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

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.

Variability of Spleen and Mesenteric Lymph Node in Control Cynomolgus Monkeys ( Macaca fascicularis) from Nonclinical Safety Studies: A Retrospective Assessment

We assessed the variability of spleen and mesenteric lymph node (MLN) microscopic observations and the correlations of these observations with other study data from 478 control cynomolgus monkeys from 53 routine nonclinical safety studies. Spleen weight parameters (absolute and relative to body or brain weights) were highly variable both within a control group on an individual study (up to 5.11-fold) and among animals with the same light microscopic observation. Grades for microscopic observations were also highly variable. The most frequent microscopic observations for spleen were changes in the size and number of germinal centers (58%), acidophilic (hyaline) material in lymphoid follicles (52%), and compound lymphoid follicles (20%). The most frequent microscopic observations in the MLN were eosinophil infiltrates (90%), changes in size and number of germinal centers (42%), and brown pigment (21%). The only meaningful relationships ( r² > 0.3) were positive correlations between reticuloendothelial hyperplasia and malarial pigment in the spleen and between each of these observations and spleen weight parameters. We conclude that determination of test article-related effects on the immune system in routine monkey toxicology studies requires careful consideration and a weight-of-evidence approach due to the low numbers of animals/group, the inherent variability in spleen and MLN parameters, and the infrequent correlation among immune system-related end points.

Maturity-related Variability of the Thymus in Cynomolgus Monkeys (Macaca fascicularis)

Terminal body weights (TBWs), thymus weight parameters, and thymus morphology were retrospectively evaluated in 453 cynomolgus monkeys assigned to control groups on nonclinical toxicity studies. Morphology of bone, ovary, and testis/epididymis were used to determine maturity status of individual animals. There was no correlation between TBW and thymus weight (absolute and/or relative to TBW or brain weight). Thymus weight parameters and grades of decreased lymphocytes in the thymus were highly variable in immature animals compared to mature animals. There was also high (up to 11-fold) variability of thymus weight parameters within a given control group on the same study (generally 3 or 4 animals per sex). Several parameters evaluated had more pronounced age-related changes in males when compared to females. Our results demonstrate the inherent variability of thymus weight parameters and morphologic observations for cynomolgus monkeys on toxicology studies. Changes in thymus parameters in cynomolgus monkeys are unreliable indicators of immunomodulation or immunotoxicity in the absence of other relevant findings. Therefore, the thymus parameters commonly evaluated in preclinical safety assessments should not be the primary data set used to determine the presence of a direct test article-related effect on the immune system.

Acute oral toxicity evaluations of some zinc(II) complexes derived from 1-(2-salicylaldiminoethyl)piperazine Schiff bases in rats

The current study described the synthesis and the in vivo acute oral toxicity evaluations in Sprague Dawley rats. The compounds were characterized by elemental analyses, LC-MS, FTIR, ¹H NMR, ¹³C NMR and UV-visible spectroscopy. In the acute toxicity study, a single administration of the compounds was performed orally to the rats at the single doses of 2000 mg/kg and they were then monitored for possible side effects, mortality or behavioral changes up to 14 days. The serum level of aspartate (AST), alanine aminotransferases (ALT), alkaline phosphate (ALP), triglyceride, high density lipoprotein (HDL), immunoglobulins (GAM) and the C-reactive proteins did not significantly change. The hematological indices white blood cells (WBC), haematocrit (HCT), red blood cells (RBC), mean corpuscular volume (MCV), mean corpuscular haemoglobin concentration (MCHC), and mean corpuscular hemoglobin (MCH) were within the normal range. The renal function indices examined were also within the reference range. Generally, the compounds exhibited low toxic effects as required for further in vivo therapeutic studies.

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.

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.

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.

Methods of evaluating immunotoxicity

Immunotoxicology is an important aspect of the safety evaluation of drugs and chemicals. Immunosuppression, (unspecific) immunostimulation, hypersensitivity and autoimmunity are the four types of immune-mediated adverse effects. However, the nonclinical assessment of immunotoxicity is at present often restricted to animal models and assays to predict unexpected immunosuppression. There is, however, no general consensus that a variety of assays can be considered depending on the compound to be tested. A major issue is whether histological examination of the thymus, spleen, lymphoid organs and Peyer's patches is a reliable predictor of immunosuppression or whether immune function should also be assessed. A T-dependent antibody response assay, either the plaque-forming cell assay or anti-keyhole limpet haemocyanin enzyme-linked immunosorbant assay, is recommended as a first-line assay. A variety of assays, including lymphocyte subset analysis, natural killer-cell activity, lymphocyte proliferation, delayed-type hypersensitivity, cytotoxic T-lymphocyte activity and macrophage/neutrophil function assays, can also be used. In certain circumstances, host resistance assays can be considered. With the exception of contact sensitisation, very few animal models and assays can reliably predict the potential for (unspecific) immunostimulation, hypersensitivity or autoimmunity. A major limitation of immunotoxicity risk assessment is the lack of human data. Immunological end points and clinical criteria to be included in clinical trials and epidemiological studies have to be carefully standardised and validated.

A tiered approach to systemic toxicity testing for agricultural chemical safety assessment

A proposal has been developed by the Agricultural Chemical Safety Assessment (ACSA) Technical Committee of the ILSI Health and Environmental Sciences Institute (HESI) for an improved approach to assessing the safety of crop protection chemicals. The goal is to ensure that studies are scientifically appropriate and necessary without being redundant, and that tests emphasize toxicological endpoints and exposure durations that are relevant for risk assessment. The ACSA Systemic Toxicity Task Force proposes an approach to systemic toxicity testing as one part of the overall assessment of a compound's potential to cause adverse effects on health. The approach is designed to provide more relevant data for deriving reference doses for shorter time periods of human exposure, and includes fewer studies for deriving longer term reference doses-that is, neither a 12-month dog study nor a mouse carcinogenicity study is recommended. All available data, including toxicokinetics and metabolism data and life stages information, are taken into account. The proposed tiered testing approach has the potential to provide new risk assessment information for shorter human exposure durations while reducing the number of animals used and without compromising the sensitivity of the determination of longer term reference doses.

Immune Function Tests for Hazard Identification: A Paradigm Shift in Drug Development

Routine immune function testing in preclinical drug development was established as a regulatory requirement in June of 2000 under the Committee of Proprietary Medicinal Products (CPMP) Note for Guidance on Repeated Dose Toxicity (CPMP/SWP/1042/99). The purpose of the more stringent approach to immunotoxicology testing was to better identify unintended immunosuppression; however, the requirement was met with much discussion and debate. At the center of the discussion was an attempt to reconcile opposing regulatory directives from agencies outside of Europe that adhere to a more selective, weight-of-evidence approach to functional evaluations. Uncertainty over the predictive value of the recommended immune function tests relative to conventional toxicology parameters prompted an investigation by the International Committee on Harmonization (ICH). The results of a preliminary, industry-wide survey indicated that only a low percentage of pharmaceuticals adversely affect immune function without alterations to standard toxicology parameters. Expected ICH guidelines will ultimately determine to what extent and for what purpose immune function tests will be conducted. In the meantime, optimization of the recommended immune function tests is ongoing. The T-cell dependent antibody response (TDAR) by either conventional Sheep Red Blood Cell (SRBC) plaque assay or by the modified ELISA method using either SRBC or keyhole limpet hemocyanin (KLH) as antigen is being extensively evaluated to determine best practices and procedures for preclinical immunotoxicity evaluations. This review addresses some aspects of the debate concerning the appropriateness of immune function tests for hazard identification, along with recommendations for optimizing TDAR methodology to ensure adequate sensitivity and predictability in risk assessments for immunotoxicity.

Harmonization of immunotoxicity guidelines in the ICH process--pathology considerations from the guideline Committee of the European Society of Toxicological Pathology (ESTP)

As part of the ICH process of harmonization of testing guidelines for immunotoxicity, the European Society of Toxicologic Pathology (ESTP) has contributed to the scientific discussion on methods and evaluation of immunotoxicity studies with technical and scientific recommendations on toxicologic pathology. The weighing and sampling of immune organs is discussed taking into consideration specifically the value of lymph node weighing and the selection of appropriate lymph nodes for the detection of local and systemic effects. The different techniques of bone marrow preparation are considered for routine and extended investigations. Criteria are given for the gross and histopathological detection of effects in Peyer's patches. For the histopathological evaluation it is strongly recommended that each compartment within the different lymphoid organs is investigated separately and semiquantitatively since this approach has shown to increase the sensitivity and specificity of immunohistopathology.

Inhalation exposure to methylene chloride does not induce systemic immunotoxicity in rats

Methylene chloride (dichloromethane) is used in a variety of industrial applications. To date, there has been no formal assessment of immunotoxicity attributed to methylene chloride. Studies were undertaken to examine whether methylene chloride has any potential to influence the integrity of immune function. For this purpose, Sprague-Dawley rats of both genders were exposed by inhalation to a single high dose (5000 ppm) of methylene chloride for 6 h/d, 5 d/wk for 28 d. This was considered the relevant route of administration, as not only is inhalation a primary route for human exposure to methylene chloride, but, also, the chemical is absorbed rapidly via the lungs. Under these conditions of exposure, methylene chloride failed to influence absolute or relative thymus weights in either gender and produced a significant reduction in relative, but not absolute, spleen weight in female rats only. Immunocompetence was measured as a function of the ability of treated animals to mount immunoglobulin M (IgM) antibody responses to sheep red blood cells (SRBC) as determined by enzyme-linked immunosorbent assay (ELISA). Exposure to methylene chloride did not affect antibody production. Evidence indicates that under these conditions of exposure, methylene chloride did not compromise immune function.

Effects of sevoflurane general anesthesia: immunological studies in mice

Based on the immunomodulatory effects of anesthesia and surgery, a study was undertaken to assess the effect of sevoflurane anesthesia on the immune system in a murine model without surgery. Adult male mice were anesthetized with 3% sevoflurane (1.2 minimal alveolar concentration, MAC) in oxygen for 40 min, whereas nontreated animals served as controls. After sevoflurane anesthesia, peripheral blood leukocyte counts, the splenic composition and in vitro macrophage phagocytic activity and lymphoproliferative response were assessed. The in vivo specific immune response to sheep red blood cells (SRBC), a conventional T-dependent antigen was determined. In addition, liver, spleen, thymus and kidney histopathology and also hepatic and renal functions after anesthesia were studied. Sevoflurane diminished the number of peripheral blood lymphocytes and splenic B-cell counts, enhancing CD4+ lymphocytes in spleen. The in vitro functionality of macrophages and the mitogen-induced lymphoproliferative response were preserved, while the in vivo immune response to SRBC was enhanced in treated animals. Microscopic studies revealed conserved architecture of the spleen, thymus, lymph node, liver and kidney, and there were no differences in serum parameters of hepatic and renal functions between treated and control groups. Our results suggest that 3 days after the anesthetic exposure, animals treated with sevoflurane modulated their peripheral blood leukocyte counts, splenic lymphoid composition and the characteristics of the specific response to SRBC, while there was no evidence of hepatic or renal toxicity.

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.

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.

A local lymph node assay to analyse immunosuppressive effects of topically applied drugs

Topical glucocorticosteroids represent the mainstay of antiinflammatory therapy in the treatment of inflammatory skin diseases. Their clinical use, however, is limited by local and systemic side-effects. Thus, in dermatopharmacology there is a large demand for alternative non-steroidal antiinflammatories. Other than transplantation models, most of the frequently used in vivo test systems for assessment of drug-induced immunosuppression measure changes in inflammatory skin responses by means of skin erythema and edema after challenge of sensitized animals. The aim of this study was to develop an alternative mouse model to detect and analyse immunosuppressive effects of topically applied drugs. On the basis of a modified local lymph node assay, we analysed effects of topical hydrocortisone, dexamethasone, mometasone furoate and FK506 (tacrolimus) during the induction phase of contact hypersensitivity. On 4 consecutive days, NMRI mice were treated on the dorsal surfaces of both ears with increasing concentrations of test compound. During the last 3 days, the mice received in addition the contact sensitizer, oxazolone (1%). On day 5, draining auricular lymph nodes were removed in order to assess lymph node cell counts and perform flow cytometric analysis of lymph node cell subpopulations (CD4+/CD25+, Ia+/CD69+, Ia+/B220+). All test compounds proved to exert significant immunosuppressive effects after topical application, but showed differences in their immunomodulatory potential. In conclusion, the local lymph node assay serves as an appropriate model to characterize immunosuppressive effects of topically applied drugs by measuring immunologically relevant end-points.