Effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) treatment in vivo on thymocyte functions in mice after activation in vitro

Effect of 2,3,7,8-Tetrachloro-di-benzo-p-dioxin on T Cell Subpopulations in the Thymus and Spleen of Mice with Chronic Toxoplasma gondii Infection

In the current study, the effect of exposure to the environmental pollutant, 2,3,7,8-tetrachloro-di-benzo- p-dioxin (TCDD), on mice having chronic infection with Toxoplasma gondii was investigated. For this purpose, four groups of mice were used—mice treated with vehicle, mice treated with TCDD alone, mice infected with T. gondii alone, and mice receiving a combination of TCDD treatment and T. gondii infection. Histological examination and tissue cyst enumeration were performed to indicate the level of infection of the brain. The immune status was studied by enumerating the cellularity as well as the percentages and absolute numbers of the lymphocyte subsets based on the expression of CD4 and CD8 markers in the thymus and spleen. Our studies demonstrated that there was a significant decrease in the total number of thymocytes in TCDD-treated mice that were either uninfected or infected with T. gondii when compared to vehicle controls. However, there was no significant difference observed in thymic cellularity in mice that were infected with T. gondii alone when compared to the uninfected vehicle controls. In addition, the ratio and the total numbers of CD4+, CD8+, CD4–CD8–(double negative, DN) and CD4+CD8+ (double positive, DP) T cell subsets in the thymus from various groups were determined. There was no change in the percentages of T cell subsets in TCDD-treated mice or T. gondii-infected mice when compared to the vehicle controls. However, there was a decrease in the percentage of DPT cells and an increase in the DN and CD8+ T cells in mice that received a combination of TCDD-treatment and T. gondii infection when compared to mice receiving the vehicle or TCDD-treatment alone or infection with T. gondii alone. There was also a decrease in the absolute numbers of the DP and CD4+ T cells and an increase in the CD8+ T cells in the thymus of mice receiving the combination of TCDD-treatment and T. gondii infection when compared to vehicle controls. The splenic cellularity as well as the percentage and absolute numbers of the CD4+ and CD8+ T cell subsets and the non-T cells were not altered in all the groups tested. The natural history of T. gondii infection was not altered following TCDD treatment as demonstrated by no significant differences in brain lesion scores and the number of tissue cysts in the brains of these mice.

Primary peripheral T cells become susceptible to 2,3,7,8-tetrachlorodibenzo-p-dioxin-mediated apoptosis in vitro upon activation and in the presence of dendritic cells

Although the toxic effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) on T cells in vivo have been well characterized, attempts to reproduce these findings in vitro have not been successful. In the current study, we examined whether activation or the presence of dendritic cells (DCs) would make primary naive T cells from C57BL/6 mice susceptible to TCDD-induced apoptosis in vitro. Although nonactivated primary T cells cultured with 10 to 1000 nM TCDD were relatively resistant to apoptosis, they became sensitive to apoptosis upon activation with concanavalin A (ConA). Moreover, ConA-activated T cells cultured in the presence of DCs showed highest levels of TCDD-induced apoptosis. Likewise, primary T cells from OT.II.2a mice cultured with specific ovalbumin peptide and syngeneic DCs showed higher levels of apoptosis compared with similar nonactivated T cells. T-cell activation led to up-regulation of aryl hydrocarbon receptor (AhR), Fas, and Fas-ligand (FasL) expression. In addition, DC maturation and culture with TCDD caused significant induction of FasL. TCDD-mediated apoptosis in activated peripheral T cells was AhR-dependent. Analysis of why nonactivated T cells are more resistant, whereas activated T cells are sensitive to TCDD-induced apoptosis revealed that TCDD treatment of activated but not nonactivated T cells led to down-regulation of cellular FLICE inhibitory protein (c-FLIP), an inhibitor of apoptosis. Moreover, down-regulation of c-FLIP using small interfering RNA in nonactivated T cells made them sensitive to TCDD-induced apoptosis. The current study demonstrates for the first time that TCDD can induce apoptosis in vitro in peripheral T cells upon activation and in the presence of DCs and that this may be mediated by down-regulation of c-FLIP.

Detection of a novel population of fetal thymocytes characterized by preferential emigration and a TCRγδ+ T cell fate after dioxin exposure

T cell maturation into TCRalphabeta(+) or TCRgammadelta(+) cells from common immature CD4(-)CD8(-)(DN) precursors occurs in the thymus, and is controlled through ordered regulation of genes. The aryl hydrocarbon receptor (AHR), a latent cytoplasmic transcription factor, affects thymocyte maturation and differentiation at several stages, also including DN cells. We analyzed in murine fetal thymus organ cultures (FTOC) the outcome of AHR-signaling and found a higher frequency of DN TCRgammadelta(+) cells in the presence of the AHR-activating ligand TCDD. We detected a novel population of CD25(int/lo)CD44(hi) cells associated with preferential emigration and a TCRgammadelta(+) T cell fate of thymocytes. Sorted DN TCRgammadelta(+) emigrants could proliferate if IL-2 was available. Moreover, they suppressed the proliferation of co-cultivated, activated CD4(+) T cells. Gene expression profiles of purified DN emigrants from TCDD*FTOC revealed 295 modulated genes, 10% of which are genes of the immune system. For instance, RAG-1, TdT, and Gfi-1 were downregulated, yet genes indicative of mature thymocytes were upregulated. In conclusion, we have detected changes in the differentiation programme of fetal DN thymocytes after ligand-activation of the AHR. In particular, we observed a higher frequency of DN TCRgammadelta(+) cells with high emigration potential, and possible regulatory functions.

Aqueous exposure to Aroclor 1254 modulates the mitogenic response of Atlantic salmon anterior kidney T-cells: indications of short- and long-term immunomodulation

Polychlorinated biphenyls (PCBs) exist as persistent organic pollutants in numerous river systems in the United States. Unfortunately, some of these rivers are sites of active Atlantic salmon restoration programs, and polychlorinated biphenyls have been implicated as ancillary factors contributing to failed salmon restoration. Here, we investigate the immediate and chronic effects of intermediate duration aqueous PCB exposure (1 or 10 microgL-1 Aroclor 1254) on the mitogen-stimulated lymphoproliferative response of Atlantic salmon anterior kidney leukocytes (AKLs). A short-term study was designed to examine immunomodulation in Atlantic salmon smolts immediately following 21 days of aqueous exposure, while a long-term study evaluated chronic impacts in the mitogen response in parr 15 months post-exposure as larvae. The proliferative response of AKLs to the mitogens concanavalin A (CON A), phytohemaglutinnin-P (PHA-P), pokeweed mitogen (PWM), and lipopolysaccharide were used as an indice of immunomodulation. The proliferative response to the T-cell mitogens CON A and PHA-P was significantly increased in the 10 microgL-1 group (n=10; P=0.043 and 0.002, respectively) immediately following exposure of smolts. Additionally, The PHA-P response was significantly increased in the 1 microgL-1 exposure group (n=10, P=0.036). In fish treated as larvae and tested 15 months later, the PHA-P sensitive populations exhibited elevated proliferation in the 1 and 10 microgL-1 groups (n=12, P<0.04) relative to the vehicle control while the PWM response was significantly increased (n=12, P=0.036) only in the 10 microgL-1 treated groups. These results demonstrate an immunomodulatory effect of PCBs on T-cell mitogen sensitive populations of lymphocytes in Atlantic salmon as well as long-term immunomodulation in PHA-P and PWM sensitive populations.

Signaling via the AHR leads to enhanced usage of CD44v10 by murine fetal thymic emigrants: possible role for CD44 in emigration

Signaling via the endogenous arylhydrocarbon receptor (AHR) affects proliferation, differentiation, function and gene expression of thymocytes. In the present study, we show that treatment of mouse fetal thymus lobes in organ culture (FTOC) with AHR ligands results in (a) a drastic decrease in the emigration of thymocytes in terms of numbers and types of cells, and (b) preferential emigration of CD4-CD8- (DN) cells expressing CD44v7- and CD44v10-containing isoforms on the cell surface. Moreover, a higher level of transcripts of various other CD44 variant isoforms (CD44v) could be detected by RT-PCR in emigrants from fetal thymi exposed to either AHR-agonist during culture. Expression of CD44v9-10-containing isoforms could be exclusively detected in DN thymic emigrants. Thus, signaling via AHR by ligands alters CD44v expression patterns in a thymocyte subpopulation. Furthermore, emigration could be decreased by the addition of anti-panCD44 antibodies to TCDD-treated FTOCs, suggesting a role for CD44 in emigration.

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

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

Comparative assessment of metabolic enzyme levels in macrophage populations of the F344 rat

The immune system is a direct target for toxic insult by a number of drugs and other chemicals, many of which require activation to toxic metabolites by drug-metabolizing enzymes. We compared the induction of drug-metabolizing enzymes, including cytochrome P450 1A1 (CYP1A1) and aldehyde dehydrogenase (ALDH), which are differentially expressed in various macrophage populations following treatment of F344 rats with the inducer 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). Kupffer cells, alveolar macrophages and splenic macrophages from TCDD-treated animals expressed elevated levels of inducible CYP1A1 as compared to other macrophage subpopulations or cells from control rats. TCDD treatment also resulted in increased ethoxyresorufin-O-deethylase (EROD) activity and total cytochrome P450 content in tissue-derived macrophages. Immunoreactive protein and mRNA transcripts for CYP1A1 were not detectable in resident peritoneal macrophages or peripheral blood monocytes. Examination of aromatic hydrocarbon receptor (AhR) levels in macrophage populations suggests that the ability of TCDD to induce metabolic enzymes in specific cell types correlates well with AhR expression. In vivo activation of macrophages, using either Bacillus of Calmette and Guérin, Mycobacterium tuberculosis (BCG) or polyinosinic:polycytidylic acid (Poly I:C), caused no significant alteration in the levels of induction of CYP1A1. ALDH-3 induction was similar in all macrophage populations examined. These studies indicate that macrophages, particularly those from portals of entry, may be induced to produce increased levels of specific enzymes, and the induction is dependent upon their maturational stage rather than their activation state. The metabolism of xenobiotics to toxic intermediates by immune cells and its role in immunosuppression are discussed.

Lack of direct immunosuppressive effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) on human peripheral blood lymphocyte subsets in vitro

A wide variety of immunosuppressive effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) in experimental animals has been documented. In contrast, the impact of dioxin on the human immune system remains controversial, although adverse health effects have been reported in humans after occupational or accidental exposure to dioxin. Recently, Neubert et al. (1991) found that a dose-dependent decrease of peripheral blood lymphocyte (PBL) subpopulations in humans and non-human primates, including helper-inducer/memory cells (CD4+CD29+) and B cells (CD20+) occurred in pokeweed mitogen (PWM) stimulated cultures at concentrations as low as 10(-12)-10(-14) M TCDD. Therefore, the direct effects of dioxin on human PBL subpopulations have been studied, in order to determine their usefulness as sensitive biomarkers for human dioxin exposure. Lymphocyte cultures from healthy individuals were treated with 10(-7) M-10(-14) M TCDD in the absence and presence of stimulation with pokeweed mitogen (PWM) or anti-CD3 monoclonal antibody (moAb; OKT3) for 3 days. Cytochrome P450 (CYP1A1) enzyme induction, one of the best studied direct biological effects of TCDD on numerous cell types, was assayed in parallel by ethoxyresorufin-O-deethylase (EROD) activity. Percentages of the different lymphocytes subsets, including CD2 (T cells); CD4; CD45 RA (suppressor-inducer/virgin T cells); CD4 CD29; CD8; CD19 (B cells) as well as interleukin 2 (IL-2) receptor (CD25) and class II antigen (HLA-DR) expression, were analyzed by flow cytometry. DNA synthesis was determined by 3H-thymidine uptake after 3 days of culture.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) inhibits the activation of antigen-specific T-cells in mice

There are conflicting data in the literature regarding target cells in 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)-induced immunotoxicity. In the present study, adult male C57BL/6 mice were exposed to TCDD (50 micrograms/kg) 4 days prior to immunization with ovalbumin (OVA). The effect of TCDD on the specific immune response in vivo was determined by T-cell proliferation and IL-2 production in response to either OVA or anti-mouse-CD3 antibodies plus PMA in vitro. The antigen-specific T-cell proliferation and IL-2 production in response to OVA were significantly suppressed by TCDD, while the polyclonal response to anti-CD3 antibodies plus PMA was not affected. This indicates that even at a high dose of TCDD the intra T-cell signalling pathways in resting cells are not disturbed, but TCDD selectively impairs the antigen-specific activation of T-cells. Since activated T-cells are required in antibody responses to T-dependent antigens, the low number of such cells observed in the present study, may well explain the suppressive effects of TCDD on humoral immunity reported previously.

Reversibility of thymic atrophy induced by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and bis(tri-n-butyltin)oxide (TBTO)

We studied the reversibility of thymic atrophy induced by intubation of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), 10 days after a single dose of 50 micrograms/kg, or bis(tri-n-butyltin)oxide (TBTO), 4 days after a single dose of 75 mg/kg. This was done by an experimental design in which the atrophic thymus was placed in an in vivo situation in which the toxic chemical was no longer present, e.g. by transplantation of atrophic thymic lobes in untreated normal rats with connection to the vasculature of the recipient. At 20 days after the transplantation, the atrophic thymus showed the morphology and architecture of a normal uninvoluted thymus: lymphocyte counts and phenotypic expression of markers on lymphocytes, epithelium, and macrophages in the transplanted lobe did not differ from those in untreated donor rats or those in the normal uninvoluted thymus. Considering the mechanism of action of the toxic chemical, TBTO has been claimed to affect preferentially (passenger) lymphocytes in the thymus: the recovery after transplantation therefore is explained on the mere influx of newly-recruited precursor cells from the bone marrow. For TCDD a toxic action on the stationary epithelial component of the thymus has been claimed. We conclude that this epithelial damage is reversible within the 3-week period of the present experiment, with respect to both the morphology and immunologic phenotype of epithelium and other cell populations, as well as the recruitment of lymphocytes.

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) induced suppression of the local immune response

TCDD suppressed the normal immune response in popliteal and inguinal lymph nodes, when administered i.p. (50 micrograms/kg) to C57BL/6 mice, 4 days before immunization with the T-dependent antigen ovalbumin (10 micrograms/pad) in the hind foot pads. A hampered increase in lymph node cell number and a reduced frequency of antigen-specific B-cells were observed, despite the fact that cell proliferation in vivo was normal. While the restimulation of lymph node cells in vitro with ConA or LPS was normal, suggesting that the APC function was largely unaffected, the OVA-induced proliferation was greatly reduced. The anti-OVA antibody (ab) concentration both in serum and in supernatants of cultured lymph node cells was lower than in controls. In contrast, the production of anti-BSA ab upon LPS stimulation was normal. This indicates that the ability of the B-cells to produce ab and to proliferate was not disturbed. The DTH assay clearly showed an impaired T-cell function in TCDD-treated animals. Since APC or B-cells have appeared normal in their functions tested in this study, we propose that TCDD disturbed T-cell functions, leading to an impaired activation of B-cells.

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

2,3,7,8-tetrachlorodibenzo-p-dioxin, more popularly called dioxin or TCDD and referred to in this review as 2,3,7,8-TCDD, is considered the prototype of the polychlorinated dibenzo-p-dioxins (PCDD). The PCDD are true contaminants and are formed primarily as byproducts in the manufacture of materials requiring the use of chlorinated phenols and during the combustion of chlorinated chemical products. From an environmental perspective, the PCDD have been most closely associated with the use of a number of phenolic herbicides, including Agent Orange, which is a 1:1 mixture of the butyl esters of 2,4-dichlorophenoxyacetic acid (2,4-D) and 2,4,5-trichlorophenoxyacetic acid (2,4,5-T). 2,3,7,8-TCDD and related PCDD are not produced commercially except in small amounts for research purposes and to date, have no known human benefit. 2,3,7,8-TCDD has been demonstrated to be the most potent and the most biologically active congener among the halogenated aromatic hydrocarbons (HAH), which include polychlorinated and polybrominated biphenyls (PCB and PBB, respectively) and the polychlorinated dibenzofurans (PCDF), in addition to the PCDD. An updated review on the effects of 2,3,7,8-TCDD on immunocompetence is timely from a number of perspectives. First, effects on immune function have been demonstrated to be among the earliest and most sensitive indicators of 2,3,7,8-TCDD-induced toxicity. Second, recent evidence indicates that exposure to 2,3,7,8-TCDD causes changes in innate immunity in addition to the changes in acquired immunity (i.e., which include effects on both cell-mediated and humoral immunity) previously shown to be associated with this chemical. Third, effects on immune function are almost universally observed among the animal species in which it has been evaluated, including some non-human primates. Fourth, effects of 2,3,7,8-TCDD on specific indicators of immune function have been correlated with changes in host resistance capabilities, which are often considered to be more holistic indicators of immunocompetence. Fifth, there are several reports which describe possible effects of 2,3,7,8-TCDD and related compounds (i.e., primarily PBB and PCB) on immune function in humans. It is important to emphasize at the onset that these studies have triggered much controversy, both political and scientific. However, it is equally important to speculate that at least part of the controversy associated with man's sensitivity to the immunological effects of 2,3,7,8-TCDD may be that the most appropriate approaches have heretofore not been applied. This possibility is discussed further in this review.(ABSTRACT TRUNCATED AT 400 WORDS)