the serum level of corticosterone was increased dose-dependently. To investigate the possible role of corticosterone in ethyl carbamate-induced immunosuppression, the antibody response to SRBCs and the subpopulation changes of splenocytes and thymocytes were determined in naive, sham-operated and adrenalectomized (ADX) female BALB/c mice. When the mice were treated intraperitoneally with 400 mg/kg ethyl carbamate, the antibody response was significantly suppressed by ethyl carbamate in naive and sham-operated mice in accompanying the decrease in spleen and thymus weights and/or the increase in the level of serum corticosterone. Meanwhile, the antibody response was not suppressed by ethyl carbamate in the ADX mice. The splenic numbers of total cells, macrophages, B and T cells, and CD4(+) cells were decreased by ethyl carbamate in naive and sham-operated mice. Meanwhile, each cell number was comparable with control in the ADX mice. The flow cytometric analyses on thymocytes did not show obvious differences as seen in the spleen. Finally, when the ADX mice were treated intraperitoneally with 25 mg/kg corticosterone, the antibody response was significantly suppressed. Taken together, our present results suggested that corticosterone might be, at least partially, responsible for ethyl carbamate-induced immunosuppression in female BALB/c mice.

Effects of peroxisome proliferators on the thymus and spleen of mice

The effects of peroxisome proliferators on the immune system of male C57B1/6 mice have been investigated. Significant atrophy of the thymus and spleen was observed in animals treated with potent peroxisome proliferators (e.g. perfluorooctanoic acid (PFOA), di(2-ethylhexyl)phthalate (DEHP), Wy-14643 and nafenopin), whereas the effects of a moderate peroxisome proliferator (i.e. acetylsalicylic acid (ASA)) were relatively weak. The time course of thymic and splenic atrophy caused by PFOA was found to resemble the time course of the increase in liver weight and of peroxisome proliferation. Analysis of the numbers and phenotypes of thymocytes and splenocytes from PFOA-treated mice revealed the following: (i) the numbers of thymocytes and splenocytes were decreased > 90% and about 50%, respectively, by PFOA treatment; (ii) although all populations of thymocytes were decreased, the immature CD4+CD8+ population was decreased most dramatically; (iii) the numbers of both T and B cells in the spleen were decreased by PFOA treatment. Analysis of the cell cycle of thymocytes indicated that the thymic atrophy caused by PFOA in mice results, at least in part, from inhibition of thymocyte proliferation. Interestingly, in vitro exposure to PFOA for up to 24 h did not produce analogous effects in either thymocytes or splenocytes. Thus, the thymic and splenic atrophy caused by PFOA appears to involve an indirect pathway.

Disruption of Th1/Th2 cytokine balance by cocaine is mediated by corticosterone

Cocaine has been shown to affect immune function through the release of corticosterone. Acute administration of both cocaine and corticosterone produces an enhancement of the T-dependent antibody response to sheep erythrocytes. The T-independent antibody response to DNP-ficoll is not enhanced under identical conditions, suggesting that the T-cell is involved as a cellular target. We examined T-helper cell cytokine production following in vivo cocaine administration and found an increase in IL-4 and IL-10; while IL-2 and IFN-gamma were unaffected. The rise in Th2 cytokines is consistent with an enhanced T-dependent antibody response, a measure of humoral immunity. Because previous results showed that the enhancement by cocaine is mediated via corticosterone, the direct effects of corticosterone on Th1/Th2 in vitro cytokine production were investigated. Th1 cytokines, IL-2 and IFN-gamma, were dose-dependently suppressed by corticosterone at physiologic concentrations. In contrast Th2 cytokines, IL-4 and IL-10, exhibited a biphasic dose response curve, whereby an enhancement was observed at low doses, followed by suppression at higher doses. In order to determine the consequences of this apparent shift towards a Th2 response on a Th1 response, we looked at the delayed-type hypersensitivity response to sheep erythrocytes. This measure of cell-mediated immunity was not significantly affected by acute cocaine, however, corticosterone administration resulted in a significant suppression. These results indicate that corticosterone can produce a shift towards a Th2 predominate response, possibly at the expense of Th1-mediated responses.

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.

Role of metabolism in ethyl carbamate-induced suppression of antibody response to sheep erythrocytes in female Balb/C mice

A possible role of metabolism by cytochrome P450 (P450) in ethyl carbamate-induced suppression of the antibody response to a T-cell-dependent antigen, sheep erythrocytes (SRBCs), was investigated in female Balb/C mice. When mice were treated with ethyl carbamate intraperitoneally for 14 consecutive days at 25, 50, 100, 200 and 400 mg/kg, the antibody response was significantly suppressed from 200 mg/kg. These doses also caused a decrease in thymus weight. An acute dosing of ethyl carbamate at 1 g/kg also caused not only a significant suppression of the antibody response, but also a decrease in thymus weight. The antibody response was most likely to be the IgM antibody response, which was demonstrated in a haemagglutination study. When mice were pretreated with phenobarbital (80 mg/kg) for 3 days to induce P450 enzymes, followed by administration of ethyl carbamate intraperitoneally for 7 consecutive days, the antibody response was more suppressed than in saline-pretreated controls. Moreover, a study using aminoacetonitrile, a P450 inhibitor, showed that the antibody response suppressed by ethyl carbamate was completely recovered by the inhibitor. The present results suggest that metabolism of ethyl carbamate by P450 may be the critical pathway to produce metabolites capable of suppressing the antibody response.

Role of RT6+ T lymphocytes in mercury-induced renal autoimmunity: experimental manipulations of "susceptible" and "resistant" rats

Brown Norway (BN) rats, "susceptible" to the autoimmune effects of mercury, experience a decrease of peripheral RT6.2+ T lymphocytes after the injection of relatively low doses of mercuric chloride. This change coincides with the appearance of circulating autoantibodies to renal antigens (e.g., laminin). Lewis (LEW) rats, "resistant" to the autoimmune effects of mercury, do not show significant decreases of RT6+ T cells. It is possible that BN rats are particularly sensitive to stress induced by mercury and that secretion of adrenocortical hormones decreases levels of RT6+ T cells in this rat strain. Alternatively, mercury may induce a graft-versus-host-like syndrome in BN rats, resulting in higher levels of corticosteroids capable of affecting RT6+ lymphocytes. To eliminate the possible influence of adrenocortical hormones, we have adrenalectomized BN rats prior to administration of mercury. Autoimmune responses to renal antigens were not affected by this experimental manipulation. Similarly, adrenalectomized rats exposed to mercury showed a significant decrease of RT6+ T lymphocytes in cervical lymph nodes. Overall, these observations do not support the hypothesis that increases in adrenocortical hormones play a major role in mercury-induced changes of RT6+ T cells. We have also explored whether experimental depletion of RT6+ T lymphocytes would result in autoimmunity. Gamma irradiation of BN rats led to a decrease of RT6+ T splenocytes, but by itself (i.e., without exposure to mercury) did not cause autoimmune responses to renal antigens. In addition, gamma-irradiated BN rats treated with mercury had autoimmune responses similar to those observed in mercury-treated nonirradiated controls. Depletion of RT6+ T cells in LEW rats through the use of a monoclonal antibody against the RT6.1 alloantigen did not by itself cause renal autoimmunity in this "resistant" strain. Depletion followed by administration of mercury also failed to induce renal autoimmunity. The lack of autoimmune effects in RT6-depleted BN and LEW rats suggests that a combination of several factors may be necessary to break self-tolerance and cause mercury-induced autoimmunity. Such factors likely comprise both environmental (mercury) and endogenous, genetically determined components. The latter include regulatory T cells (possibly RT6+), major histocompatibility complex (MHC), and T-cell receptors (TCR). Thus, BN rats with decreased percentages of immunoregulatory RT6+ T lymphocytes require additional immunotoxic and/or toxic effects of mercury for autoimmunity to occur. On the other hand, LEW rats depleted of regulatory T cells may still be unable to develop renal autoimmunity after exposure to mercury because they lack the appropriate MHC and TCR.