A review of 2,3,7,8-tetrachlorodibenzo-p-dioxin-induced changes in immunocompetence: 1991 update

A Constitutively Active Arylhydrocarbon Receptor Induces Growth Inhibition of Jurkat T Cells through Changes in the Expression of Genes Related to Apoptosis and Cell Cycle Arrest

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) is known to suppress T cell-dependent immune reactions through the activation of the arylhydrocarbon receptor (AhR). Our previous findings suggest that TCDD inhibits the activation and subsequent expansion of T cells following antigen stimulation in mice, leading to a decreased level of T cell-derived cytokines involved in antibody production. In the present study, we investigated the effects of activated AhR on T cells by transiently expressing a constitutively active AhR (CA-AhR) mutant in AhR-null Jurkat T cells. In agreement with our previous findings, CA-AhR markedly inhibited the growth of Jurkat T cells. The inhibited cell growth was found to be concomitant with both an increase in the annexin V-positive apoptotic cells and the accumulation of cells in the G(1) phase. The growth inhibition was also shown to be mediated by both xenobiotic response element (XRE)-dependent and -independent mechanisms, because an A78D mutant of the CA-AhR, which lacks the ability of XRE-dependent transcription, partially inhibited the growth of Jurkat T cells. Furthermore, we demonstrated that CA-AhR induces expression changes in genes related to apoptosis and cell cycle arrest. These expression changes were shown to be solely mediated in an XRE-dependent manner, because the A78D mutant of the CA-AhR did not induce them. To summarize, these results suggest that AhR activation causes apoptosis and cell cycle arrest, especially through expression changes in genes related to apoptosis and cell cycle arrest by the XRE-dependent mechanism, leading to the inhibition of T cell growth.

Persistent suppression of contact hypersensitivity, and altered T-cell parameters in F344 rats exposed perinatally to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)

The outcome of perinatal low-level TCDD exposure on the T-cell-mediated contact hypersensitivity (CHS) response in adult F344 rats was investigated. Suppression of the 2,4-dinitrofluorobenzene (DNFB)-specific contact hypersensitivity reponse occurred in mature offspring of dams dosed by gavage with 1microg or 3microg TCDD/kg on gestation day (GD) 14. To determine if this effect was correlated with altered distribution or activation of major T-cell subtypes, cells of the auricular lymph node draining the hapten-treated skin were evaluated by flow cytometry for expressed phenotype, including activation markers, 24h after challenge. Six-month-old female offspring with significantly decreased CHS and born to dams given 3microg TCDD/kg, had significantly greater proportion of CD4(+) T cells expressing a naive phenotype marker, CD45RC(hi), in their draining nodes. The greater relative frequency of this CD4(+) subset in peripheral lymphoid tissues associated with a reduced CHS in these rats may be attributed to a reduction in the proportion of CD4(+) T cells maintaining or recruited into an activated state. The CHS proved to be a valuable bioassay for investigating long-term immunotoxic effects of perinatal TCDD exposure in rats.

3-Methylcholanthrene induces lymphocyte and phagocyte apoptosis in common carp (Cyprinus carpio L) in vitro

Polycyclic aromatic hydrocarbons (PAH) are an important class of environmental pollutant that are known to be carcinogenic and immunotoxic. In mammals it was suggested that PAH compromise the immune system in part through the induction of programed cell death (apoptosis). In fish, no study has reported the importance of this physiological process in PAH-induced immunotoxicity. We have therefore investigated the capacity of 3-methylcholanthrene to induce lymphocyte and phagocyte apoptosis in carp. By three criteria (exposition of phosphatidylserine at the outer cell membrane, chromatin condensation and fragmentation, and decreased cell size) the data indicate that 3-methylcholanthrene (3-MC) treatment (from 20 to 200 microM) during 24 h produces apoptosis in both lymphocytes and phagocytes. In order to evaluate whether 3-MC induced apoptosis is related to the metabolic activation of 3-MC or 3-MC Ah-R binding, co-exposure experiments with 3-MC and alpha-naphtoflavone (alpha-NF), a compound that inhibits metabolic activation of 3-MC and 3-MC Ah-R binding were performed. While alpha-NF did not prevent 3-MC-induced apoptosis, the compound itself was found to be a strong inducer of apoptosis. There results might indicate that metabolic activation of 3-MC or 3-MC Ah-R binding is not causally linked to apoptosis. However, since 3-MC, alpha-NF and 3-MC + alpha-NF treatments produce the same sustained increase (3 h minimum) in intracellular calcium level, it is possible that this phenomenon is implicated in the induction of programmed cell death by these hydrocarbons.

2,3,7,8-tetrachlorodibenzo-p-dioxin, an exogenous modulator of the 3'alpha immunoglobulin heavy chain enhancer in the CH12.LX mouse cell line

Transcriptional regulation of the Ig heavy chain gene involves several regulatory elements, including the 3'alpha enhancer, which is composed of four distinct regulatory domains. DNA binding sites for several transcription factors, including B cell-specific activator protein, nuclear factor for immunoglobulin kappa chain in B cells, and octamer have been identified within the 3'alpha enhancer domains and are believed to be important in regulating 3'alpha enhancer activity. We have identified an additional DNA binding motif, the dioxin-responsive element (DRE), which can contribute to 3'alpha enhancer regulation. 2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD), a known disrupter of B cell differentiation (i.e., decreased plasma cell formation, inhibition of micro heavy chain expression, and suppression of IgM secretion), induces binding of the aryl hydrocarbon receptor (AhR) nuclear complex to DREs. TCDD also induces AhR binding to the hypersensitive (hs)4 domain of the 3'alpha enhancer. Interestingly, TCDD enhances LPS-induced activation of the hs4 domain but profoundly inhibits LPS-induced activation of the complete 3'alpha enhancer. Furthermore, site-directed mutational analysis demonstrated that a DRE and kappaB element in the hs4 domain is modulated by TCDD in lipopolysaccharide-activated B cells. We propose that the AhR is a novel transcriptional regulator of the 3'alpha enhancer, which can mediate, at least in part, the effects of TCDD on the 3'alpha enhancer and its domains, putatively contributing to a marked suppression of IgM production.

Inhibition of Human Polymorphonuclear Cell Oxidative Burst by 17-β-estradiol and 2,3,7,8-tetrachlorodibenzo-p-dioxin

PROBLEM

Polymorphonuclear cell (PMN) function may be directly influenced by 17-beta-estradiol and the endocrine disruptor, 2,3,7,8-tetrachloro-dibenzo-p-dioxin (TCDD). This may have significant consequences on PMN function within the female reproductive tract. This study evaluated the effects of 17-beta-estradiol and TCDD on PMN oxidative burst.

METHOD OF STUDY

Peripheral blood PMN were isolated from normal male donors. Following treatment with 17-beta-estradiol, TCDD or both, PMN were stimulated with phorbol 12-myristate 13-acetate. Superoxide production was measured by lucigenin-enhanced chemiluminescence.

RESULTS

Following 24-hr culture with either 17-beta-estradiol or TCDD, PMN superoxide production was significantly reduced, however, no such inhibition was observed when PMN were cultured with both estradiol and TCDD. Using antagonists, the estradiol and TCDD effects on PMN superoxide production was shown to be estrogen and aryl hydrocarbon receptor mediated.

CONCLUSIONS

Estradiol and TCDD influence PMN oxidative burst through receptor mediated events. Such altered PMN function may have profound effects upon the normal endometrial cycle.

Embryonic exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin in chickens: effects of dose and embryonic stage on hatchability and growth

Chicken embryos (Gallus domesticus) were injected with 0, 8, 20 or 50 ng tetrachlorodibenzo-p-dioxin (TCDD) per egg at embryonic day (ED) 4, 8 or 12 to investigate the effects of differential periods of sensitivity to TCDD exposure. At hatch, all chicks were weighed, sexed and examined macroscopically to identify possible malformations. Liver, bursa, heart and spleen masses were recorded from a number of chicks. The remaining chicks were raised until 6 weeks of age and body and organ masses, plasma concentrations of thyroid hormones, triglycerides and glucose were measured. Dose and stage during embryonic development at which injection was performed affected hatchability. Fifty nanogram of TCDD was highly toxic for 4-day-old chicken embryos. TCDD was less toxic for chicken embryos of 8- and especially 12-days old. One-day-old chick and organ weights were not different between TCDD doses at all injection days. However, injection performed at ED4 or ED8 with 20 and 50 ng, respectively, significantly depressed post-hatch body mass gain. Moreover, body mass gain in males was more depressed than in females. The delayed growth in TCDD treated chickens was accompanied by changes in T(3)/T(4) ratio that at some ages were significantly higher compared to control animals. No pronounced changes in plasma triglycerides or glucose concentrations during postnatal life were observed. Absolute and relative organ masses of 6-week-old chickens showed no remarkable changes.

Suppressive effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) on the high-affinity antibody response in C57BL/6 mice

In the humoral immune response to an invasion of foreign antigens, B cells differentiate into low-affinity antibody-forming cells (AFCs) that mainly secrete IgM or, through germinal center (GC) formation, into high-affinity AFCs that secrete IgG-class antibodies with a higher affinity for the antigen. Previous studies have established the suppressive effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) on low-affinity antibody responses to antigens. However, whether and how TCDD affects the high-affinity antibody response to antigens has not yet been clarified. In this paper we investigate the effects of TCDD on GC formation, high-affinity AFC generation, and high-affinity antibody production in the primary humoral immune response. C57BL/6 mice were orally administered 0 or 20 microg/kg of TCDD and subsequently immunized with alum-precipitated ovalbumin (OVA) on day 0. Then the GC formation in the spleen and OVA-specific antibodies in the plasma, was evaluated until day 14 postimmunization. TCDD exposure reduced the production of OVA-specific IgG1 on days 10 and 14. GC formation in the spleen was also suppressed by TCDD exposure, and the suppression persisted from day 7 until day 14. In TCDD-administered mice, on day 7, cellular proliferation in the GCs was significantly suppressed, although apoptosis was not markedly affected. In order to measure high-affinity antibody and high-affinity AFCs, the mice were administered TCDD followed by immunization with alum-precipitated (4-hydroxy-3-nitrophenyl) acetyl linked to chicken gamma-globulin (NP-CG). The frequency of high-affinity NP-specific AFCs that bind to low-haptenated antigen was clearly shown to be reduced in the spleen on days 10 and 14. Furthermore, the high-affinity anti-NP IgG1 levels on days 10 and 14 postimmunization were significantly reduced by TCDD exposure. Taken together, the results of this paper demonstrate that TCDD exposure inhibits the generation of high-affinity AFCs and high-affinity antibody production during the primary humoral immune response and suggest that these alterations were caused by the suppression of antigen-responding B-cell proliferation induced by TCDD during GC formation.

Functional activation of arylhydrocarbon receptor (AhR) in primary T cells by 2,3,7,8-tetrachlorodibenzo-p-dioxin

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) exerts diverse adverse health effects by activating the transcription factor arylhydrocarbon receptor (AhR). The activated AhR induces the expression of various genes having xenobiotic responsive elements (XREs) in their enhancer regions, such as the gene for cytochrome P-450 1A1 (CYP1A1). The immune system is sensitively affected by TCDD, while the precise mechanism of how TCDD acts in each immune cell type remains to be determined. The results of previous studies on AhR activation in B cell lines, T cell clones, and thymocytes, which mainly consist of immature T cells, suggested that AhR in mature T cells is inactive, whereas that in B cells and immature T cells act functionally. In the present study, we investigated whether or not TCDD induces the CYP1A1 gene by functionally activating AhR in primary mature T cells in mice. When the splenocytes that contain mature T and B lymphocytes as their predominant cell types or the thymocytes were cultured in the presence of TCDD, each of them showed a similar magnitude of CYP1A1 induction with a peak induction at 4 h. Both mature T cells and B cells that had been separated from total splenocytes also showed CYP1A1 induction at the same magnitude with a peak induction at 4 h. Gene expression of CYP1A1 was observed at 0.1 nM or greater concentrations of TCDD in splenocytes and separated T cells. The induction of CYP1A1 in T cells was confirmed in mice exposed to TCDD. These results indicate that TCDD induces the functional activation of AhR in primary mature T cells in mice.

3-methylcholanthrene inhibits lymphocyte proliferation and increases intracellular calcium levels in common carp (Cyprinus carpio L)

Polycyclic aromatic hydrocarbons (PAHs) are an important class of environmental pollutants that are known to be carcinogenic and immunotoxic. Many authors have focused on macrophage activities in fish exposed to PAHs. However, fewer studies have reported decrease in specific immunity in such fish. We investigated the intracellular mechanisms by which the 3-methylcholanthrene (3-MC) decreased lymphocyte proliferation in carp. T- and B-lymphocyte proliferation induced by Concanavalin A (Con A) and lipopolysaccharide (LPS) were inhibited by 3-MC (0.5-50 microM). 3-MC also produced a rapid and a sustained increase in intracellular calcium concentration ([Ca(2+)](i)) (2 h minimum). However, the cytochrome p450 1A and Ah receptor inhibitor, alpha-naphtoflavone (a-NF), also inhibited lymphocyte proliferation and did not reverse the effects of 3-MC. Moreover, since a-NF and 3-MC increased [Ca(2+)](i) and inhibited lymphocyte proliferation it was possible that calcium release played a role in 3-MC-inhibited lymphocyte proliferation. The rise in [Ca(2+)](i) induced by 3-MC was potentiated by the inhibitor of the endoplasmic reticulum calcium ATPases, thapsigargin. Treating cells with 3-MC decreased calcium mobilization caused by thapsigargin. These results suggest that 3-MC acts on the endoplasmic reticulum, perhaps directly on calcium ATPases, to increase intracellular calcium levels in carp leucocytes.

A constitutively active aryl hydrocarbon receptor causes loss of peritoneal B1 cells

The dioxin/aryl hydrocarbon (Ah) receptor functions as a ligand-activated transcription factor that mediates toxicity of dioxins and related environmental pollutants. We have developed a transgenic mouse model that expresses a constitutively active Ah receptor. The immune system is one of the most sensitive target organs for dioxin toxicity and we have therefore investigated alterations of different lymphocyte populations in these mice. The population of mature bone-marrow derived B cells was enlarged, consistent with previous findings in dioxin exposed mice. In contrast, the peritoneal population of CD5-expressing B cells (B1 cells) was significantly diminished. This is the first study that demonstrates the effect of an activated Ah receptor on B1 cells. Since these cells are important mediators of innate immunity against pathogens such as Influenza virus, these results may explain the decreased resistance against infections that has been documented after dioxin exposure.

Pathogenesis of endometriosis

Endometriosis is a common gynecologic disorder characterized by the presence of endometrial tissue outside the uterine cavity. Various theories have been put forth to explain the mechanisms for the development of this disease. Although no single theory can explain all cases of endometriosis, the retrograde menstruation theory has gained the widest acceptance. This theory proposes that viable endometrial tissue is refluxed through the fallopian tubes during menstruation and implants on peritoneal surface or pelvic organs. Retrograde menstruation occurs in 76% to 90% of women. The much lower prevalence of endometriosis suggests that additional factors determine susceptibility to endometriosis. Once in the peritoneal cavity, the survival and implantation of endometrial cells seem to be mediated by abnormal MMP and TIMP expression, altered immune milieu, aberrant local aromatase activity, and genetic and environmental factors.

Feasibility of human trials to assess developmental immunotoxicity, and some comparison with data on New World monkeys

Procedures to reveal 'immunotoxic' potentials of chemicals in animal experiments (mostly in rodents) have been recommended, but the selection of test systems is rather arbitrary. The predictive power of extrapolations to the possible situation in humans is unknown because human studies to confirm or to reject clues from animal data are largely lacking. End points selected in animal studies and those expected to be relevant in humans are not identical. Results of animal experiments are based on doses, generally ignoring the important species differences in pharmacokinetics. This unfavorable situation is especially pronounced when attempting to evaluate 'environmental chemicals'. Because much more information is available on many medicinal drugs, exposures can be defined and pharmacokinetic data are available or obtainable. The situation is even more complicated when attempting to assess possible adverse effects on the developing immune system: in addition to the problems mentioned, numerous different developmental periods with varying susceptibilities must be considered, and species differences in the immune response are superimposed with large differences in pre-, early post-, and later postnatal development. Simultaneously, the kinetic variables are continuously changing with time (with additional variability among species). Different results, even between rats and mice, are bound to occur. Extrapolation to the situation possibly relevant for human exposure will be almost impossible, especially from rodent data. The majority of such effects induced peri- or early postnatally may be expected to be reversible. It must also be assessed whether qualitatively different adverse effects are likely to be induced during 'development', which cannot be revealed (accepting quantitative differences) by more easily performed tests on the adult organism. Considering the intrinsic difficulties, the most promising approach would be to directly obtain data from human trials. This is feasible for medicinal drugs. Alternatively, data on nonhuman primates, the species phylogenetically closest to man, may provide useful information. The status quo for such a strategy and the possible pitfalls are discussed in this overview.

Aryl hydrocarbon receptor-dependent inhibition of AP-1 activity by 2,3,7,8-tetrachlorodibenzo-p-dioxin in activated B cells

B cells have been identified as sensitive cellular targets responsible for 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)-mediated suppression of humoral immunity. In previous studies, TCDD was shown to produce a significant inhibition of IgM secretion and mu gene expression in LPS-activated CH12.LX B cells (AhR expressing) but not in BCL-1 B cells (AhR deficient). The present studies extend these previous findings by investigating the effect of TCDD on AP-1 and nuclear factor (NF)-kappaB, both of which play an important role in B-cell activation, differentiation, and immunoglobulin (Ig) gene expression. Electrophoretic mobility shift assays and chloramphenicol acetyl transferase reporter gene experiments demonstrated that lipopolysaccharide (LPS)-induced DNA binding and transcriptional activity of AP-1 was markedly inhibited by TCDD at 24, 48, and 72 h after cellular activation of CH12.LX cells. Conversely, TCDD treatment produced no significant change on the activity of NF-kappaB. Two AhR antagonists, alpha-naphthoflavone and 2,2',5,5'-tetrachlorobiphenyl, attenuated TCDD-induced inhibition of AP-1 binding in CH12.LX cells. Concordant with this result, TCDD did not inhibit LPS-induced AP-1 activity in BCL-1 B cells. Moreover, supershift analysis revealed the major component of the AP-1 complex in LPS-activated CH12.LX cells was c-Jun. Additional studies revealed that the nuclear c-jun and c-jun steady-state mRNA expression was inhibited by TCDD treatment. Collectively, these results suggest that TCDD-induced inhibition of IgM expression by B cells may be mediated, at least in part, through a down-regulation of AP-1 activity in an AhR-dependent manner.

Circulating lysozyme and hepatic CYP1A activities during a chronic dietary exposure to tributyltin (TBT) and 3,3',4,4',5-pentachlorobiphenyl (PCB-126) mixtures in channel catfish, Ictalurus punctatus

Exposure to planar congeners of halogenated aromatic hydrocarbons (HAHs) leads to a myriad of toxicities, including developmental, reproductive, and immunotoxic effects. Halogenated aromatic hydrocarbons possessing structural similarity to TCDD, such as 3,3',4,4',5-pentachlorobiphenyl (PCB-126), are more prevalent in the environment than TCDD, and they elicit similar toxic effects, primarily through the cytosolic aromatic hydrocarbon receptor (AhR). While polychlorinated biphenyls (PCBs) are ubiquitous pollutants, they rarely exist alone in the environment. The aquatic biocide tributyltin (TBT) is also a widespread environmental contaminant, and numerous studies indicate that it has reproductive, developmental, and immunotoxic effects in a variety of organisms. Unlike planar HAHs, TBT is not associated with any known cellular receptor. The induction of cytochrome P-4501A (CYP1A) activity in most vertebrates is a classical physiological response to planar HAH exposure. TBT has been shown to inhibit the induction of cytochrome P-4.50s at high doses. Recent studies demonstrate that low levels of TBT potentiate PCB-126-associated CYP1A induction in channel catfish, Ictalurus punctatus, and in rodents following intraperitoneal injections. In this study, the effects of dietary exposures to TBT, PCB-126, and mixtures of the two on channel catfish hepatic CYP1A activity, as well as plasma lysozyme activity, were examined. Circulating lysozyme, a marker of proinflammatory responses, was monitored to determine the relative specificity of treatments for CYP1A induction. Plasma lysozyme levels were examined along with hepatic CYP1A protein induction and EROD activity following exposure to nominal doses of 1 and 100 ppb dietary TBT, PCB-126, or mixtures of the two. As expected, the highest level of PCB-126 induced CYP1A, and TBT did not. In mixtures, the low level of TBTpotentiated the ability of the high PCB-126 dose to induce CYP1A. Plasma lysozyme levels were suppressed by both concentrations of TBT and by the low concentrations of PCB- 126 during the initial phase of the response to Vibrio anguillarum. The normal and expected pattern of initial increases in circulating lysozyme levels following immunization, with subsequent return to baseline level, was disrupted by TBT. High levels of PCB-126 potentiated the lysozyme response. As seen with CYP1A activities, exposures to mixtures of TBT and PCB-126 resulted in a potentiation of plasma lysozyme levels. The data show that dietary TBT modulates PCB- 126-induced CYP1A activities and that these mixtures may have potent proinflammatory properties as well.

Effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) on T cell-derived cytokine production in ovalbumin (OVA)-immunized C57Bl/6 mice

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) is known to suppress both cellular and humoral immunity. Effector T cell-derived type-2 cytokines, including IL-4 and IL-5, play pivotal roles in humoral immunity. Herein, we studied whether TCDD affects type-2 cytokine productions during the immune response. C57Bl/6 mice were intraperitoneally immunized with ovalbumin (OVA) and orally administered 5 or 20 microg TCDD/kg on Day 0, and then challenged with OVA on Day 21. Seven days later (Day 28), antigen-specific antibodies in plasma, and T cell-derived cytokines produced by splenocytes and proliferation of splenocytes upon ex vivo re-stimulation with OVA were investigated. The quantities of IgM class and IgG1 class OVA-specific antibodies in plasma were reduced by 5 or 20 microg TCDD/kg and by 20 microg TCDD/kg, respectively. While thymus weight and cellularity were reduced by 20 microg TCDD/kg, spleen weight and cellularity were not changed by either 5 or 20 microg TCDD/kg. The proportions of B and T cells in the spleen were not affected by TCDD exposure. On the other hand, splenocytes from mice treated with 5 or 20 microg TCDD/kg were shown to produce less IL-4 or IL-5 upon ex vivo re-stimulation with OVA. Production of the T cell growth factor IL-2 was also decreased in splenocytes from TCDD-treated mice. In contrast, the type-1 cytokine IFN-gamma was increased by TCDD. Twenty micrograms of TCDD/kg suppressed OVA- or T cell mitogen (Con A)-stimulated proliferation of splenocytes, but did not affect B cell mitogen (LPS)-stimulated proliferation. These results suggested compromised T cell activation and suppressed type-2 cytokine production by T cells to be involved in the impaired humoral immunity associated with TCDD exposure.

Effect of low-dose 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) on influenza A virus-induced mortality in mice

Dioxins, including the most toxic congener, 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), exert diverse biological effects in humans and animals. Host resistance, especially to virus infections, is considered one of the most sensitive targets of TCDD-toxicity, while a recent study showed that the vulnerability to TCDD of host resistance to viruses varied form experiment to experiment. Burleson et al. [Fundam. Appl. Toxicol. 29 (1996) 40] reported that a single oral dose as low as 10 ng TCDD/kg increased the mortality of mice infected with influenza A virus. If this value had been adopted as the basis for the tolerable daily intake (TDI) of dioxins, the TDI of 1-4 pg toxic equivalent (TEQ)/kg per day recommended by WHO would have to be lower. In the present study, we used the same experimental protocol described by Burleson et al. to determine whether low-dose TCDD consistently compromises the host resistance of mice infected with influenza A virus. To do so, we investigated the effect of TCDD in the dose range of 0-500 ng/kg on the mortality of virus-infected female B6C3F1 mice. We also investigated the sex- and strain-dependency of host resistance in male B6C3F1 mice and in female C57Bl/6, Balb/c, and DBA/2 mice by administering the same dose range of TCDD. The results showed that TCDD doses up to 500 ng/kg did not increase the mortality of virus-infected mice in any of the strains. Further studies on the mechanism underlying the toxicity of TCDD are needed to assess the risk of exposure to this compound in influenza A virus infection.

Enhanced activation-induced cell death as a mechanism of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)-induced immunotoxicity in peripheral T cells

T cells upon activation undergo apoptosis, a process termed activation-induced cell death (AICD). In the current study, we investigated whether 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) increases AICD and whether this constitutes one of the mechanisms by which TCDD induces immunotoxicity. To this end, C57BL/6+/+, C57BL/6 gld/gld (Fas ligand-defective) and C57BL/6 lpr/lpr (Fas-deficient) mice were injected with TCDD (50 microg/kg body weight, ip) or the vehicle (corn oil) and with anti-CD3 mAbs into the footpads. 3 days later, inguinal and popliteal lymph node cells were harvested, pooled and enumerated. Cells were cultured in vitro with anti-CD3 mAbs and cell proliferation was measured. Also, such cells were studied for their ability to undergo apoptosis upon in vitro culture with either tissue culture medium alone or with anti-CD3 mAbs. The data demonstrated that lymph nodes from TCDD-treated wild-type (+/+) mice showed a decrease in cellularity and the T cells exhibited decreased responsiveness to anti-CD3 mAbs when compared to the vehicle-treated control group. Furthermore, such cells from TCDD-treated mice exhibited increased levels of apoptosis upon in vitro culture when compared to the cells from vehicle-treated mice. In contrast, activated lymph nodes from TCDD-treated C57BL/6 gld/gld and C57BL/6 lpr/lpr mice showed normal cellularity and T cell responsiveness to anti-CD3 stimulation when compared to the vehicle controls. In addition, the activated lymph node T cells from the TCDD-treated C57BL/6 gld/gld and C57BL/6 lpr/lpr mice failed to exhibit increased apoptosis when compared to the controls. The current study demonstrates that the immunotoxic effects of TCDD in activated peripheral T cells may result from increased AICD mediated through Fas-Fas ligand interactions.

Aging and resistance to Trichinella spiralis infection following xenobiotic exposure

Aging is accompanied by well-documented physiological changes, including alterations in the immune system that can lead to reduced resistance to a variety of infectious agents. We tested the hypothesis that immunosenescence exacerbates the immunosuppressive effect of xenobiotics. If proven true, a given dose of an immunosuppressive xenobiotic would cause greater suppression of host resistance in an aged population.

2,3,7,8-tetrachlorodibenzo-p-dioxin and natural immunity: lack of an effect on the complement system in a guinea pig model

We examined the complement system as a potential target for toxicity of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) in normal Hartley guinea pigs. Guinea pigs were injected intravenously with 0.5 microg/kg TCDD and bled at weekly intervals up to 1 month following injection. Serum samples were collected at each time point and assayed for total hemolytic complement activity (CH50), alternative pathway complement activity (AH50) and complement component C4 functional activity. Injection of TCDD led to a wasting syndrome, as observed by a lower body weight gain in TCDD-treated animals when compared to control animals. However, at this dose, TCDD failed to induce any significant change in complement activity as determined by all three methods used in this study. The results indicate that, at 0.5 microg/kg body weight, a dose slightly lower than the LD50 value in guinea pigs, TCDD fails to affect the complement system.

Adrenocorticotropin (ACTH) and corticosterone secretion by perifused pituitary and adrenal glands from rodents exposed to 2,3,7, 8-tetrachlorodibenzo-p-dioxin (TCDD)

Although in utero maternal stress has been shown to have lasting effects on rodent offspring, fetal effects of chemically-induced alterations of the maternal hypothalamic-pituitary-adrenal axis (HPA) have not been well studied. This study examined the effects of in vivo 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) exposure on pituitary-adrenal function in the male rat, pregnant female rat and pregnant female mouse. The secretion of adrenocorticotropin (ACTH) and corticosterone (CORT) in pituitary and adrenal glands, respectively, was assessed in ex vivo perifusion cultures. Male and pregnant female (gestation day 8) Sprague-Dawley rats were gavaged once with 10 microgram/kg TCDD, pregnant female mice once with 24 microgram/kg TCDD, and euthanized 10 days later. Hemi-pituitary (rat) or whole anterior pituitaries (mice) and right adrenal glands from the same animal were quartered, perifused under baseline and stimulated conditions. In both males and pregnant females, TCDD did not affect corticotropin releasing hormone (CRH)-stimulated ACTH secretion. Neither total pituitary ACTH nor plasma ACTH was altered in either sex or species by TCDD treatment. ACTH-stimulated CORT secretion was not affected by TCDD in either sex or species, and adrenal tissue and plasma CORT levels were unchanged in males and pregnant females by TCDD. However, the plasma ACTH:CORT ratio was decreased about 46% in male rats treated with TCDD. Plasma CORT levels were 23-fold higher and plasma ACTH levels were 1.5-fold higher in pregnant females than in male rats. In male versus female rats, adrenal CORT and anterior pituitary ACTH tissue levels were about 7.5- and 1.75-fold higher and ACTH, respectively. Female mouse adrenal tissue CORT was about 4-fold greater than female rat. The reduced plasma ACTH:CORT ratio in the male rat suggests that TCDD disturbs HPA function. Exposure of male rat to a 5-fold higher dose in earlier studies clearly demonstrated effects of TCDD on male rat HPA. The present study identified substantial HPA performance differences between male and pregnant female rats. The failure to detect a response to TCDD in pregnant female rat and mouse could be a function of both TCDD dose and the high level of secretion of both ACTH and CORT in pregnant animals. For the rat or mouse, a single exposure to TCDD during pregnancy does not appear sufficient to induce maternally-mediated developmental, reproductive and behavioral toxicity via the HPA axis.

Differential effects of diethylstilbestrol and 2,3,7,8-tetrachlorodibenzo-p-dioxin on thymocyte differentiation, proliferation, and apoptosis in bcl-2 transgenic mouse fetal thymus organ culture

Both the estrogenic drug diethylstilbestrol (DES) and the pervasive environmental contaminant 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) inhibit thymocyte development. The mechanisms by which either agent induces thymic atrophy are still undetermined. We previously found that TCDD and DES inhibited C57BL/6 murine fetal thymocyte organ cultures (FTOC) at different stages of development. Now, using bcl-2 transgenic (TG) mice, we have further investigated their effects on FTOC proliferation, differentiation, maturation, and apoptosis. As with C57BL/6 mice, thymocyte development in C3H/bcl-2 FTOCs was inhibited by either TCDD (10 nM) or DES (20 microM) in both bcl-2 TG- and TG+ littermates. However, the percentage reduction of cell number induced by DES in bcl-2 TG+ FTOCs was significantly less than the level of inhibition in TG- FTOCs. There was no difference in the level of reduction from TCDD-exposed TG+ or TG- FTOC. Whereas TCDD increased production of mature CD8 cells in either strain, DES mainly yielded cells in the CD4(-)CD8(-)(DN) stage in TG- mice. The anti-apoptotic bcl-2 transgene overcame some DES blocking of DN thymocyte development, allowing more cells to differentiate into CD4 single-positive cells. Analysis of cell cycle showed that TCDD inhibited entry into S phase, whereas DES blocked cell cycling in the G2/M phase. TCDD did not induce detectable apoptosis in FTOC. However, unlike the effects of 17 beta-estradiol (E2) in vivo, DES induced apoptosis in the TG- FTOC, and these apoptotic cells were mainly in the DN subpopulation. This apoptosis could be prevented by the overexpression of bcl-2 in the TG+ mice. Our results demonstrate that, in addition to inhibition of fetal thymocytes at different stages of development by TCDD and DES, DES also induces thymic atrophy by both bcl-2-inhibitable apoptosis and by inducing cell cycle arrest in G2/M in the latest stage in the stem cell compartment. TCDD, on the other hand, does not induce apoptosis, but inhibits entry into cell cycle in the earliest stage in the stem cell compartment.

The potential role of environmental toxins in the pathophysiology of endometriosis

Environmental contaminants that are known to disrupt steroid action can influence the development of reproductive diseases. Our group has focused on whether 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD or dioxin) can disrupt steroid regulation of endometrial matrix metalloproteinase (MMP) expression. The MMPs regulate extracellular matrix turnover in normal tissues, but the inappropriate expression of these enzymes is associated with numerous disease states that involve invasive processes. We have previously shown that secretion of MMPs by human endometrium is critical for establishment of ectopic lesions in a nude mouse model of experimental endometriosis. In this report, we show that TCDD exposure promotes establishment of experimental endometriosis by interfering with the ability of progesterone to suppress endometrial MMP expression. Copyrightz1999S. KargerAG,Basel

Generation of alphabeta T-cell receptor+ CD4- CD8+ cells in major histocompatibility complex class I-deficient mice upon activation of the aryl hydrocarbon receptor by 2,3,7,8-tetrachlorodibenzo-p-dioxin

Gene-targeted mice lacking the beta2 microglobulin gene (beta2m-/- mice), and hence functional major histocompatibility complex (MHC) class I molecules, do not develop CD4- CD8+ cells. We show here that both in vitro and in vivo treatment with 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), a trans-activating ligand of the endogenous aryl hydrocarbon receptor (Ah-R), bypasses the need for MHC class I molecules for selection into the CD4- CD8+ cell pool. When beta2m-/- dams were given a single dose of 50 microg of TCDD, approximately 13% of CD4- CD8+ thymocytes could be detected in their newborn pups. In TCDD-exposed fetal thymus organ cultures of beta2m-/- mice, approximately 35% CD4- CD8+ thymocytes were detectable. About 16% of these CD4- CD8+ cells bore the alpha beta T-cell receptor (TCR) and approximately 33% bore CD3. Only a minority of the CD8+ cells were heat-shock antigen positive. The cells possessed killing activity as shown using the 51Cr-release assay comprising gamma delta TCR- CD4- CD8+ thymocytes from 3 to 4-day-old b2m-/- mice. Thus, TCDD leads to a significant increase of mature CD4- CD8+ thymocytes in relative and absolute numbers. High numbers of CD4- CD8+ thymocytes developed also in organ cultures from thymi, lacking both MHC class I and class II molecules, exposed to TCDD. A 10-fold transient increase of Notch1 mRNA in thymocytes from fetal thymus organ culture, exposed for 4 days to TCDD, was detected in CD4+ CD8+ cells compared with controls. We suggest that TCDD affects thymic selection and directs the lineage commitment of CD4+ CD8+ thymocytes towards CD4- CD8+ cells, possibly via up-regulation of the Notch1 gene.

Thymocyte development in Ah-receptor-deficient mice is refractory to TCDD-inducible changes

The arylhydrocarbon receptor (AhR), a ligand-activated transcription factor, is differentially distributed in tissues and abundant in the thymus epithelium. The activated AhR can induce the transcription of an array of genes, including genes of cell growth and differentiation. Neither the physiological function of the AhR nor its putative natural ligand is known. 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) is a xenobiotic high-affinity activator of the AhR, and appears to be essential for most of the multifold toxic effects of TCDD. Activation of the AhR by even low doses of TCDD results in general immunosuppression and thymus hypoplasia. TCDD exposure interferes with thymocyte development; for instance, it reduces the proliferation rate of the very immature (CD4- CD8- and CD4- CD8+ HSA+) thymocytes, leads to preferential emigration of very immature cells, and drastically skews the differentiation of thymocyte subpopulations towards mature CD4- CD8+ alphabeta TCRhigh thymocytes. As shown here, in fetal thymi of AhR-deficient mice, thymocyte differentiation kinetics as defined by CD4 and CD8 surface markers, was comparable to AhR+/+ C57BL/6 mice. Also, the cell emigration characteristics were similar to AhR+/+ mice. These parameters were refractory to TCDD exposure in the AhR-/- mice, but not in the C57BL/6 mice. However, in AhR deficient mice at gestation day 15 more CD4- CD8- immature cells bore high amounts of the (alphabeta-T-cell receptor. Also, fetal thymocyte numbers were significantly lower, as compared to strain C57BL/6. Thus, the AhR is the mediator of thymotoxic effects of TCDD.

Effects of aging on resistance to Trichinella spiralis infection in rodents exposed to 2,3,7,8-tetrachlorodibenzo-p-dioxin

Immune function, including resistance to infection, decreases as humans and rodents age. We have shown that preinfection exposure of young (9-11 weeks) mice or rats to TCDD decreased resistance to Trichinella spiralis (Ts) infection, expressed as delayed onset or completion of parasite elimination and as increased muscle burdens of larvae. It has also been shown that aged mice express lower constitutive levels of resistance to Ts infection, compared to young adult animals. This study tested the hypothesis that the age-related decrease in constitutive levels of resistance to Ts infection exacerbates the decreased resistance to infection that follows TCDD exposure. This hypothesis addresses the concern that TCDD may pose a greater threat to the elderly than to the population at large. Animals were given a single oral dose of 1, 10, or 30 microg TCDD/kg, 7 days before infection. Eleven days later, young (approximately 10 weeks) control rodents had eliminated a greater proportion of the original parasite burden from the intestine than aged control animals. Nevertheless, parasite elimination was decreased by TCDD exposure only in young rodents. The effect of TCDD exposure on numbers of encysted larvae was evaluated only in rats. Increased larvae burdens occurred in young rats at 30 microg TCDD/kg and at 10 or 30 microg TCDD/kg in aged rats. Parasite-specific splenocyte and lymph node cell proliferation was suppressed following dioxin exposure in young mice; cells from aged mice were markedly less responsive to stimulation, yet less sensitive to TCDD exposure. The response to parasite antigens was not affected in aged rats exposed to TCDD, although elevated mitogen-driven B-cell proliferation was observed. These results indicate that age-related constitutive immunosuppression did not exacerbate TCDD-induced suppression of T-cell mediated adult parasite expulsion; rather, advanced age provided some degree of protection. On the other hand, a lower dose of TCDD was required in aged rats to suppress the combined humoral and cellular responses that limit the burden of encysted larvae, compared to young rats. These model-dependent results preclude acceptance or rejection of the tested hypothesis.

Expression of dioxin-responsive genes in human endometrial cells in culture

To investigate expression of dioxin-responsive genes in human endometrial cells with exposure to 2,3,7, 8-tetrachlorodibenzo-p-dioxin (TCDD), human endometrial stromal cells immortalized with temperature-sensitive SV40 T antigen were used for the experiments. Cells were treated with 0.1% DMSO or 0.1, 1, 10, or 100 nM TCDD for 24 h. Induction of interleukin-1beta (IL-1beta) and plasminogen activator inhibitor-2 (PAI-2) mRNAs was analyzed by reverse-transcription polymerase chain reaction. Expression of IL-1beta or PAI-2 mRNA in response to TCDD was increased in a dose-dependent fashion. The maximum increases of PAI-2 and IL-1beta mRNAs were observed at 100 and 10 nM TCDD, respectively. While cycloheximide treatment did not show a significant difference of PAI-2 mRNA levels between control and TCDD-treated cells, mRNA stability assay using actinomycin D showed that PAI-2 mRNA in TCDD-treated cells was about twofold more stable than the control cells. While expression of CYP1A1 mRNA was not detected and levels of ARNT mRNA were not altered by TCDD exposure, the amount of AhR mRNA was decreased dose dependently. The present study represents an initial attempt to determine the responses of dioxin-responsive genes in human endometrial cells following TCDD exposure. The results demonstrated that IL-1beta and PAI-2 genes are induced dose dependently in human endometrial cells with exposure to TCDD and expression of PAI-2 mRNA is controlled at the posttranscriptional level.

Controlling persistent organic pollutants-what next?

Within the context of current international initiatives on the control of persistent organic pollutants (POPs), an overview is given of the scientific knowledge relating to POP sources, emissions, transport, fate and effects. At the regional scale, improvements in mass balance models for well-characterised POPs are resulting in an ability to estimate their environmental concentrations with sufficient accuracy to be of help for some regulatory purposes. The relevance of the parameters used to define POPs within these international initiatives is considered with an emphasis on mechanisms for adding new substances to the initial lists. A tiered approach is proposed for screening the large number of untested chemical substances according to their long-range transport potential, persistence and bioaccumulative potential prior to more detailed risk assessments. The importance of testing candidate POPs for chronic toxicity (i.e. for immunotoxicity, endocrine disruption and carcinogenicity) is emphasised as is a need for the further development of relevant SAR (structure activity relationship) models and in vitro and in vivo tests for these effects. Where there is a high level of uncertainty at the risk assessment stage, decision-makers may have to rely on expert judgement and weight-of-evidence, taking into account the precautionary principle and the views of relevant stake-holders. Close co-operation between the various international initiatives on POPs will be required to ensure that assessment criteria and procedures are as compatible as possible.

Immunological markers among workers exposed to 2,3,7,8-tetrachlorodibenzo-p-dioxin

OBJECTIVES

To examine the association of immune cell number and function with occupational exposure to substances contaminated with 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD).

METHODS

A cross sectional medical survey. The exposed participants were employed at two chemical plants between 1951 and 1972 in the manufacture of 2,4,5-trichlorophenate and its derivatives. The reference group consisted of people with no occupational exposure to phenoxy herbicides who lived within the communities of the workers. Data from a total of 259 workers and 243 unexposed referents were included in the analysis of immune function. Laboratory tests for immune status included enumeration of circulating leukocyte and lymphocyte populations, proliferative responses of circulating lymphocytes to mitogens and antigens, and serum concentrations of the major immunoglobulins and complement factor C3.

RESULTS

The workers had substantial exposure to substances contaminated with TCDD, as indicated by a lipid adjusted mean serum TCDD concentration of 229 ppt compared with a mean of 6 ppt in the unexposed referents. Workers were divided into categories based on their serum TCDD concentration. For all categories except the lowest, with values of serum TCDD comparable with the unexposed referents, there were increased odds of having lower counts of CD26 cells (activated T cells) (odds ratio (OR) 1.0, 95% confidence interval (95% CI) 0.5 to 1.8 for TCDD < 20 ppt; OR 1.6, 95% CI 0.8 to 3.2 for TCDD 20-51 ppt; OR 2.7, 95% CI 1.4 to 5.1 for TCDD 52-125 ppt; OR 2.6, 95% CI 1.4 to 4.9 for TCDD 125-297 ppt; OR 2.4, 95% CI 1.3 to 4.6 for TCDD 298-3389 ppt). A less consistent finding was decreased spontaneous proliferation of cultured lymphocytes. However, increases were found in proliferation of lymphocytes in response to concanavalin and pokeweed in workers in the high TCDD category. Age, cigarette smoking, and alcohol were significant predictors of several immunological outcomes.

CONCLUSIONS

Associations between serum TCDD concentration and both a decrease in circulating CD26 cells and decreased spontaneous background proliferation were the major findings of this study. These results are unlikely to be of clinical importance but may reflect limited evidence for an association between immunological changes in workers and high serum concentrations of TCDD, or chance findings resulting from the evaluation of multiple immunological variables.

Analysis of gene expression in lung and thymus of TCDD treated C57BL/6 mice using differential display RT-PCR

The differential display RT-PCR technique offers a means to identify genes which are regulated by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). Treatment of female C57BL/6 mice with 10 micrograms TCDD/kg body weight followed by DDTR-PCR analysis with a set of different primers revealed differential expression for a number of mRNAs. RNA slot blot analysis of six of these clones confirmed differential expression. Further characterization of one of these clones obtained from lung tissue demonstrated that the corresponding mRNA is about 1 kb and shows significant TCDD induced expression in Northern blot analysis. A Genbank search of this clone showed no similarity to known sequences.

Differential induction of apoptosis in activated and resting T cells by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and its repercussion on T cell responsiveness

TCDD is well known for its immunotoxic effects on T cells, although the exact mechanism of toxicity remains unknown. In the current study, we investigated the effect of TCDD administration on resting and activated T cells within the same animal. To this end, C57BL/6 mice were injected intraperitoneally with either TCDD (50 microg/kg body weight) or the vehicle and were injected with anti-CD3 mAbs into the rear footpads to polyclonally activate T cells in the popliteal lymph nodes (LN). Axillary LN cells harvested from the same groups of mice served as a source of resting T cells. When the LN cells were tested for their proliferative responsiveness to stimulation with anti-CD3 mAbs in vitro, the activated popliteal LN, but not the resting axillary LN cells from TCDD-treated mice exhibited a significant decrease in responsiveness when compared to the vehicle controls. Inasmuch as TCDD has been shown to induce apoptosis in thymocytes, we addressed whether TCDD triggered apoptosis in LN cells, using the terminal deoxynucleotidyl transferase (TdT)-mediated FITC-dUTP nick end labeling (TUNEL) method. The axillary and popliteal LN cells from TCDD-treated mice failed to exhibit significant levels of apoptosis when freshly harvested. However, upon in vitro culture for 24 h with either tissue culture medium alone or with anti-CD3 mAbs, activated popliteal LN cells from TCDD-treated mice showed a significant increase in apoptosis when compared to similar cells from vehicle-treated mice. In contrast, resting axillary LN cells from TCDD-treated mice, similarly cultured in vitro, exhibited decreased levels of apoptosis when compared to the controls. Using a double-staining technique, the activated popliteal LN cells undergoing increased apoptosis in TCDD-treated animals were confirmed to be CD3+ T cells. Together, these data demonstrate that TCDD exerts differential effects on activated and resting T cells, even within the same animal, by inhibiting the proliferative responsiveness of activated, but not resting, T cells. Furthermore, this effect may be mediated by the ability of TCDD to induce increased apoptosis in activated, but not resting, T cells.

Experimental studies on immunosuppression: how do they predict for man?

The ultimate goal of any animal model in immunotoxicity testing is that it be a sensitive predictor of xenobiotic-induced immune dysfunction in humans. Such models should be capable of identifying the target(s) within the immune system affected by the xenobiotic. In particular the tier testing models have been successfully used to identify and characterize a variety of different immunotoxicants in animals as it pertains to immunosuppression and reduced resistance to infectious diseases. These tier models in mice and rats have been validated in interlaboratory studies. Although these protocols were designed for studies of rats and mice, some have been applied successfully for studying immunotoxicity in other animal species, including non-human primates. A great amount of data has been generated by the application of these models, which demonstrate that xenobiotics alter the immune system of animals. In man, the database on chemical-induced immunosuppression is limited, as the use of markers of immunotoxicity has received little attention in clinical and epidemiological studies. Such studies have not been performed frequently, and their interpretation often does not permit unequivocal conclusions to be drawn, due for instance to the presence of confounding factors and the uncontrolled nature of exposure. Also, testing possibilities in humans are limited and immune function changes by chemical exposure are often subtle. In humans, a number of agents have been shown to have immunosuppressive properties (including PCBs, PCDDs, PCDFs, oxidant gases, and ultraviolet radiation), but the strongest evidence stems from the clinical use of immunosuppressant drugs in transplant patients. These human data do in general terms confirm the data gained with experimental animals. Immunotoxicity assessment in rodents therefore adequately forms the basis for human risk assessment. Knowledge on the predictability of these animal models and immune assays can be further improved by comparison of the human and animal data obtained in the development of drugs.

Methylcholanthrene causes increased thymocyte apoptosis

The polycyclic aromatic hydrocarbon (PAH), methylcholanthrene (MCA), is a well studied carcinogen and a teratogen. MCA and other PAH cause immune suppression of B cell and T cell responses in mice and MCA had been reported to induce thymus atrophy. Here we show that MCA treatment causes thymus atrophy in adrenalectomized mice and in C57BL/6 and DBA/2 mice which differ in aryl hydrocarbon receptor (AhR) expression. This indicates that MCA-mediated thymus atrophy is mediated, at least in part, by glucocorticoid hormone receptor- and aryl hydrocarbon receptor-independent mechanisms. Assay of thymocytes, both in situ and ex vivo, demonstrate that MCA induces thymocyte apoptosis. Apoptotic thymocytes can be found within or adjacent to thymic Mphi, suggesting rapid phagocytosis. Mice that are deficient in tumor necrosis factor-alpha receptor-1 or p53, or that overexpress bcl-2 are susceptible to MCA-mediated thymus atrophy.

P450 isoforms in a murine macrophage cell line, RAW264.7, and changes in the levels of P450 isoforms by treatment of cells with lipopolysaccharide and interferon-gamma

The presence of P450 in a murine macrophage cell line, RAW264.7, was investigated to clarify the biological role and regulation of P450. Microsomes of RAW264.7 cells were isolated and subjected to immunoblotting with anti-rat CYP2A1, 2B1, and 4A2 antibodies. The microsomes gave staining bands with all these antibodies, suggesting the presence of mouse Cyp2a, 2b, and 4a isoforms in RAW264.7. RAW264. 7 cells were treated with typical inducers of P450 (phenobarbital, clofibrate, beta-naphthoflavone and 3-methylcholanthrene). None of these chemicals induced these P450s. Stimulation of RAW264.7 cells with lipopolysaccharide (LPS) and interferon-gamma (INF-gamma) which increase inducible nitric oxide synthase (iNOS) and cytokines in cells decreased Cyp4a protein but not Cyp2a and 2b proteins. To identify P450 isoforms in RAW264.7, we used polymerase chain reaction (PCR) primers for mouse Cyp2a4, 2a12, 2b9/10, 4a10, and 4a12. Total RNA was isolated from these cells and converted to cDNA by reverse transcriptase. PCR was done with these primers and the amplified nucleotides were analyzed by a DNA sequencer. Only Cyp2b9/10 and 4a12 primers gave clear bands, although all primers gave clear bands from liver total RNA. Nucleotide sequences of these products amplified by PCR were identical with Cyp2b9 and 4a12. These findings indicate that Cyp2b9 and 4a12 were present in a macrophage cell line, RAW264.7, and the regulation of P450 by inducers and cytokine differed from that in liver.

2,3,7,8-Tetrachlorodibenzo-p-dioxin co-stimulates staphylococcal enterotoxin beta (SEB) cytokine production and phenotypic cell cycling in Long-Evans rats

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) is an environmental contaminant that is considered to be a potent immunotoxicant. In the present study, we examined the effect of 25 micrograms/kg TCDD on cytokine production and T lymphocyte phenotype, cell cycling and receptor activity in female Long-Evans rats that had been injected with 50 micrograms of Staphylococcal Enterotoxin B (SEB). In the SEB-injected rats, TCDD increased the serum levels of interleukin-2 (IL-2) but did not affect the serum levels of interleukin-1 (IL-1), interleukin-6 (IL-6) or tumor necrosis factor (TNF). The ability of spleen cells and peritoneal cells to produce cytokines in response to SEB restimulation in vitro was also evaluated. TCDD exposure significantly enhanced IL-2 production by spleen cells from SEB-primed rats after 6 h or 24 h in cultures co-stimulated with SEB in vitro. However, TCDD treatment did not alter the production of IL-6 and TNF in these cultures. Although TCDD did not influence the production of IL-6 and TNF in peritoneal cells from SEB-primed rats with SEB restimultion in vitro, IL-1 production was significantly increased at 2 h. Both the kinetics and extent of SEB-induced IL-2 receptor (IL-2R) and T-cell receptor (TCR) expression on CD4+ cells was unaffected by TCDD. TCDD did not significantly alter the percentage or the total numbers of CD4+ and CD8+ subpopulations at various times after SEB injection. However, flow cytometric analysis showed that TCDD exposure increased the percentage of both CD4+ and CD8+ cells cycling in the S and G2M phase. TCDD, in the absence of SEB priming, did not affect any of the immune parameters tested. Nevertheless, collectively these results showed that TCDD can enhance the production of IL-2 and the percentage of CD4+ and CD8+ cells cycling in SEB-exposed Long-Evans rats. Histopatholgically, there were not observable effects of SEB on lymphoid organs while thymic atrophy and diffuse hepatocellular hypertrophy was observed in the TCDD-treated animals.

Down-regulation of major histocompatibility complex Q1b gene expression by 2,3,7,8-tetrachlorodibenzo-p-dioxin

We analyzed mouse hepatoma cells using differential display to discover new genes that respond to the environmental contaminant 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). We identified a class I major histocompatibility complex (MHC) gene, which we designated as MHC Q1b, whose expression decreases in the presence of TCDD. TCDD-induced down-regulation of MHC Q1b requires both the aromatic hydrocarbon receptor and the aromatic hydrocarbon receptor nuclear translocator, transcription factors that up-regulate other genes in response to TCDD. Down-regulation of MHC Q1b by TCDD appears to involve both transcriptional and post-transcriptional regulatory events; the post-transcriptional destabilization of MHC Q1b mRNA is probably a secondary response to TCDD. Our findings reveal new mechanistic aspects of gene regulation by TCDD. In addition, our observations suggest a mechanism that might account for some of TCDD's immunotoxic effects.

Middle-ear disease in children exposed prenatally to polychlorinated biphenyls and polychlorinated dibenzofurans

During 1978 and 1979, an episode of poisoning from ingestion of rice oil contaminated with polychlorinated biphenyls and polychlorinated dibenzofurans occurred in central Taiwan. We followed-up children who had been born between June 1978 and March 1985, as well as matched unexposed children. The mothers of exposed children had consumed contaminated oils before the children were born. In 1993, otolaryngologists examined the middle ear of each child with a pneumatic otoscope, and they measured the middle-ear pressure by tympanometry with a Rion RS20 impedance audiometer. The exposed children had a significantly higher prevalence of middle-ear diseases than their matched controls. The exposed children who had ear diseases had higher serum levels of 2,3,4,7,8-pentachloro- and 1,2,3,4,7,8-hexachloro-dibenzofurans than the children who did not have similar diseases. Therefore, in this study, children exposed to polychlorinated biphenyls and polychlorinated dibenzofurans had a higher incidence of middle-ear diseases than their controls.

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.

Modulated cytokine production from cervical lymph node cells treated with B[a]P and PCB

To investigate the effects of Benzo[a]pyrene (B[a]P) and polychlorinated biphenyls (PCB) on in vitro cytokine production, we isolated cervical lymph node cells from ovalbumin (OA)-primed mice and the lymph node cells were cultured with various concentrations of B[a]P or PCB in the presence of antigen presenting cells (APC) and OA. B[a]P significantly increased the levels of interleukin-4 (IL-4) and -6(IL-6) in the culture supernatants. However, the levels of interferon-gamma (IFN-gamma) were decreased by treatment with PCB. These findings indicate that in vitro treatment with B[a]P or PCB modulate cytokine production from lymph node cells.

Identification of functional aryl hydrocarbon receptor and aryl hydrocarbon receptor nuclear translocator in murine splenocytes

The objective of the present studies was to determine whether the aryl hydrocarbon receptor (AhR) and AhR nuclear translocator (ARNT) protein are present and functional in B6C3F1 (C57BL/6 x C3H) mouse splenocytes. Northern analysis of poly(A) RNA isolated from splenocytes revealed transcripts of approximately 6.6 kb which hybridized to the AhR complementary DNA (cDNA) probe. Anti-AhR antibodies identified two major cytosolic forms of the AhR in splenocytes, approximately 95 and 104 kDa, corresponding to the codominately expressed Ahrb alleles in the B6C3F1 mice. Northern analysis utilizing an oligomer probe for ARNT identified three messenger RNA (mRNA) transcripts, approximately 5.6, 2.0, and 1.1 kb, in spleen which was consistent with the banding pattern observed in the B6C3F1 mouse liver. Western blotting confirmed the presence of the approximately 87 kDa ARNT protein in splenocytes. Protein quantitation by slot blot analysis demonstrated approximately 2.0-fold more AhR in liver than in splenocytes. Interestingly, ARNT was approximately 2.4-fold more abundant in splenocytes than in liver. Consistent with these results, comparison by quantitative reverse transcriptase-polymerase chain reaction analysis of AhR and ARNT transcripts in liver and splenocytes demonstrated approximately 2.3-fold more AhR transcripts in liver than in splenocytes and approximately 3.2-fold more ARNT transcripts in splenocytes than in liver. In addition, comparisons between AhR and ARNT transcripts isolated from the liver and splenocytes indicated a greater number of ARNT transcripts as compared with AhR in both preparations. TCDD treatment of splenocytes induced binding of the AhR nuclear complex to the dioxin-responsive enhancer (DRE) as detected by the electrophoretic mobility shift assay. These findings confirm that the AhR and ARNT are present in mouse splenocytes and are capable of binding to the DRE.

Immunotoxicity of a reconstituted polynuclear aromatic hydrocarbon mixture in B6C3F1 mice

Previous studies on the immunotoxicity of a complex mixture of polynuclear aromatic hydrocarbon (PAH) by-products from a manufactured gas plant indicated possible synergistic interactions which were investigated by determining the immunosuppressive effects of a reconstituted PAH mixture in female B6C3F1 mice challenged with TNP-haptenated sheep red blood cells (SRBCs) (T-cell-dependent) or trinitrophenyl-lipopolysaccharide (TNP-LPS) (T-cell-independent) antigens. The reconstituted PAH mixture contained the following 17 congeners: 2-rings (indan, naphthalene, 1- and 2-methylnaphthalene), 3-rings (acenaphthylene, acenaphthene, dibenzofuran, fluorene, phenanthrene and anthracene), and > or = 4-rings (pyrene, fluoranthene, benz[a]anthracene, chrysene, benzo[b]fluoranthene, benzo[k]fluoranthene and benzo[a]pyrene), and resembled mixtures identified as by-products from manufactured gas plants. The reconstituted mixture and the 2-, 3- and > or = 4-ring PAH fractions all caused a dose-dependent decrease in the splenic plaque-forming cell (PFC) response to SRBCs or TNP-LPS, and their ED50 values for the four treatment groups were 86, 354, 145, and 23 or 163, 439, 637 and 31 mg/kg, respectively. The corresponding ED50 values for decreased serum anti-TNP IgM levels for these same mixtures were (TNP-haptenated SRBCs, T-cell-dependent) 144, 231, 42 and 27 units, respectively, and (TNP-LPS, T-cell-independent) 161, 406, 312 and 69 units, respectively. The suppression of anti-TNP IgM titers was similar to the suppression of the PFC response and shows that antigen-specific immunoglobulin titer can be used as a biomarker of PAH exposure. A direct comparison of the immunotoxic responses of the reconstituted PAH mixture and the corresponding dose of the > or = 4-ring PAHs indicated that the latter fraction was primarily responsible for the activity of the reconstituted mixture.

Identification of dioxin-responsive elements (DREs) in the 5' regions of putative dioxin-inducible genes

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) is an exogenous ligand for the cytosolic aryl hydrocarbon receptor (AhR), a ligand-inducible transcription factor whose exact physiological role remains elusive. TCDD has been shown to modulate the expression of a large array of genes, albeit often indirectly, by demonstration of protein or mRNA upregulation. Here, by computer analysis of available promoter sequences, we identify dioxin-responsive elements in the promoter regions of many putative AhR regulated and therefore dioxin-inducible genes.

Effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin on humoral and cell-mediated immunity in Sprague-Dawley rats

There is much discussion about the occurrence of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)-induced effects on the human immune system. Extensive studies have been conducted in mice, but those results cannot explain some of the epidemiological data obtained in exposed humans. Therefore, studies in other laboratory animal species are needed. The aim of these experiments was to examine effects of TCDD on cell- and humoral-mediated immunity in male Sprague-Dawley (SD) rats. A delayed-type hypersensitivity (DTH) assay was used to examine cell-mediated immunity. A time-course study demonstrated that TCDD treatment on day -5 relative to immunization (day 0) produced the greatest effect on cell-mediated immunity. In a dose-response experiment, rats were treated with 1, 3, 10, 20, 30, 40 and 90 micrograms TCDD/kg The effect of TCDD on cell-mediated immunity displayed an inverted U-shaped dose-response curve, in that low doses enhanced and high doses suppressed this immune function. This is the first study to demonstrate an U-shaped dose-response curve of TCDD on the immune system. Primary antibody response to sheep red blood cells (SRBC) was used as endpoint to study the effect of TCDD on humoral immunity. Serum anti-SRBC IgM and IgG levels were measured using an enzyme-linked immunosorbent assay (ELISA). In the dose range examined (10, 20 and 40 micrograms TCDD/kg), serum IgM levels were not affected by TCDD compared to controls at 7 and 14 days after immunization. In contrast, serum IgG levels were dose-dependently elevated both 7 and 14 days after immunization, with a maximum increase of 59% over controls.

Thymus atrophy and changes in thymocyte subpopulations of BN rats with mercury-induced renal autoimmune disease

Administration of low doses of mercury induces autoantibodies to laminin and autoimmune glomerulonephropathy in BN, MAXX and DZB rats as well as in (BN x LEW)F1 hybrids. LEW strain rats are resistant to these immunotoxic effects. Susceptible rats also show lymphoid hyperplasia in spleen and lymph nodes and severe thymic atrophy. It is still uncertain whether these mercury-induced changes have any role in the induction of autoimmune responses to laminin. In the present study, we have examined the effects of mercury on the thymus of susceptible and resistant rats. Histological analysis of thymuses from BN rats revealed extensive disorganization within 15 days following mercury treatment, with loss of demarcation between cortex and medulla. Numbers of thymus cells were significantly decreased in both BN and (BN x LEW)F1 hybrid rats injected with HgCl2. There was no apparent increase in apoptotic cells in the thymus of these animals. By flow cytometry we detected a relative and absolute loss of double-positive CD4+ CD8+ thymocytes in BN (but not in LEW rats) within 15 days of mercury treatment. There was a corresponding increase in the relative proportion of single-positive (CD4+ or CD8+) and double-negative CD4- CD8- thymocytes in mercury-treated BN rats. Absolute increases in the number of CD4+ single-positive thymocytes were also observed. In contrast, mercury-treated LEW rats had no changes in thymus architecture or significant decreases in cell numbers. Since the thymus is important in both position and negative selection of developing thymocytes, immunotoxic effects of mercury on its structure and thymocyte subpopulations may have multiple consequences. Alternatively, we suggest the hypothesis that autoimmunity (and in particular autoantibodies to laminin) may be responsible for the changes observed in the thymus.

Evidence for the promotion of positive selection of thymocytes by Ah receptor agonist 2,3,7,8-tetrachlorodibenzo-p-dioxin

2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) is a ligand for the arylhydrocarbon receptor (Ah receptor), abundant in the murine thymus. In the thymus immunocompetent T cells develop. Upon exposure of murine fetal thymi in organ cultures to TCDD the distribution of mature and immature thymocytes is skewed towards apparently mature, prospective cytotoxic cells of the CD4-CD8+T cell receptor+ phenotype. The normally abundant CD4+ CD8+ cells are decreased. Proliferation of the most immature thymocyte subpopulations is inhibited and maturation of thymocytes appears accelerated by TCDD. Eventually the thymocyte number is significantly decreased. Selective treatment of stroma cells showed them to be the primary target cells of TCDD action. Thymus stroma plays a pivotal role in thymocyte maturation and is indispensable for the selection of thymocytes bearing T cell receptors specific for foreign antigen in the context of self. We tested whether the effects of TCDD on thymocyte differentiation and maturation has further consequences for the selection processes by analysing (a) the repertoire of V beta genes used as a measure for negative selection and (b) the expression of CD69 and bcl-2 by thymocytes as a parameter of positive selection. Our data indicate that TCDD does not cause gross disturbance of negative selection but provide evidence for more cells auditioning for positive selection by TCDD exposure.

Markers for immunotoxic effects in rodents and man

There is growing interest and concern in society about the capacity of chemicals to impair immune responses and trigger autoimmune disease. Methods to investigate chemical-induced immunosuppression have been developed and validated in the mouse and rat. Animal models that are suitable to investigate the ability of chemicals to induce autoimmune disease are virtually lacking. From the plethora of tests to assess immunity in man, panels of biomarkers to study immunotoxicity in humans have been proposed. Such studies in humans are considerably more complex than in animals as non-invasive tests are limited, responses in the population are heterogeneous, and exposure levels are often low. Human risk assessment is therefore mostly based on animal studies. As examples of compounds affecting biomarkers in animals and man, 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and hexachlorobenzene are discussed.