Correlations between polychlorinated biphenyl immunotoxicity, the aromatic hydrocarbon locus, and liver microsomal enzyme induction in C57BL/6 and DBA/2 mice

3,3',4,4'-tetrachlorobiphenyl (PCB77) enhances human Kv1.3 channel currents and alters cytokine production

Polychlorinated biphenyls (PCBs) were once used throughout various industries; however, because of their persistence in the environment, exposure remains a global threat to the environment and human health. The Kv1.3 and Kv1.5 channels have been implicated in the immunotoxicity and cardiotoxicity of PCBs, respectively. We determined whether 3,3',4,4'-tetrachlorobiphenyl (PCB77), a dioxin-like PCB, alters human Kv1.3 and Kv1.5 currents using the Xenopus oocyte expression system. Exposure to 10 nM PCB77 for 15 min enhanced the Kv1.3 current by approximately 30.6%, whereas PCB77 did not affect the Kv1.5 current at concentrations up to 10 nM. This increase in the Kv1.3 current was associated with slower activation and inactivation kinetics as well as right-shifting of the steady-state activation curve. Pretreatment with PCB77 significantly suppressed tumor necrosis factor-α and interleukin-10 production in lipopolysaccharide-stimulated Raw264.7 macrophages. Overall, these data suggest that acute exposure to trace concentrations of PCB77 impairs immune function, possibly by enhancing Kv1.3 currents.

Dioxin-like Activity in Pregnant Women and Indices of Fetal Growth: The ACCEPT Birth Cohort

Exposure to lipophilic persistent organic pollutants (lipPOPs) elicits a number of species- and tissue-specific toxic responses, many of which involve the aryl hydrocarbon receptor (AhR). This study aims to measure the combined serum dioxin-like activity of lipPOPs in Greenlandic Inuit pregnant women and the associations with fetal growth indices. The combined dioxin-like activity of serum lipPOPs extracts was determined using the AhR reporter gene bioassay and expressed as pico-gram (pg) TCDD equivalent (TEQ) per gram serum lipid [AhR-TEQ (pg/g lipid)]. Significant AhR-TEQ was found in >87% of serum samples with the median level of 86.2 pg TEQ/g lipid. The AhR-TEQ level positively correlated with the marine food intake biomarker n-3/n-6 polyunsaturated fatty acids ratio, while negatively correlated with body mass index and parity. Women giving birth to infants with low birth weight (<2500 g) and length (<50 cm) had higher AhR-TEQ level compared to those with normal weight and length infants. For previous smokers, we found significant inverse associations between maternal AhR-TEQ level and fetal growth indices. In conclusion, exposure of Greenlandic Inuit pregnant women to dioxin-like compounds through traditional marine food can adversely influence the fetal growth via induced AhR activity. Smoking might have modifying effects.

Effects of an environmentally relevant PCB-mixture on immune function, clinical chemistry, and thyroid hormone levels in adult female B6C3F1 mice

Polychlorinated biphenyls (PCBs) have been assessed for immunotoxicity; however, humans and wildlife are exposed to multiple PCBs environmentally. Therefore, the aim of this study was to examine the effects of a complex 37 PCB congener mixture identified in blubber specific to dolphins residing in the estuarine waters of Charleston, South Carolina. Immunotoxicity was determined in adult female B₆C₃F₁ mice by assessing lymphocyte proliferation, splenic and thymic immunophenotypes, and IgM production. Mice were exposed via oral gavage to the PCB-mixture (0, 1.8, 3.6, 7.1, or 14.3 mg/kg/day) for 28 days to yield a targeted total administered dose (TAD) 0, 50, 100, 200, or 400 mg/kg. Significant increased liver weight occurred at the highest treatment. IgM production was suppressed compared to control for all treatments. Numbers of thymic CD4+/CD8+, CD4-/CD8-, and CD4+/CD8- cells were not altered, but numbers of thymic CD4-/CD8+ cells were significantly increased in the highest treatment. Lymphocyte proliferation was not markedly affected by any treatment. The numbers of splenic CD4/CD8 T-cells or MHCII+ cells were not significantly changed. Humoral immunity using the plaque-forming cell assay for determining the specific IgM antibody-forming cell response appeared to be the most sensitive endpoint affected. As the lowest concentration tested resulted in decreased IgM production and total and free thyroxine (T₄) serum levels a NOAEL was not identified. The calculated ED₅₀ for suppression of IgM production was 2.4 mg/kg/day.

Targeting Aryl hydrocarbon receptor for next-generation immunotherapies: Selective modulators (SAhRMs) versus rapidly metabolized ligands (RMAhRLs)

Aryl Hydrocarbon Receptor (AhR) constitutes a major network hub of genomic and non-genomic signaling pathways, connecting host's immune cells to environmental factors. It shapes innate and adaptive immune processes to environmental stimuli with species-, cell- and tissue-type dependent specificity. Although an ever increasing number of studies has thrust AhR into the limelight as attractive target for the development of next-generation immunotherapies, concerns exist on potential safety issues associated with small molecule modulation of the receptor. Selective AhR modulators (SAhRMs) and rapidly metabolized AhR ligands (RMAhRLs) are two classes of receptor agonists that are emerging as interesting lead compounds to bypass AhR-related toxicity in favor of therapeutic effects. In this article, we discuss SAhRMs and RMAhRLs reported in literature, covering concepts underlying their definitions, specific binding modes, structure-activity relationships and AhR-mediated functions.

Environmental Immunology: Lessons Learned from Exposure to a Select Panel of Immunotoxicants

Exposure to environmental contaminants can produce profound effects on the immune system. Many classes of xenobiotics can significantly suppress or enhance immune responsiveness depending on the levels (i.e., dose) and context (i.e., timing, route) of exposure. Although defining the effects that toxicants can have on the immune system is a valuable component to improving public health, environmental immunology has greatly enhanced our understanding of how the immune system functions and has provided innovative avenues to explore new immunotherapies. This Brief Review focuses on three examples of how immunotoxicology has benefitted the field of immunology, presenting information on the aryl hydrocarbon receptor signaling pathway, the immunomodulatory effects of nanomaterials, and the impact of xenobiotic exposure on the developing immune system. Collectively, contributions from immunotoxicology have significantly enhanced public health and spurred seminal advances in both basic and applied immunology.

The role of the aryl hydrocarbon receptor in normal and malignant B cell development

The aryl hydrocarbon receptor (AhR) is a ligand-activated transcription factor historically studied for its role in environmental chemical-mediated toxicity and carcinogenicity. In the last 5 years, however, it has become clear that the AhR, presumably activated by endogenous ligand(s), plays an important role in immune system development and function. Other articles in this edition summarize AhR function during T cell and antigen-presenting cell development and function, including the effects of AhR activation on dendritic cell function, T cell skewing, inflammation, and autoimmune disease. Here, we focus on AhR expression and function during B cell differentiation. Studies exploiting immunosuppressive environmental chemicals to probe the role of the AhR in humoral immunity are also reviewed to illustrate the multiple levels at which a “nominally activated” AhR could control B cell differentiation from the hematopoietic stem cell through the pro-B cell, mature B cell, and antibody-secreting plasma cell stages. Finally, a putative role for the AhR in the basic biology of B cell malignancies, many of which have been associated with exposure to environmental AhR ligands, is discussed.

The Relationship Between Noncoplanar PCB-Induced Immunotoxicity and Hepatic CYP1A Induction in a Fish Model

Abundant literature exists demonstrating the immunomodulating effects of polychlorinated biphenyls (PCBs). To date, most of the research has focused on dioxin-like coplanar PCB congeners because of their high affinity for the aryl hydrocarbon receptor (AhR) and cytochrome P450-inducing capability. For this study, the impact of two structurally different PCB congeners on the immune responsiveness of bluegill sunfish (Lepomis macrochirus) was examined to evaluate the immunotoxic potential of each congener (as separate entities) and to relate effects on immune function with hepatic CYP1A induction. Fish received a single intraperitoneal injection of the: coplanar congener, PCB 126 (0.01 or 1.0 mug/g BW); noncoplanar PCB 153 (5.0 or 50.0 mug/g BW); or, the corn oil vehicle. PCB-induced effects on innate and cell-mediated immune parameters, and on hepatic CYP1A protein induction were evaluated in fish sacrificed 1, 3, 7, 14 or 21 days post-injection. In the absence of CYP1A induction, PCB 153 increased kidney phagocyte-mediated superoxide production 3 d post-injection, and at the highest dose suppressed B- and T-lymphocyte proliferation after 3 and 7 days, respectively. Treatment of fish with PCB 126 had no effect on oxyradical production, but altered B-lymphocyte proliferation after 1 day, also in the absence of CYP1A induction. Hepatic CYP1A was only induced in fish exposed to the highest PCB 126 dose; protein induction appeared at 3 d post-injection and persisted for up to 21 days. Taken together, these results demonstrate that exposure to different PCB congeners can alter immune function in the absence of CYP1A induction, suggesting that mechanisms other than the AhR pathway may play a role in PCB-induced immunotoxicity, particularly for the noncoplanar congeners.

Pre- and postnatal polychlorinated biphenyl concentrations and longitudinal measures of thymus volume in infants

BACKGROUND

Previously, we reported an association between higher maternal polychlorinated biphenyl (PCB) concentrations and smaller thymus volume in newborns in a birth cohort residing in eastern Slovakia.

OBJECTIVE

In the present report we address whether thymus volume at later ages is influenced by prenatal and early postnatal PCB exposure.

METHODS

At the time of delivery, 1,134 mother-infant pairs were enrolled. Maternal and 6- and 16-month infant blood samples were collected and analyzed for 15 PCB congeners. Thymus volume was measured in infants shortly after birth and at ages 6 and 16 months using ultrasonography.

RESULTS

Higher maternal PCB concentration was associated with reduced thymus volume at birth [a 0.21 SD reduction in thymus volume for an increase in total maternal PCB concentration from the 10th to the 90th percentile; 95% confidence interval (CI): -0.37, -0.05], whereas maternal PCB concentration was not predictive of 6- and 16-month thymus volume. Six-month infant PCB concentration was associated with a 0.40 SD decrease in 6-month thymus volume (95% CI: -0.76, -0.04). There was also some suggestion that thymus volume at 16 months was positively associated with concurrent infant PCB concentration.

CONCLUSIONS

The potential adverse effects of in utero PCB exposure on thymic development may extend beyond the neonatal period. Results from this highly exposed cohort provide suggestive evidence that postnatal PCB concentrations may be influential, but a smaller set of 6-month PCB measurements limited statistical power at that time point. Implications regarding impaired immunologic maturation or long-term clinical implications remain to be determined.

A cohort study of developmental polychlorinated biphenyl (PCB) exposure in relation to post-vaccination antibody response at 6-months of age

BACKGROUND

Extensive experimental data in animals indicate that exposure to polychlorinated biphenyls (PCBs) during pregnancy leads to changes in offspring immune function during the postnatal period. Whether developmental PCB exposure influences immunologic development in humans has received little study.

METHODS

The study population was 384 mother-infant pairs recruited from two districts of eastern Slovakia for whom prospectively collected maternal, cord, and 6-month infant blood specimens were available. Several PCB congeners were measured in maternal, cord, and 6-month infant sera by high-resolution gas chromatography with electron capture detection. Concentrations of IgG-specific anti-haemophilus influenzae type b, tetanus toxoid, and diphtheria toxoid were assayed in 6-month infant sera using ELISA methods. Multiple linear regression was used to estimate the relation between maternal, cord, and 6-month infant PCB concentrations and the antibody concentrations evaluated at 6-months of age.

RESULTS

Overall, there was little evidence of an association between infant antibody concentrations and PCB measures during the pre- and early postnatal period. In addition, our results did not show specificity in terms of associations limited to a particular developmental period (e.g. pre- vs. postnatal), a particular antibody, or a particular PCB congener.

CONCLUSIONS

At the PCB concentrations measured in this cohort, which are high relative to most human populations today, we did not detect an association between maternal or early postnatal PCB exposure and specific antibody responses at 6-months of age.

Evolution of tolerance to PCBs and susceptibility to a bacterial pathogen (Vibrio harveyi) in Atlantic killifish (Fundulus heteroclitus) from New Bedford (MA, USA) harbor

A population of the non-migratory estuarine fish Fundulus heteroclitus (Atlantic killifish) resident to New Bedford (NB), Massachusetts, USA, an urban harbor highly contaminated with polychlorinated biphenyls (PCBs), demonstrates recently evolved tolerance to some aspects of PCB toxicity. PCB toxicology, ecological theory, and some precedence supported expectations of increased susceptibility to pathogens in NB killifish. However, laboratory bacterial challenges of the marine pathogen Vibrio harveyi to wild fish throughout the reproductive season and to their mature laboratory-raised progeny demonstrated comparable survival by NB and reference killifish, and improved survival by NB males. These results are inconsistent with hypothesized trade-offs of adaptation, and suggest that evolved tolerance in NB killifish may include mechanisms that minimize the immunosuppressive effects of PCBs. Compensatory strategies of populations persisting in highly contaminated environments provide a unique perspective for understanding the long-term ecological effects of toxic chemicals.

Effects of PCBs 52 and 77 on Th1/Th2 Balance in Mouse Thymocyte Cell Cultures

Polychlorinated biphenyls (PCBs) are industrial chemicals that have been released into the environment, resulting in widespread and persistent contamination. PCBs exist as 209 different congeners depending on the chlorine substitution on the biphenyl rings, and the physical properties and toxic effects of different PCB congeners are structure-dependent. We have tested an ortho-substituted, noncoplanar congener, 2,2',5,5'-tetrachlorobiphenyl (PCB 52), and a non-ortho-substituted coplanar congener, 3,3',4,4'-tetrachlorobiphenyl (PCB 77), for their effects on the T helper 1 (Th1) and T helper 2 (Th2) lymphocyte balance. The experiments were performed in 10 microg/ml concanavalin A (Con A)-stimulated and nonstimulated thymocytes for determination of cytokine production profiles. Interferon-gamma (produced by Th1 cells) and interleukin (IL)-10 (produced by Th2 cells) concentrations were measured in the supernatants at 12 and 24 hr after treatment with PCBs. PCBs 52 and 77 caused significant increases in interferon-gamma levels at 12 and 24 hr in both Con A-stimulated and nonstimulated media. IL-10 levels were significantly reduced at two interval periods by PCBs 52 and 77 in cultures with and without Con A. Our results show that the cytokine production profile was significantly shifted to Th1 by both ortho-substituted and coplanar PCB congeners in mouse thymocyte cultures.

Genotoxic effects of PCB 52 and PCB 77 on cultured human peripheral lymphocytes

Polychlorinated biphenyls (PCBs) are known to be carcinogenic, but the mechanisms of this action are uncertain. Most, but not all, studies have concluded that PCBs are not directly mutagenic, and that much if not all of the carcinogenic activity resides in the fraction of the PCB mixture that contains congeners with dioxin-like activity. The present study was designed to determine genotoxic effects of an ortho-substituted, non-coplanar congener, 2,2',5,5'-tetrachlorobiphenyl (PCB 52), and a non-ortho-substituted coplanar congener with dioxin-like activity, 3,3',4,4'-tetrachlorobiphenyl (PCB 77) on cultured human peripheral lymphocytes. DNA damage was assessed by use of the comet assay (alkaline single-cell gel electrophoresis). After cell cultures were prepared, test groups were treated with different concentrations of PCB 52 (0.2 and 1 microM) and PCB 77 (1 and 10 microM) for 1h at 37 degrees C in a humidified carbon dioxide incubator, and compared to a DMSO vehicle control group. The cells were visually classified into four categories on the basis of extent of migration such as undamaged (UD), low damage (LD), moderate damage (MD) and high damage (HD). The highest concentration of PCBs 52 and 77 significantly increased DNA breakage in human lymphocytes (p<0.001). Our results indicate that both the non-coplanar PCB 52 and coplanar PCB 77 cause DNA damage, and that the ortho-substituted congener was significantly more potent than the dioxin-like coplanar congener.

Polychlorinated biphenyls (PCBs) detection in milk samples by an electrochemical magneto-immunosensor (EMI) coupled to solid-phase extraction (SPE) and disposable low-density arrays

An electrochemical immunosensor for polychlorinated biphenyl (PCB) detection based on graphite screen-printed low-density arrays and on magnetic beads is reported. The immunological reaction for the detection of PCBs is based on a direct competitive assay using alkaline phosphatase (AP) as enzymatic label. After the immunochemical recognition, the modified magnetic beads are captured by a magnet on the surface of the graphite working electrode. The electrochemical detection is thus achieved through the addition of the AP substrate (alpha-naphthyl-phosphate). Two different antibodies (sIgG anti-PCB28 and rIgG anti-PCB77) were tested and compared in terms of sensitivity and ability to recognise different congeners. The developed electrochemical magneto-immunosensor (EMI) was successfully combined with solid-phase extraction (SPE) for the analysis of PCBs in milk samples. In spiked samples a recovery of 80% was obtained. The proposed strategy offers great promise for rapid, simple, cost-effective, and on-site analysis of clinical, food and environmental samples, considering also that low-density arrays allow the simultaneous analysis of different processed samples.

Cytochrome P450s and short-chain dehydrogenases mediate the toxicogenomic response of PCB52 in the nematode Caenorhabditis elegans

Although non-coplanar PCBs are ubiquitous organic chemicals known to induce numerous biological responses and thus are toxic to man and wildlife, little is known about the toxic mode of action. Using PCB52, an ortho-substituted, 2,2',5,5'-tetrachlorobiphenyl, it was possible to pinpoint the relationship between induced gene expression and observed toxicity in the model nematode Caenorhabditis elegans. On the basis of the calculated EC20 for brood size (5 mg/l), whole genome DNA microarray experiments were performed to identify differentially expressed genes. Gene knockdown by RNAi was used to determine the consequences in reproductive fitness in the presence and in the absence of PCB52. On the basis of altered phenotype, several gene classes were identified to have a pivotal role in PCB52 toxicogenesis, most notably cytochrome P450s, short-chain dehydrogenases and lipases. In addition to this, four of six selected cytochrome P450s were shown to be involved in fat storage, with PCB52 exposure increasing the fat content in N2 wild-type as indicated by staining with Nile red. Furthermore, exposure to PCB52 induces a general detoxification response via small heat-shock proteins and caspases. Our data provide strong evidence of the molecular mechanisms that underlie the toxicity of non-coplanar PCBs, and confirms that, despite the ability to metabolize PCB, alterations in lipid metabolism and storage are major factors that drive the toxic effect of PCB52.

Organochlorine Exposures During Pregnancy and Infant Size at Birth

BACKGROUND

Organochlorines, including polychlorinated biphenyls (PCBs) and pesticides, are environmentally persistent contaminants that concentrate in the food chain as well in human adipose tissue and readily cross the placenta.

METHODS

To follow up on studies suggesting an association of organochlorine exposure with reduced birth size, we investigated the association of PCBs and organochlorine pesticides (including p,p'-dichlorodiphenyl dichloroethene [p,p'-DDE], the major degradation product of p,p'-dichlorodiphenyl trichloroethane [p,p'-DDT], and hexachlorobenzene [HCB]), with birth weight, crown-heel length, and head circumference. We evaluated a cohort of 722 infants born between 1993 and 1998 to mothers residing near a PCB-contaminated harbor and Superfund site in New Bedford, Massachusetts.

RESULTS

Small negative associations were observed for PCBs and birth weight; associations were weaker for birth length and head circumference. There was evidence for effect modification by smoking during pregnancy on the association between PCBs and birth weight. No associations were found with p,p'-DDE or HCB for any measures of birth size.

CONCLUSIONS

This study supports the growing literature that demonstrates at most a weak association between very low-level organochlorine exposure and birth size.

The risk of infection from polychlorinated biphenyl exposure in the harbor porpoise (Phocoena phocoena): a case-control approach

The objective of this study was to determine whether the risk of mortality from infectious disease in harbor porpoise in U.K. waters increased with high exposure to polychlorinated biphenyls (PCBs), using a case-control study design. This is the first time that data from a long-term marine mammal strandings scheme have been used to estimate any increase in risk. The exposure odds ratio (OR) from a logistic regression model with infectious disease deaths as cases and physical trauma deaths as controls, after controlling for the effect of confounding factors, was 1.048 [95% confidence interval (CI), 1.02-1.07]. To further adjust for the difference in energetic status between cases and controls and account for the negative relationship between PCBs (sum of 25 chlorobiphenyl congeners) and blubber mass, we also "standardized" the blubber PCBs to an optimal blubber mass. This lowered the OR to 1.02 (95% CI, 1.00-1.03). Thus, for each 1 mg/kg increase in blubber PCBs, the average increase in risk of infectious disease mortality was 2%. A doubling of risk occurred at approximately 45 mg/kg lipid. In this study, we have endeavored to avoid selection bias by using controls that died of physical trauma as representative of the exposure prevalence in the population that gave rise to the cases. In addition, we controlled for the effect of variation in energetic status among the cases and controls. However, as with case-control studies in human and veterinary epidemiology, unforeseen misclassification errors may result in biased risk estimates in either direction.

Effects of organochlorine contaminants on loggerhead sea turtle immunity: comparison of a correlative field study and in vitro exposure experiments

Several laboratory and field studies indicate that organochlorine contaminants (OCs), such as polychlorinated biphenyls (PCBs) and pesticides, modulate immune responses in rodents, wildlife, and humans. In the present study we examined the effects of OCs on immunity in free-ranging loggerhead sea turtles (Caretta caretta). Mitogen-induced lymphocyte proliferation responses, lysozyme activity, and OC concentrations were measured from blood samples. Mitogens chosen in the lymphocyte proliferation assay were phytohemagglutinin (PHA) and concanavalin A (ConA) for T-lymphocyte stimulation, and lipopolysaccharide (LPS) and phorbol 12,13-dibutyrate (PDB) for B-lymphocyte stimulation. Lysozyme activity was significantly and negatively correlated with whole-blood concentrations of 4,4 -dichlorodiphenyldichloroethylene (4,4 -DDE) and the sum of chlordanes. Lymphocyte proliferation responses stimulated by PHA, LPS, and PDB were significantly and positively correlated with concentrations of the sum of PCBs measured in whole blood. LPS- and PDB-induced proliferation were also significantly and positively correlated with 4,4 -DDE blood concentrations. These correlative observations in free-ranging turtles suggest that current, chronic exposure to OCs may suppress innate immunity and enhance certain lymphocyte functions of loggerhead sea turtles. To further test this hypothesis, lymphocyte proliferation was measured after in vitro exposure of peripheral blood leukocytes from 16 turtles to Aroclor 1254 (0-13.5 microg/mL) or 4,4 -DDE (0-13.4 microg/mL). Both contaminants increased PHA- and PDB-induced proliferation at concentrations below those that affected cell viability. Moreover, the concentrations that enhanced PDB-induced proliferation in vitro were similar to concentrations measured in turtles with the highest proliferative responses. The similarities between the in vitro experiments and the correlative field study suggest that OC exposure modulates immunity in loggerhead turtles.

The effects of 3-methylcholanthrene on lymphocyte proliferation in the common carp (Cyprinus carpio L.)

The sensitivity of lymphocyte proliferation as bioindicator of pollution stress was evaluated in the common carp (Cyrinus carpio L.). The time course response of peripheral blood leukocyte proliferation in response or not to mitogens was measured from 1 to 7 days after peritoneal injection of 3-methylcholantrene (3-MC), and compared to the time course response of a highly sensitive biomarker, induction of cytochrome P450. 3-Methylcholanthrene (40 mg kg(-1)) inhibited both B- and T-lymphocyte proliferation in response to lipopolysaccharide (LPS) and concanavalin A (Con A). Studies with alpha-naphtofiavone, suggest the lack of metabolic processes. 3-Methylcholanthrene alone strongly stimulated resting peripheral blood leukocytes (PBLs) proliferation. This effect was not transient. The induction of lymphocyte proliferation paralleled the increase in cytochrome P450 content in the liver. The specificity of polycyclic aromatic hydrocarbon (PAH)-induced lymphocyte proliferation suggests that this immune activity may be an early marker of exposure to PAHs in aquatic environments. The capacity of 3-MC to induce rapid lymphocyte proliferation may be related to PAH-induced rapid clonal expansion in mammals. These results strongly suggested that the underlying mechanism might be the same in both models. More studies are needed in fish to explain this phenomenon and may be helpful in understanding the occurrence of neoplastic epizootics in fish associated with PAH exposition.

Inhibition of LPS-induced splenocyte proliferation by ortho-substituted polychlorinated biphenyl congeners

Polychlorinated biphenyls (PCBs) are persistent environmental contaminants, and their ubiquitous nature has prompted studies of their potential health hazards. As a result of their lipophilic nature, PCBs accumulate in breast milk and subsequently affect the health of offspring of exposed individuals. Biological effects of PCBs in animals have mostly been attributed to coplanar congeners, although effects of ortho congeners also have been demonstrated. To investigate the relationship of immunotoxicity and chlorine substitution pattern, the effects of PCB congeners and mixtures of ortho and non-ortho-substituted constituents of Aroclor 1242 on splenocytes from C57B1/6 mice were examined. The immunotoxic endpoints investigated included splenocyte viability, lipopolysaccharide (LPS)-induced splenocyte proliferation, and LPS-induced antibody secretion. Congeners with multiple ortho chlorines preferentially inhibited splenocyte proliferation as compared with non- or mono-ortho-substituted congeners. However, mixtures of non- and mono-ortho-substituted congeners and multi-ortho-substituted congeners inhibited LPS-induced splenocyte proliferation and antibody secretion at similar concentrations. Exposure of splenocytes to these mixtures did not activate the aryl hydrocarbon receptor (AhR) signal transduction pathway. These results suggest individual multi-ortho-substituted congeners preferentially inhibit LPS-induced splenocyte proliferation, while congeners not exhibiting an effect individually may have additive effects in a mixture to produce an immunotoxic response through an AhR-independent pathway.

The flame retardants tetrabromobisphenol A and tetrabromobisphenol A-bisallylether suppress the induction of interleukin-2 receptor alpha chain (CD25) in murine splenocytes

Polybrominated flame retardants (PBF) are frequently used additives in electronical equipment. They are ubiquitous environmental contaminants which bioaccumulate with several health effects for humans and the environment. This study investigated immunotoxic effects of the PBF tetrabromobisphenol A (TBBP A), tetrabromobisphenol A-bisallylether (TBBP A-AE), tetrabromobisphenol A-bis-(2,3-dibromopropyl-ether) (TBBP A-PE), decabromodiphenylether (DBDE), and 2,4,6-tribromophenol (TBP) in vitro. The structurally related polychlorinated aromatic hydrocarbon 3,4,3',4'-tetrachlorobiphenyl (PCB77) and dioxins mediate their immunotoxicity via the Ah-receptor gene complex. A highly relevant function of the Ah receptor, the induction of CYP 1A1 in hepatocytes of C57BL/6 mice by the established inducers 3-methylcholanthrene (MC) and PCB77 was compared to the effect of PBF by measurement of ethoxyresorufin-o-deethylase (EROD) activity. The PBF did not show any induction of CYP 1A1, while EROD activity of hepatocytes exposed to MC and PCB77 was induced 10.8- and 8.7-fold, respectively. To investigate immunotoxic effects of the flame retardants, splenocytes of C57BL/6 mice were incubated with subtoxic doses of the flame retardants and PCB77 and activated by concanavalin A (Con A). The flame retardants TBBP A and TBBP A-AE significantly inhibited the expression of interleukin-2 receptor alpha chain (CD25) in contrast to TBBP A-PE, DBDE, TBP, and PCB77 as shown by immunohistochemistry and quantitative analysis by laser scanning cytometry. None of the substances had any effect on the Con A-induced production of cytokines. Hence, TBBP A and TBBP A-AE may act as immunotoxic compounds by specifically inhibiting the expression of CD25.

Impact of polychlorinated biphenyls (PCBs) on the immune function of fish: age as a variable in determining adverse outcome

Polychlorinated biphenyls (PCBs) are a major contaminant of global extent in water resources and aquatic biota. Due to its high lipid solubility, PCBs fail to be degraded and, therefore, continue to bioaccumulate throughout the environment and food chain. To determine the impact of PCBs on the immune system of aged and juvenile Japanese medaka (Oryzias latipes), fish were injected with the coplanar PCB congener 126 and examined after 3 and 14 days. PCB 126 produced oxidative stress in both age groups of fish 14 days post-injection; however, juvenile medaka appeared more susceptible than aged fish. Humoral immunity, as determined by antibody forming cell (AFC) numbers, was significantly depressed for up to 14 days post-injection in both age groups. These results demonstrate the sensitivity of the fish immune response for predicting PCB-induced immunotoxicity and identify age as a variable in determining adverse outcome.

The effects of 3-methylcholanthrene on macrophage respiratory burst and biotransformation activities in the common carp (Cyprinus carpio L.)

The sensitivity of phagocytic cell function as a bioindicator of pollution stress by polycyclic aromatic hydrocarbons was evaluated in the common carp (Cyprinus carpio L). The time course response of the head-kidney macrophage respiratory burst was measured 1, 2, 3, 5 and 7 days after intraperitoneal injection of a prototypical Cyp 1A inducer (3-methylcholanthrene). This immune activity was compared to the rate of induction of total cytochrome P450, ethoxyresorufin O-deethylase activity (EROD) and glutathione S-transferase activity (GST) in the liver and head-kidney. 3-methylcholanthrene (40 mg kg(-1)) caused a rapid increase in the macrophage respiratory burst. This response was maximal at day 3 post exposure and coincided with maximum induction of cytochrome P450 and EROD activity in liver and head-kidney. Moreover, alpha-naphtoflavone, which functions as both an Ah receptor antagonist and an inhibitor of cytochrome P450 1A activity, reversed the 3-methylcholanthrene induction of immune and enzymatic parameters measured, suggesting metabolic processes. Taken together these results suggest that the induction of macrophage oxidative function may be an equally sensitive marker of exposure to polycyclic aromatic hydrocarbon as the induction of biotransformation activities and confirm that responses mediated by the Ah receptor are similar, if not identical, to those of mammals.

Aryl hydrocarbon receptor-deficient mice generate normal immune responses to model antigens and are resistant to TCDD-induced immune suppression

The aryl hydrocarbon receptor (AhR) is a ligand-activated transcription factor that mediates many of the toxic effects induced by exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), a high-affinity AhR ligand and a potent immunotoxicant. AhR-deficient mice have been constructed, and there are reports that the animals display altered splenic architecture and cellularity with an apparent increased incidence of infection. These observations have led to speculation that the immune system of these animals might be compromised, however, their functional immune response has not been directly tested. In the studies presented here, we examined the immune response of two strains of 8- to 10-week-old AhR-deficient mice. Mice were challenged with model antigens, allogeneic P815 tumor cells, or sheep red blood cells, and their ability to generate cell-mediated and humoral immune responses was examined. In addition, to address the obligatory role of the AhR in TCDD-induced immune suppression, we examined the immune response of the AhR-null animals following exposure to an immunosuppressive dose of TCDD. Results from these studies showed that AhR-deficient mice were able to mount normal productive immune responses to both model antigens and that neither the cellular nor the humoral response was suppressed by exposure to TCDD. Interestingly, however, we found that the immune response of heterozygous AhR(+/-) mice was less sensitive to TCDD than homozygous AhR(+/+) mice. The results of these studies suggest that the absence of the AhR does not impact the function of the immune system, but confirm the findings of previous studies that have indicated the AhR plays an obligatory role in TCDD-induced immune suppression.

Polychlorinated biphenyl mixtures (Aroclors) inhibit LPS-induced murine splenocyte proliferation in vitro

The immune system is believed to be a sensitive indicator for adverse polychlorinated biphenyl (PCB)-induced health effects. Four commercial PCB mixtures (Aroclors) or six individual PCB congeners were evaluated for their effect on splenocyte viability and lipopolysaccharide (LPS)-induced splenocyte proliferation in vitro in two strains of mice, C57B1/6 (high affinity aromatic hydrocarbon receptor (AhR) complex) and DBA/J (low affinity AhR complex). All four Aroclors, the selected individual noncoplanar congeners, or two tertiary mixtures containing one congener from each class significantly decreased the in vitro LPS-induced proliferation of murine splenocytes in either strain of mice without inducing a significant decrease in viability. In contrast, selected individual coplanar or mono-ortho-coplanar congeners did not inhibit splenocyte proliferation or viability at any concentration. These results suggest that mixtures of PCBs and/or congener class (specifically, noncoplanar congeners) may be more highly immunotoxic than individual planar and mono-ortho-coplanar congeners alone. Thus, this in vitro assay has revealed a more complex pattern of immunotoxicity of Aroclors versus individual congeners than has previously been reported or anticipated based on both in vivo derived immunotoxic data and standard comparisons to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). These results have important practical significance since mixtures of PCB congeners were used industrially and now contaminate the environment.

Interactions between estrogen, tamoxifen, octylphenol, and two polychlorinated biphenyls in murine splenocytes

Prior exposure of cultured murine splenocytes to 17beta-estradiol (E) protects them from the membrane disrupting effects of the xenoestrogen 4-tert-octylphenol (OP). Using splenocytes isolated from male Balb/c mice, we tested whether (a) the xenoestrogen, 2', 3', 4', 5'-tetrachloro-4-biphenylol (PCB-OH), or the polychlorinated biphenyl, 3, 3', 4, 4'-tetrachlorobiphenyl (PCB 77), which displays both estrogenic and anti-estrogenic actions, would compromise the membrane integrity of the cells and (b) E or tamoxifen (TX), another ligand for the E receptor, would protect the membranes of cells exposed to the agents. We also examined possible interactions between OP, PCB-OH, and PCB 77 on the cells. Splenocytes were cultured for 24 hr. Concentrations of OP (10(-5)-10(-9) M), PCB-OH (10(-6)-10(-16) M), or PCB 77 (10(-8)-10(-12) M) significantly compromised the membrane integrity of the cultured splenocytes in a dose response manner. Concentrations of E as high as 10(-5) M or TX as high as 10(-7) M were without effect. Incubation of splenocytes in medium containing E or TX at 10(-7) M for 2 hr prior to the subsequent addition of either OP, PCB-OH or PCB 77 (final concentrations of 10(-7), 10(-7), or 10(-8) M, respectively) blocked the membrane disrupting effects. Incubation of splenocytes in medium containing 10(-7) M E starting 2 hr after the addition of OP or PCB 77 or incubation of splenocytes in medium containing 10(-7) M TX starting 2 hr after the addition of OP or PCB-OH did not block the damaging effects of OP, PCB 77, or PCB-OH on the cell membranes. No interactions were observed when various combinations of OP, PCB-OH, or PCB 77 were used. These data suggest that: (a) TX acts like E in this system, (b) a prior response of splenocytes to E or TX can protect them from the potential cytotoxic effects of OP, PCB-OH, or PCB 77; and, (c) OP, PCB-OH, and PCB 77 were not additive in their actions.

Ah receptor and NF-kappaB interactions, a potential mechanism for dioxin toxicity

The Ah receptor (AhR) mediates many of the toxic responses induced by polyhalogenated and polycyclic hydrocarbons (PAHs) which are ubiquitous environmental contaminants causing toxic responses in human and wildlife. NF-kappaB is a pleiotropic transcription factor controlling many physiological functions adversely affected by PAHs, including immune suppression, thymus involution, hyperkeratosis, and carcinogenesis. Here, we show physical interaction and mutual functional repression between AhR and NF-kappaB. This mutual repression may provide an underlying mechanism for many hitherto poorly understood PAH-induced toxic responses, and may also provide a mechanistic explanation for alteration of xenobiotic metabolism by cytokines and compounds that regulate NF-kappaB.

Induction of PreB cell apoptosis by 7,12-dimethylbenz[a]anthracene in long-term primary murine bone marrow cultures

Numerous studies demonstrate that polycyclic aromatic hydrocarbons (PAH) suppress immunity by modifying the function of both B and T cells. Relatively few studies have assessed the effects of these common environmental chemicals on immature lymphocytes. In the present study, long-term primary bone marrow cultures were employed to investigate the effects of a prototypic PAH and aryl hydrocarbon receptor (AhR) agonist, 7,12-dimethylbenz[a]anthracene (DMBA), on immature B lymphocytes. In this system, immature preB cells are maintained in a supportive microenvironment provided by bone marrow stromal cells. Results presented here demonstrate that (1) exposure of primary bone marrow cultures to DMBA results in preB cell death by apoptosis; (2) notably low doses of DMBA (> or = 10(-8) M) induce preB cell apoptosis; (3) in long-term cultures, bone marrow stromal cells, but not preB cells, express AhR mRNA and protein as determined by in situ hybridization, RT-PCR, and immunoblotting; (4) freshly isolated unfractionated bone marrow cells, but not purified bone marrow B cells, express AhR protein as assessed by immunohistochemistry; (5) alpha-naphthoflavone, a competitive AhR inhibitor and cytochrome P450 antagonist, completely blocks DMBA-induced preB cell apoptosis in primary bone marrow cultures; and (6) DMBA or benzo[a]pyrene injection in vivo results in bone marrow cell apoptosis consistent with the death of hematopoietic cells clustered around stromal elements. The results implicate programmed cell death as a mechanism underlying DMBA-mediated immunosuppression and suggest that preB cell death is influenced by local interactions with AhR+ bone marrow stromal cells.

Effects of Aroclor 1254 on the thyroid gland, immune function, and hepatic cytochrome P450 activity in mallards

Adult male mallards were exposed to 0, 4, 20, 100, 250, and 500 mg/kg Aroclor 1254 by gavage twice per week for 5 weeks. Immunotoxic effects, as measured by antibody titers to sheep erythrocytes, natural killer cell activity and lymphocyte mitogenesis to phytohemagglutinin, were not detected as a consequence of polychlorinated biphenyl (PCB) exposure. Hepatic cytochrome P450 activities were measured as microsomal dealkylations of ethoxyresorufin (EROD) and pentoxyresorufin (PROD). Significant elevations in EROD and PROD were noted at 20 mg/kg and peaked in birds treated with 100 mg/kg. Total P450 was induced beginning at 100 mg/kg and peaked at 250 mg/kg. Relative liver weights were dose-dependently increased following treatment with 100 mg/kg or more. Thyroid weights were significantly increased in PCB-treated birds treated with 100 mg/kg or greater, but no significant histological abnormalities were observed, except at the highest dose. Plasma total triiodothyronine (T3) was decreased in a dose-dependent manner, with a significant lowest-observed-adverse-effect level (LOAEL) of 20 mg/kg. T3 was decreased following 7 days treatment with 100 mg/kg. The no-observed-adverse-effect level (NOAEL) was 4 mg/kg for decreased T3. Plasma glucose levels were decreased on days 28 and 35 in mallards treated with 500 mg/kg, while other clinical plasma biochemistry parameters were unaltered by PCB treatment. Plasma corticosterone levels were unchanged by PCB treatment. These results indicate that thyroid hormone levels and P450 activity in mallards are sensitive to subchronic PCB exposure in the absence of gross toxic effects and immunotoxicity.

Apoptosis-mediated immunotoxicity of polychlorinated biphenyls (PCBs) in murine splenocytes

Polychlorinated biphenyls (PCBs) exhibited immunotoxicity on antibody forming response to T-dependent antigen of sheep red blood cells, primary activation of T cells by mixed lymphocyte response, and lymphocyte proliferation induced by various mitogens. These immunosuppressions were related with the loss of lymphocyte viability which was determined by the propidium iodide method, and this death was proven to be linked with apoptosis which showed DNA fragmentation detected by the diphenylamine method and agarose gel electrophoresis. The degree of DNA fragmentation was increased in a dose- and time-dependent manner in PCB-treated splenocytes. In conclusion, it was assumed that apoptosis was attributable to the immunotoxicity of PCBs in murine splenocytes.

Epidemiological and laboratory evidence of PCB-induced neurotoxicity

The purpose of this review is to provide a selective, but critical, assessment of important findings derived from both epidemiological and laboratory studies suggesting that: (1) exposure to polychlorinated biphenyls (PCBs) and related halogenated aromatic hydrocarbons induces significant neurological and behavioral dysfunctions in humans and laboratory animals, particularly following exposure during gestation and lactation; (2) the neurochemical actions of PCBs depend on their structure and the developmental status of the animal at the time of exposure; and (3) the mechanisms responsible for these changes may involve alterations in basic cellular signaling processes and endocrine function that influence the synthesis and activity of important central nervous system neurotransmitters, the organization of the developing brain, and the behavioral responses to these environmental contaminants.

Immunotoxicity of PCBs (Aroclors) in relation to Great Lakes

Polychlorinated biphenyls (PCBs) are among the most widespread environmental pollutants and a prominent contaminant of the Great Lakes basin. Due to their resistance to biodegradation and lipophilic properties, PCBs bioaccumulate in fish tissues and in fish-eating humans. PCBs are also known to cross the placenta and to be excreted into the mother's milk, thus predisposing the infant to potentially adverse health effects. For example, a higher incidence of bacterial infections was reported for breast-fed infants born to mothers who consumed large amounts of Great Lakes fish compared to the incidence in control infants whose mothers ingested low amounts of fish. While data regarding the PCB-induced immunotoxic effects in humans are scarce, data derived from the use of experimental animals, including nonhuman primates, indicate that the immune system is a potential target for the immunotoxic effects of PCBs. Such studies have used the commercially available PCB mixtures alone. However, PCBs have the potential of partially antagonizing the effects of other structurally related compounds including the highly toxic dioxins, which are also present in small amounts in the Great Lakes. Thus, to fully evaluate the magnitude of the immunotoxic risk PCBs pose to humans, consideration should be given to investigations in which the interactive effects of PCBs are combined with other contaminants present in the Great Lakes.

Halogenated aromatic hydrocarbon-induced suppression of the plaque-forming cell response in B6C3F1 splenocytes cultured with allogenic mouse serum: Ah receptor structure activity relationships

The immunosuppressive effects of halogenated aromatic hydrocarbons (HAHs) were investigated in B6C3F1 female mice and in mouse splenocytes cultured with allogenic mouse serum using the Mishell-Dutton model for in vitro immunization to trinitrophenyl-lipopolysaccharide (TNP-LPS). Exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), 2,3,4,7,8-pentachlorodibenzofuran (PeCDF), 1,2,3,7,8-PeCDF, 1,3,6,8-tetrachlorodibenzofuran (TCDF), 3,3',4,4',5-pentachlorobiphenyl (pentaCB), or 3,3',4,4',5,5'-hexaCB resulted in a dose-dependent suppression of the splenic plaque-forming cell (PFC) response both in vivo and in vitro. The effective dose required to decrease 50% (ED50) of the response to 2,3,7,8-TCDD, 2,3,4,7,8-PeCDF, 1,2,3,7,8-PeCDF, 1,3,6,8-TCDF, 3,3',4,4',5-pentaCB, or 3,3',4,4',5,5'-hexaCB in vivo was 14.1, 5.5, 1695, 34,800, 21, and 19 nmol/kg, respectively, and in vitro was 7.0, 10.6, 149, 2325, 9.1 and 9.1 nM, respectively. There was an excellent rank order and linear correlation between the in vivo versus in vitro activities for these HAHs (r < 0.99) and the relative immunosuppressive potencies of these compounds paralleled their binding affinities for the aryl hydrocarbon (Ah) receptor. These results show that splenocytes cultured with allogenic mouse serum is an Ah-responsive in vitro assay which can be used for quantitating the immunosuppressive effects of HAHs.

An enzyme-linked immunosorbent assay (ELISA) specific for antibodies to TNP-LPS detects alterations in serum immunoglobulins and isotype switching in C57BL/6 and DBA/2 mice exposed to 2,3,7,8-tetrachlorodibenzo-p-dioxin and related compounds

An enzyme-linked immunosorbent assay (ELISA) was developed to detect IgM and IgG antibodies specific for trinitrophenyl-lipopolysaccharide (TNP-LPS). Treatment of C57BL/6 and DBA/2 mice with 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and other aryl hydrocarbon (Ah) receptor agonists followed by immunization with TNP-LPS resulted in a dose-dependent decrease in serum IgM which paralleled the decrease in the splenic PFC response. The ED50 values for the IgM and splenic PFCs in C57BL/6 mice for 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), 3,3',4,4',5-pentachlorobiphenyl (pentaCB) and 3,3',4,4',5,5'-hexaCB were 2.8 and 1.6, 11 and 14, and 25 and 20 micrograms/kg, respectively; in the less Ah-responsive DBA/2 mice, the ED50 values were 8.5 and 10, 61 and 69, and 73 and 71 micrograms/kg, respectively. In addition, treatment of C57BL/6 mice with TCDD resulted in alterations of serum IgG relative to IgM and a delay of isotype switching was observed after immunization and boosting with TNP-LPS. This ELISA may prove to be a useful tool in monitoring immune function during long-term exposure of mice to TCDD and related compounds and exploring the mechanism of Ah receptor-mediated immunosuppression.

Review of the safety assessment of polychlorinated biphenyls (PCBs) with particular reference to reproductive toxicity

1. The methods used to evaluate the toxicological effects of PCBs in animals have been reviewed. 2. The data show that Toxic Equivalency Factors (TEFs) could be developed to assess the potential toxicity of PCB mixtures for certain specific target organ effects (such as the liver and immune system) but would be inappropriate for other effects (e.g. thyroid function and neurochemical effects). More data on a wider range of individual PCB congeners and a method for systematically balancing toxicodynamic and toxicokinetic data are required before the TEF approach can be fully evaluated. 3. With the exception of the teratogenic effects seen in mice and the anti-oestrogenic effects reported in in vitro studies, there are insufficient data on individual PCB congeners to evaluate the structure-activity relationships for the effects of PCBs on reproduction. The data also show that individual PCBs may have opposing effects on a particular aspect of reproduction (for example individual PCB congeners may have either oestrogenic or anti-oestrogenic effects). Studies with individual PCB congeners have shown both enhancement and antagonism of the teratogenic effects of 2, 3, 7, 8-tetrachloro dibenzo-p-dioxin (TCDD) in the mouse. It is not possible to use TEFs to evaluate the reproductive effects of PCBs. 4. The mechanism(s) responsible for the effects of PCBs on postnatal neurobehavioural development in rodents and monkeys have not been elucidated. At least two groups of PCBs which might be responsible for the observed effects have been identified in this review, one affecting the dopaminergic system and the other group affecting thyroid hormone levels. Considerably more research would be required before the TEF approach could be applied to the effects of PCBs on postnatal neurobehavioural development. This would include research on an appropriate animal model to determine whether the critical toxicological mechanism is mediated through the Ah receptor. 5. The reproductive toxicity of complex PCB mixtures such as those found in foods will depend on the identifies and relative proportions of individual PCB congeners in the mixture. It is not possible to give an accurate estimate of a NOAEL or LOAEL from the reproduction studies using commercial PCB mixtures which could be readily applied to the safety assessment of PCBs present as contaminants in food. 6. It is concluded that the data presented in this paper support the hypothesis that there is no satisfactory method derived from the available studies in laboratory animals for evaluating the potential risk of adverse effects on reproduction posed by contamination of foods with PCBs.

Immunologic and endocrine effects of the flame-retardant pentabromodiphenyl ether (DE-71) in C57BL/6J mice

Polybrominated diphenyl ethers are manufactured for use as flame retardants in commercial plastics and textiles in Europe and North America. These studies investigated the acute and subchronic immunotoxicity and endocrine effects of a commercial pentabromodiphenyl either mixture, DE-71, in female C57BL/6 mice. Mice were orally exposed to acute single doses of DE-71 of 0, 0.8, 4.0, 20, 100, or 500 mg/kg, or to subchronic daily doses totaling 0, 250, 500, or 1000 mg/kg over a 14 day period. Immunotoxicity was assessed by measuring the plaque-forming cell response to sheep erythrocytes (SRBC) and natural killer cell (NKC) activity (basal and poly I:C stimulated) to YAC-1 target cells. Liver cytochrome P450 content and activities (ethoxyresorufin-o-deethylase (EROD) and pentoxyresorufin-o-deethylase (PROD)) as well as corticosterone (CS) and thyroxine (T4) concentrations were also measured. PROD activity was induced 3-5-fold in mice exposed acutely or subchronically to DE-71 at doses > 250 mg/kg. EROD activity and total microsomal cytochrome P450 content were significantly induced only in mice treated subchronically with DE-71; maximum induction of EROD was 3.3-fold. Total serum T4 concentrations were significantly lower in mice treated acutely with DE-71 at all doses except the 100 mg/kg dose. Total and free T4 concentrations were dose-dependently decreased in DE-71-treated mice following subchronic exposure. Plasma CS levels were elevated following subchronic exposure to DE-71. The elevation of CS was correlated with order of capture at necropsy, suggesting an interactive effect of DE-71 and stress. In regard to immunotoxicity, significant suppression of the anti-SRBC response was seen only in mice exposed subchronically to 1000 mg DE-71/kg, an exposure that also resulted in decreased thymus weight. NKC activity was not altered by exposure to DE-71.

Polychlorinated biphenyls (PCBs): environmental impact, biochemical and toxic responses, and implications for risk assessment

Commercial polychlorinated biphenyls (PCBs) and environmental extracts contain complex mixtures of congeners that can be unequivocally identified and quantitated. Some PCB mixtures elicit a spectrum of biochemical and toxic responses in humans and laboratory animals and many of these effects resemble those caused by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and related halogenated aromatic hydrocarbons, which act through the aryl hydrocarbon (Ah)-receptor signal transduction pathway. Structure-activity relationships developed for PCB congeners and metabolites have demonstrated that several structural classes of compounds exhibit diverse biochemical and toxic responses. Structure-toxicity studies suggest that the coplanar PCBs, namely, 3,3',4,4'-tetrachlorobiphenyl (tetraCB), 3,3',4,4',5-pentaCB, 3,3',4,4',5,5'-hexaCB, and their monoortho analogs are Ah-receptor agonists and contribute significantly to the toxicity of the PCB mixtures. Previous studies with TCDD and structurally related compounds have utilized a toxic equivalency factor (TEF) approach for the hazard and risk assessment of polychlorinated dibenzo-p-dioxin (PCDD) and polychlorinated dibenzofuran (PCDF) congeners in which the TCDD or toxic TEQ = sigma([PCDFi x TEFi]n)+sigma([PCDDi x TEFi]n) equivalent (TEQ) of a mixture is related to the TEFs and concentrations of the individual (i) congeners as indicated in the equation (note: n = the number of congeners). Based on the results of quantitative structure-activity studies, the following TEF values have been estimated by making use of the data available for the coplanar and monoortho coplanar PCBs: 3,3',4,4',5-pentaCB, 0.1; 3,3',4,4',5,5'-hexaCB, 0.05; 3,3',4,4'-tetraCB, 0.01; 2,3,3',4,4'-pentaCB, 0.001; 2,3',4,4',5-pentaCB, 0.0001; 2,3,3',4,4',5-hexaCB, 0.0003; 2,3,3',4,4',5'-hexaCB, 0.0003; 2',3,4,4',5-pentaCB, 0.00005; and 2,3,4,4',5-pentaCB, 0.0002. Application of the TEF approach for the risk assessment of PCBs must be used with considerable caution. Analysis of the results of laboratory animal and wildlife studies suggests that the predictive value of TEQs for PCBs may be both species- and response-dependent because both additive and nonadditive (antagonistic) interactions have been observed with PCB mixtures. In the latter case, the TEF approach would significantly overestimate the toxicity of a PCB mixture. Analysis of the rodent carcinogenicity data for Aroclor 1260 using the TEF approach suggests that this response is primarily Ah-receptor-independent. Thus, risk assessment of PCB mixtures that uses cancer as the endpoint cannot solely utilize a TEF approach and requires more quantitative information on the individual congeners contributing to the tumor-promoter activity of PCB mixtures.

Immunosuppressive effects of highly chlorinated biphenyls and diphenyl ethers on T-cell dependent and independent antigens in mice

The dose-dependent effects of 2,2',3,3',4,4',5,5',6-nonachlorobiphenyl (nonaCB), 2,2',3,3',4,4',5,6,6'-nonaCB, 2,2',3,3',4,5,5',6,6'-nonaCB and decaCB on the suppression of the splenic plaque-forming cell (PFC) response to the T-cell-dependent antigen, sheep red blood cells (SRBCs) and the T-cell-independent antigen, trinitrophenyl-lipopolysaccharide (TNP-LPS), were determined in genetically inbred mice. In addition, the induction of hepatic microsomal ethoxyresorufin O-deethylase (EROD) activity was also measured. The highly chlorinated biphenyls suppressed the splenic PFC response to SRBCs in C57BL/6 and DBA/2 mice and were relatively more active in the former strain. The C57BL/6 mice are more responsive to aryl hydrocarbon (Ah) receptor agonists than DBA/2 mice and these data support a possible role for the Ah receptor in mediating this response. However, previous studies with polychlorinated biphenyls (PCBs) indicate that congeners with 3 or 4 ortho-chloro substituents are inactive as Ah receptor agonists and this was consistent with the minimal induction of hepatic microsomal EROD activity by the highly chlorinated biphenyls in both strains of mice. Thus, the results suggest that the inhibition of the splenic PFC response to SRBCs observed in this study was primarily an Ah receptor-independent response. Some of the highly chlorinated diphyenyl ethers namely decachlorodiphenyl ether and 2,2',3,3',4,4',5,6,6'-nonachlorodiphenyl ether, inhibited the antigenic response to TNP-LPS in C57 BL/6 mice. The results indicate that the suppression of the TNP-LPS-mediated immune response may be a more reliable indicator of the Ah receptor-dependent immunotoxicity of halogenated hydrocarbons.

Teratogenicity and immunotoxicity of 3,3',4,4',5-pentachlorobiphenyl in C57BL/6 mice

Administration of 3,3',4,4',5-pentachlorobiphenyl (pentaCB) to female C57BL/6 mice at doses from 130.5 to 522 micrograms/kg body weight resulted in the dose-dependent formation of fetal cleft palate and hydronephrosis. The estimated relative potency of 3,3',4,4',5-pentaCB compared to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) was in the range of < 0.07-0.04. The immunotoxicity of 3,3',4,4',5-pentaCB and two structurally-related congeners, 3,3',4,4'-tetraCB and 3,3',4,4',5,5'-hexaCB, was investigated in male C57BL/6 mice by determining their suppression of the splenic plaque-forming cell response to sheep red blood cells. The potencies of these compounds relative to TCDD were determined from the ratios of their corresponding ED50 values and were 0.77-0.55 (3,3',4,4',5-pentaCB), 1.1-0.29 (3,3',4,4',5,5'-hexaCB) and 0.14-0.03 (3,3',4,4'-tetraCB). These results demonstrate that the immunosuppressive activities of the PCB congeners relative to TCDD were much higher than observed for many other TCDD-like responses in mice and other laboratory animals.

Mechanisms by which benzo[a]pyrene, an environmental carcinogen, suppresses B cell lymphopoiesis

The capacity for polycyclic aromatic hydrocarbons (PAH) to suppress immune cell function has been well documented. Nevertheless, mechanisms responsible for PAH immunosuppression and potential effects of PAH on lymphocyte development (lymphopoeisis) remain poorly defined. Murine bone marrow cultures were used in the present studies to determine if and by what mechanism(s) benzo[a]pyrene (B[a]P), a prototypic and highly carcinogenic PAH, suppresses B cell lymphopoiesis. Emphasis was placed on similarities between the processes leading to transformation and immunosuppression and on a possible role for programmed cell death (apoptosis) in B[a]P lymphotoxicity. Data presented herein indicate that: (1) B[a]P suppresses B cell lymphopoiesis in bone marrow cultures at extremely low concentrations (10(-8) M); (2) benzo[e]pyrene, the relatively noncarcinogenic congener of B[a]P, is approximately 1000 times less potent than B[a]P in suppressing B cell lymphopoiesis; (3) bone marrow cells from PAH-resistant DBA/2 mice are less sensitive to B[a]P than cells from C57BL/6 mice; (4) B[a]P induces preB cell apoptosis; and (5) alpha-naphthaflavone, an inhibitor of Ah-receptor dependent, P450 isoenzyme activity, blocks B[a]P-mediated preB cell apoptosis and inhibits B[a]P-dependent suppression of lymphopoiesis. The results support the hypothesis that B[a]P suppression of B cell lymphopoiesis is mediated at least in part by the induction of programmed cell death and that the Ah receptor and/or P450 isoenzymes are involved in this process. The results suggest the potential for PAH to affect development of the B lymphocyte repertoire.

Fluoranthene induces programmed cell death and alters growth of immature B cell populations in bone marrow cultures

Many studies have demonstrated that polycyclic aromatic hydrocarbons adversely affect mature mammalian immune systems. However, little is known about the cellular mechanism(s) mediating this immunosuppression or the potential for these ubiquitous environmental chemicals to similarly compromise lymphocyte development (lymphopoiesis). Murine bone marrow cultures were exploited in the present studies to evaluate the potential for fluoranthene, a mutagenic, cocarcinogenic, polycyclic aromatic hydrocarbon, to modulate B cell lymphopoiesis. In this well characterized system, interactions between immature bone marrow-derived precursor B (preB) cells and bone marrow-derived stromal cells closely mimic preB-stromal cell interactions in vivo and resemble interactions between other bone marrow-derived hematopoietic cells and their supporting stroma. Data presented herein indicate that: (i) fluoranthene suppresses B lymphopoiesis within 2 days in bone marrow cultures; (ii) fluoranthene suppresses lymphopoiesis at least in part by direct interactions with preB cells; (iii) fluoranthene lymphotoxicity is mediated by rapid induction of DNA fragmentation characteristic of programmed cell death (apoptosis) and (iv) preB cell populations surviving the initial death signal or preB cell populations exposed to lower doses of fluoranthene (0.5-5 micrograms/ml) exhibit altered growth and survival characteristics. The data suggest several levels at which fluoranthene could compromise B lymphopoiesis.

Halogenated aryl hydrocarbon-induced suppression of the in vitro plaque-forming cell response to sheep red blood cells is not dependent on the Ah receptor

The immunosuppressive effects of the halogenated aromatic hydrocarbons (HAHs), 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), 2,3,4,7,8-pentachlorodibenzofuran (PeCDF), 2,3,7,8-tetrachlorodibenzofuran (TCDF), 1,2,3,7,9-PeCDF and 1,3,6,8-TCDF were investigated utilizing the Mishell-Dutton model for in vitro immunization. The selected polychlorinated dibenzofuran congeners and 2,3,7,8-TCDD caused a concentration-dependent suppression of the splenic plaque-forming cell response to sheep red blood cells using cell cultures derived from C57BL/6 (Ah responsive) mice. Previous studies showed that there was up to a 14,900-fold difference in the in vivo immunotoxicity of these compounds, however in the in vitro studies, their immunosuppressive potencies were comparable. In addition, these congeners also exhibited similar potencies using spleen cell cultures from DBA/2 (Ah-nonresponsive) mice. Previous research demonstrated that alpha-naphthoflavone was relatively inactive in the in vitro splenic assay system and that co-treatment of cells from C57BL/6 mice with alpha-naphthoflavone (10 microM) plus 2,3,7,8-TCDD (20 mM) resulted in a significant inhibition of the immunotoxicity of 2,3,7,8-TCDD. In these studies, comparable interactive effects were also observed in cells treated with alpha-naphthoflavone plus 1,3,6,8-TCDF, a weak in vivo Ah receptor agonist. Collectively, the results from this study suggest that there may be mechanism(s) of action for HAH-induced suppression of the in vitro murine humoral response to sheep red blood cells which are independent of the Ah receptor protein.

Detection and characterization of Ah receptor in tissue and cells from human tonsils

Ah receptor was identified and characterized in cytosol and nuclear extracts from human tonsils obtained at surgery from children 2 to 6 years of age. Ah receptor was found in cytosol prepared from whole-tonsil homogenates as well as in cytosol and nuclear fractions prepared from tonsil lymphocytes or tonsil fibroblasts grown in primary culture. Cytosolic Ah receptor was detectable in tonsillar tissue with either halogenated (2,3,7,8-[3H]tetrachlorodibenzo-p-dioxin (TCDD)) or nonhalogenated (3-[3H]methylcholanthrene and [3H]benzo[a]pyrene) aromatic hydrocarbons and sedimented at approximately 9 S after velocity sedimentation on sucrose gradients. The apparent binding affinity (Kd) of [3H]TCDD for Ah receptor ranged from 3 to 12 nM in cytosols from seven different donors. The same analyses indicated a concentration of Ah receptor in human tonsils of approximately 100-300 fmol/mg cytosolic protein. Incubation of either tonsil lymphocytes or tonsil fibroblasts with [3H]TCDD resulted in transformation of cytosolic Ah receptor to a nuclear binding form which could be detected as a specifically labeled peak sedimenting at approximately 6 S on sucrose gradients. These data demonstrate the existence of Ah receptor in human tonsils and suggest that this immune organ may be an appropriate model for further studies on the mechanism and manifestation of aromatic hydrocarbon-induced immunotoxicity in man.

Immunosuppressive and monooxygenase induction activities of polychlorinated diphenyl ether congeners in C57BL/6N mice: quantitative structure-activity relationships

The dose-response effects of several polychlorinated diphenyl ether (polyCDE) congeners on the inhibition of the splenic plaque-forming cell (PFC) response to sheep red blood cell antigen and the induction of hepatic microsomal aryl hydrocarbon hydroxylase (AHH) and ethoxyresorufin O-deethylase (EROD) activities were determined in male C57BL/6 mice. The immunotoxic potencies for the polyCDE congeners (ED50 values for the suppression of PFCs/spleen and PFCs/10(6) cells) followed the order 2,3,3',4,4',5-hexaCDE (0.5 and 2.2 mumols/kg) greater than 3,3',4,4',5-pentaCDE (8.8 and 5.1 mumols/kg) greater than 2,3',4,4',5-pentaCDE (21.8 and 14.2 mumols/kg) greater than 3,3',4,4'-tetraCDE (50.6 and 28.7 mumols/kg) greater than 2,2',4,4',5,5'-hexaCDE (81.2 and 56.5 mumols/kg) greater than 2,2',4,5,5'-pentaCDE (258 and 228 mumols/kg) greater than 2,2',4,4',5,6'-hexaCDE (greater than 400 mumols/kg for both responses). The potencies of the polyCDE congeners as inducers of hepatic microsomal AHH and EROD activities were similar to their immunotoxicities and only one compound, namely, 2,3',4,4',5,5'-hexaCDE, did not cause dose-response immunosuppressive effects in the mice. The structure-activity relationships for the polyCDEs exhibited both differences and similarities. For example, the coplanar 3,3',4,4'-tetraCDE and 3,3',4,4',5-pentaCDE congeners were less immunotoxic than their monoortho 2,3',4,4',5-pentaCDE and 2,3,3',4,4',5-hexaCDE analogs, respectively, and similar results were also observed for their enzyme induction potencies. For the corresponding polychlorinated biphenyls (PCB) congeners the coplanar compounds were significantly more active than their monoortho analogs. In addition, two diortho-substituted compounds, namely, 2,2',4,5,5'-pentaCDE and 2,2',4,4',5,5'-hexaCDE, were also immunotoxic at a dose of 400 mumols/kg whereas, their PCB analogs were inactive. These studies clearly demonstrate that for the polyCDE congeners, increasing ortho-chloro substitution is less effective in reducing the activity of these congeners compared to the well-recognized ortho effects reported for the PCBs. The differences in the structure-activity relationships between polyCDEs and PCBs are related to the ether bridge in the polyCDEs in which the resultant increased bond length between the two phenyl rings thereby diminishes the effects of ortho substituents on the biochemical and toxic potencies of these compounds.

Influence of the Ah locus on the humoral immunotoxicity of 2,3,7,8-tetrachlorodibenzo-p-dioxin: evidence for Ah-receptor-dependent and Ah-receptor-independent mechanisms of immunosuppression

There are conflicting reports in the literature regarding the role of the Ah locus in 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) immunotoxicity. The present studies have utilized two congenic strains of C57Bl/6 mice that differ only at this locus to assess its influence on TCDD-induced suppression of antibody responses. Mice were given a single oral dose of TCDD 2 days prior to challenge with sheep red blood cells (SRBC) or trinitrophenyl-lipopolysaccharide (TNP-LPS). The subsequent dose-dependent effects of TCDD on the amount of antibody produced by splenic plasma cells were measured using the hemolytic antibody isotope release assay. In addition, the relative importance of the Ah genotype of lymphoid versus nonlymphoid tissue was examined in adoptive transfer experiments. Aryl hydrocarbon hydroxylase (AHH) activity was significantly induced in Ahbb mice by a dose of 0.5 micrograms/kg TCDD and maximally induced by a dose of 2 micrograms/kg. Ahdd mice required 10-fold higher doses of TCDD to induce comparable levels of AHH. The degree of thymic involution and liver hypertrophy induced by TCDD was also influenced by the Ah genotype of the animals. Both Ahbb and Ahdd mice exhibited dose-dependent suppression of the anti-TNP response following TCDD exposure. The ID50 was 7.0 micrograms/kg in Ahbb mice and 30.8 micrograms/kg in Ahdd mice. Suppression of the antibody response to SRBC was also dependent on the Ah locus. The ID50 in Ahbb mice was 0.6 micrograms/kg TCDD. However, an apparent biphasic dose response for suppression of the anti-SRBC response in Ahdd mice suggested the involvement of an Ah-independent component of suppression as well. In adoptive transfer studies, lymphocytes were identified as an Ah-dependent component of the response. The Ah-independent component of the response was not identified, and could be either lymphoid or nonlymphoid in nature. The possibility that T helper cells represent the Ah-independent component is discussed.

Immunosuppressive activities of polychlorinated biphenyls in C57BL/6N mice: structure-activity relationships as Ah receptor agonists and partial antagonists

The immunosuppressive activity of polychlorinated biphenyl (PCB) congeners is structure-dependent and 2 classes of compounds, namely the coplanar (class I) and monoortho coplanar (class II) congeners exhibit immunotoxicity. This study extends the structure-immunotoxicity relationships for PCBs by investigating representative congeners from the following structural classes of PCBs: monoortho coplanar (2,3,3',4,4',5-hexachlorobiphenyl, class II); monoortho coplanar minus a single parachloro group (2,3,3',4,5,5'-hexachlorobiphenyl and 2,3,3',4,5'-pentachlorobiphenyl, class III); diortho coplanar (2,3',4,4',5',6-hexachlorobiphenyl, class IV); triortho coplanar (2,2',4,4',5,6'-hexachlorobiphenyl, class V) and a tetraortho-substituted PCB (2,2',4,4',6,6'-hexachlorobiphenyl, class VI). The effects of these compounds on the splenic plaque forming cell response to sheep red blood cells was determined in 7-8 week old male C57BL/6N mice. The results showed that the class II-IV congeners were immunotoxic and with only one exception these compounds also induced hepatic microsomal aryl hydrocarbon hydroxylase and ethoxyresorufin O-deethylase activities and displaced [3H]-2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) from the cytosolic aryl hydrocarbon (Ah) receptor in competitive binding assays. These results thus extend the structure-activity relationships for PCBs as Ah receptor agonists. The interaction of these PCB congeners with an ED70-90 dose of TCDD (3.7 nmol/kg) showed that only one structural class of compounds, namely class III, partially antagonized TCDD-mediated immunotoxicity.