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

Immunoresponsiveness in endometriosis: implications of estrogenic toxicants

Endometriosis is a reproductive disease characterized by the growth of endometrial cells at sites outside the uterus. This disease is a serious disorder associated with chronic pain and infertility, which may be present in 6 million women in this country. Traditional medical therapy has consisted of hormonal regimens that limit the action of endogenous estrogen. The etiology of endometriosis is unknown, but studies suggest that soluble factors known as cytokines play a role in disease pathogenesis. 2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD or dioxin) is an environmental toxicant that alters the action of estrogen in reproductive organs and adversely affects immunocompetence. The incidence of endometriosis was determined in rhesus monkeys that were chronically exposed to dioxin for a period of approximately 4 years. Ten years after termination of dioxin treatment, the presence and severity of endometriosis was assessed by surgical laparoscopy. The incidence of endometriosis correlated with dioxin exposure and disease severity was dependent upon the dose administered. Moderate to severe endometriosis was not found in control animals but was documented in three of seven animals exposed to 5 ppt dioxin (43%) and in five of seven animals exposed to 25 ppt dioxin (71%). The frequency of spontaneous disease in the control group was 33%, similar to an overall prevalence of 30% in 304 rhesus monkeys with no history of dioxin exposure. This study indicates that endometriosis may be associated with dioxin exposure in the rhesus. In view of overwhelming evidence that cytokines participate in the mediation of reproductive-endocrine phenomena and regulation of endometrial growth, future assessment of the effects of environmental toxicants on reproductive health may depend upon our understanding of the bidirectional cytokine network between the immune and endocrine systems.

Impaired cellular immune response in harbour seals (Phoca vitulina) feeding on environmentally contaminated herring

In a 2.5-year immunotoxicological study, two groups of captive harbour seals (Phoca vitulina) were fed herring from the heavily polluted Baltic Sea or from the relatively uncontaminated Atlantic Ocean. Blood samples were collected at regular intervals, and functional immunological parameters were monitored. T cell mitogen and mixed lymphocyte-induced proliferative responses of peripheral blood mononuclear cells (PBMC) obtained from seals fed Baltic herring were significantly reduced over the course of experiment. Upon immunization with rabies virus antigen (RV) and tetanus toxoid (TT), specific proliferative responses of PBMC from the seals fed Baltic herring were also significantly reduced. Impairment of T cell-mediated immune responses became especially apparent during the second year on the respective diets, and correlated significantly to 2,3,7,8-tetrachloro-dibenzo-p-dioxin toxic equivalent levels in blubber biopsies taken from the seals after 2 years on the respective diets. Humoral immune responses, including lipopolysaccharide (LPS)-induced lymphoproliferative responses, in vitro immunoglobulin production by PBMC, as well as RV-, TT-and poliovirus-specific serum antibody responses following immunization, remained largely unaffected. We conclude that suppression of the cellular immune response in the seals fed Baltic herring was induced by the chronic exposure to immunotoxic environmental contaminants accumulated through the food chain. Since cellular immune responses are known to be of crucial importance in the clearance of morbillivirus infections, these results suggest that environmental pollution-related immunosuppression may have contributed to the severity and extent of recent morbillivirus-related mass mortalities among marine mammals.

Immunomodulatory effect of 2,3,7,8-tetrachlorodibenzo-p-dioxin tested by the popliteal lymph node assay

Drugs and other chemicals that have the potential to induce or exacerbate systemic autoimmune diseases in humans are of great concern. The aim of this study was to examine the immune-disregulating potential of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) by using the popliteal lymph node (PLN) assay. Chlorpromazine (CPZ) was used as a reference compound for 2 reasons: (a) CPZ is known to elicit a positive response in this assay, and (b) CPZ is a structural analogue of TCDD. Male Sprague-Dawley rats were injected subcutaneously with either TCDD or CPZ into the right hind footpad, whereas vehicle alone was injected into the contralateral footpad. Control rats were injected with vehicle in both hind footpads. Animals were sacrificed on day 7, and their PLNs were removed, weighed, and immersed in 10% formalin. The PLN weight index (the weight ratio of right PLN over left PLN) was significantly higher in both CPZ- and TCDD-treated rats than in controls. Histological examinations of PLNs in the CPZ- and TCDD-treated rats revealed similar morphological changes in both groups (e.g., mild follicular hyperplasia with no evidence of an acute inflammatory response). These results indicate that TCDD has the potential to induce or exacerbate autoimmune-like reactions. Results also suggest that drugs may be useful surrogates to study the mechanism of toxicity of environmental chemicals that cannot be administered to humans.

High consumption of fatty fish from the Baltic Sea is associated with changes in human lymphocyte subset levels

Fatty fish from the Baltic Sea accumulate immunotoxic persistent organochlorine compounds. In a previous study we found inverse correlations between such fish consumption and natural killer (NK) cell levels in a Swedish population. The present study concerns 68 Latvian subjects with high, low or intermediate fish consumption. High fish consumption correlated positively with B cell levels and CD4+/CD8+ ratios, but negatively with levels of cytotoxic (CD8+) T cells. Furthermore, NK cell levels correlated inversely with plasma selenium, one of several strong correlates with fish intake. A high fish diet includes a set of possible immunomodulating agents. It is presently not possible to pinpoint the cause for the observed subset deviations or to establish their possible biological importance.

Suppression of cytotoxic T lymphocyte activity by 2,3,7,8-tetrachlorodibenzo-p-dioxin occurs in vivo, but not in vitro, and is independent of corticosterone elevation

Previous studies have shown that 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) is a potent immunosuppressive compound. In our laboratory, TCDD and structurally related polychlorinated biphenyls (PCBs) have been shown to suppress alloantigen-specific cytotoxic T lymphocyte (CTL) activity in C57B1/6 mice. PCB-induced CTL suppression occurs coincident with significant elevation of plasma glucocorticoid (GC) levels (> 500 ng/ml). Since GC elevation can cause immune suppression, this study was conducted to determine if TCDD-induced CTL suppression is correlated with elevation of plasma corticosterone (CS), the major GC in mice. Single oral doses of TCDD (2.5-40 micrograms/kg) induced a dose-dependent suppression of CTL activity with a calculated 50% immunosuppressive dose (ID50) occurring at 7.2 micrograms/kg. When total lytic units (LU)/spleen were calculated, the ID50 was 2.8 micrograms/kg. In contrast, plasma CS levels were not significantly altered at doses below 40 micrograms/kg. These data suggest that TCDD-induced CTL suppression is not dependent on CS elevation. The direct effect of TCDD on CTL generation was tested by adding TCDD at 10(-13)-10(-9) M to in vitro mixed lymphocyte-tumor cell (MLTC) cultures. No alteration of CTL activity was observed after 5 days of culture at any TCDD concentration. In contrast, CS alone significantly suppressed CTL activity in vitro. CS-induced CTL suppression in vitro was neither enhanced nor inhibited by the presence of TCDD. These results suggest that TCDD causes CTL suppression in vivo by a mechanism that does not involve CS.

The effects of acute exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin on glioma-specific cytotoxic T-cell activity in Fischer 344 rats

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) is one of the most potent environmental immunomodulating agents identified so far. Historically, mice have been used to model mammalian immunobiology and most of the data gathered on the immunotoxicity of TCDD has been obtained from studies with mice. However, rats have been used more extensively in toxicological research to establish human risk assessment criteria. A need exists, therefore, to develop a database using the rat model in immunotoxicology so that complete animal toxicity studies can be conducted. We have treated female Fischer 344 rats with a single i.p. dose of 0.3, 3.0, or 30.0 micrograms/kg TCDD or corn oil vehicle and examined cytotoxic T-cell (CTL) activities 24 days following treatment. Syngeneic in vivo tumor-specific CTLs were generated that model cell-mediated immune reactions against neoplastically transformed self antigens. RT2, a virally-induced Fischer 344 rat glioma, and D74, a ethylnitrosurea-induced Fischer 344 rat glioma were used as targets. This immunological parameter was compared to body, thymic, and liver weights as well as liver ethoxyresorufin deethylase (EROD) activity on day 24 post-TCDD treatment. The results indicate that Fischer 344 rats are very sensitive to TCDD as indicated by severe thymic atrophy and EROD induction at all three doses. In contrast, CTL activity was only marginally affected by these same doses of TCDD with only a modest suppression noted at the highest dose. These results indicate that the CTL response in rats may not be useful in characterizing the effects of this xenobiotic on immunocompetence in the rat.

Enhanced human IgE production results from exposure to the aromatic hydrocarbons from diesel exhaust: direct effects on B-cell IgE production

Epidemiologic and experimental studies suggest that air pollution, and particularly diesel exhaust particles (DEPs) may play a role in the increasing prevalence and severity of airway allergic disease. We show that the extract of polyaromatic hydrocarbons (PAHs) from DEPs (PAH-DEP) enhances human IgE production from purified B cells. Interleukin-4 plus CD40 monoclonal antibody-stimulated IgE production was enhanced 20% to 360% by the addition of PAH-DEP over a period of 10 to 14 days. This effect was increased when PAH-DEP was added 2 to 5 days after cultures were initiated. PAH-DEP itself did not induce IgE production or synergize with interleukin-4 alone to induce IgE from purified B cells, suggesting that it was enhancing ongoing IgE production rather than inducing germline transcription or isotype switching. The prototype nonmetabolized aromatic hydrocarbon 2,3,7,8 tetracholorodibenzo-p-dioxin, which functions solely through activation of the cytosolic aromatic hydrocarbon receptor complex, also increased IgE production. Additionally, the pattern of mRNAs coding for distinct isoforms of the epsilon chain was altered by PAH-DEP, and B-cell expression of the low-affinity IgE receptor was upregulated by PAH-DEP. Enhanced IgE production in the human airway, resulting from exposure to PAH-DEP, may be an important factor in the increase in airway allergic disease.

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.

3,3',4,4'-Tetrachlorobiphenyl inhibits proliferation of immature thymocytes in fetal thymus organ culture

The environmental pollutant 3,3',4,4'-tetrachlorobiphenyl (TCB) leads to thymic atrophy and immunosuppression, the former possibly causing the latter. TCB binds to the cytosolic aryl-hydrocarbon receptor (AhR) and transforms it into a DNA-binding state. The development of fetal thymocytes is severely affected by TCB and other AhR-binding xenobiotics, leading to a skewed pattern of thymocyte maturation stages. Murine thymocyte proliferation after exposure to TCB was studied in fetal thymus organ culture (FTOC). C57BL/6 fetus thymic lobes from day 15 of gestation were explanted and grown for 2, 4, 6, and 8 days in organ culture in the presence or absence of 3.3 microM TCB. Subsets of thymocytes were defined by CD4 and CD8 surface markers, and their cell cycle was analysed by DNA staining with 7-amino-actinomycin D (7-AAD). Exposure of fetal thymi in vitro to 3.3 microM TCB significantly reduced the total number of thymocytes, and fewer thymocytes were in S/G2M phase. The inhibition of cell proliferation induced by TCB treatment affected mainly the CD4-CD8- (double-negative, DN) and CD4-CD8+ (single-positive, SP) subsets, and these inhibition appeared mainly in more immature thymocytes, i.e. DNCD3- and CD8+CD3- subpopulations, whereas no effect of TCB on CD4+CD8+ (double-positive, DP) cell proliferative activity was observed. Analysis of the relation of cell proliferation and development of subsets in differentiating fetal thymocytes suggests that TCB enhanced thymocyte differentiation into mature CD8+ cells.

Evaluation of the effect of low-level 2,3,7,8-tetrachlorodibenzo-p-dioxin exposure on cell mediated immunity

The immunotoxicity of TCDD in the mouse has been well documented. To date, the most sensitive endpoint to TCDD-induced toxicity in mice is that reported by Clark et al. (Clark, D. A., Gauldie, J., Szewczuk M. R. and Sweeney, G. (1981) Proc. Soc. Exper. Biol. Med. 168, 290.) who found that TCDD suppressed the murine cytotoxic T lymphocyte (CTL) response following four weekly doses of 4 ng TCDD/kg/week. However, these results have never been corroborated, as other laboratories have been unable to detect immunosuppression by TCDD at such low levels. In this study, we evaluated the effect of TCDD on the in vivo- and in vitro-generated CTL response to P815 mastocytoma cells in adult C57BL/6J female mice via a 51Cr release assay. Mice were given weekly intraperitoneal injections of TCDD or vehicle for 4 weeks at dosages ranging from 0.01 to 3.00 micrograms/kg/week. No statistically significant suppression of the in vivo- or in vitro-generated CTL response was detected at any dosage. As expected, significant increases in liver weights and decreases in thymus weights were observed at TCDD dosages of 1.0 and 3.0 micrograms/kg/week. Likewise, suppression of the antibody plaque-forming cell response to sheep erythrocytes was observed at dosages of 1.0 and 3.0 micrograms TCDD/kg/week. Although expected humoral immunosuppression and organ effects were observed, our data do not support suppression of murine CTL responses at the TCDD doses employed in this study.

Parameters of immunological competence in subjects with high consumption of fish contaminated with persistent organochlorine compounds

Consumption of fatty fish species, like salmon and herring, from the Baltic Sea is an important source of human exposure to persistent organochlorine compounds, e.g. polychlorinated dioxins (PCDDs), dibenzofurans (PCDFs) and biphenyls (PCBs). Many of these compounds show immunotoxic and hepatotoxic effects in animals. We have now studied immunological competence, including lymphocyte subsets, in 23 males with a high consumption of fish from the Baltic Sea and in a control group of 20 males with virtually no fish consumption. The high consumers had lower proportions and numbers of natural killer (NK) cells, identified by the CD 56 marker, in peripheral blood than the non-consumers. Weekly intake of fatty fish correlated negatively with proportions of NK cells (rs = -0.32, P = 0.04). There were also, in a subsample of 11 subjects, significant negative correlations between numbers of NK cells and blood levels of a toxic non-ortho-PCB congener (IUPAC 126; rs = -0.68, P = 0.02) and a mono-ortho congener (IUPAC 118; rs = -0.76, P = 0.01). A similar correlation, in 12 subjects, was seen for p,p'-DDT (rs = -0.76, P = 0.01). The corresponding negative correlation, in 13 subjects, with blood levels of PCDD/Fs was not significant (rs = -0.57, P = 0.07). No significant association was seen between organic mercury in erythrocytes and NK cells. Fish consumption was not associated with levels of any other lymphocyte subset. Neither were there any correlations with plasma immunoglobulins or liver enzyme activities. Our study indicates that accumulation of persistent organochlorine compounds in high consumers of fatty fish may adversely affect NK cell levels.

Direct exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) increases infectivity of human erythrocytes to a malarial parasite

Direct exposure to 10 nM 2,3,7,8-TCDD caused a 75% increase and a 2-fold increase in the infectivity of isolated human erythrocytes to P. falciparum after 48 hours when the parasites were in an unsynchronized or synchronized state of growth, respectively. Treatment of human erythrocytes with 10 microM sodium orthovanadate (NaOV), an inhibitor of plasma membrane Ca-ATPase and phosphotyrosine phosphatase, decreased parasitemia by 30%. Co-treatment of RBCs with TCDD and NaOV completely blocked the TCDD-induced increase in parasitemia. Because erythrocytes are anucleated, these results are discussed as evidence for biochemical changes by TCDD without requiring the activation of gene products.

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.

Mouse Hepa 1c1c7 hepatoma cells produce complement component C3; 2,3,7,8-tetrachlorodibenzo-p-dioxin fails to modulate this capacity

Previous studies from this laboratory have shown that 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) decreased complement component C3 levels in female B6C3F1 mouse serum following in vivo acute or subchronic exposure (White et al., 1986). Since TCDD is a hepatotoxic compound and more than 90% of serum C3 is produced by the liver, studies were undertaken using mouse Hepa 1c1c7 (Hepa 1) hepatoma cell line to determine if TCDD acts directly on hepatocytes to inhibit C3 production. The C3-producing capacity of Hepa 1 cells was first examined. When confluent Hepa 1 cell monolayers were cultured in 24-well plates with serum-free medium, a detectable amount of C3 (14.1 +/- 0.8 ng/ml) was secreted as early as 1 h after culture and reached a plateau at 12 h (68.3 +/- 4.9 ng/ml). Furthermore, the sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) analysis demonstrated that the molecular weight of C3 in culture supernatant corresponded to that present in mouse serum. Human recombinant IL-1 beta (hrIL-1 beta), a known inducer of complement C3, at doses as low as 1 unit/ml increased the C3 production to 158% of control after 24 h of incubation. The effect of hrIL-1 beta was dose dependent, and the maximum tested dose of 10 units/ml increased C3 production to 256% of control. When cells were directly exposed to TCDD at concentrations from 10(-10) to 10(-6) M, there was no inhibitory effect on production of C3. TCDD also failed to block the stimulatory effect of 10 units/ml hrIL-1 beta added to the culture 1 h later. To verify that cultured Hepa 1 cells were able to respond to TCDD, 7-ethoxyresorufin O-deethylase (EROD) activity was measured under the same conditions. TCDD dose-dependently increased EROD activity of Hepa 1 cells at 24 h following exposure. The activity reached 56.7 +/- 3.0 pmol/min/mg protein with 10(-9) M TCDD, compared with 10.7 +/- 1.7 pmol/min/mg protein of vehicle-exposed cells. Our results indicate that the direct interaction of TCDD with Hepa 1 cells does not affect their C3-producing capacity, although EROD activity, a characteristic response mediated by the cellular TCDD/Ah receptor, was induced. The lack of effect of TCDD in vitro suggests that the decrease of serum C3 levels observed in vivo may result from an indirect effect of TCDD on hepatocytes.

Modulation of liver intracellular C3 in mice by 2,3,7,8-tetrachlorodibenzo-p-dioxin

Earlier studies from this laboratory have shown that the complement system, especially the component C3, in female B6C3F1 mice is suppressed following TCDD exposure in vivo. However, the direct exposure of TCDD in vitro does not affect the C3-producing capacity of two types of hepatoma cells, as well as mouse primary hepatocytes. To investigate the effect of TCDD on C3 production by the liver following in vivo exposure, liver intracellular C3 levels and pro-C3, the precursor of the secreted C3, were examined in the present study. The results demonstrated that there was a dose-dependent increase of liver intracellular C3 levels (from 138% to 175% of control) immediately following TCDD (from 10 to 40 micrograms/kg) exposure. This increase was rapid (4 h after exposure), but transient (less than 2 h), and was not accompanied by an alteration of serum C3 levels. Studies using sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) analysis showed that the increase in liver intracellular C3 levels resulted from, at least partially, an increase in intracellular pro-C3. Serum C3 levels did not decrease until d 3 after exposure, when both liver intracellular C3 levels and pro-C3 in TCDD-treated mice were not different from those of the control mice. These results indicated that the modulation of liver intracellular C3 by TCDD did not correlate with the suppression of serum C3 levels following exposure.

Characterization of protein phosphorylation by 2,3,7,8-tetrachlorodibenzo-p-dioxin in murine lymphocytes: indirect evidence for a role in the suppression of humoral immunity

Studies were undertaken to more thoroughly characterize 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)-induced stimulation of kinase activity in murine lymphocytes. In female B6C3F1 mice, TCDD-induced phosphorylation of 29, 45, 52 and 63 KDa proteins was selective for B cells, with little or no enhancement observed in T cells. When B cells were purified and separated by density on a percoll gradient, phosphorylation was only observed in the band composed of activated B cells, and was not enhanced in the band composed of resting B cells. TCDD-stimulated phosphorylation was associated with both the cytosol (45 and 52 KDa species) and membrane (52 KDa species) fractions. Purified B cells from both DBA/2 (Ahdd) and C57B16 (Ahbb) mice demonstrated equivalent enhancement of phosphorylation in response to TCDD. Administration of human gamma interferon (Hu-IFNg) at concentrations from 0.5 to 500 Units/ml produced a dose-related reversal of TCDD-induced suppression of in vitro antibody responses to both the polyclonal B cell activator, LPS, and the T-dependent antigen, sRBC in whole splenocytes isolated from female B6C3F1 mice. These concentrations of Hu-IFNg did not affect the magnitude of either response in the absence of TCDD, and did not reverse dexamethasone-induced suppression of either in vitro antibody response. TCDD-induced suppression of the T-dependent response was reversed only when Hu-IFNg was added to culture within the first 18 hours after treatment with TCDD and sRBC. These studies demonstrate that Hu-IFNg can reverse TCDD-induced in vitro Ab response suppression if it is administered during the period of susceptibility to TCDD. TCDD-induced phosphorylation in isolated B cells was also antagonized following co-incubation with Hu-IFNg. The profile of TCDD-induced increases in protein phosphorylation, including the selective effect on activated B cells, the general involvement of both cytosolic and membrane proteins, the lack of segregation with the Ah-dependent processes, and the ability of Hu-IFNg to reverse both the suppression of the Ab response and the increase in phosphorylation, supports the interpretation that such phosphorylation is involved in TCDD-induced suppression of the Ab response.

Aryl hydrocarbon or dioxin receptor: biologic and toxic responses

1. The AhR represents a ligand-activated transcription factor. Receptor agonists include planar aromatic compounds, a variety of heterocyclic plant constituents, and PCDD/PCDF. The latter lead to persistent activation of the receptor due to their strong binding affinity and long biologic half-life of over 10 years in human blood and fat. Practically every person on earth is exposed to these compounds via the diet (> 90%) and by high concentrations in mother's milk. PCDD/PCDF produced toxic responses in exposed people (primarily chloracne and immunosuppression) in the past. However, the present PCDD/PCDF levels (basal levels) in the general population are below those warranting toxicologic concern. 2. The AhR has been characterized as a helix-loop-helix transcription factor related to the Drosophila developmental genes sim and per. The cytosolic form of the receptor is present as an inactive complex with two subunits of HSP90. After ligand binding HSP90 is released and the receptor enters the nucleus as a heterodimer together with a related protein ARNT. It binds with high affinity to certain enhancer elements in the upstream region of several genes such as cytochrome P4501A1 (CYP1A1). The AhR transcriptionally activates several drug-metabolizing enzymes and proteins involved in growth/differentiation, such as the plasminogen activator inhibitor PAI-2 and IL-1 beta. In addition, it modulates the action of a number of other nuclear transcription factors such as receptors of the steroid hormone receptor superfamily and of cell surface receptors such as EGF. With the exception of CYP1A1 induction, little is known about the mechanism of transcriptional activation of the AhR-controlled genes. Many AhR-modulated biologic responses (such as modulation of the estrogen and EGF receptor) appear to be indirect. 3. Persistent activation of the AhR is probably responsible for toxic responses in experimental animals and humans. They are markedly tissue and species specific. In rodents a wasting syndrome, immunosuppression, teratogenicity, chloracne, and carcinogenicity/tumor promotion have been well studied. There is good evidence for an involvement for the AhR in these responses. However, the chain of events from receptor activation to the diverse toxic endpoints is largely unknown. Alteration of growth and differentiation of epithelial tissues may underlie most of the toxic responses. A lot has already been achieved, mostly by characterizing the AhR and transcriptional activation of CYP1A1. Still more work lies ahead of us, for example, elucidation of the physiologic roles of the AhR and of the chains of events from receptor activation to the various biologic and toxic endpoints.

Human immune toxicity

This review examines xenobiotic toxicity to the immune system, stressing in particular those aspects of most relevance to humans. Immunotoxicity is examined especially from three points of view: by what immunological component is affected, by classes of foreign agents that adversely affect the human immune system and by critical evaluation of human case reports and epidemics. Mechanisms by which xenobiotics interrupt cytokine networks are emphasized. The concept that microbial agents, both environmental as well as infectious, may act as immunotoxicants, either alone or in synergism with conventional agents is introduced. Instances of human immunotoxicology are critically evaluated in terms of clinical relevance, i.e. whether increased susceptibility to opportunistic infections or tumor emergence takes place in the affected individuals.