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

2,3,7,8-tetrachlorodibenzo-p-dioxin slows the progression of experimental cutaneous Leishmaniasis in susceptible BALB/c and SCID mice

In a model of experimental cutaneous leishmaniasis, pre-exposure of Leishmania major-resistant mice to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), an aryl hydrocarbon receptor agonist, causes suppression of the protective anti-parasite T helper 1 response while paradoxically also reducing parasite burdens in those animals. In this study, we examined if TCDD exposure could also reduce parasite burdens in L. major-susceptible BALB/c mice. In the highest dose group (160 µg/Kg), TCDD treatment caused a significant reduction of parasite burdens by 10-fold after three weeks while also causing a significant lymphoid atrophy indicating suppression of the non-protective T helper 2 response. A dose-dependent delay of foot lesion progression was also observed such that lesion size in the highest dose group was less than half that of controls after 35 days of infection. Importantly, although TCDD exposure initially reduced disease severity and prolonged the course of disease by as much as three fold in some animals, this effect was transitory and TCDD did not induce resistance to L. major infection. Because TCDD exposure reduced L. major burdens in both resistant and susceptible mice, we hypothesized that TCDD reduces L. major burdens in mice by a mechanism that does not involve adaptive immunity. To test this, severe combined immunodeficient (SCID) mice were used. In mice infected with a moderate number of L. major (10,000), TCDD treatment caused a time- and dose-dependent decrease of parasite burdens by nearly 100-fold after six weeks in the highest dose group (200 µg/Kg). A significant and dose-dependent delay of foot lesion progression was also observed in these animals. These results indicate that TCDD exposure can reduce the severity of leishmanial disease in mice independent of adaptive immunity.

Effects of lactational and/or in utero exposure to environmental contaminants on the glucocorticoid stress-response and DNA methylation of the glucocorticoid receptor promoter in male rats

Perinatal events can reprogram the hypothalamo-pituitary-adrenal axis for the entire lifespan leading to abnormal glucocorticoid stress-response (GSR) in adulthood: a phenomenon reported to be mediated by changes in DNA methylation of the glucocorticoid receptor (GR) gene promoter. We examined whether in utero and/or lactational exposure to mixtures of environmental contaminants can also induce abnormal GSR during adulthood. The experiment included nine treatment groups. From gestation day (GD) 0 until postnatal day (PND) 20, dams were fed daily with a cookie laced with corn oil (control) or a chemical mixture (M) [polychlorinated biphenyls (PCBs), organochlorine pesticides, and methylmercury] at 0.5 or 1.0mg/kg/day (0.5M, and M). At birth, some control (C) and M litters were cross-fostered to create four groups with the following in utero/postnatal exposure: C/C, M/C, C/M, M/M. Other dams received 1.8ng/kg/day of a mixture of aryl hydrocarbon receptor (AhR) agonists (non-ortho PCBs, PC-dibenzodioxins and PC-dibenzofurans) without or with 0.5M (0.5MAhR). In adult male offspring the abundance of GR in treated groups was not different from the control, but the AhR and M groups were significantly different from each other with opposite effects in the hippocampus and liver. There was no change in DNA methylation of the GR promoter (exon-17 and -110). Abnormal GSRs were detected in the AhR, 0.5MAhR, CM, and MM groups. The literature associates abnormal GSR with metabolic and mental health impairments, thus these results support further investigation of the influence of developmental exposure to environmental contaminants and predisposition to stress-induced diseases.

Protective effect of curcumin on immune system and body weight gain on rats intoxicated with 2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD)

BACKGROUND AND AIM

In this study, the negative effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) on the immune system and body weight gain of rats and the preventive effects of curcumin were examined.

MATERIAL AND METHODS

For this purpose, 128 3-4-month-old Wistar albino rats with 280-310 g body weights were used. The 2 microg/kg dose of 2,3,7,8-TCDD and 100 mg/kg dose of curcumin were dissolved in corn oil and orally given to the rats found in the experimental and control groups. Then, the serum samples were taken from all rats at 15, 30, 45 and 60 days to analyzed for the determination of TNF-alpha, IFN-gamma, IL-12 and IL-13 levels by ELISA method. The data of body weight gain was measured at 15, 30, 45 and 60 days.

RESULTS

The results indicated that 2,3,7,8-TCDD caused a significant increase (p<0.05) in serum TNF-alpha level. However, it caused significant (p<0.05) decreases in the levels of IFN-gamma, IL-12 and IL-13 in rats. On the contrary, curcumin increased IFN-gamma, IL-12 and IL-13 levels, but decreased TNF-alpha level in rats. Additionally, TCDD caused significant (P<0.01) reductions in the body weight gain. However, curcumin reversed this effect of TCDD.

CONCLUSION

2,3,7,8-TCDD significantly suppressed the humoral immunity and body weight gain in rats at doses of 2 microg/kg. However curcumin, which was found in some plants, eliminated the effect of TCDD on immune system and body weight when it was given together with 2,3,7,8-TCDD. It is thought that this effect may have occurred via curcumin and TCDD binding aryl hydrocarbon receptor (AhR) competitively.

The elderly as a sensitive population in environmental exposures: making the case

The US population is aging. CDC has estimated that 20% of all Americans will be 65 or older by the year 2030. As a part of the aging process, the body gradually deteriorates and physiologic and metabolic limitations arise. Changes that occur in organ anatomy and function present challenges for dealing with environmental stressors of all kinds, ranging from temperature regulation to drug metabolism and excretion. The elderly are not just older adults, but rather are individuals with unique challenges and different medical needs than younger adults. The ability of the body to respond to physiological challenge presented by environmental chemicals is dependent upon the health of the organ systems that eliminate those substances from the body. Any compromise in the function of those organ systems may result in a decrease in the body's ability to protect itself from the adverse effects of xenobiotics. To investigate this issue, we performed an organ system-by-organ system review of the effects of human aging and the implications for such aging on susceptibility to drugs and xenobiotics. Birnbaum (1991) reported almost 20 years ago that it was clear that the pharmacokinetic behavior of environmental chemicals is, in many cases, altered during aging. Yet, to date, there is a paucity of data regarding recorded effects of environmental chemicals on elderly individuals. As a result, we have to rely on what is known about the effects of aging and the existing data regarding the metabolism, excretion, and adverse effects of prescription medications in that population to determine whether the elderly might be at greater risk when exposed to environmental substances. With increasing life expectancy, more and more people will confront the problems associated with advancing years. Moreover, although proper diet and exercise may lessen the immediate severity of some aspects of aging, the process will continue to gradually degrade the ability to cope with a variety of injuries and diseases. Thus, the adverse effects of long-term, low-level exposure to environmental substances will have a longer time to be manifested in a physiologically weakened elderly population. When such exposures are coupled with concurrent exposure to prescription medications, the effects could be devastating. Public health officials must be knowledgeable about the sensitivity of the growing elderly population, and ensure that the use of health guidance values (HGVs) for environmental contaminants and other substances give consideration to this physiologically compromised segment of the population.

Aryl Hydrocarbon Receptor Activation Impairs the Priming but Not the Recall of Influenza Virus-Specific CD8+T Cells in the Lung

The response of CD8+ T cells to influenza virus is very sensitive to modulation by aryl hydrocarbon receptor (AhR) agonists; however, the mechanism underlying AhR-mediated alterations in CD8+ T cell function remains unclear. Moreover, very little is known regarding how AhR activation affects anamnestic CD8+ T cell responses. In this study, we analyzed how AhR activation by the pollutant 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) alters the in vivo distribution and frequency of CD8+ T cells specific for three different influenza A virus epitopes during and after the resolution of a primary infection. We then determined the effects of TCDD on the expansion of virus-specific memory CD8+ T cells during recall challenge. Adoptive transfer of AhR-null CD8+ T cells into congenic AhR(+/+) recipients, and the generation of CD45.2AhR(-/-)-->CD45.1AhR(+/+) chimeric mice demonstrate that AhR-regulated events within hemopoietic cells, but not directly within CD8+ T cells, underlie suppressed expansion of virus-specific CD8+ T cells during primary infection. Using a dual-adoptive transfer approach, we directly compared the responsiveness of virus-specific memory CD8+ T cells created in the presence or absence of TCDD, which revealed that despite profound suppression of the primary response to influenza virus, the recall response of virus-specific CD8+ T cells that form in the presence of TCDD is only mildly impaired. Thus, the delayed kinetics of the recall response in TCDD-treated mice reflects the fact that there are fewer memory cells at the time of reinfection rather than an inherent defect in the responsive capacity of virus-specific memory CD8+ cells.

T cell-derived IL-5 production is a sensitive target of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)

The immune system is one of the organs most vulnerable to the toxicity of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). Among the various immunotoxic effects of TCDD, the thymus involution and suppression of IgM antibody production are well known sensitive reactions of the thymocytes and B cells affected by TCDD. Recently, we reported that TCDD greatly inhibits the production of type-2 helper T (Th2) cell-derived cytokines, especially IL-5, by the splenocytes in mice immunized with ovalbumin (OVA). In the present study, we investigated the dose-dependency of these TCDD immunotoxic effects in OVA-immunized mice to identify the most sensitive target. Mice of two age groups, 6 weeks old and 3 weeks old, were dosed with 0.3, 1.0, or 3.0 microg TCDD/kg and immunized with OVA using alum as an adjuvant. Seven days later, the thymus weight, thymocyte population, antigen-specific IgM in the plasma, and IL-5 production by the splenocytes were examined. Among them, IL-5 production was significantly suppressed by all three doses of TCDD and reduced to about 30% by even a small dose of 0.3 microg TCDD/kg in both age groups. The thymus weight was significantly reduced by 1.0 microg or 3.0 microg TCDD/kg, but IgM production was not affected by up to 3.0 microg/kg of TCDD in both age groups. Taken together, the Th2 cell-derived IL-5 production was the most sensitive endpoint detecting TCDD toxicity among those examined. Our results also suggest that effector T cells are targets more vulnerable to TCDD toxicity than thymocytes or antibody-producing B cells in the OVA-immunized mice.

An approach to evaluation of the effect of bioremediation on biological activity of environmental contaminants: dechlorination of polychlorinated biphenyls

The effectiveness of bioremediation efforts is assessed traditionally from the loss of the chemical of interest. In some cases, analytical techniques are coupled with evaluation of toxicity to organisms representative of those found in the affected environment or surrogate organisms. Little is known, however, about the effect of remediation of environmental chemicals on potential toxicity to mammalian organisms. We discuss both an approach that employs mammalian cell system bioassays and the criteria for selection of the assays. This approach has been used to evaluate the biological response to mixtures of polychlorinated biphenyls (PCBs) before and after remediation by reductive dechlorination. The dechlorination process used results in accumulation of congeners substituted in only the ortho and para positions and containing fewer chlorines than the starting mixtures. Evaluation of the dechlorinated mixture reveals a loss of biological activity that could be ascribed to coplanar PCBs not containing chlorine in the ortho positions. Conversely, biological activity associated with ortho-substituted PCB congeners is unaffected or increased by remediation. Thus, the results of the bioassays are consistent with the remediation-induced change in the profile of PCB congeners and the known mechanisms of action of PCBs. The results emphasize a need for evaluation of the products of remediation for biological activity in mammalian systems. Furthermore, the approach outlined demonstrates the potential to assess the impact of remediation on a range of biological activities in mammalian cells and thus to estimate positive and negative effects of remediation strategies on toxicity. Future needs in this area of research include assays to evaluate biological effects under conditions of exposure that mimic those found in the environment and models to extrapolate effects to assess risk to people and wildlife.

Protein kinase C? activity is involved in the 2,3,7,8-tetrachlorodibenzo-p-dioxin-induced signal transduction pathway leading to apoptosis in L-MAT, a human lymphoblastic T-cell line

The aromatic hydrocarbon receptor (AhR)-dependent pathway involved in 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)-induced immunotoxicity has been studied extensively, but the AhR-independent molecular mechanism has not. In previous studies we found that the AhR is not expressed in L-MAT, a human lymphoblastic T-cell line. In this report, we provide the following evidence that the protein kinase C (PKC)theta activity is functionally involved in the AhR-independent signal transduction mechanism that participates in the TCDD-induced L-MAT cell apoptosis. First, only rottlerin, a novel PKC (nPKC)-selective inhibitor, blocked the apoptosis completely, in a dose-dependent manner. Second, PKCtheta was the major nPKC isoform (compared to PKCdelta) expressed in the L-MAT cell line. Third, a cell-permeable myristoylated PKCtheta pseudosubstrate peptide inhibitor also blocked the apoptosis completely, in a dose-dependent manner. Fourth, both rottlerin and myristoylated PKCtheta pseudosubstrate peptide inhibitor completely inhibited PKCtheta kinase activity in vitro at doses that effectively blocked TCDD-induced L-MAT cell apoptosis. TCDD treatment induced a time-dependent activation of nPKC kinase activity in L-MAT cells, and moreover, TCDD induced a translocation of PKCtheta from the cytosolic fraction to the particulate fraction in L-MAT cells. Finally, transient over-expression of a dominant negative PKCtheta (a kinase-dead mutant, K/R 409) in L-MAT cells conferred significant protection against TCDD-induced apoptosis.

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

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

Functional alterations in CD11b(+)Gr-1(+) cells in mice injected with allogeneic tumor cells and treated with 2,3,7,8-tetrachlorodibenzo-p-dioxin

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) exposure results in an increased percentage of CD11b(+) (Mac-1(+)) cells in the spleens of mice challenged with P815 tumor cells, coincident with a failure of the mice to generate allospecific CD8(+) CTL activity. Since CD11b(+)Gr-1(+) myeloid suppressor cells (MSC) have been described as that which prevent cytotoxic T lymphocyte (CTL) development in a variety of disease states, we hypothesized that TCDD promoted MSC development, leading to suppression of CTL activity. The purpose of the present studies was to compare the phenotypic and functional characteristics of CD11b(+) cells in vehicle- and TCDD-treated mice during the P815 tumor allograft response to determine their potential to function as MSC. Initial studies showed that virtually all splenic CD11b(+) cells in both vehicle- and TCDD-treated mice co-expressed Gr-1. Consistent with MSC activity, CD11b(+)Gr-1(+) cells isolated from TCDD- but not vehicle-treated mice suppressed the development of CTL activity when added in vitro to mixed lymphocyte-P815 tumor cell cultures. Also consistent with MSC activity, this suppressive effect in vitro required cell-to-cell contact. Surprisingly, however, in vivo depletion of CD11b(+)Gr-1(+) cells failed to affect TCDD-induced suppression of the CTL response, arguing against an immunoregulatory role for the cells in vivo. Immunohistochemical analysis of the spleen showed that CD11b(+)Gr-1(+) cells were localized in the red pulp, and physically separated from the T cells in the white pulp. The localization of CD11b(+)Gr-1(+) cells in the red pulp was indicative of extramedullary myelopoiesis and suggested that TCDD enhanced myelopoiesis. A significantly enhanced neutrophilia in the blood of TCDD-treated mice supported this conclusion. CD11b(+)Gr-1(+) cells isolated from the blood or spleen of TCDD-treated mice produced up to fivefold higher levels of superoxide following PMA stimulation when compared with cells from vehicle-treated mice. However, unlike vehicle-treated mice, CD11b(+)Gr-1(+) cells from TCDD-treated mice were unable to kill YAC-1 target cells. These results indicate that TCDD exposure alters the host response to allogeneic tumor growth, resulting in enhanced myelopoiesis perhaps as a compensatory response to the suppressed T cell-mediated immunity in the face of an increasing P815 tumor burden. Furthermore, within the context of the P815 response, TCDD appears to alter the functional capabilities of mature neutrophils, by enhancing their oxidative burst capacity but reducing their tumoricidal response.

Recent advances in understanding the mechanisms of TCDD immunotoxicity

TCDD is a highly immunosuppressive chemical that induces potent suppression of immune responses in laboratory animals. However, apart from the requisite role of the AhR and the identification of bone-marrow-derived cells as critical AhR-expressing targets, the specific cells and the underlying biochemical mechanisms by which TCDD disrupts immunological functions remain unclear. Recent data suggest that a new paradigm for the mechanism of immunotoxic action of TCDD may be more accurate, moving from one focused on the suppression of immune functions to one focused on the inappropriate activation of cells, leading to anergy or death, and the consequent premature termination of the immune response. Enhanced activation of B cells, DC and CD4+ T cells by TCDD has been described as well as the earlier disappearance of the latter two populations from peripheral lymphoid organs. Although much remains to be learned about how inappropriate cellular activation via the AhR induces immune suppression, deducing this mechanism of action and the signaling pathways involved, should lead to new insight into basic mechanisms of immune regulation.

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

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

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.

2,3,7,8-Tetrachlorodibenzo-p-dioxin affects the number and function of murine splenic dendritic cells and their expression of accessory molecules

Primary T cell-mediated immune responses are highly susceptible to suppression by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) exposure, yet direct effects of TCDD on T cells have been difficult to demonstrate. Since the activation of naive T cells has been shown to be initiated primarily by dendritic cells (DC), these cells represent a potential target for TCDD immunotoxicity. In this report, we have examined the influence of TCDD exposure on splenic DC phenotype and function in the absence of antigenic stimulation. Results showed that DC from TCDD-treated mice expressed higher levels of several accessory molecules including ICAM-1, CD24, B7-2, and CD40, whereas the expression of LFA-1 was significantly reduced. These effects were dose-dependent and persisted for at least 14 days after exposure. The effects were also dependent upon the aryl hydrocarbon receptor (AhR), as similar effects were observed in AhR+/+ C57Bl/6 and Balb/c mice but not in AhR-/- mice. When DC from TCDD-treated mice were cultured with allogeneic T cells, the proliferative response and production of IL-2 and IFN-gamma by the T cells were increased. Production of IL-12 by the DC was likewise enhanced in comparison to cells from vehicle-treated mice. Interestingly, however, the number of DC recovered from TCDD-treated mice was significantly decreased. Taken together, these results suggest that, in the absence of antigen, TCDD provides an activation stimulus to DC that may lead to their premature deletion. Since the survival of DC has been shown to influence the strength and duration of the immune response, these results suggest a possible novel mechanism for TCDD-induced immune suppression.

CTL hyporesponsiveness induced by 2,3,7, 8-tetrachlorodibenzo-p-dioxin: role of cytokines and apoptosis

Studies have shown that blocking B7-mediated costimulation induces T cell tolerance via anergy or apoptosis. Provision of exogenous IL-2 can reverse or prevent the induction of tolerance. We have previously shown that TCDD-induced suppression of the CTL response to allogeneic P815 tumor cells is accompanied by decreased expression of CD86 (B7-2) as well as suppressed IL-2 and IFNgamma production. In the present studies, the role of IL-2 and IFNgamma and the analysis of inappropriate deletion of CD8(+) cells was examined. Administration of IL-2 on days 7-9 relative to the injection of P815 tumor cells dose-dependently increased the CTL activity and the generation of CD8(+) CTL effector cells in TCDD-treated mice. This increased CTL response was not due to recruitment of naive CTL precursors (CTLp), suggesting that a small pool of activated CTLp in TCDD-treated mice could respond to the IL-2. A much larger pool of activated CTLp in control mice was also expanded by IL-2 treatment. In contrast, treatment with IFNgamma during the same time period did not alter CTL activity in control or TCDD-treated mice. To address the possibility that insufficient IL-2 early in the response was responsible for the reduced pool of activated CTLp in TCDD-treated mice, IL-2 was administered on days 1-3 after P815 injection. However, not only did early treatment with IL-2 fail to restore the response in TCDD-treated mice, it suppressed the CTL response of non-TCDD-treated mice. To test whether exposure to TCDD induced apoptosis of activated CD8(+) T cells, phosphatidylserine (PS) expression was measured on various days after P815 tumor challenge. Surprisingly, the percentage of apoptotic CD8(+) T cells was significantly lower in TCDD-treated mice compared to controls throughout the allograft response. Similarly, exposure to TCDD failed to enhance peripheral deletion of Vbeta3(+)CD8(+) T cells after injection of the superantigen Staphylococcal enterotoxin A (SEA). Taken together, the data indicate that TCDD induces an early defect in CTLp activation that is not due to insufficient IL-2 or deletion of CD8(+) cells and may implicate a novel mechanism by which ligands of the Ah receptor disrupt CTL precursor activation.

Novel phenotype associated with in vivo activated CTL precursors

A previously undefined phenotype of CD8(+) cells that appears to represent in vivo activated CTL precursors (CTLP*) has been identified in the spleens of C57Bl/6 mice responding to a P815 tumor allograft. This population was first evident by the transient expression of very high levels of CD28 and CD44 on day 5 of the allograft response and reached maximal levels on days 7 and 8 before declining on day 9. A transient increase in CD69 expression was also observed on these cells on day 5. In contrast, CTL effectors (CTLE), identified by their CD8(+)CD44(hi)CD62LloCD45RBlo phenotype, were not appreciably detected in the spleen until day 8 and reached maximal levels on day 10. Further characterization of CTLP* on day 7 revealed that they represented blasting cells by increased light scatter and also expressed very high levels of CD54 but not CD122, CD152, or CD154. In addition, the cells had already up-regulated CD49d, asialo GM1, CD11a, and CD95L, and down-regulated their expression of CD62L. A small percentage of these cells also expressed CD25. Day 7 CTLP* sorted on the basis of their CD44(xhi) and CD54(xhi) phenotype did not exhibit cytolytic activity in a standard chromium release assay but became cytotoxic when they were cultured in the presence of exogenous murine IL-2 for 5 days. Granzyme B activity, however, was detected in CTLP* on day 7 at levels equivalent to CTLE on day 10. In order to establish a potential precursor relationship between CTLP* and CTLE, mice were treated with various doses of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), a chemical that has been shown to dose-dependently suppress the in vivo generation of CTLE to P815 tumor cells by altering an early stage of CTLP activation. Results indicated that CTLP* were suppressed by TCDD on day 7 to the same degree that CTLE were suppressed on day 10. Importantly, for controls and for all doses of TCDD, there were approximately 12.5 CTLE on day 10 for every CTLP* detected on day 7. These results suggested that TCDD acted identically across all doses to inhibit the early stages of activation of CTLP but did not affect the final stages of differentiation and expansion to CTLE. This interpretation supports the previous observation that TCDD exposure had to occur within the first 3 days of the allograft response in order to induce suppression of CTLE activity. Taken together, these results support the conclusion that in vivo activated CTLP can be identified by their unique expression of very high levels of CD44, CD28, and/or CD54 prior to their full maturation and clonal expansion to functional CTLE.

Altered cytokine (receptor) mRNA expression as a tool in immunotoxicology

Molecular immunotoxicology is aimed at analysing exposure effects on the temporal expression of important immunoregulatory genes. Cytokines play key roles in the immune system and thus molecular immunotoxicology has focused on the analysis of cytokine (expression) levels. These targets offer important new avenues to explore both in terms of mechanistic understanding of immunotoxicity and in terms of developing new assays and tests for predicting the immunotoxic potential of novel compounds. Effects on cytokine levels can be analysed on two different levels, these being mRNA and protein. The choice essentially depends on the aim of the study. Proteins comprise the biological activity so they are a more direct measure than mRNA. mRNA on the other hand, measures at a specific point in time within a tissue or organ, whereas protein is measured in a body fluid, possibly as a spill-over from tissue, or in a supernatant as a summation over a culture period. mRNA levels are assayed using Northern or dot blotting that both comprise hybridisation and using reverse transcription-polymerase chain reaction (RT-PCR). Although the latter technique has both enormous sensitivity and relative ease of operation as important advantages, it requires much more effort in terms of quantitation. References to the nucleic acid sequences of human, murine, and rat cytokines and their receptors are presented (with accession numbers). Examples in which molecular techniques were successfully employed to assess immunotoxicity and (in some cases) understand mechanisms of action are also presented.

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.