Direct suppression of antibody responses by chlorinated dibenzodioxins in cultured spleen cells from (C57BL/6 x C3H)F1 and DBA/2 mice

Evolutive emergence and divergence of an Ig regulatory node: An environmental sensor getting cues from the aryl hydrocarbon receptor?

One gene, the immunoglobulin heavy chain (IgH) gene, is responsible for the expression of all the different antibody isotypes. Transcriptional regulation of the IgH gene is complex and involves several regulatory elements including a large element at the 3' end of the IgH gene locus (3'RR). Animal models have demonstrated an essential role of the 3'RR in the ability of B cells to express high affinity antibodies and to express different antibody classes. Additionally, environmental chemicals such as aryl hydrocarbon receptor (AhR) ligands modulate mouse 3'RR activity that mirrors the effects of these chemicals on antibody production and immunocompetence in mouse models. Although first discovered as a mediator of the toxicity induced by the high affinity ligand 2,3,7,8-tetracholordibenzo-p-dioxin (dioxin), understanding of the AhR has expanded to a physiological role in preserving homeostasis and maintaining immunocompetence. We posit that the AhR also plays a role in human antibody production and that the 3'RR is not only an IgH regulatory node but also an environmental sensor receiving signals through intrinsic and extrinsic pathways, including the AhR. This review will 1) highlight the emerging role of the AhR as a key transducer between environmental signals and altered immune function; 2) examine the current state of knowledge regarding IgH gene regulation and the role of the AhR in modulation of Ig production; 3) describe the evolution of the IgH gene that resulted in species and population differences; and 4) explore the evidence supporting the environmental sensing capacity of the 3'RR and the AhR as a transducer of these cues. This review will also underscore the need for studies focused on human models due to the premise that understanding genetic differences in the human population and the signaling pathways that converge at the 3'RR will provide valuable insight into individual sensitivities to environmental factors and antibody-mediated disease conditions, including emerging infections such as SARS-CoV-2.

Role of Programmed Cell Death Protein-1 and Lymphocyte Specific Protein Tyrosine Kinase in the Aryl Hydrocarbon Receptor- Mediated Impairment of the IgM Response in Human CD5+ Innate-Like B Cells

Innate-like B cells (ILBs) are a heterogeneous population B cells which participate in innate and adaptive immune responses. This diverse subset of B cells is characterized by the expression of CD5 and has been shown to secrete high levels of immunoglobulin M (IgM) in the absence of infection or vaccination. Further, CD5+ ILBs have been shown to express high basal levels of lymphocyte specific protein tyrosine kinase (LCK) and programmed cell death protein-1 (PD-1), which are particularly sensitive to stimulation by interferon gamma (IFNγ). Previous studies have demonstrated that activation of the aryl hydrocarbon receptor (AHR), a cytosolic ligand-activated transcription factor, results in suppressed IgM responses and is dependent on LCK. A recent study showed that CD5+ ILBs are particularly sensitive to AHR activation as evidenced by a significant suppression of the IgM response compared to CD5- B cells, which were refractory. Therefore, the objective of this study was to further investigate the role of LCK and PD-1 signaling in AHR-mediated suppression of CD5+ ILBs. In addition, studies were conducted to establish whether IFNγ alters the levels of LCK and PD-1 in CD5+ ILBs. We found that AHR activation led to a significant upregulation of total LCK and PD-1 proteins in CD5+ ILBs, which correlated with suppression of IgM. Interestingly, treatment with recombinant IFNγ reduced LCK protein levels and reversed AHR-mediated IgM suppression in CD5+ ILBs in a similar manner as LCK inhibitors. Collectively, these results support a critical role for LCK and PD-1 in AHR-mediated suppression of the IgM response in human CD5+ ILBs.

Lymphocyte-Specific Protein Tyrosine Kinase (LCK) is Involved in the Aryl Hydrocarbon Receptor-Mediated Impairment of Immunoglobulin Secretion in Human Primary B Cells

The aryl hydrocarbon receptor (AHR) is a cytosolic ligand-activated transcription factor involved in xenobiotic sensing, cell cycle regulation, and cell development. In humans, the activation of AHR by 2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD), a high affinity AHR-ligand, impairs the secretion of immunoglobulin M (IgM) to suppress humoral immunity. However, the mechanisms bridging the activation of AHR and the impairment of IgM secretion by human primary B cells remain poorly understood. Recent transcriptomic analysis revealed upregulation of lymphocyte-specific protein tyrosine kinase (LCK) in AHR-activated human primary B cells. LCK is a well-characterized tyrosine kinase that phosphorylates critical signaling proteins involved in activation and cytokine production in T cells. Conversely, the role of LCK in human primary B cells is not well understood. In the current studies, we have verified the transcriptomic finding by detecting AHR-mediated upregulation of LCK protein in human primary B cells. We also confirmed the role of AHR in the upregulation of LCK by using a specific AHR antagonist, which abolished the AHR-mediated increase of LCK. Furthermore, we have confirmed the role of LCK in the AHR-mediated suppression of IgM by using LCK specific inhibitors, which restored the IgM secretion by human B cells in the presence of TCDD. Collectively, the current studies demonstrate a novel role of LCK in IgM response and provide new insights into the mechanism for AHR-mediated impairment of immunoglobulin secretion by human primary B cells.

Suppression of the IgM Response by Aryl Hydrocarbon Receptor Activation in Human Primary B Cells Involves Impairment of Immunoglobulin Secretory Processes

Aryl hydrocarbon receptor (AHR) activation by 2, 3, 7, 8-tetrachlorodibenzo-p-dioxin (TCDD) is well established at suppressing humoral immunity. Previous studies in mouse B cells revealed that decreased IgM production was due to a significant suppression in the mRNA levels of the immunoglobulin M components (IgH, IgJ, and Igκ chains) and subsequent decrease in IgM synthesis. In contrast, the current study shows that activation of AHR in human B cells also results in a significant suppression of the number of IgM-secreting cells, but this is not due to a decrease in the transcription or translation of IgH, IgJ, and Igκ chains. Instead, the reduced humoral response is due to the impairment of IgM secretion. This is further evidenced by an accumulation of intracellular IgM in human B cells, which indicates that activation of AHR alters distinct regulatory pathways in human and mouse B cells leading to the suppressed primary IgM response. Collectively, these results demonstrate that although AHR activation mediates suppression of humoral immune responses across many different animal species, the mechanism of action is not necessarily conserved across species.

SHP-1 is directly activated by the aryl hydrocarbon receptor and regulates BCL-6 in the presence of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)

The environmental contaminant 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), which is a strong AHR agonist, causes significant suppression of human B cell activation and differentiation. The current studies describe the identification of Src homology phosphatase 1 (SHP-1) encoded by the gene PTPN6 as a putative regulator of TCDD-mediated suppression of B cell activation. Shp-1 was initially identified through a genome-wide analysis of AHR binding in mouse B cells in the presence of TCDD. The binding of AHR to the PTPN6 promoter was further confirmed using electrophoretic mobility shift assays in which, specific binding of AHR was detected at four putative DRE sites within PTPN6 promoter. Time-course measurements performed in human B cells highlighted a significant increase in SHP-1 mRNA and protein levels in the presence of TCDD. The changes in the protein levels of SHP-1 were also observed in a TCDD concentration-dependent manner. The increase in SHP-1 levels was also seen to occur due to a change in early signaling events in the presence of TCDD. We have shown that BCL-6 regulates B cell activation by repressing activation marker CD80 in the presence of TCDD. TCDD-treatment led to a significant increase in the double positive (SHP-1^hi BCL-6^hi) population. Interestingly, treatment of naïve human B cells with SHP-1 inhibitor decreased BCL-6 protein levels suggesting possible regulation of BCL-6 by SHP-1 for the first time. Collectively, these results suggest that SHP-1 is regulated by AHR in the presence of TCDD and may, in part through BCL-6, regulate TCDD-mediated suppression of human B cell activation.

All-or-none suppression of B cell terminal differentiation by environmental contaminant 2,3,7,8-tetrachlorodibenzo-p-dioxin

Many environmental contaminants can disrupt the adaptive immune response. Exposure to the ubiquitous aryl hydrocarbon receptor (AhR) ligand 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and other agonists suppresses the antibody response. The underlying pathway mechanism by which TCDD alters B cell function is not well understood. The present study investigated the mechanism of AhR-mediated pathways and mode of suppression by which TCDD perturbs terminal differentiation of B cells to plasma cells and thereby impairs antibody production. An integrated approach combining computational pathway modeling and in vitro assays with primary mouse B cells activated by lipopolysaccharide was employed. We demonstrated that suppression of the IgM response by TCDD occurs in an all-or-none (binary) rather than graded mode: i.e., it reduces the number of IgM-secreting cells in a concentration-dependent manner without affecting the IgM content in individual plasma cells. The mathematical model of the gene regulatory circuit underpinning B cell differentiation revealed that two previously identified AhR-regulated pathways, inhibition of signaling protein AP-1 and activation of transcription factor Bach2, could account for the all-or-none mode of suppression. Both pathways disrupt the operation of a bistable-switch circuit that contains transcription factors Bcl6, Prdm1, Pax5, and Bach2 and regulates B cell fate. The model further predicted that by transcriptionally activating Bach2, TCDD might delay B cell differentiation and increase the likelihood of isotype switching, thereby altering the antibody repertoire. In conclusion, the present study revealed the mode and specific pathway mechanisms by which the environmental immunosuppressant TCDD suppresses B cell differentiation.

Regulation of Bach2 by the aryl hydrocarbon receptor as a mechanism for suppression of B-cell differentiation by 2,3,7,8-tetrachlorodibenzo-p-dioxin

Exposure to the aryl hydrocarbon receptor (AHR) agonist, 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) alters B-cell differentiation and suppresses antibody production. Previous genomic studies in mouse B cells identified Bach2 as a direct target of the AHR. Bach2 is known to repress expression of Prdm1, a key transcription factor involved in B-cell differentiation, by binding to Maf elements (MAREs) in the regulatory regions of the gene. Chromatin immunoprecipitation followed by quantitative PCR in TCDD-treated lipopolysaccharide (LPS)-activated B cells showed increased binding of the AHR within the first intron in the Bach2 gene. The binding was further confirmed by electrophoretic mobility shift assay (EMSA). TCDD also induced expression of Bach2 in activated as well as resting B cells from 2 to 24h post-treatment in a time- and concentration-dependent manner. Expression of Prdm1 was decreased by TCDD at 24h and was consistent with repression by Bach2. Increased DNA binding activity to the intron 5 MARE with increasing TCDD concentrations was observed by EMSA. Supershifts identified the presence of Bach2 in the DNA binding complex associated with the intron 5 MARE of Prdm1. Functional validation of the role of Bach2 in the suppression of B-cell differentiation by TCDD was performed using RNA interference (RNAi). Knockdown of Bach2 showed approximately 40% reversal in the TCDD-induced suppression of IgM secretion when compared to controls. The results suggest that the transcriptional regulation of Bach2 by the AHR is one of the mechanisms involved in the suppression of B-cell differentiation by TCDD.

The long winding road toward understanding the molecular mechanisms for B-cell suppression by 2,3,7,8-tetrachlorodibenzo-p-dioxin

Suppression of humoral immune responses by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) was first reported in the mid-1970s. Since this initial observation, much effort has been devoted by many laboratories toward elucidation of the cellular and molecular mechanisms responsible for the profound impairment of humoral immune responses by TCDD, which is characterized by decreased B cell to plasma cell differentiation and suppression of immunoglobulin production. These efforts have led to a significant body of research demonstrating a direct effect of TCDD on B-cell maturation and function as well as a requisite but as yet undefined role of the aryl hydrocarbon receptor (AhR) in these effects. Likewise, a number of molecular targets putatively involved in mediating B-cell dysfunction by TCDD, and other AhR ligands, have been identified. However, our current understanding has primarily relied on findings from mouse models, and the translation of this knowledge to effects on human B cells and humoral immunity in humans is less clear. Therefore, a current challenge is to determine how TCDD and the AhR affect human B cells. Efforts have been made in this direction but continued progress in developing adequate human models is needed. An in-depth discussion of these advances and limitations in elucidating the cellular and molecular mechanisms putatively involved in the suppression of B-cell function by TCDD as well as the implications on human diseases associated in epidemiological studies with exposure to TCDD and dioxin-like compounds is the primary focus of this review.

An integrated genomic analysis of aryl hydrocarbon receptor-mediated inhibition of B-cell differentiation

The aryl hydrocarbon receptor (AHR) agonist 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) alters differentiation of B cells and suppresses antibody production. A combination of whole-genome, microarray-based chromatin immunoprecipitation (ChIP-on-chip), and time course gene expression microarray analysis was performed on the mouse B-cell line CH12.LX following exposure to lipopolysaccharide (LPS) or LPS and TCDD to identify the primary and downstream transcriptional elements of B-cell differentiation that are altered by the AHR. ChIP-on-chip analysis identified 1893 regions with a significant increase in AHR binding with TCDD treatment. Transcription factor binding site analysis on the ChIP-on-chip data showed enrichment in AHR response elements. Other transcription factors showed significant coenrichment with AHR response elements. When ChIP-on-chip regions were compared with gene expression changes at the early time points, 78 genes were identified as potential direct targets of the AHR. AHR binding and expression changes were confirmed for a subset of genes in primary mouse B cells. Network analysis examining connections between the 78 potential AHR target genes and three transcription factors known to regulate B-cell differentiation indicated multiple paths for potential regulation by the AHR. Enrichment analysis on the differentially expressed genes at each time point evaluated the downstream impact of AHR-regulated gene expression changes on B-cell-related processes. AHR-mediated impairment of B-cell differentiation occurred at multiple nodes of the B-cell differentiation network and potentially through multiple mechanisms including direct cis-acting effects on key regulators of B-cell differentiation, indirect regulation of B-cell differentiation-related pathways, and transcriptional coregulation of target genes by AHR and other transcription factors.

Induction of the aryl hydrocarbon receptor-responsive genes and modulation of the immunoglobulin M response by 2,3,7,8-tetrachlorodibenzo-p-dioxin in primary human B cells

Past studies in rodent models identified the suppression of primary humoral immune responses as one of the most sensitive sequela associated with 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) exposure. Yet, the sensitivity of humoral immunity to TCDD in humans represents an important toxicological data gap. Therefore, the objectives of this investigation were two-fold. The first was to assess the induction of known aryl hydrocarbon receptor (AHR)-responsive genes in primary human B cells as a measure of early biological responses to TCDD. The second was to evaluate the direct effect of TCDD on CD40 ligand-induced immunoglobulin M (IgM) secretion by human primary B cells. The effects of TCDD on induction of AHR-responsive genes and suppression of the IgM response were also compared with B cells from a TCDD-responsive mouse strain, C57BL/6. AHR-responsive genes in human B cells exhibited slower kinetics and reduced magnitude of induction by TCDD when compared with mouse B cells. Evaluation of B-cell function from 12 donors identified two general phenotypes; the majority of donors exhibited similar sensitivity to suppression by TCDD of the IgM response as mouse B cells, which was not attributable to decreased B-cell proliferation. In a minority of donors, no suppression of the IgM response by TCDD was observed. Although donor-to-donor variation in sensitivity to TCDD was observed, human B cells from the majority of donors evaluated showed impairment of effector function by TCDD. Collectively, data presented in this series of studies demonstrate that TCDD impairs the humoral immunity of humans by directly targeting B cells.

2,3,7,8-tetrachlorodibenzo-p-dioxin-mediated suppression of toll-like receptor stimulated B-lymphocyte activation and initiation of plasmacytic differentiation

2,3,7,8-Tetrachlordibenzo-p-dioxin (TCDD) is a potent suppressor of humoral immunity, disrupting antibody production in response to both T cell-dependent and T cell-independent antigens. Among the cell types required for humoral responses, the B cell is highly, and directly, sensitive to TCDD. B cells become antibody-secreting cells via plasmacytic differentiation, a process regulated by several transcription factors, including activator protein-1, B-cell CLL/lymphoma 6 (BCL-6), and B lymphocyte-induced maturation protein 1 (Blimp-1). The overarching conceptual framework guiding experimentation is that TCDD disrupts plasmacytic differentiation by altering the expression or activity for upstream regulators of Blimp-1. Multiparametric flow cytometry was used to investigate TCDD-induced alterations in both activation marker and transcription factor expression following lipopolysaccharide (LPS) activation of purified B cells. TCDD significantly impaired LPS-activated expression of major histocompatibility complex class II, cluster of differentiation (CD)69, CD80, and CD86. Immunosuppressive concentrations of TCDD also suppressed LPS-activated Blimp-1 and phosphorylated c-Jun expression, whereas elevating BCL-6 expression. Because BCL-6 and c-Jun are directly and indirectly regulated by the kinases AKT, extracellular signal-regulated kinase (ERK), and Jun N-terminal kinase (JNK), it was hypothesized that TCDD alters toll-like receptor-activated kinase phosphorylation. TCDD at 0.03 and 0.3 nM significantly impaired phosphorylation of AKT, ERK, and JNK in CH12.LX B cells activated with LPS, CpG oligonucleotides, or resiquimod (R848). In primary B cells, R848-activated phosphorylation of AKT, ERK, and JNK was also impaired by TCDD at 30 nM. These results suggest that impairment of plasmacytic differentiation by TCDD involves altered transcription factor expression, in part, by suppressed kinase phosphorylation.

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.

Involvement of Blimp-1 and AP-1 dysregulation in the 2,3,7,8-Tetrachlorodibenzo-p-dioxin-mediated suppression of the IgM response by B cells

B cell differentiation and humoral immune responses are markedly suppressed by the persistent environmental contaminant, 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). The suppression of humoral immune responses by TCDD occurs by direct actions on the B cell and involves activation of the aryl hydrocarbon receptor. Transcriptional regulation of paired box gene 5 (Pax5), an important regulator of B cell differentiation, is altered by TCDD in concordance with the suppression of B cell differentiation and humoral immunoglobulin M response. We hypothesized that TCDD treatment leads to dysregulation of Pax5 transcription by interfering with the basic B cell differentiation mechanisms and aimed to determine the effects of TCDD on upstream regulators of Pax5. A critical regulator of B cell differentiation, B lymphocyte-induced maturation protein-1 (Blimp-1) acts as a transcriptional repressor of Pax5. In lipopolysaccharide (LPS)-activated murine B cell lymphoma, CH12.LX, Blimp-1 messenger RNA, and DNA-binding activity within the Pax5 promoter were suppressed by TCDD. Furthermore, LPS activation of CH12.LX cells upregulated DNA-binding activity of activator protein 1 (AP-1) at three responsive element-like motifs within the Blimp-1 promoter. TCDD treatment of LPS-activated CH12.LX cells suppressed AP-1 binding to these motifs between 24 and 72 h, in concordance with the suppression of Blimp-1 by TCDD. A more comprehensive analysis at 72 h demonstrated that the suppression of AP-1 binding within the Blimp-1 promoter by TCDD was concentration dependent. In summary, our findings link the TCDD-mediated suppression of Blimp-1 through AP-1 to the dysregulation of Pax5, which ultimately leads to the suppression of B cell differentiation and humoral immune responses.

2,3,7,8-Tetrachlorodibenzo-p-dioxin-mediated impairment of B cell differentiation involves dysregulation of paired box 5 (Pax5) isoform, Pax5a

The persistent environmental contaminant and immunotoxicant, 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), markedly suppresses humoral immune responses. We recently reported impaired down-regulation of paired box 5 (Pax5), a repressor of B cell differentiation and concomitant suppression of the IgM response by TCDD in the murine CH12.LX B cell line. The objectives of the current study were to determine the impact of TCDD treatment on molecular outcomes characteristic of terminal B cell differentiation and to assess the role that Pax5 isoforms plays in the suppression of B cell differentiation by TCDD. In this study, we show that the highly abundant full-length Pax5 isoform, Pax5a, and at least two additional modestly expressed Pax5 isoforms were expressed in CH12.LX and splenic B cells. In lipopolysaccharide (LPS)-activated B cells, all of the identified Pax5 isoforms were synchronously down-regulated, and in the presence of TCDD cotreatment they were abnormally and synchronously elevated, suggesting a common mechanism of regulation. Furthermore, B cell differentiation markers X-box protein-1 and major histocompatibility complex class II showed that the levels to which Pax5 was derepressed by TCDD were sufficient to impair B cell differentiation and immunoglobulin gene expression. Confirming the involvement of Pax5, ectopic expression of Pax5a in the LPS-activated CH12.LX cells closely mimicked the suppression of the IgM response by TCDD. In summary, our results demonstrate that Pax5a has a critical role in both the TCDD-mediated impairment of B cell differentiation and the suppression of the humoral immune response.

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.

Effects of the ethanol extract of Cichorium intybus on the immunotoxicity by ethanol in mice

Effects of the ethanol extract of Cichorium intybus (CIEE) on the immunotoxicity of ethanol (EtOH) were investigated in ICR mice. Mice were divided into four groups, and CIEE at dose of 300 mg/kg was orally administered to mice daily for 28 consecutive days, and normal mice were given vehicle. Mice treated with EtOH were given freely with 20% w/v EtOH solution. The results of this study are summarized as follows: The combination of CIEE and EtOH showed significant increases in the circulating leukocytes and the relative weights of liver, spleen and thymus, as compared with those in mice treated with EtOH alone. However, the body weight gain was not affected. Splenic plaque forming cells (PFC) and hemagglutination (HA) titers to sheep red blood cells (SRBC), and the secondary IgG antibody response to bovine serum albumin (BSA) were markedly enhanced by CIEE plus EtOH treatment as compared with the treatment of EtOH alone. In mice receiving the combination of CIEE and EtOH when compared with EtOH alone-treated mice, there were also significant increases in delayed-type hypersensitivity (DTH) reaction, phagocytic activity, natural killer (NK) cell activity and cell proliferation as well as interferony (IFN-gamma) secretion. In the case of interleukin-4 (IL-4) content, however, an insignificant induction observed by CIEE plus EtOH treatment. These findings indicate that the immunotoxicity induced by EtOH is significantly restored or prevented by CIEE treatment.

Potentiation of the immunotoxicity of ethanol by acetaminophen in mice

To investigate the influences of acetaminophen (APP) on the immunotoxicity of ethanol (EtOH) in ICR mice, APP at a dose of 100 mg/kg was orally administered to mice daily for 28 consecutive days. Mice treated with EtOH were given freely with 20% w/v EtOH during the experimental period, and normal mice were given vehicle. The results of this study are summarized as follows: the combination of APP and EtOH significantly decreased the circulating leukocytes and the relative weights of liver, spleen and thymus, compared with the treatment of EtOH alone. In mice receiving the combination of AAP and EtOH when compared with the treatment of EtOH alone, there were also significant reductions in the splenic plaque forming cells (PFC) and hemagglutination (HA) titers to sheep red blood cells (SRBC), and the secondary IgG antibody response to bovine serum albumin (BSA). A tendency toward suppression of delayed-type hypersensitivity (DTH) reaction and phagocytic activity was also observed in the combination of AAP and EtOH. In addition, the combination of AAP and EtOH greatly increased serum alanine aminotransaminase (ALT) and total protein levels, compared with the treatment of EtOH alone. Significant decreases in serum albumin and A/G ratio were observed in EtOH alone-fed mice compared with those in normal animals, and their reductions were further induced in mice treated with AAP and EtOH. These findings indicate that EtOH-induced immunotoxicity is aggravated by the combination of APP and EtOH.

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.

Effect of biphenyl dimethyl dicarboxylate on the humoral immunosuppression by ethanol

The present study was undertaken to investigate the effect of biphenyl dimethyl dicarboxylate (PMC) on the humoral immunosuppression by ethanol (EtOH) in ICR mice. PMC at a dose of 6 mg/kg was orally administered to mice daily for 28 consecutive days, and the control mice were given vehicle. Mice treated with EtOH were given freely with 20% EtOH instead of water. The results of this study are summarized as follows; a gain of body weight and the relative weights of spleen and liver were significantly increased by combination of PMC and EtOH, as compared with those in mice treated with EtOH alone. Splenic plaque forming cells (PFC) and hemagglutination (HA) titers to sheep red blood cells (SRBC), and the secondary IgG antibody response to bovine serum albumin (BSA) were decreased by the treatment of EtOH alone, then restored to normal level by PMC treatment. The elevations of serum glutamic-pyruvic transaminase (S-GPT) and total protein levels caused by EtOH were reduced to normal level by the combination of PMC and EtOH. In addition, lower serum albumin and A/G ratio were also increased to normal level. These findings indicate that PMC has a protective effect against EtOH-induced humoral immunosuppression.

Influence of melatonin on immunotoxicity of lead

The results suggested that immunotoxicity induced by lead [Pb, as Pb(NO(3))(2)] was significantly restored or prevented by melatonin (MLT). MLT (10 or 50 mg/kg) was orally administered to ICR mice daily for 28 days, and Pb was also administered at 35 mg/kg in the same way 2 h after the administration of MLT, and the normal mice were given vehicle. Within the Pb plus MLT-treated group, the body weight gains and the relative thymus weights were significantly increased when compared with the treatment of Pb alone. The relative spleen and liver weights were increased by the treatment of Pb alone, and then restored to normal value by MLT treatment. Hemagglutination (HA) titer, plaque-forming cell response to sheep red blood cell (SRBC), and secondary IgG antibody response to BSA were significantly enhanced in the Pb plus MLT-treated mice, as opposed to when compared with the treatment of Pb alone. The mitogenic response of splenic T cell to concanavalin A and that of B cells to lipopolysaccharide was remarkably increased by MLT treatment when compared with treatment of Pb alone. Splenic CD4(+)cells were significantly increased by MLT treatment when compared with treatment of Pb alone. In case of CD8(+) cells, the slight enhancement was observed in MLT treatment. Splenic T and B cells were significantly increased by MLT treatment when compared with the treatment of Pb alone. The natural killer cell, phagocytic activity and the number of peripheral leukocytes were significantly enhanced in Pb plus MLT-treated mice when compared with the treatment of Pb alone.

Influence of melatonin on immunotoxicity of cadmium

The results suggested that immunotoxicity induced by Cd was significantly restored or prevented by MLT. MLT (10 or 50 mg/kg) was orally administered to ICR mice daily for 28 consecutive days, and cadmium (Cd, as [Cd(AC)(2)]) was also administered at 25 mg/kg by the same route 2 h after the administration of MLT, and the normal mice were given vehicle. Within the Cd plus MLT-treated group, the body weight gains and relative thymus weights were significantly increased when compared with the treatment of Cd alone. The relative spleen and liver weights were increased by treatment of Cd alone, then restored to normal value by MLT treatment. Hemagglutination (HA) titer, primary IgM antibody response to SRBC, and secondary IgG antibody response to BSA was significantly increased with the Cd plus MLT-treated mice, as opposed to when compared with treatment of Cd alone. The NK cell and phagocytic activity used for evaluation of non-specific immunocompetence was significantly increased in Cd plus MLT-treated mice when compared with the treatment of Cd alone. The number of peripheral leukocytes was significantly increased in Cd plus MLT-treated mice when compared with treatment of Cd alone.

Effect of a single or repeated dose of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) on T cell subpopulations in the Long-Evans rat

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) exposure results in adverse effects on the immune system of experimental animals. The purpose of this study was to compare the effects of a single and repeated dosing of TCDD on splenic T-cell subpopulations in Long Evans rats 9 days post-exposure to TCDD. A single dose (25 microg/kg body weight) of TCDD resulted in reduced body weight. The percentage and total number of CD4+ or CD8+ subsets and percentage of CD4+ or CD8+ cell cycling in the S and G2M phases were similar in the single dosed (25 microg/kg body weight) TCDD group compared with the vehicle control. A repeated dose (5 microg/kg/day for 5 days) of TCDD also resulted in a significant reduction in body weight. However, multiple doses of TCDD significantly decreased the percentage of the CD4+ subset and the percentage of CD4+ cells cycling in the S and G2M phases. No significant change occurred in the CD8+ cell subpopulation after single or multiple dosing with TCDD. These results demonstrated that repeated dosing of TCDD decreased the total percentage of CD4+ cells and the percentage of CD4+ cells cycling 9 days post-exposure, while an analogous single dose of TCDD failed to affect the CD4+ cell subpopulation. The difference in biological responses to a single versus 'equivalent' multiple (cumulative) dose of TCDD is discussed.

Ahr null alleles: distinctive or different?

Two independent laboratories have generated Ahr "null" or "knockout" mice that share some common characteristics but also have distinct phenotypes. In this Commentary, we will discuss our view of the candidate variables that might account for these differences. More importantly, we hope that this discussion can identify important parameters to be assessed by investigators in the process of characterizing their own modified loci. The variables that we have considered include the possibility that different targeting strategies can result in altered products with unsuspected function or that the targeting event itself can alter the function of neighboring genes. Further, genetic background can have an important influence on phenotype, and differences in genome can be introduced during derivation by the type of embryonic stem cells used and by the random segregation of parental genes in the F2 generation of line propagation. In addition, phenotype may be acutely sensitive to environmental variables, such as pathogen and chemical exposure and stress introduced by crowding and disease. Finally, we discuss approaches to resolving differences between null mice and propose a partial solution, the institution of a repository for detailed information on targeted alleles that may not typically be allowed in today's "fast paced" scientific publications.

Ah receptor signaling pathways

The aryl hydrocarbon (Ah) receptor has occupied the attention of toxicologists for over two decades. Interest arose from the early observation that this soluble protein played key roles in the adaptive metabolic response to polycyclic aromatic hydrocarbons and in the toxic mechanism of halogenated dioxins and dibenzofurans. More recent investigations have provided a fairly clear picture of the primary adaptive signaling pathway, from agonist binding to the transcriptional activation of genes involved in the metabolism of xenobiotics. Structure-activity studies have provided an understanding of the pharmacology of this receptor; recombinant DNA approaches have identified the enhancer sequences through which this factor regulates gene expression; and functional analysis of cloned cDNAs has allowed the characterization of the major signaling components in this pathway. Our objective is to review the Ah receptor's role in regulation of xenobiotic metabolism and use this model as a framework for understanding the less well-characterized mechanism of dioxin toxicity. In addition, it is hoped that this information can serve as a model for future efforts to understand an emerging superfamily of related signaling pathways that control biological responses to an array of environmental stimuli.

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

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

Immunotoxic effects of prolonged dietary exposure of male rats to 2,3,7,8-tetrachlorodibenzo-p-dioxin

The effects of low level exposure of rats to 2,3,7,8-tetrachlorodibenzo-p- dioxin (TCDD) on their immune system was investigated Dietary administration to young adult male Leeds strain rats of a total dose of 3 micrograms/kg body weight of TCDD resulted in an exposure duration-dependent reduction of in vitro lipopolysaccharide-induced production of interleukin (IL)-1 in cultures of their splenic macrophages. A 30-day exposure produced approximately 30% suppression and 180-day exposure produced approximately 52% suppression. This reduction did not negatively influence lipopolysaccharide- induced proliferation of B cells, instead an enhancement of B cell proliferation was observed after 30 days exposure. A 180 day exposure significantly suppressed the generation of IL-2 by either concanavalin A or phorbol myristate acetate/calcium ionophore stimulation, and reduced the lectin-induced proliferation of splenic T cells. The 30-day TCDD exposure showed no such immunotoxicity. TCDD at both exposure durations suppressed the expression of the alpha chain of the IL-2 receptor in concanavalin A-activated T cells, without affecting the CD4+/CD8+ ratio. The results suggest that exposure to a low dietary dose of TCDD suppresses the functions of several T cell subsets, some of the immunotoxic effects being produced early, while others require a longer exposure also down-regulates the IL-1 production function of macrophages. A common mechanism of TCDD immunotoxicity may be on the multifunctional signal transduction pathways downstream to the activation of protein kinase C and Ca2+ flux.

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.

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)

Serum modulation of the effects of TCDD on the in vitro antibody response and on enzyme induction in primary hepatocytes

We have recently reported that the effects of TCDD on the in vitro antibody response can vary considerably depending on the serum conditions used in the culture media. To further investigate this phenomenon, studies were performed to compare the effects of TCDD on both splenocyte antibody responses and P450 enzyme induction (EROD) in primary hepatocytes (HPTC) derived from B6C3F1 and DBA/2 mice when evaluated in the presence of either fetal bovine serum (FBS), newborn calf serum (NBCS) or normal mouse serum (NMS). The latter studies with NMS also included crossovers where splenocytes and HPTC from B6C3F1 mice were cultured in the presence of DBA/2 serum and vice versa. Results with NBCS showed comparable suppression of antibody responses by TCDD in splenocytes from B6C3F1 and DBA/2 mice where we detected IC50 values of 3.0 and 2.8 nM, respectively. In contrast, responses in the presence of NMS showed an Ah-dependency that was characterized by a dose-related suppression of antibody responses by B6C3F1 splenocytes, but a lack of suppression in the responses by DBA/2 splenocytes. Distribution studies with radiolabelled TCDD indicated that the observed profile of activity could not be attributed to a differential uptake of the chemical into splenocytes from B6C3F1 or DBA/2 under the various serum conditions. Serum was also found to modulate the TCDD-induced EROD activity in primary HPTC and the profile of activity was identical to the effects of TCDD on in vitro antibody responses. We observed an enhanced induction of EROD in the presence of NBCS (immunosuppressive conditions) and a lower induction in the presence of FBS (non-immunosuppressive conditions), each giving the same relative magnitude of induction regardless of the mouse strain used as the source of HPTC. In contrast, induction in the presence of NMS showed an Ah-dependency and resulted in a dose-related enhancement in EROD activity in B6C3F1 HPTC but decreased activity in the DBA/2 HPTC. Cross-over studies further showed that the pattern of effects on both splenocytes and HPTC was not altered by changing the strain of mouse used as the source of serum, where each gave equivalent results. These findings demonstrate that the Ah-dependency for the effects of TCDD on both the in vitro antibody response and P450 enzyme induction are modulated by the serum environment in which the cells are exposed. The studies with NMS indicate that it is the genotype of the lymphocyte (i.e., or the HPTC), and not the strain-specific hormone environment, which confers sensitivity to TCDD.(ABSTRACT TRUNCATED AT 400 WORDS)

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.

Direct effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) on responses to lipopolysaccharide (LPS) by isolated murine B-cells

Previous results from this laboratory have indicated that the B-cell is the primary cell target responsible for the suppression of humoral immunity by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). While other laboratories have confirmed that the B-cell is affected, there is not a clear understanding of TCDD's relative effects on B-cell proliferation and differentiation. In the present study, we isolated B-cells from whole spleen cell suspensions from female B6C3F1 mice and further separated them according to density on a percoll density step gradient. Cell cycle analysis by propidium iodide indicated that both high (i.e., 1.092-1.079 density interface) and low (i.e., 1.079-1.070 density interface) density B-cells were predominantly cells in the G0/G1 phase of the cell cycle. Based on acridine orange staining, low-density B-cells exhibited slightly higher RNA levels than high-density B-cells indicating they are an "activated" population of cells, probably somewhere in G1A. Confirmation of these results was the observation that the high density, small, resting B-cells had negligible background proliferation and immunoglobulin secretion, while the low density, larger, activated B-cells had marked background proliferative and antibody responses. Direct addition of TCDD (0.3-30.0 nM) produced a significant, dose-related and comparable suppression of both parameters in the low density B-cells. Similar results were obtained when low density B-cells were stimulated with LPS and exposed to TCDD. In contrast, neither the proliferation nor the antibody response by high density B-cells stimulated with LPS were affected by TCDD.(ABSTRACT TRUNCATED AT 250 WORDS)

The role of chemical-induced stress responses in immunosuppression: a review of quantitative associations and cause-effect relationships between chemical-induced stress responses and immunosuppression

Although there is an increasing awareness that drugs and chemicals can modulate the immune system by indirect mechanisms, few compounds have been thoroughly evaluated in this regard. Several environmentally relevant chemicals induce stresslike responses, as indicated by elevated glucocorticoid levels. Comparable glucocorticoid levels induced by physical or psychological stressors are consistently associated with suppression of one or more immunological parameters. Thus, it seems likely that stress-related neuroendocrine mechanisms are important in immunosuppression by some environmental chemicals. Distinguishing direct and indirect (stress-related) mechanisms of immunosuppression is generally possible, and this could be done as a routine part of immunotoxicity assessment. Although it is clear that glucocorticoids can contribute to such immunosuppression, it is also clear that several other neuroendocrine mediators associated with stress responses can be immunomodulatory. Thus, correlation between glucocorticoid levels and immunosuppression does not conclusively demonstrate a cause-effect relationship. Demonstrating such relationships has been difficult, but it has been done in a few cases of drug-induced thymic hypoplasia by monitoring several parameters known to be affected by glucocorticoids and by measuring the ability of a glucocorticoid antagonist (RU 486) or adrenalectomy to block changes in these parameters. A similar strategy might be useful for evaluation of the role of glucocorticoids in drug- or chemical-induced suppression of a variety of immune functions, but the effects of RU 486 on neuroendocrine feedback circuits and the possibility of consequent immunological changes must be considered when the data are interpreted. This approach could also be applied to evaluation of the roles in chemical-induced immunosuppression of other neuroendocrine mediators for which antagonists or agents that block the synthesis or release of the mediator are available. However, it is likely that a comprehensive (and perhaps predictive) understanding of the relationship between chemically induced stress responses and immunosuppression will require more detailed and quantitative elucidation of the mechanisms and regulation of neuroendocrine-immune interactions.

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.

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.

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.

Effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) on humoral immunity: I. Similarities to Staphylococcus aureus Cowan Strain I (SAC) in the in vitro T-dependent antibody response

We have determined that suppression of the in vitro T-dependent humoral immune response by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) is dependent on the type and concentration of serum used in the culture media. Only five out of 23 commercial lots of serum (screened at both 5 and 10%) could support a suppression in the presence of 30 nM TCDD, with the remaining lots demonstrating an apparent 'protective-like' effect against the TCDD exposure. When log dose response curves were established with TCDD (0.3, 3.0, and 30 nM) in media containing each of the serum lots supporting a suppression (at both 5 and 10%), we determined that only three lots could support a full dose-responsive suppression. Subsequently, in a comparison study between the effects of TCDD and the polyclonal B-cell activator Staphylococcus aureus Cowan Strain I (SAC) on the in vitro T-dependent humoral immune response, we have found that SAC suppresses the antibody response to SRBC and demonstrates the same serum dependency for this effect as was previously noted for TCDD. Under serum-free culturing conditions, TCDD (30 nM) caused a 15-fold increase in the AFC response to SRBCs over controls, suggesting that direct addition of TCDD to whole splenocyte cultures in the absence of serum-derived growth factors results in an increase in B-cell activation. Likewise, under serum-free conditions, SAC dose-dependently increased the AFC response over media controls, and at doses which achieved the same degree of suppression of the humoral response aa TCDD. Taken together, these studies suggest that TCDD has actions that are similar to a T cell independent polyclonal B cell activator such as SAC, and selectively acts on the B cell to suppress the T-dependent humoral immune response by a mechanism which is unique to this series of compounds. This effect however, is only detectable under appropriate serum-supported (or serum-deficient) culture conditions as described.

Effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) on humoral immunity: II. B cell activation

Previous reports have indicated similarities in the actions of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and the polyclonal B cell activator Staphylococcus aureus Cowan Strain I (SAC) on the in vitro T-dependent antibody response to SRBC. This finding has suggested that B cell activation is likely to be an important component in the direct effects of TCDD on lymphocyte function. In the current investigation, various techniques were employed to determine whether TCDD could cause the activation of splenic-derived dense resting B cells in the absence of antigen and to determine if the modulatory effect of serum-derived growth factors is the result of a direct action on the B lymphocyte. TCDD (30 and 60 nM) caused an increase in proliferation of dense resting B cells at both 72 and 96 h following addition. This action of TCDD was demonstrated to possess a serum dependency that was based on the lot of serum in which the cells were cultured. Under similar conditions, TCDD (30 nM) stimulated an increase in total IgM secretion as measured on day 7 of culture. A similar profile of activity was observed in vivo, where splenic-derived dense resting B cells from animals treated with 1 microgram/kg TCDD for 5 days, but unsensitized to SRBC, demonstrated a 10-fold increase in proliferation on day 3 of culture which likewise occurred in a serum dependent manner. In addition, we observed that mice treated with 1 microgram/kg TCDD for 5 days and sensitized with SRBCs, sustained a complete loss in their splenic-derived dense B cell populations (day 4 after sensitization). The loss of this B cell population is indicative of a movement of these cells into a blastogenic state of activation and is not observed in matched corn oil-treated controls. These findings support our previous observations and give evidence that TCDD is capable of causing the direct activation of resting B cells. This activation is dependent on the type of serum present during in vitro culture and appears to be intensified in the presence of antigen in vivo.

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.

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)

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.

Elucidation of cellular targets responsible for tetrachlorodibenzo-p-dioxin (TCDD)-induced suppression of antibody responses: II. The role of the T-lymphocyte

Various immunological assays have been applied by our laboratory in an attempt to assess the role of the T-cell in the TCDD-induced suppression of the antibody response by murine B6C3F1 splenic lymphocytes. Animals were treated in vivo (via gavage) with 1.0 microgram/kg TCDD in corn oil for 5 days before in vitro analysis of splenocyte immunocompetence and T-cell function. To study the effects on T-helper cell function, alterations in the proliferative responses of T-cells following TCDD exposure were investigated. Results show no significant difference in [3H]thymidine uptake between vehicle- and TCDD-treated whole splenocytes 24 h after in vitro stimulation with the T-cell mitogen Con A. This is consistent with the finding that IL-2 production at either 24 or 48 h after Con A stimulation of TCDD-treated lymphocytes was not significantly different from that of vehicle-treated controls. The possibility of the induction of a suppressor T-cell by TCDD was also investigated. Titration of T-cells from TCDD-treated mice into naive splenocyte cultures did not suppress the humoral response to either a T-dependent (SRBC) or a T-independent (DNP-Ficoll) antigen. In contrast, titration of cells stimulated in vitro with Con A for 48 h (a positive control for the induction of a suppressor T-cell) inhibited humoral responses of naive cells to both types of antigen. Likewise, T-cells plus macrophages from TCDD-treated mice did not suppress the in vitro humoral responsiveness of naive B-cells plus macrophages to a T-independent antigen (DNP-Ficoll). These results would indicate that an alteration in T-cell function following TCDD exposure does not play a role in the suppression of the antibody response elicited by antigen stimulation of murine B6C3F1 splenocytes.

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

Thymocytes from 15-day old C57BL/6 mice, pretreated with 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) 4 days before sacrifice, showed an earlier response and a higher maximal cell proliferation than thymocytes from control mice upon stimulation by concanavalin A in vitro. This is partly in contrast to the conclusions from earlier published studies. IL-2 content--as measured by growth of CTLL cells--was equally high in TCDD and in control cultures at day 1. At day 2, TCDD cultures had decreased dramatically in IL-2 content, possibly due to a high rate of consumption. At this point in time, the controls still contained a high concentration of IL-2, although less than at day 1. In contrast to the increased sensitivity to mitogen stimulation, thymocytes from TCDD-treated mice induced B-cells less avidly with respect to antibody production, and could also inhibit the T-cell help of thymocytes from untreated animals, a phenomenon which could be reversed if TCDD-treated thymocytes were irradiated before culture.

Elucidation of cellular targets responsible for tetrachlorodibenzo-p-dioxin (TCDD)-induced suppression of antibody responses: I. The role of the B lymphocyte

The objective of these studies was to identify the primary cellular target(s) responsible for TCDD (tetrachlorodibenzo-p-dioxin)-induced suppression of antibody production. Responses to T-independent and T-dependent antigens (administered in vivo or directly to splenic culture) were suppressed in a dose-related fashion in female B6C3F1 mice dosed for 5 consecutive days with 0.1, 1.0 and 10.0 micrograms/kg of 2,3,7,8-TCDD. When nonadherent (B and T) and adherent (macrophage) cells from vehicle- and TCDD-treated mice were combined in various combinations and immunized with either DNP-Ficoll (T-independent) or sRBCs (T-dependent), it was demonstrated that nonadherent cells, but not adherent cells, were functionally affected by TCDD. Similarly, as various combinations of B + macrophage and T + macrophage populations were immunized with sRBCs, the B cell was shown to be the primary target. Using LPS as the stimulus, an inhibition of the antibody response with no effect on the mitogenic response further indicated that the primary target of the TCDD-induced suppression of IgM antibody production is the B lymphocyte at the level of cell differentiation.

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) enhances antibody production and protein kinase activity in murine B cells

Treatment of murine spleen cells with 30 nM TCDD resulted in an approximately 3 fold increase in unstimulated antibody production after 3 days in culture. This response was not accompanied by increased cellular proliferation and may represent an effect of TCDD on B cell activation or differentiation. Since PMA is capable of activating B cells, presumably via PKC, we have compared the effects of PMA and TCDD on protein kinase activation and phosphorylation of endogenous proteins in a highly purified preparation of B cells. In contrast to a reduction of cytosolic PKC activity, the expected effect of PMA, TCDD caused an increase in basal kinase activity with no effect on PKC activity. Addition of either PMA or TCDD resulted in enhanced phosphorylation of a similar profile of proteins, including proteins of Mr 12.2, 14.6, 29.2, 52.3 and 62.7 KDa. Addition of TCDD also resulted in the increased phosphorylation of a protein of Mr 45.2, which was unaffected by PMA. Combined treatment with PMA and TCDD resulted in additive responses. The additive effects of PMA and TCDD suggest an interaction at the level of protein phosphorylation which is mediated by different kinases. Therefore, TCDD may be stimulating B cells via an early effect on an unidentified protein kinase.