Histopathology of the immune system as a tool to assess immunotoxicity

Toxicologic Pathology Forum: Considerations Regarding Determination of Adversity for Immunopathology Findings in Nonclinical Toxicology Studies with Immune-Modulating Therapeutics

The evaluation of changes in the immune system serves to determine the efficacy and potential immunotoxicologic effects of new products under development. Toxicologic pathologists play critical roles in identifying immune system changes that drive the immunosafety determination. Standard pathology evaluations of therapies and chemicals remain similar; however, biopharmaceutical therapies have moved from simply affecting the immune system to being specifically developed to modify the immune system, which can impact interpretation. Recent explosive growth in immunomodulatory therapies presents a challenge to the toxicologic pathologist, toxicologist, and regulatory reviewer in terms of evaluating the clinical relevance and potential adversity of immune system changes. Beyond the recognition of such changes, there is an increasing expectation to evaluate, describe, and interpret how therapies affect complex immune system pathways for both immunomodulatory therapies and non-immunomodulatory drugs with off-target immunotoxic effects. In this opinion piece, considerations regarding immune system evaluation, the current landscape of immunomodulatory therapies, a brief description of immunotoxicologic (and immunopathologic) endpoints, the importance of integrating such immunosafety data, and relevance to adversity determination are discussed. Importantly, we describe how the current paradigm of determining adversity for immune system changes may be challenging or insufficient and propose a harmonized and flexible approach for assessing adversity.

Enhanced Histopathology Evaluation of Lymphoid Organs

Enhanced histopathology is a tool that the pathologist can use as a screening test to identify immunomodulatory compounds. This assessment is based on the assumption that chemically induced alterations may result in qualitative or quantitative changes in the histology of the lymphoid organs. It involves the histological evaluation of various lymphoid organs and their respective tissue compartments to identify specific cellular and architectural changes. Although this methodology cannot directly measure immune function, it does have the potential to determine whether or not a specific chemical causes suppression or enhancement of the immune system. As with all screening tests, evaluation of and comparison with control tissues are crucial in order to establish the range of normal tissue changes for a particular group of animals. Laboratory animals include species other than rat and mouse; therefore, recognition of species differences in the structure and function of the immune system should be noted as well as identification of which differences are biologically relevant for the endpoint being considered. Consideration should also be given to the nutritional status, antigen load, age, spontaneous lesions, steroid hormone status, and stress for each strain and group of animals. General guidelines for the examination of each of the lymphoid organs are provided in this chapter.

Measuring fetal thymus size: a new method for diabetes screening in pregnancy

Objective: To investigate the correlation between fetal thymus size and diabetes in pregnancy.Method: Fetal thymus size was assessed in 160 pregnant women with gestational age of 19-39 weeks. They included 80 diabetic (investigation group) and 80 nondiabetic (control group) women. Fetal thymus size was measured by thymic-thoracic ratio. We did this with dividing the thymus' anteroposterior diameter by anteroposterior of mediastinum.Results: Thymic-thoracic ratio was significantly smaller in fetuses of diabetic mothers compared to the nondiabetic group (p = .001). It remained significant after subgrouping diabetic mothers into overt diabetes, insulin-dependent gestational diabetes, and noninsulin-dependent gestational diabetes.Conclusion: Although thymus size was smaller in fetuses of diabetic pregnant women compared to nondiabetic pregnant women, it seems that thymic-thoracic ratio can be a predictor of diabetes and its other related adverse effects during pregnancy.

Immune responses in the aquatic gastropod Lymnaea stagnalis under short-term exposure to pharmaceuticals of concern for immune systems: Diclofenac, cyclophosphamide and cyclosporine A

This is a pioneering study in the ecotoxicological assessment of immunotoxic effects of the three selected drugs of concern to a freshwater gastropod species. Lymnaea stagnalis was exposed in the laboratory for 3 days to three drugs used for immune systems: diclofenac (nonsteroidal anti-inflammatory drug), cyclophosphamide (anti-cancer immunosuppressive drug) or cyclosporine A (anti-xenograft immunosuppressive drug). Exposure ranges included environmental realistic (1-10μgL^-1) and therapeutic concentrations (100-1000μgL^-1). At the end of exposure times, the immune parameters of individual snails were measured: hemocyte density and viability, hemocyte phagocytosis capacity and hemocyte-related oxidative activities (basal and NADPH-oxidase stimulated with zymosan particles). Diclofenac and cyclosporine A induced immune responses, although the effects were not strong. No immunosuppression was observed. Such subtle immunomodulations bring further interrogations regarding their long-term immunotoxicity and possible resulting tradeoffs with life-history traits. On the other hand, the prodrug cyclophosphamide did not induce significant immune responses. Since metabolism pathways differ greatly between vertebrates and invertebrates, this study also suggests that relevant vertebrate metabolites should be included in the immunotoxicity assessment of pharmaceuticals in non-target invertebrate species. Finally, the possible interactive effects of these pharmaceuticals sharing similar modes of action or effects features should also be explored.

Ectopic Tertiary Lymphoid Tissue in Inflammatory Bowel Disease: Protective or Provocateur?

Organized lymphoid tissues like the thymus first appeared in jawed vertebrates around 500 million years ago and have evolved to equip the host with a network of specialized sites, strategically located to orchestrate strict immune-surveillance and efficient immune responses autonomously. The gut-associated lymphoid tissues maintain a mostly tolerant environment to dampen our responses to daily dietary and microbial products in the intestine. However, when this homeostasis is perturbed by chronic inflammation, the intestine is able to develop florid organized tertiary lymphoid tissues (TLT), which heralds the onset of regional immune dysregulation. While TLT are a pathologic hallmark of Crohn's disease (CD), their role in the overall process remains largely enigmatic. A critical question remains; are intestinal TLT generated by the immune infiltrated intestine to modulate immune responses and rebuild tolerance to the microbiota or are they playing a more sinister role by generating dysregulated responses that perpetuate disease? Herein, we discuss the main theories of intestinal TLT neogenesis and focus on the most recent findings that open new perspectives to their role in inflammatory bowel disease.

Immunotoxicology—Regulatory and Risk Assessment Concepts

Disclaimer: This article was written by Susan D. Wilson in her private capacity. No official support or endorsement by the Food and Drug Administration is intended or should be inferred. Address correspondence to James Blank, Pfizer Central Research (Mailbox #8014), Eastern Point Road, Groton, CT 06340, USA. E-mail: james_a_blank@groton.pfizer.com This article provides a review of presentations given at the symposium on Immunotoxicology: Regulatory and Risk Assessment Concepts held at the American College of Toxicology meeting in Orlando, Florida, in November, 1998. Immune system alterations have typically been assessed by histopathology of select lymphoid tissue, clinical pathology, clinical chemistry, plaque forming cell assay for humoral immunity, and allergic contact hypersensitivity. Advances in immunology and molecular biology have led to various activities to optimize hazard identification and risk assessment processes and strategies for immunotoxicants. With such advances, regulatory agencies have been either implementing immunotoxicology guidance as part of the safety of medical devices, evaluating environmental chemicals, or considering immunotoxicologic criteria for nonclinical assessments. Reviews of the guidance document provided by the Food and Drug Administration (FDA)/Center for Device and Radiological Health and concepts being considered by the FDA/Center for Drug Evaluation and Research are presented. In addition, a review of the process for evaluation of the murine local lymph node assay by the Interagency Coordinating Committee on the Validation of Alternative Methods and the state of risk assessment for chemical-induced autoimmunity are presented.

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.

How does human-induced environmental change influence host-parasite interactions?

Host-parasite interactions are an integral part of ecosystems that influence both ecological and evolutionary processes. Humans are currently altering environments the world over, often with drastic consequences for host-parasite interactions and the prevalence of parasites. The mechanisms behind the changes are, however, poorly known. Here, we explain how host-parasite interactions depend on two crucial steps – encounter rate and host-parasite compatibility – and how human activities are altering them and thereby host-parasite interactions. By drawing on examples from the literature, we show that changes in the two steps depend on the influence of human activities on a range of factors, such as the density and diversity of hosts and parasites, the search strategy of the parasite, and the avoidance strategy of the host. Thus, to unravel the mechanisms behind human-induced changes in host-parasite interactions, we have to consider the characteristics of all three parts of the interaction: the host, the parasite and the environment. More attention should now be directed to unfold these mechanisms, focusing on effects of environmental change on the factors that determine encounter rate and compatibility. We end with identifying several areas in urgent need of more investigations.

Isolated Lymphoid Follicles are Dynamic Reservoirs for the Induction of Intestinal IgA

IgA is one of the most important molecules in the regulation of intestinal homeostasis. Peyer's patches have been traditionally recognized as sites for the induction of intestinal IgA responses, however more recent studies demonstrate that isolated lymphoid follicles (ILFs) can perform this function as well. ILF development is dynamic, changing in response to the luminal microbial burden, suggesting that ILFs play an important role providing an expandable reservoir of compensatory IgA inductive sites. However, in situations of immune dysfunction, ILFs can over-develop in response to uncontrollable enteric flora, resulting in ILF hyperplasia. The ability of ILFs to expand and respond to help control the enteric flora makes this dynamic reservoir an important arm of IgA inductive sites in intestinal immunity.

Enhanced histopathology of the immune system: a review and update

Enhanced histopathology (EH) of the immune system is a tool that the pathologist can use to assist in the detection of lymphoid organ lesions when evaluating a suspected immunomodulatory test article within a subchronic study or as a component of a more comprehensive, tiered approach to immunotoxicity testing. There are three primary points to consider when performing EH: (1) each lymphoid organ has separate compartments that support specific immune functions; (2) these compartments should be evaluated individually; and (3) semiquantitative descriptive rather than interpretive terminology should be used to characterize any changes. Enhanced histopathology is a screening tool that should be used in conjunction with study data including clinical signs, gross changes, body weight, spleen and thymus weights, other organ or tissue changes, and clinical pathology. Points to consider include appropriate tissue collection, sectioning, and staining; lesion grading; and diligent comparison with concurrent controls. The value of EH of lymphoid organs is to aid in the identification of target cell type, changes in cell production and cell death, changes in cellular trafficking and recirculation, and determination of mechanism of action.

Diuron exposure induces systemic and organ-specific toxicity following acute and sub-chronic exposure in male Wistar rats

Diuron [3-(3,4-dichlorophenyl)-1,1-dimethylurea] is a substitute urea herbicide widely used on agricultural crops with potential mutagenic, teratogenic, reproductive and carcinogenic effects. Nonetheless, its toxic potential on the immune system needs a detailed assessment. Thus, in order to evaluate the adverse effect of this herbicide on lymphohematopoietic organs and macrophage activity, male Wistar rats were orally treated with Diuron at 125, 1250 and 2500 ppm for 14, 28 or 90 days. General signs of toxicity were observed in Diuron-treated groups (1250 and 2500 ppm), including reduced food intake and body weight gain, as well as higher relative weights for spleen, kidneys and liver (28 and 90-day toxicity studies) and elevated serum levels of ALT, albumin, total protein, creatinine and urea (28-day toxicity study). Diuron exposure caused a severe depletion of splenic white pulp compartments and cellularity, followed by a decreased number of CD4(+) T lymphocytes, increased extramedullary hematopoiesis and deposition of hemosiderin in red pulp. Despite alteration in macrophage spreading, the macrophagic activity was not significantly affected by the herbicide. Under these experimental conditions, the results suggest that Diuron exerts systemic and target-organ toxicity, mainly at higher concentration.

Enhanced histopathology evaluation of lymphoid organs

Enhanced histopathology is a tool that the pathologist can use as a screening test to identify -immunomodulatory compounds. This assessment is based on the assumption that chemically induced alterations may result in qualitative or quantitative changes in the histology of the lymphoid organs. It involves the histological evaluation of various lymphoid organs and their respective tissue compartments to identify specific cellular and architectural changes. Although this methodology cannot directly measure immune function, it does have the potential to determine whether or not a specific chemical causes suppression or enhancement of the immune system. As with all screening tests, evaluation of, and comparison with, control tissues are crucial in order to establish the range of normal tissue changes for a particular group of animals. Laboratory animals include species other than rat and mouse; therefore, recognition of species differences in the structure and function of the immune system should be noted as well as identification of which differences are biologically relevant for the endpoint being considered. Consideration should also be given to the nutritional status, antigen load, age, spontaneous lesions, steroid hormone status, and stress for each strain and group of animals. General guidelines for the examination of each of the lymphoid organs are provided in this chapter.

Tiered toxicity testing: evaluation of toxicity-based decision triggers for human health hazard characterization

A set of biologically-based toxicity testing decision triggers was developed and analyzed within a tiered testing and decision-making framework for evaluating potential human health hazards and risks associated with chemical exposures. The proposed three-tiered toxicity testing approach starts from a base set of toxicity studies (acute toxicity, in vitro genetic toxicity, in vitro cytogenetics, repeat dose/subchronic toxicity, developmental toxicity, reproductive toxicity) and then uses the toxicity triggers to identify which specific additional tests are needed to adequately characterize a substance's hazard potential. The toxicity triggers were initially evaluated using published information for eight chemicals, representing diverse classes. A retrospective validation study was then conducted using seven chemicals which had completed the USEPA's Voluntary Children's Chemical Evaluation Program (VCCEP). The toxicity triggers were shown to identify appropriate higher tier tests and to be reasonably predictive of the results expected in higher tiered tests. Employing these toxicity triggers within a tiered testing framework could lead to a reduction in the number of laboratory animals without diminishing the degree of scientific certainty necessary for hazard evaluations. The toxicity triggers appear to be suitable for identifying which specific endpoints and tests warrant further evaluation, and which do not, and for documenting the scientific basis for such decisions.

Screening of xenobiotics for direct immunotoxicity in an animal study

It has now been recognised that the immune system as a whole can be the target for xenobiotic induced toxicity. The discipline of immuntoxicology encompasses non specific direct immunotoxicity and immunostimulation, and specific responses like hypersensitivity and autoimmunity. Direct immunotoxicity can be determined in tiered studies, TIER 1 being a general toxicity study with emphasis on evaluation of organs belonging to the immune system, TIER 2 investigating the effects of xenobiotics on immune functionality in immunological challenge experiments. In the TIER 1 study, organ weights and histopathological evaluation of immune organs like spleen, thymus, lymph nodes, blood and bone marrow may detect the occurrence of direct immunotoxicity. The follow up studies in the TIER 2 phase can then determine the extent of the immunosuppression and identify which specific parts or cellular components of the immune system are involved. In view of the complexity of the immune system and the multitude of interactions within the immune system in vivo animal experiments are needed to investigate xenobiotics for their potential immunotoxicity. In vitro assays with in vivo exposed cells of the immune system may present additional information on the mechanisms involved in the observed direct immunotoxicity.

Histopathology of the thymus

The thymus is a primary lymphoid organ that manifests dynamic physiological changes as animals age in addition to being exquisitely sensitive to stress and toxic insult. It is typically the first lymphoid tissue to respond to immunotoxic xenobiotics, with the first change being loss of cortical lymphocytes by apoptosis. This is followed by removal of the apoptotic cellular debris and, in the absence of recovery, may lead to loss of the cortico-medullary demarcation and organ atrophy. Nonneoplastic proliferative changes include focal lymphoid hyperplasia and proliferation of medullary epithelial cells, often with formation of ribbons, cords, or tubules. Thymomas are relatively rare tumors that exhibit a wide spectrum of morphologic types but do not metastasize. Thymic lymphomas are common in some mouse strains and can become leukemic with hematogenous spread throughout the body.

Normal structure, function and histology of the thymus

The thymus, a primary lymphoid organ and the initial site for development of T cell immunological function, is morphologically similar across species. It is actually an epithelial organ in which its epithelial cells provide a framework containing T cells as well as smaller numbers of other lymphoid cells. A symbiotic interaction exists between the thymic microenvironment and developing T cells, and the specificity of T cell release into the systemic circulation is under thymic control. The thymic cortex in a young animal is heavily populated by developing T cells along with a smaller proportion of associated epithelial cells. Larger, more mature T cells are found in the medulla where epithelial and other cell types are more abundant. Understanding normal morphological features of the thymus and their perturbations provides a cornerstone to assessing immune system function.

Normal structure, function, and histology of mucosa-associated lymphoid tissue

The mucosa-associated lymphoid tissue (MALT) initiates immune responses to specific antigens encountered along all mucosal surfaces. MALT inductive sites are secondary immune tissues where antigen sampling occurs and immune responses are initiated. Effector sites, present as diffuse lymphoid tissue along all mucosal surfaces are the sites of IgA transport across the mucosal epithelium. Though there are many differences between inductive sites in various organs, they all contain the same basic compartments-follicles, interfollicular regions, subepithelial dome regions, and follicle-associated epithelium. The morphologic differences between MALT and other secondary lymphoid tissues, between the MALT sites of differing anatomic locations, and species differences among laboratory animals are described. The morphologic changes in MALT associated with aging, route of nutrition, and genetic mutation (i.e., the nude and SCID mutations) are also discussed. MALT tissues comprise the mucosal immune system which can function independently of the systemic immune system and are, therefore, an important and often overlooked aspect of immunopathology.

Effects of Feed Restriction on the Nucleolar Structure and Function in Lymphocytes

Since malnutrition leads to adverse effects of several drugs on haemopoiesis and blood morphology, we tested if malnutrition itself could affect the nucleolar structure and function in rat lymphocytes. We report changes in the proportion of different nucleolar types that were dependent on the severity of feed restriction. The observed changes resemble those seen with cytostatics suggesting a possible link between feed restriction and haemotoxicity.

Revised guides for organ sampling and trimming in rats and mice--Part 3. A joint publication of the RITA and NACAD groups

This is the third part of a series of three articles on trimming instructions of rat and mouse protocol organs and tissues in regulatory type toxicity studies, covering the urinary, nervous, musculoskeletal, cardiovascular, and lymphoreticular systems. The article is based on the experience of the European RITA and American NACAD working groups and is an extended revision of trimming guides published in 1995 (BAHNEMANN et al.). The optimum localization for tissue preparation, the sample size, the direction of sectioning and the number of sections to be prepared is described organ by organ. These descriptions are illustrated for each organ by a schematic drawing and/or a macro-photograph showing the plane of section as well as a low magnification of the H&E stained slide demonstrating the optimum "end-product". The objectives of this work, as addressed in detail in the first part (Ruehl-Fehlert et al. 2003), are to standardize tissue sampling and trimming for comparison of historical data obtained from different studies and different laboratories, ensure the presence of all relevant target sites for histopathological evaluation and provide technical advice for preparatory techniques during necropsy, fixation and trimming (Crissman et al. 2004).

Immunotoxicological effects of piperine in mice

The immunotoxicological effects of piperine were investigated in Swiss male mice, gavaged at a dose of 1.12, 2.25 or 4.5 mg/kg body weight for five consecutive days. All the dose levels had no overt toxic effect and the liver gained weight normally. Treatment at highest dose, however, resulted in significant decrease in the weight of spleen, thymus and mesenteric lymph nodes, but not of peripheral lymph nodes. All the dose levels suppressed the cellular population of lymphoid organs, except for the spleen, where the doses of 1.12 and 2.25 caused an increase. Haematologically, doses of 2.25 and 4.5 mg/kg caused a significant reduction in total leucocyte counts and differential leucocyte counts showed an increase in the percentage of neutrophils. The higher doses of 2.25 and 4.5 mg/kg suppressed the mitogenic response of B-lymphocyte to lipopolysaccharide. The number of primary antibody (IgM) forming cells in the spleen and the level of primary antibody in serum, was decreased. The doses of 1.12 and 2.25 mg/kg suppressed the mitogenic response of T-lymphocytes to phytohaemagglutinin and the nitroblue tetrazolium (NBT) dye reducing activity of peritoneal exudate cells (PECs). Since the lowest dose of 1.12 mg of piperine per kg body weight had no immunotoxic effect, it may be considered as immunologically safe "no observed adverse effect level (NOAEL)" dose.

Chemically induced immunotoxicity in a medium-term multiorgan bioassay for carcinogenesis with Wistar rats

A variety of chemicals can adversely affect the immune system and influence tumor development. The modifying potential of chemical carcinogens on the lymphoid organs and cytokine production of rats submitted to a medium-term initiation-promotion bioassay for carcinogenesis was investigated. Male Wistar rats were sequentially initiated with N-nitrosodiethylamine (DEN), N-methyl-N-nitrosourea (MNU), N-butyl-N-(4hydroxybutyl)nitrosamine (BBN), dihydroxy-di-n-propylnitrosamine (DHPN), and 1,2-dimethylhydrazine (DMH) during 4 weeks. Two initiated groups received phenobarbital (PB) or 2-acetylaminofluorene (2-AAF) for 25 weeks and two noninitiated groups received only PB or 2-AAF. A nontreated group was used as control. Lymphohematopoietic organs, liver, kidneys, lung, intestines, and Zymbal's gland were removed for histological analysis. Interleukin (IL)-2, IL-12, interferon gamma (IFN-gamma), tumor necrosis factor alpha (TNF-alpha), IL-10, and transforming growth factor beta1 (TGF-beta1) levels were determined by ELISA in spleen cell culture supernatants. At the fourth week, exposure to the initiating carcinogens resulted in cell depletion of the thymus, spleen and bone marrow, and impairment of IL-2, IL-12, and IFN-gamma production. However, at the 30th week, no important alterations were observed both in lymphoid organs and cytokine production in the different groups. The results indicate that the initiating carcinogens used in the present protocol exert toxic effects on the lymphoid organs and affect the production of cytokines at the initiation step of carcinogenesis. This early and reversible depression of the immune surveillance may contribute to the survival of initiated cells facilitating the development of future neoplasia.

Gender-related early immune changes in mice exposed to airborne suspended matter

Immunotoxicity of the airborne suspended matter (ASM) from Upper Silesia (Poland) was investigated. The effect of the mixture of toxic components from the air suspended matter on the distribution and expression of several surface markers on lymphoid cells from thymus, spleen and axillary lymph nodes of female and male mice were examined. Corticosterone, a stress related hormone and sex hormones concentration in the plasma of animals were evaluated to discuss the interplay between the immune and endocrine system. The route, dose and length of application of ASM were examined. Thymus was shown to be a very sensitive organ to the effect of airborne suspended matter. Profound changes were shown in the distribution of CD4+ CD8+, CD4+ and CD8+ thymocytes subsets and TCRbeta and CD3 expression on thymic cells in young mice of both sexes exposed to ASM. Gender-related differences between female and male thymuses are rather quantitative than qualitative. Changes observed in the peripheral lymphoid organs were not so dramatic. The decrease of the percentage of CD19+ and Thy-1.2- in female spleens was noted. None changes in the plasma level of corticosterone and testosterone was observed in mice exposed to ASM, while a profound decrease of estradiol in female mice was noted. An interpretation all of these documented changes is attempted.

Combination of fixation using PLP fixative and embedding in paraffin by the AMeX method is useful for histochemical studies in assessment of immunotoxicity

To establish a method for processing lymphoid organs suited to morphological, immunohistochemical and enzyme histochemical analyses for assessment of immunotoxicity, we examined a combination of fixation with periodate-lysine-paraformaldehyde (PLP) fixative and embedding in paraffin by the AMeX method (PLP-AMeX method). Spleen and thymus removed from monkeys and rats were fixed in PLP fixative for 6 hours at 4 degrees C. After fixation, specimens were processed and embedded in paraffin by the AMeX method. In hematoxylin and eosin-stained sections, tissue architecture was well preserved. In immunohistochemical staining, markers of T lymphocytes (CD3, CD4, CD8), B lymphocytes (monkey: CD20cy, rat: CD45RA) and macrophage (monkey; CD68, rat: ED-1) were well identified according to their specificities, although the staining intensity of CD8 in the monkey and CD4 in the rat were somewhat weaker in PLP-AMeX-prepared sections than in those frozen. In enzyme histochemical staining, alkaline phosphatase activity was well preserved in neutrophils. In toluidine blue- and Giemsa-stained sections, eosinophil granules and the metachromasia of granules in basophil/mast cells were clearly detectable. These findings suggest that the PLP-AMeX method is a powerful tool for assessment of immunotoxicity.

Immune responses: adverse versus non-adverse effects

The adaptive immune system in vertebrates has evolved to provide host resistance to infectious microorganisms and malignant disease. Normal immune function and the induction of specific immune responses require the orchestrated interaction between cells and molecules both within and outside the lymphoid system. Immunotoxicology can be defined as the study of adverse health effects that may result from the interaction of xenobiotics with the immune system. In general terms such effects can take one of two forms. The first of these is immunotoxicity (or immunosuppression) where there is a perturbation of, or damage to, one or more components of the immune system resulting in impaired immune function and reduced host resistance. The design and interpretation of experimental immunotoxicity studies and the investigation of clinical immunosuppression require consideration of the relationship between changes in the structure and/or function of discrete components of the immune system and holistic changes in the susceptibility to infectious and malignant disease. The other main way in which chemicals may cause adverse health effects secondary to interaction with the immune system is through stimulation of specific immune responses that result in allergic disease. Allergy to chemicals and proteins can take many forms, including allergic contact dermatitis, allergic sensitization of the respiratory tract (associated with rhinitis and/or asthma), systemic allergic reactions (associated frequently with drug treatment), and gastrointestinal disease. Here there is a need to distinguish between immunogenic responses per se and those immune responses that are of sufficient vigor and of the quality necessary to provoke allergic sensitization. The purpose of this article is to explore the extent to which distinctions can be drawn between adverse and nonadverse effects in the context of immunotoxicity and allergy.

Modulation of cell death in the rat thymus. Light and electron microscopic investigations

In mammals, the thymus is the primary central organ of the lymphoid system; after birth, it progressively diminishes in size, undergoing gradual atrophy. Physiological maturation and/or involution of the thymus may be accelerated by endogenous or exogenous factors. Exposure to extremely low frequency EMF seems to interfere with thymic cell death. Data suggest that, in the rat model, a prolonged exposure to 50 Hz electric and magnetic fields, independently from field strength, seems to affect thymic cell death and possibly thymic physiology, since alterations in the balance of cell death and other parameters such as mitoses might interfere with the positive and negative selection of thymocytes and with the immunosurveillance properties of the thymus.

Efficacy of histopathology in detecting petrochemical-induced toxicity in wild cotton rats (Sigmodon hispidus)

A variety of chemical mixtures exist in the soil of petrochemical waste sites, and many of these compounds are known immunotoxicants that have been observed to induce immune alterations in wild rodents inhabiting many of these petrochemical waste sites. Conventional histopathological assessments have been widely used with considerable success to investigate immunotoxicity of various agents under laboratory conditions. We hypothesized that histopathologic assessments would be equally sensitive for detecting exposure to complex mixtures of toxicants in cotton rats (Sigmodon hispidus) residing in contaminated habitats. Histopathological parameters were examined from a total of 624 cotton rats that were seasonally collected from 13 petrochemical-contaminated waste sites and 13 ecologically matched reference sites in Oklahoma over a 3-year period. Histopathological examination did not reveal any lesion associated with exposure to petrochemical wastes except renal inclusion bodies. Prevalence and severity of histologic lesions in liver and kidneys of cotton rats were significantly influenced by season, where prevalence and severity were lower in winter than summer on all study sites. These results suggest that the evaluation of toxicity from exposure to contaminants in the soil of industrial waste sites using histopathological assessments is not sensitive enough to detect exposure to the low levels of environmental contaminants present on most waste sites.

US FDA "Redbook II" immunotoxicity testing guidelines and research in immunotoxicity evaluations of food chemicals and new food proteins

The rapid advances in the field of immunology and an understanding of the potential adverse effects of xenobiotics on the immune system have resulted in the development of a discipline in toxicology now referred to as immunotoxicology. This discipline has evolved steadily over the last 2 decades as a result of research in the national and international communities. Various US, European, and Japanese regulatory agencies have recognized a need to promulgate testing guidelines for immunotoxicity in support of the approval process involving toxicological testing. The US Food and Drug Administration "Redbook II" guidelines and some of the research conducted in support of the concepts and testing strategies are presented here. Concerns raised with regard to these guidelines are included, as are on-going initiatives in development of experimental approaches for assessing allergic potential and/or hypersensitivity responses to new foods and food constituents.

Age-dependent remodeling of rat thymus. Morphological and cytofluorimetric analysis from birth up to one year of age

Structural and phenotypic modifications of rat thymocytes from birth up to one year of age, i.e. during maturation and at the beginning of the involutive process of the thymus are described. Since the biological significance and the mechanisms of thymic involution are still a matter of debate, this study aims at clarifying the complexity of the compensatory events occurring during this relatively neglected period of time. Thymuses from Sprague-Dawley rats were analyzed morphologically and morphometrically by light and electron microscopy. At the same time, thymocyte subsets, isolated from the same animals, were characterized by flow cytometry according to physical parameters and phenotypic markers. Results indicate that major changes occur during the first month from birth and from six months onward. In particular, already during the first weeks after birth, thymocytes undergo a slight reduction of mitoses associated with an increased number of apoptoses. Moreover, during the same period of time, flow cytometry revealed an expansion of small thymocytes and changes in thymocyte subsets such as increase of CD4+CD8+ and CD5+alpha(beta)TCR- and a decrease of CD4-CD8-, CD4-CD8+ cells. The thymus of adult rats was characterized by time-dependent decrease of both mitoses and apoptoses, progressive physical disconnection among cells, increase of necrotic areas and fibrosis. Around one year of age tissue changes were associated with a dramatic reduction of the population of large thymocytes and the rise of numerous small thymocytes that were unexpectedly negative for all tested markers. By contrast, medium-size thymocytes exhibited a marked decrease of CD4+CD8+ and CD5+alpha(beta)TCR- subsets. In conclusion, our data indicate that thymus undergoes, with time, a complex remodeling and suggest that thymic involution is not only a simple shrinkage of the organ but rather the result of a series of compensatory mechanisms among different cell populations in a setting of progressive involution.

Further evaluation of the incorporation of an immunotoxicological functional assay for assessing humoral immunity for hazard identification purposes in rats in a standard toxicology study

A previous study (Ladics et al., 1995) conducted in our laboratory using the known immunosuppressant agent, cyclophosphamide, indicated that a functional assay for assessment of humoral immunity may be conducted in rats in a standard toxicology study. The objective of this study was to further examine the feasibility of conducting an immunotoxicological assay for assessing humoral immunity in rats in a standard toxicology study using a chemical, carbon tetrachloride (CCl4), whose principal target organ of toxicity is not the immune system. Specifically, the previous study and this study were done to determine whether the conduct of an assay for assessing humoral immune function would affect standard toxicological endpoints. Male CD rats were untreated or dosed orally for 30 or 90 days, excluding weekends, with vehicle or 12.5 or 25 mg/kg CCl4. Six days prior to sacrifice, selected rats were injected intravenously with sheep red blood cells (SRBC) for assessment of humoral immune function. One day prior to necropsy, blood for hematological and clinical chemical measurements was collected from each rat. On the day of necropsy standard protocol tissues were collected, weighed, processed to slides, and later examined microscopically. One-half of each spleen was used to assess spleen cell numbers and quantitate lymphocyte subsets (Thelper; Tcyt/sup; total T- and B-cells) by flow cytometry. Serum was analyzed for anti-SRBC IgM antibody by using an enzyme-linked immunosorbent assay. Administration of 12.5 and 25 mg/kg CCl4 for 30 days decreased SRBC-specific serum IgM levels 42 and 45%, respectively, while 25 mg/kg CCl4 for 90 days increased SRBC-specific IgM levels by 50%. CCl4 did not alter splenic lymphocyte subset numbers nor the weight nor morphology of lymphoid organs. Exposure to 25 mg/kg CCl4 did increase liver weight and serum sorbitol dehydrogenase levels, as well as produce centrilobular fatty change. SRBC administration did not alter any hematological or clinical chemistry parameters, nor lymphocyte subset numbers. With the expected exception of the spleen (slight increase in number and size of germinal centers), administration of SRBC did not significantly alter the weights or morphology of routine protocol tissues. Furthermore, administration of SRBC did not mask the rather mild hepatotoxic effects of CCL4 exposure observed in this study. Based on these and previous findings, it appears that a functional assay for assessing humoral immunity may be conducted in animals on standard toxicology study without altering standard toxicological endpoints.

Immunotoxicology: hazard identification and risk assessment

In summary, immunotoxicology is a relatively new science that can be defined as the study of the consequences of exposure to drugs, chemicals, and environmental toxicants on the structure and function of the immune system. Laboratory animal studies over the last 20 years have clearly demonstrated as association between suppressed immune function and altered host defense. Furthermore, rodent-based screening approaches, even with their limitations, have been reasonably successful and have added to this knowledge base. The challenges for the future lie in using these data to design better prospective human exposure studies and to improve the basis for immunotoxicology risk assessment.

Differential effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin, bis(tri-n-butyltin) oxide and cyclosporine on thymus histophysiology

Recent advances in the histophysiology of the normal thymus have revealed its complex architecture, showing distinct microenvironments at the light and electron microscopic level. The epithelium comprising the major component of the thymic stroma is not only involved in the positive selection of thymocytes, but also in their negative selection. Dendritic cells, however, are more efficient than epithelial cells in mediating negative selection. Thymocytes are dependent on the epithelium for normal development. Conversely, epithelial cells need the presence of thymocytes to maintain their integrity. The thymus rapidly responds to immunotoxic injury. Both the thymocytes and the nonlymphoid compartment of the organ can be targets of exposure. Disturbance of positive and negative thymocyte selection may have a major impact on the immunological function of the thymus. Suppression of peripheral T-cell-dependent immunity as a consequence of thymus toxicity is primarily seen after perinatal exposure when the thymus is most active. Autoimmunity may be another manifestation of chemically mediated thymus toxicity. Although the regenerative capacity of thymus structure is remarkable, it remains to be clarified whether this also applies to thymus function. In-depth mechanistic studies on chemical-induced dysfunction of the thymus have been conducted with the environmental contaminants 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and bis(tri-n-butyltin)oxide (TBTO) as well as the pharmaceutical immunosuppressant cyclosporine (CsA). Each of these compounds exerts a differential effect on the morphology of the thymus, depending on the cellular targets for toxicity. TCDD and TBTO exposure results in cortical lymphodepletion, albeit by different mechanisms. An important feature of TCDD-mediated thymus toxicity is the disruption of epithelial cells in the cortex. TBTO primarily induces cortical thymocyte cell death. In contrast CsA administration results in major alterations in the medulla, the cortex remaining largely intact. Medullary epithelial cells and dendritic cells are particularly sensitive to CsA. The differential effects of these three immunotoxicants suggest unique susceptibilities of the various cell types and regions that make up the thymus.

Sensitivity of mouse lymphoid and nonlymphoid organs to Silesian air pollutants

The toxic effect of Silesian air pollutants to mouse organs was examined. Histological changes were found in the examined lymphoid organs (thymus, spleen) as well nonlymphoid organs (liver, kidneys). The alterations in weight indexes of lymphoid organs were also observed. Considerable changes in cellularity, weight index, and histology of the thymus in the mice exposed to air pollutants suggest the atrophy of this organ, which may lead to extrathymic T-cell differentiation and even acceleration of thymocytes maturation, which may lead to certain allergic or auto-immune pollutants of all investigated mouse organs in the following order: thymus, liver, kidneys, and spleen.

Immunotoxicological effects of benzene inhalation in male Sprague-Dawley rats

The inhalation of benzene is toxic to various components of the immunologic system in rodents. Spleen and thymus weights, total spleen and femur marrow cell counts, enumeration of spleen B- and T-lymphocytes, and an assessment of humoral immunocompetence, were used to evaluate the immunotoxicity of benzene in male Sprague-Dawley rats. Rats were exposed to 0, 30, 200 or 400 ppm benzene for 6 h/day, 5 days/week for 2 or 4 weeks. An early indicator of immunotoxicity was a reduction in the number of B-lymphocytes after 2 weeks of 400 ppm. After 4 weeks of 400 ppm, there was a reduction in thymus weight and spleen B-, CD4+/CD5+ and CD5+ T-lymphocytes. Rats exposed to 30, 200 or 400 ppm benzene for 2 or 4 weeks and challenged with sheep red blood cells developed a humoral response comparable to that of the control (0 ppm) animals. Enumeration of spleen T- and B-lymphocytes in rats exposed to benzene and challenged with SRBC showed only a transient reduction in spleen B-lymphocytes after 2 weeks of exposure to 400 ppm. These data suggest that there are no immunotoxicological effects of exposure to 200 ppm benzene or less, in rats exposed for 6 h/day, 5 days/week for 2 or 4 weeks.

Dose-response relationships of cytotoxicity, PFC response and histology in the spleen in rats treated with alkylating agents

Our previous study revealed the appropriate conditions for the plaque- forming cell (PFC) assay in rats. Using this assay in the present study, dose-response relationships of cytotoxicity, PFC response and histology in the spleen were evaluated in rats receiving alkylating agents. Rats were given a single intravenous administration of cyclophosphamide (CY) at a dose of 3, 10 or 30 mg/kg. Spleen weights and cellularity were decreased in the rats treated with 30 mg/kg. Suppressed PFC response wes observed in the rats receiving 10 mg/kg or more. In the rats treated with CY at 1, 3 or 10 mg/kg for 7 days, spleen weights and cellularity and PFC response were reduced at doses of 3 mg/kg or more. Treatment with the other alkylating agents, however, had a different consequence. Namely, in the rats treated with nitromin once or for 7 days, spleen weights and cellularity were decreased at a dose lower than that causing a reduction in the PFC response. In the rats treated with melphalan or chlorambucil, the weights and cellularity of the spleen tended to be decreased at a dose lower than that suppressing the PFC response. Histologically, in the case of CY, the marginal zone was narrow with cellular depletion in the rates receiving 3 mg/kg, whereas little change was seen in the periarteriolar lymphoid sheath (PALS). At a dose of 10 mg/kg, the marginal zone was markedly atrophied and slight atrophy of the PALS was seen. On the other hand, in the rats treated with nitromin, a dose-related decrease in the size of the spleen was seen without changes in the tissue architecture. Melphalan caused atrophy of both the marginal zone and the PALS at a dose suppressing PFC response. Regarding the red pulp, the extramedullary hematopoiesis disappeared with melphalan and nitromin, but not with CY. These results indicate that the decreases in weights and cellularity and histological changes in the spleen caused by the alkylating agents are detectable at the dose suppressing PFC response except for CY, which has a marked immunosuppressive action. Furthermore, the observed histological findings in the spleen were characteristic of each alkylating agent.

Pathology of the thymus: changes induced by xenobiotics and gene targeting

Studies on the thymus in pathologic conditions have been of great help in the elucidation of the function of the organ in T-cell development. The first examples come from congenital immunodeficiency states in man and laboratory animals. A number of toxic substances affect different components of the thymus already at exposure levels where there is no effect on the peripheral immune system. In some cases, this thymotoxic effect has been causally related to defects in the peripheral immune system (immunodeficiency and autoimmunity). In recent years immunodeficient states have been created in mouse by disruption of genes coding immunologically relevant molecules. Studies on such gene 'knock-out' mice have shown that a number of molecules are indispensable for appropriate T-cell development at different stages in the thymus, whereas others are dispensable. It is concluded that the experimental approach combining gene targeting and exposure to thymotoxic xenobiotics will present interesting tools for further studies in thymus research.

Pathology considerations for, and subsequent risk assessment of, chemicals identified as immunosuppressive in routine toxicology

Several proposals have been made with the aim of assisting in the early identification of chemicals with immunotoxic potential. The Organisation for Economic Cooperation and Development is now likely to incorporate enhanced immunopathology into the test guideline for the 28-day rat study, which may be regarded as a Tier I investigation. However, no guidelines have yet been proposed either for how the new data generated will be evaluated, or for how a subsequent risk assessment will be made. In this paper, considerations for the immunopathological assessment of the thymus, spleen, lymph nodes and bone marrow are described, together with comments on haematological and organ weight changes that may be associated with immunotoxicity. Their interpretation will depend on the doses at which changes are manifest, the quantity and quality of the effects observed and the presence and severity of other forms of toxicity. Lastly, risk assessment and the approach to Tier II testing in immunotoxicity is discussed. It is concluded that much of this work must be on a case-by-case basis, but should not in principle differ from the approach adopted for any other type of toxicity identified ina 28-day study.

Possible incorporation of an immunotoxicological functional assay for assessing humoral immunity for hazard identification purposes in rats on standard toxicology study

The objective of this study was to examine the feasibility of conducting an immunotoxicological assay for assessing humoral immunity in rats on standard toxicology study. Male CD rats were untreated or dosed intraperitoneally daily for 30 or 90 days, excluding weekends, with vehicle or 2 mg/kg cyclophosphamide (CY). Six days prior to sacrifice, selected rats were injected intravenously with sheep red blood cells (SRBC). One day prior to necropsy, blood samples for hematological and clinical chemical measurements were collected from each rat. On the day of necropsy standard protocol tissues were collected, weighed, processed to slides, and examined microscopically. One-half of each spleen was used to prepare a single cell suspension in order to assess spleen cell numbers. Serum was analyzed for anti-SRBC IgM antibody using an enzyme-linked immunosorbent assay. A second set of studies was performed to examine further the effect of SRBC administration on lymphoid organ weights using 30- and 90-day study age-equivalent naive male CD rats. Exposure of animals to 2 mg/kg CY for 30 or 90 days resulted in a 28% and 61% decrease, respectively, in SRBC-specific serum IgM levels. CY treatment also caused mild alterations in some leukocytic parameters, with significant decreases of 35% and 33% in white blood cell and lymphocyte counts, respectively, observed in 30-day CY-treated animals receiving SRBC. Injection of SRBC alone did not alter hematological or clinical chemistry parameters. With the expected exception of the spleen (increased number and size of germinal centers), administration of SRBC did not significantly alter the weights or morphology of routine protocol tissues. Furthermore, administration of SRBC did not mask the immunosuppressive effects of CY treatment under the conditions of this study. Based on our preliminary findings, a functional assay for assessing humoral immunity may be conducted in animals on standard toxicology study.