Hexachlorobenzene-induced immunomodulation and skin and lung lesions: a comparison between brown Norway, Lewis, and Wistar rats

B-1 cells in immunotoxicology: Mechanisms underlying their response to chemicals and particles

Since their discovery nearly 40 years ago, B-1 cells have continued to challenge the boundaries between innate and adaptive immunity, as well as myeloid and lymphoid functions. This B-cell subset ensures early immunity in neonates before the development of conventional B (B-2) cells and respond to immune injuries throughout life. B-1 cells are multifaceted and serve as natural- and induced-antibody-producing cells, phagocytic cells, antigen-presenting cells, and anti-/pro-inflammatory cytokine-releasing cells. This review retraces the origin of B-1 cells and their different roles in homeostatic and infectious conditions before focusing on pollutants comprising contact-sensitivity-inducing chemicals, endocrine disruptors, aryl hydrocarbon receptor (AHR) ligands, and reactive particles.

Modeling trauma in rats: similarities to humans and potential pitfalls to consider

Trauma is the leading cause of mortality in humans below the age of 40. Patients injured by accidents frequently suffer severe multiple trauma, which is life-threatening and leads to death in many cases. In multiply injured patients, thoracic trauma constitutes the third most common cause of mortality after abdominal injury and head trauma. Furthermore, 40-50% of all trauma-related deaths within the first 48 h after hospital admission result from uncontrolled hemorrhage. Physical trauma and hemorrhage are frequently associated with complex pathophysiological and immunological responses. To develop a greater understanding of the mechanisms of single and/or multiple trauma, reliable and reproducible animal models, fulfilling the ethical 3 R's criteria (Replacement, Reduction and Refinement), established by Russell and Burch in 'The Principles of Human Experimental Technique' (published 1959), are required. These should reflect both the complex pathophysiological and the immunological alterations induced by trauma, with the objective to translate the findings to the human situation, providing new clinical treatment approaches for patients affected by severe trauma. Small animal models are the most frequently used in trauma research. Rattus norvegicus was the first mammalian species domesticated for scientific research, dating back to 1830. To date, there exist numerous well-established procedures to mimic different forms of injury patterns in rats, animals that are uncomplicated in handling and housing. Nevertheless, there are some physiological and genetic differences between humans and rats, which should be carefully considered when rats are chosen as a model organism. The aim of this review is to illustrate the advantages as well as the disadvantages of rat models, which should be considered in trauma research when selecting an appropriate in vivo model. Being the most common and important models in trauma research, this review focuses on hemorrhagic shock, blunt chest trauma, bone fracture, skin and soft-tissue trauma, burns, traumatic brain injury and polytrauma.

The transferability from rat subacute 4-week oral toxicity study to translational research exemplified by two pharmaceutical immunosuppressants and two environmental pollutants with immunomodulating properties

Exposure to chemicals may have an influence on the immune system. Often, this is an unwanted effect but in some pharmaceuticals, it is the intended mechanism of action. Immune function tests and in depth histopathological investigations of immune organs were integrated in rodent toxicity studies performed according to an extended OECD test guideline 407 protocol. Exemplified by two immunosuppressive drugs, azathioprine and cyclosporine A, and two environmental chemicals, hexachlorobenzene and benzo[a]pyrene, results of subacute rat studies were compared to knowledge in other species particular in humans. Although immune function has a high concordance in mammalian species, regarding the transferability from rodents to humans various factors have to be taken into account. In rats, sensitivity seems to depend on factors such as strain, sex, stress levels as well as metabolism. The two immunosuppressive drugs showed a high similarity of effects in animals and humans as the immune system was the most sensitive target in both. Hexachlorobenzene gave an inconsistent pattern of effects when considering the immune system of different species. In some species pronounced inflammation was observed, whereas in primates liver toxicity seemed more obvious. Generally, the immune system was not the most sensitive target in hexachlorobenzene-treatment. Immune function tests in rats gave evidence of a reaction to systemic inflammation rather than a direct impact on immune cells. Data from humans are likewise equivocal. In the case of benzo[a]pyrene, the immune system was the most sensitive target in rats. In the in vitro plaque forming cell assay (Mishell-Dutton culture) a direct comparison of cells from different species including rat and human was possible and showed similar reactions. The doses in the rat study had, however, no realistic relation to human exposure, which occurs exclusively in mixtures and in a much lower range. In summary, a case by case approach is necessary when testing immunotoxicity. Improvements for the translation from animals to humans related to immune cells can be expected from in vitro tests which offer direct comparison with reactions of human immune cells. This may lead to a better understanding of results and variations seen in animal studies.

Macrophages are involved in hexachlorobenzene-induced adverse immune effects

Hexachlorobenzene (HCB) is a persistent environmental pollutant that causes adverse immune effects in man and rat. The Brown Norway (BN) rat is very susceptible to HCB-induced immunopathology and oral exposure causes inflammatory skin and lung lesions, splenomegaly, lymph node (LN) enlargement, and increased serum levels of IgE and anti-ssDNA IgM. T cells play an important role but do not account for all adverse effects induced by HCB. Macrophages are probably also important and the relationship between macrophages and T cells was further investigated. To eliminate macrophages clodronate-liposomes were used. Furthermore, a kinetic study was performed to obtain insight in the early phase of the HCB-induced immune response. Also, experiments were performed to detect specific memory T cells. Therefore, an adoptive transfer study was performed. Our results indicate that macrophages are indeed involved in HCB-induced skin lesions, lung eosinophilia, and elevation of IgM against ssDNA. Kinetics showed that both skin and lung lesions appeared early after exposure. Moreover, immune effects could not be adaptively transferred. Thus, both macrophages and T cells are involved in HCB-induced immune effects but HCB exposure does not lead to specific T cell sensitization. Presumably, HCB exposure induces macrophage activation, thereby generating adjuvant signals that polyclonally stimulate T cells. Together, these events may lead to the observed immunopathology in BN rats.

Regulatory immunotoxicology: does the published evidence support mandatory nonclinical immune function screening in drug development?

Recent immunotoxicity guidance documents from the EU CHMP and the US FDA apply significantly different weightings to immune function testing; whereas the former mandates (as a starting point) incorporation of immune function tests (IFTs) to screen for immunotoxic potential in sub-chronic rodent toxicity studies, the more cautious 'for cause' FDA approach recommends the use of IFTs only when warranted by evidence obtained from conventional nonclinical and/or clinical studies. Conclusions from detailed evaluations of several key drugs, including salmeterol and some opioids, challenge the notion that data on these examples support the need for IFTs to detect unintended immunosuppression. Given the virtual absence of convincing pharmaceutical examples and the rarity of unintended immunosuppression, routine immune function testing of all new pharmaceuticals is not considered justified. Resources currently being employed in this manner in an attempt to detect a seemingly rare phenomenon would appear to be better applied to the development of reliable predictive assays for drug hypersensitivity, which is known to cause significant patient morbidity. Any moves towards a globally harmonised guideline that recommends the use of concern-based IFTs, need ideally to be accompanied by the establishment of appropriate historical control reference intervals and interpretation criteria to support a reliable weight-of-evidence approach to data evaluation.

Toxicogenomics of subchronic hexachlorobenzene exposure in Brown Norway rats

Hexachlorobenzene (HCB) is a persistent environmental pollutant with toxic effects in man and rat. Reported adverse effects are hepatic porphyria, neurotoxicity, and adverse effects on the reproductive and immune system. To obtain more insight into HCB-induced mechanisms of toxicity, we studied gene expression levels using DNA microarrays. For 4 weeks, Brown Norway rats were fed a diet supplemented with 0, 150, or 450 mg HCB/kg. Spleen, mesenteric lymph nodes (MLN), thymus, blood, liver, and kidney were collected and analyzed using the Affymetrix rat RGU-34A GeneChip microarray. Most significant (p < 0.001) changes, compared to the control group, occurred in spleen, followed by liver, kidney, blood, and MLN, but only a few genes were affected in thymus. This was to be expected, as the thymus is not a target organ of HCB. Transcriptome profiles confirmed known effects of HCB such as stimulatory effects on the immune system and induction of enzymes involved in drug metabolism, porphyria, and the reproductive system. In line with previous histopathological findings were increased transcript levels of markers for granulocytes and macrophages. New findings include the upregulation of genes encoding proinflammatory cytokines, antioxidants, acute phase proteins, mast cell markers, complements, chemokines, and cell adhesion molecules. Generally, gene expression data provide evidence that HCB induces a systemic inflammatory response, accompanied by oxidative stress and an acute phase response. In conclusion, this study confirms previously observed (immuno)toxicological effects of HCB but also reveals several new and mechanistically relevant gene products. Thus, transcriptome profiles can be used as markers for several of the processes that occur after HCB exposure.

Immunomodulatory effects of tetrachlorobenzoquinone, a reactive metabolite of hexachlorobenzene

Hexachlorobenzene (HCB) is an environmental pollutant that causes autoimmune-like effects in humans and rats. It is not completely clear whether T cells are involved and, if so, how they are stimulated after oral exposure to HCB. HCB as a rather inert chemical is not likely to bind covalently to macromolecules. The oxidative metabolite of HCB, tetrachlorobenzoquinone (TCBQ), which is in a redox equilibrium with tetrachlorohydroquinone (TCHQ), can bind to macromolecules, hence may form hapten-carrier complexes in vivo. We have assessed in the reporter antigen-popliteal lymph node assay whether HCB or TCHQ and TCBQ are able to induce a 2,4,6-trinitrophenyl (TNP) specific IgG1 response to the T cell-independent antigen TNP-Ficoll, which is indicative of neoantigen specific T cell help. To this end, these compounds and silica were injected into the footpad of Balb/c mice. Silica was included as an inert model compound, which causes autoimmune-like effects by activating macrophages. Seven days later, cell number and TNP specific antibody-secreting cells (ASC) in the popliteal lymph node (PLN) were determined. Furthermore, a secondary PLNA was performed to find out if TCHQ was capable of eliciting a memory response. Silica, TCHQ, and TCBQ, but not HCB, increased PLN cellularity and the number of IgM-producing ASC by ELISPOT. Both oxidative metabolites were able to induce the formation of germinal centers as assessed by immunohistochemistry and an IgG1 response to TNP-Ficoll. In the secondary PLNA, only mice primed with TCHQ and challenged with TCHQ together with TNP-Ficoll showed a significant increase in TNP specific IgG1 ASC. Present data show that TCHQ and TCBQ are capable of inducing neoantigen specific T cell help and that TCHQ can induce a compound specific memory response.

Chemical-specific properties co-determine the type of adverse immune response

Many drugs but also environmental pollutants may cause adverse reactions in susceptible individuals that are reminiscent of autoimmune syndromes. Apart from a number of predisposing often inherent, idiosyncratic determinants, chemical-specific properties might be involved as well. Notably, reactive chemicals or metabolites may provoke formation or release of immunosensitizing neo-antigens (a.o. hapten-carrier complexes or cryptic epitopes). In addition reactive chemicals but also certain inert chemicals may trigger macrophages and other inflammatory cells to release proinflammatory products that, via elicitation of costimulatory help, support hapten- or neo-antigen-specific T cell activation. In addition, chemicals may influence immunoregulatory processes and modulate for instance the balance between type 1 and type 2 responses. Here, we review data showing that chemically induced upregulation of second or costimulatory signals co-determines not only whether, but also what type of an adverse immune response (type 1 or type 2) is triggered.

Two immunotoxicity ring studies according to OECD TG 407-comparison of data on cyclosporin A and hexachlorobenzene

Two international ring studies were performed to develop appropriate parameters within standard toxicology study for screening of immunotoxicological potential of unknown substances. These studies followed OECD TG 407 and included a number of additional examinations. CSA was selected as model for its immunosuppressive and HCB as model for its immunostimulating effects. Reproducibility of data was defined by significant findings in at least 50% of participating laboratories. In-life clinical observations, values for WBC parameters, and changes of lymphoid organ weights suggested immune effects. Elevated IgM titers indicated increased antibody formation in HCB-exposed rats. Cellularity of T-cell compartments in thymus (medulla), spleen (PALS), and lymph nodes (paracortical zone of mesenteric and popliteal LN) were dose dependently decreased in CSA-treated rats. The numbers of follicular germinal centers were reduced in LN. HCB induced cellular proliferation in spleen marginal zones and endothelial activation in HEV of mesenteric and popliteal LN and GALT and in small pulmonary venules. Data obtained by specific immune parameters indicated immune effects; however, statistical inference was limited to low numbers of participating laboratories. In spleen, both substances decreased lymphoblast proliferation after ConA mitogen stimulation. Reduced numbers of antibody-forming cells in PFC assay indicated impaired T-cell-dependent humoral immunity by CSA, which was not seen for HCB. Altered fractions for B- and T-cell subpopulations were identified in spleen for both substances. In order to predict immunomodulatory effects of CSA or HCB, histomorphologic examination of lymphoid tissues resulted in the most reliable and sensitive data to distinguish immunosuppression and -stimulation.

Autoimmunity by pesticides: a critical review of the state of the science

The goals of this paper will be to present a critical review of the state of the science of pesticides and autoimmunity, and to discuss research that addresses the potential links between environmental chemicals and autoimmune disease. To date, the science of immunotoxicology has primarily focused on immunosuppression and hypersensitivity/allergy, and test methods are available to address these outcomes. So much progress has been made to address immunosuppression and contact sensitization that there are regulatory guidelines in the U.S. included in the registration of pesticides. In contrast, there are no validated approaches to assess autoimmunity. The overall objective of this paper will be to use pesticides as an important class of environmental chemicals to critically evaluate the state of the science for addressing chemical-induced autoimmunity. Specific examples of studies with pesticides will be discussed in the context of the following types of approaches: animal studies using standard immunotoxicological parameters; animal studies using specialized models of autoimmunity; human studies after environmental or occupational exposure; and human studies after accidental poisoning.

Hexachlorobenzene-induced eosinophilic and granulomatous lung inflammation is associated with in vivo airways hyperresponsiveness in the Brown Norway rat

We investigated whether the eosinophilic and granulomatous lung pathology that develops in Brown Norway (BN/SsNOlaHsd) rats upon feeding hexachlorobenzene (HCB) is associated with nonspecific in vivo airways hyperresponsiveness (AHR) to methacholine. To this end, female BN/SsNOlaHsd rats were exposed to diets with no supplementation or diets supplemented with 450 mg HCB per kg feed. On days 7 or 21 of exposure in vivo airways hyperresponsiveness to increasing concentrations of methacholine was assessed both by whole body plethysmography and by visual scoring. In addition, lungs were lavaged to count and differentiate lavage cells, and skin and lungs were processed for histology. Lungs of the control rats showed some scattered microgranulomas and by 3 weeks of control diet some rats showed rather extensive granuloma formation and perivascular and peribronchiolar infiltration of eosinophils, as well as increased responsiveness to methacholine. Oral exposure to HCB for 7 days caused a moderate perivasculitis, but no increase of total serum IgE levels and no AHR to methacholine was found. Prolonged HCB exposure for 21 days resulted in severe and extensive eosinophilic and granulomatous lung inflammation, a threefold increase of total serum IgE levels, and marked cholinergic AHR in all rats. Correlation analysis revealed a significant relation between the AHR and lung inflammation, as judged by granuloma formation and increased numbers of eosinophilic granulocytes in the lung interstitium, particularly around the bronchi and bronchioli. No correlation was observed between serum IgE levels and AHR. Data suggest that HCB induces AHR by stimulating eosinophilic lung inflammation and that the preexistent microgranulomas may predispose to development of the HCB-induced lung pathology.

The immune system as a potential target for environmental estrogens (endocrine disrupters): a new emerging field

It is now well known that natural (17beta-estradiol) and synthetic (e.g. diethylstilbestrol) estrogens not only affect the reproductive system, but also markedly influence the immune system. Recently, a new class of estrogens that is abundant in the environment (in industrial chemicals, pesticides, and surfactants) has been recognized. Some of these estrogenic chemicals (which are a large subgroup of endocrine disrupters) have also been shown to influence the immune system. This review assimilates growing evidence in wildlife, laboratory animals and to a limited extent in humans, which suggests that environmental chemicals may also affect the immune system. Further studies are needed to ascertain the immunological consequences of exposure to environmental estrogens, especially in humans. At the present time, it is not known whether the human immune system responds to a low dose of environmental estrogens or if environmental estrogens influence certain subsets of human populations, rather than the general population. Conceivably, an alteration of the immune system by environmental estrogens could affect the individuals' ability to mount well-regulated immune responses to microbial and vaccine antigens, allergens, self and tumor antigens. Possible changes in the immune system must be investigated routinely in toxicity studies. A comprehensive mechanistic understanding of potential immunomodulatory chemicals is needed. In this regard, relevant laboratory animals may be especially useful in identifying susceptible periods of life, whether both genders are equally affected, in analysis of changes in target lymphoid organs, and to determine the immunological effects of mixtures of chemicals.

Histopathologic approaches to detect changes indicative of immunotoxicity

Toxicologic pathology is crucial in the identification and characterization of health effects following exposure to xenobiotics, mainly in toxicity experiments in rodents. Regarding regulatory toxicology, histopathology of lymphoid organs and tissues is a cornerstone in the identification of immunotoxic compounds. A 2-tier testing system is usually employed in which the first tier is a general screen for (immuno)toxicity and the second tier consists of specific immune function studies, including host resistance tests or mechanistic studies. The role attributed to histopathology of lymphoid organs in the updated Organisation for Economic Cooperation and Development and Food and Drug Administration guidelines requires improvement and standardization of the histopathology procedures. Optimalization and standardization was started in an international collaborative immunotoxicity study (ICICIS). However, several problems were left unaddressed, mostly because of the few compounds tested in this study. Based on the results of the ICICIS study and the morphologic changes induced by immunotoxic/immunomodulatory compounds observed in other investigations, suggestions are given to further improve the identification and (semi)quantification of histopathologic changes in lymphoid organs and tissues.

The role of thymus-dependent T cells in hexachlorobenzene-induced inflammatory skin and lung lesions

The involvement of thymus-dependent T cells in the inflammatory skin and lung lesions and spleen effects induced by hexachlorobenzene (HCB) was investigated by using genetically athymic and euthymic WAG/Rij rats and Brown Norway (BN) rats with or without depletion of T cells by adult thymectomy, lethal irradiation, and bone marrow reconstitution. Rats were exposed to diets with no supplementation or diets supplemented with 150 or 450 mg HCB per kg diet for 4 (BN) or 6 (WAG/Rij) weeks. Skin lesion development and body weight gains were assessed during exposure and spleen and liver weights as well as histopathologic changes in skin, lung, and spleen were assessed after exposure. Oral HCB exposure of athymic and euthymic rats of both rat strains resulted in a dose-dependent increase of relative liver weight at doses of 150 and 450 mg/kg HCB and increased relative spleen weights at a dose of 450 mg/kg. HCB exposure of both strains further resulted in inflammatory changes in skin, lungs, and splenic red pulp independent of the T cell status except for skin lesions in the BN strain. HCB-exposed T cell-competent BN rats showed faster skin lesion development than the T cell-depleted rats, although qualitatively and quantitatively similar skin pathology was observed at the end of the 4-week exposure in both groups. In the WAG/Rij strain skin lesions could not be comparatively assessed due to preexistent inflammatory skin pathology in the nude rats. This study showed that thymus-derived T cells are not required for the induction of skin and lung pathology and splenic changes by HCB and therefore it is suggested that HCB acts differently from many allergenic and autoimmunogenic low molecular weight compounds that trigger pathology via thymus-dependent mechanisms. A role for mononuclear phagocytes and, in BN rats, eosinophilic granulocytes, in the HCB-induced pathology is suggested since these cells were prominently present in the HCB-induced lesions.

The role of the immune system in hexachlorobenzene-induced toxicity

Hexachlorobenzene (HCB) is a persistent environmental pollutant. The toxicity of HCB has been extensively studied after an accidental human poisoning in Turkey and more recently it has been shown that HCB has immunotoxic properties in laboratory animals and probably also in man. Oral exposure of rats to HCB showed stimulatory effects on spleen and lymph node weights and histology, increased serum IgM levels, and an enhancement of several parameters of immune function. Moreover, more recent studies indicate that HCB-induced effects in the rat may be related to autoimmunity. In Wistar rats exposed to HCB, IgM antibodies against several autoantigens were elevated; in the Lewis rat, HCB differently modulated two experimental models of autoimmune disease. Oral exposure of rats to HCB induces skin and lung pathology in the rat. Recently several studies have been conducted to investigate whether these skin and lung lesions can be related to HCB-induced immunomodulation, and these studies will be discussed in this review. HCB-induced skin and lung lesions probably have a different etiology; pronounced strain differences and correlation of skin lesions with immune parameters suggest a specific involvement of the immune system in HCB-induced skin lesions. The induction of lung lesions by HCB was thymus independent. Thymus-dependent T cells were not likely to be required for the induction of skin lesions, although T cells enhanced the rate of induction and the progression of the skin lesions. No deposition of autoantibodies was observed in nonlesional or lesional skin of HCB-treated rats. Therefore, we concluded that it is unlikely that the mechanism by which most allergic or autoimmunogenic chemicals work, i.e., by binding to macromolecules of the body and subsequent T- and B-cell activation, is involved in the HCB-induced immunopathology in the rat. Such a thymus-independent immunopathology is remarkable, as HCB strongly modulates T-cell-mediated immune parameters. This points at a very complex mechanism and possible involvement of multiple factors in the immunopathology of HCB.

Humoral and cellular immune responses in different rat strains on oral exposure to ovalbumin

No adequate enteral sensitization models are available to study food allergy and allergenicity of food proteins. Using a previously described oral sensitization protocol to sensitize Brown Norway rats (BN) to food proteins, the influence of genetically-based strain-specific characteristics of the immune system on the outcome of oral sensitization studies was investigated. BN, Hooded Lister (HL), Piebald Virol Glaxo (PVG) and Wistar rats were daily administered 1 mg of ovalbumin (OVA) by gavage dosing for 42 days without the use of an adjuvants. The highest OVA-specific IgG responses were detected in the BN rats followed by Wistar, HL and PVG rats. OVA-specific IgE responses were only detectable in the BN rats. The cellular immune response was examined by determination of delayed-type hypersensitivity (DTH) reactions in the animals. The response was most pronounced in the HL and Wistar rats. PVG and BN rats showed comparable DTH responses but the responses were significantly weaker than those observed in HL and Wistar rats. It was concluded that the genetic make-up of different rat strains influences the outcome of oral sensitization studies. In addition, using the described oral sensitization protocol, the BN rat seems to be the most suitable strain for inducing oral sensitization.

UVB exposure-induced systemic modulation of Th1- and Th2-mediated immune responses

Exposure to ultraviolet light, especially UVB wavelengths, can impair immune responses in animals and humans. It is remarkable that this immunomodulation is not restricted to the exposed skin but is also found at other sites, i.e. systemic (distant) immunosuppression. A frequently proposed hypothesis is that UVB exposure inhibits, specifically, T helper 1 (Th1)-mediated immune responses. The major reason for this is that contact hypersensitivity (CHS) and delayed-type hypersensitivity (DTH), both Th1-mediated immune responses, are very sensitive to UVB. For this reason these models are frequently used for photoimmunology studies. In the present study, the effects of UVB exposure were investigated in classical models for Th1-mediated immunity, i.e. CHS models in which picrylchloride or oxazolone were used as low-molecular-weight chemical antigens. In these models, CHS responsiveness and cytokines were measured, the latter by both reverse transcription-polymerase chain reaction (RT-PCR) and enzyme-linked immunosorbent assay (ELISA). The CHS responses to both contact sensitizers (picrylchloride and oxazolone) were suppressed significantly by pre-exposure to repeated suberythemal UVB exposure. Interferon-gamma (IFN-gamma), interleukin (IL)-12 and IL-4, but not IL-10, were detectable in spleen and draining lymph nodes of sensitized BALB/c mice. Repeated UVB exposure prior to sensitization at a distant locus inhibited both IFN-gamma and IL-12 but not IL-4. In BALB/c mice sensitized with ovalbumin (OVA) in the absence of complete Freund's adjuvant, a model for Th2-mediated immunity, OVA-specific serum IgE and cytokine profiles in the spleen were analysed. Sensitization did lead to a significant increase in OVA-specific IgE serum titres. Pre-exposure to UVB resulted in a decreased OVA-specific IgE serum titre. Both RT-PCR and ELISA showed increased levels of IFN-gamma, IL-4 and IL-10 in the spleens of OVA-sensitized mice. The production of IFN-gamma and IL-4 was not affected by UVB pre-exposure. In contrast, the production of IL-10 was significantly increased. This was probably caused by an up-regulation of Th2 cells. It is remarkable that IFN-gamma is significantly suppressed by UVB in Th1-mediated immune reactions but not in Th2-mediated immune reactions where it even appears to increase. IL-10, which is up-regulated by UVB pre-exposure and produced by, among others, Th2 cells, may represent a shift from Th1- to Th2-mediated immune mechanisms. However, IL-10 can also inhibit Th2 responses, which might be the reason for a decreased IgE titre in the Th2 model. From the results of this study it is concluded that UVB exposure prior to sensitization/immunization not only inhibits Th1-mediated but also Th2-mediated immune responses.

Intestinal T lymphocytes of different rat strains in immunotoxicity

In order to study the intestinal mucosal immune cells, with emphasis on single T lymphocytes, an inventory was made of single and organized lymphocytes in the epithelium and lamina propria of the small intestines of untreated Wistar, Fischer 344, and Lewis rats. The single and organized lymphocytes were examined microscopically. In addition, the single lymphocytes in the epithelium (IEL) and lamina propria (LPL) were analyzed by flow cytometry. Next, the use of flow cytometry analysis was explored to detect changes in the IEL T-lymphocyte population in subacute oral studies with the immunomodulating agents azathioprine and hexachlorobenzene. Untreated random-bred Wistar rats exhibited a large interindividual variability in IEL composition, while the variability was small in inbred Fischer 344 and Lewis rats. The explorative study with the 2 model immunomodulating compounds demonstrated that hexachlorobenzene increased the number of intraepithelial T lymphocytes with CD8+ phenotype at the cost of T cells with CD4+ phenotype in Lewis rats. Azathioprine did not induce distinct effects on the percentages of IEL. The data indicate that the intraepithelial lymphocytes in the intestines are a potential target for orally administered immunomodulating compounds and should therefore receive more attention in toxicologic pathology studies.

Effects of in vivo exposure to bis(tri-n-butyltin)oxide, hexachlorobenzene, and benzo(a)pyrene on cytokine (receptor) mRNA levels in cultured rat splenocytes and on IL-2 receptor protein levels

Analysis of cytokine (receptor) mRNA levels has been suggested to be a sensitive technique for predicting the immunomodulatory potential of drugs and chemicals. Furthermore, this type of analysis is thought to be important in unraveling mechanisms of immunotoxicity. To study these issues, male Wistar rats were exposed to the immunotoxic environmental contaminants bis(tri-n-butyltin) oxide (TBTO; 5, 20, or 80 mg/kg diet for 6 weeks), hexachlorobenzene (HCB; 50, 150, or 450 mg/kg diet for 6 weeks), or benzo(a)pyrene (B(a)P; 3, 10, 30, or 90 mg/kg body wt for 5 weeks by a daily (5 times a week) oral intubation). Spleen cells were cultured with Con A and analyzed by dot blot hybridization for IL-2, IFN-gamma, IL-2 receptor alpha-chain (IL-2R alpha; CD25), and IL-4 mRNA levels. In addition, spleen and thymus sections of TBTO-exposed animals were assayed immunohistochemically for CD25 expression. Exposure to TBTO resulted in a dose-dependent decrease in IL-2R alpha mRNA levels from 5 mg/kg, a dose-dependent increase in IFN-gamma mRNA levels from 20 mg/kg, and increased IL-2 mRNA levels at 80 mg/kg diet. Exposure to HCB resulted in a dose-dependent increase in IL-2 and IFN-gamma mRNA levels from 150 mg/kg and increased IL-2R gamma mRNA levels at 450 mg/kg diet. Exposure to B(a)P resulted in a dose-dependent increase in IL-2 and IFN-gamma mRNA levels from 10 mg/kg and increased IL-2R alpha mRNA levels at 90 mg/kg body wt. No effects were seen on IL-4 mRNA levels. Spleen and thymus sections of TBTO-exposed animals showed reduced CD25 expression from 5 mg/kg diet. These results show that (1) the correlation between altered cytokine (receptor) mRNA levels and functional endpoints is variable, depending on the type of functional endpoint tested and the compound studied, (2) these assays are among the most sensitive ones for TBTO and HCB immunotoxicity, and among the more sensitive ones for B(a)P immunotoxicity, and (3) for TBTO, these assays provide a possible clue to a mechanism for thymus atrophy, resulting from exposure to this compound: reduced IL-2R expression may impede thymocyte maturation, resulting in thymus atrophy.