Immunotoxicological investigation using pharmaceutical drugs. In vitro evaluation of immune effects using rodent or human immune cells

Consensus on the Key Characteristics of Immunotoxic Agents as a Basis for Hazard Identification

BACKGROUND

Key characteristics (KCs), properties of agents or exposures that confer potential hazard, have been developed for carcinogens and other toxicant classes. KCs have been used in the systematic assessment of hazards and to identify assay and data gaps that limit screening and risk assessment. Many of the mechanisms through which pharmaceuticals and occupational or environmental agents modulate immune function are well recognized. Thus KCs could be identified for immunoactive substances and applied to improve hazard assessment of immunodulatory agents.

OBJECTIVES

The goal was to generate a consensus-based synthesis of scientific evidence describing the KCs of agents known to cause immunotoxicity and potential applications, such as assays to measure the KCs.

METHODS

A committee of 18 experts with diverse specialties identified 10 KCs of immunotoxic agents, namely, 1) covalently binds to proteins to form novel antigens, 2) affects antigen processing and presentation, 3) alters immune cell signaling, 4) alters immune cell proliferation, 5) modifies cellular differentiation, 6) alters immune cell-cell communication, 7) alters effector function of specific cell types, 8) alters immune cell trafficking, 9) alters cell death processes, and 10) breaks down immune tolerance. The group considered how these KCs could influence immune processes and contribute to hypersensitivity, inappropriate enhancement, immunosuppression, or autoimmunity.

DISCUSSION

KCs can be used to improve efforts to identify agents that cause immunotoxicity via one or more mechanisms, to develop better testing and biomarker approaches to evaluate immunotoxicity, and to enable a more comprehensive and mechanistic understanding of adverse effects of exposures on the immune system. https://doi.org/10.1289/EHP10800.

The Key Characteristics of Carcinogens: Relationship to the Hallmarks of Cancer, Relevant Biomarkers, and Assays to Measure Them

The key characteristics (KC) of human carcinogens provide a uniform approach to evaluating mechanistic evidence in cancer hazard identification. Refinements to the approach were requested by organizations and individuals applying the KCs. We assembled an expert committee with knowledge of carcinogenesis and experience in applying the KCs in cancer hazard identification. We leveraged this expertise and examined the literature to more clearly describe each KC, identify current and emerging assays and in vivo biomarkers that can be used to measure them, and make recommendations for future assay development. We found that the KCs are clearly distinct from the Hallmarks of Cancer, that interrelationships among the KCs can be leveraged to strengthen the KC approach (and an understanding of environmental carcinogenesis), and that the KC approach is applicable to the systematic evaluation of a broad range of potential cancer hazards in vivo and in vitro We identified gaps in coverage of the KCs by current assays. Future efforts should expand the breadth, specificity, and sensitivity of validated assays and biomarkers that can measure the 10 KCs. Refinement of the KC approach will enhance and accelerate carcinogen identification, a first step in cancer prevention.See all articles in this CEBP Focus section, "Environmental Carcinogenesis: Pathways to Prevention."

Current Concepts in Natural Killer Cell Biology and Application to Drug Safety Assessments

Natural killer (NK) cells are lymphocytes capable of cytotoxicity against virally infected cells and tumor cells. The display of effector function by NK cells is the result of interactions between germline encoded activating/inhibitory NK cell receptors and their ligands (major histocompatibility complex class I, major histocompatibility complex class I-like, viral, and cellular stress-related surface molecules) expressed on target cells. Determination of NK cell number and function is a common element of the immunotoxicology assessment paradigm for the development of certain classes of pharmaceuticals across a range of modalities. This article summarizes the evidence associating NK cell dysfunction with infectious and cancer risks, reviews emerging NK cell biology, including the impact of immunogenetics on NK cell education and function, and provides perspectives about points to consider when assessing NK cell function in different species in the context of safety assessment.

The Use of In Vitro Systems for Evaluating Immunotoxicity: The Report and Recommendations of an ECVAM Workshop

This is the report of a workshop organised by the European Centre for the Validation of Alternative Methods (ECVAM). ECVAM's main goal, as defined in 1993 by its Scientific Advisory Committee, is to promote the scientific and regulatory acceptance of alternative methods that are of importance to the biosciences and which replace, reduce or refine the use of laboratory animals. One of the first priorities set by ECVAM was the implementation of procedures that would enable it to become well informed about the state-of-the-art of non-animal test development and validation, and the potential for the possible incorporation of alternative tests into regulatory procedures. It was decided that this would be best achieved by the organization of ECVAM workshops on specific topics, at which small groups of invited experts would review the current status of various types of in vitro tests and their potential uses, and make recommendations about the best ways forward (Anonymous, 1994). The workshop on "The use of in vitro systems for evaluating Immunotoxicity" was held at ECVAM (Ispra), Italy, on 24th-26th November 2003. The participants represented academia, national organizations, international regulatory bodies and industry. The aim of the workshop was to review the state-of-the-art in the field of in vitro immunotoxicology, and to develop strategies towards the replacement of in vivo testing. At the end of this report are listed the recommendations that should be considered for prevalidation and validation of relevant and reliable procedures, that could replace the use of animals in chemical and cosmetics toxicity testing.

In vitro cytokine release from human peripheral blood mononuclear cells in the assessment of the immunotoxic potential of chemicals

Alternative methods to the use of animals in testing of chemicals are needed. We investigated if the immunotoxic potential of 12 dietary toxicants could be predicted from effects on cytokine release from human peripheral blood mononuclear cells (PBMC) after in vitro exposure. Nine cytokines were selected to reflect different types of immune responses. The toxicants were classified as immunotoxic or non-immunotoxic substances according to the published in vivo data. Isolated human PBMC were exposed for 20 h to three concentrations of each of the 12 substances in the presence of human liver S9 fraction. After further incubation of PBMC in fresh medium containing the mitogen phytohemagglutinin (PHA, 10 μg/ml) for 48 h, release of the nine selected cytokines into the supernatant as well as cell proliferation were measured by Luminex technology™ and the BrdU incorporation assay, respectively. All 12 substances investigated affected the release of one or more cytokines, and each of the substances showed different cytokine release patterns. Within the limitations of the study design, the present study suggests that the effect of the substances on mitogen-induced cytokine release from PBMC cannot predict their immunotoxic potential, but may be useful in mechanistic studies.

Immunotoxicology: Opportunities for Non-animal Test Development

At present, several animal-based assays are used to assess immunotoxic effects such as immunosuppression and sensitisation. The use of whole animals, however, presents several secondary issues, including expense, ethical concerns and relevance to human risk assessment. There is a growing belief that non-animal approaches can eliminate these issues without impairing human safety, provided that biological markers are available to identify the immunotoxic potentials of new chemicals to which humans may be exposed. Driven by the 7th Amendment to the EU Cosmetics Directive, the new EU policy on chemicals (the REACH system), proposals to update the European legislation on the protection of animals used in research, and emerging visions and strategies for predicting toxicity, such in vitro methods are likely to play a major role in the near future. The realisation that the immune system can be the target of many chemicals, resulting in a range of adverse effects on the host's health, has raised serious concerns from the public and within the regulatory agencies. Hypersensitivity and immunosuppression are considered the primary focus for developing in vitro methods in immunotoxicology. However, in vitro assays to detect immunostimulation and autoimmunity are also needed. This review of the state-of-the-art in the field of in vitro immunotoxicity, reveals a lack of cell-based immunotoxicity assays for predicting the toxicity of xenobiotics toward the immune system in a simple, fast, economical and reliable way.

In vitro tests to evaluate immunotoxicity: A preliminary study

The implementation of Registration, Evaluation and Authorisation of new and existing Chemicals (REACH) will increase the number of laboratory animals used, if alternative methods will not be available. In the meantime, REACH promotes the use of in vitro tests and, therefore, a set of appropriated alternative testing methods and assessment strategies are needed. The immune system can be a target for many chemicals including environmental contaminants and drugs with potential adverse effects on human health. The aim of this study was to evaluate the predictivity of a set of in vitro assays to detect immunosuppression. The tests have been performed on human, rat and murine cells. Different endpoints have been assessed: cytotoxicity, cytokine release, myelotoxicity and mitogen responsiveness. For each of these endpoints IC50s values have been calculated. Six chemical substances, representative of the full range of in vivo responses and for which good human and/or animal data are available either from databases or literature, have been selected: two chemicals classified as not immunotoxic (Urethane and Furosemide), and four (tributyltin chloride (TBTC), Verapamil, Cyclosporin A, Benzo(a)pyrene) with different effect on immune system. All the tests confirmed the strong immunotoxic effect of TBTC as well as they confirmed the negative controls. For one chemical (Verapamil) the IC50 is similar through the different tests. The IC50s obtained with the other chemicals depend on the endpoints and on the animal species. The clonogenic test (CFU-GM) and the mitogen responsiveness showed similar IC50s between human and rodent cells except for Cyclosporin A and TBTC. All different tests classified the compounds analyzed in the same way.

Evaluation and prevalidation of an immunotoxicity test based on human whole-blood cytokine release

Immunotoxicology is a relatively new field in toxicology, and is one of emerging importance, because immunotoxicity appears to contribute to the development of cancer, autoimmune disorders, allergies and other diseases. At present, there is a lack of human cell-based immunotoxicity assays for predicting the toxicity of xenobiotics toward the immune system in a simple, fast, economical and reliable way. Existing immunotoxicity tests are mainly performed in animals, although species differences favour human-based testing. Whole-blood cytokine release models have attracted increasing interest, and are broadly used for pharmacological in vitro and ex vivo studies, as well as for pyrogenicity testing. We have adapted those methods for immunotoxicity testing, to permit the potency testing of immunostimulants and immunosuppressants. Following stimulation with a lipopolysaccharide or staphylococcal enterotoxin B, monocytes and lymphocytes release interleukin-1beta and interleukin-4, respectively. Thirty-one pharmaceutical compounds, with known effects on the immune system, were used to optimise and standardise the method, by analysing their effects on cytokine release. The in vitro results were expressed as IC50 values for immunosuppression, and SC(4) (fourfold increase) values for immunostimulation, and compared with therapeutic serum concentrations of the compounds in patients, and in vivo LD50 values from animal studies. The in vitro results correlated well with the in vivo data, so the test appears to reflect immunomodulation. Results were reproducible (CV = 20 +/- 5%), and the method could be transferred to another laboratory (r(2) = 0.99). We therefore propose this method for further validation and for use in immunotoxicity testing strategies.

Importance of natural killer cells in the rejection of hamster skin xenografts

BACKGROUND

In the hamster to rat xenogeneic combination, antibodies, T cells, and natural killer (NK) cells have all been implicated in the process of rejection. 3.2.3 is a mouse IgG1kappa monoclonal antibody (mAb) directed against NKR-P1A on rat NK cells. The purpose of this study was to evaluate the effect of this mAb independently and in combination with other immunosuppressive agents in a hamster to rat skin graft model in order to elucidate the mechanisms involved in xenograft rejection.

METHODS

Lewis rats were recipients of hamster skin grafts. Various groups received antilymphocyte serum (ALS) (days -1, 0, and +2), rapamycin (3 mg/kg; alternate days from day +1 through day +13), and 3.2.3 mAb (days 0, +1, and +2). Anti-hamster antibody production was determined serially with a complement-dependent cytotoxicity assay. Lewis anti-hamster mixed lymphocyte reaction and cell-mediated lympholysis assays were performed within 7 days after rejection of the skin graft. NK cell function was tested using a cytotoxicity assay versus YAC-1 target cells on day 14 or day 15 after skin grafting.

RESULTS

Median graft survival in untreated animals was 7 days. There was only modest prolongation in rats treated with rapamycin alone (median survival time [MST]=9 days) or ALS alone (MST=10 days). The use of 3.2.3 mAb in untreated rats (3.2.3 alone MST=7 days) and in ALS-treated rats (ALS+3.2.3 MST=9.5 days) did not improve graft survival. The combination of ALS+rapamycin substantially improved graft survival (MST=13 days), and even greater prolongation was seen with the addition of 3.2.3 mAb (ALS+rapamycin+3.2.3 MST=18.5 days). Cytotoxic antibodies, secondary mixed lymphocyte reaction responses, cytotoxic T cells, and normal NK activity were seen at the time of rejection in untreated rats as well as those treated with 3.2.3 mAb alone, ALS alone, ALS+3.2.3 mAb, and rapamycin alone. ALS+rapamycin completely blocked the formation of anti-hamster antibodies and cytotoxic T cells but did not suppress NK activity. The use of 3.2.3 mAb produced a marked but transient suppression of NK activity in all groups.

CONCLUSION

Hamster skin xenografts can be rejected by Lewis rats in the absence of cytotoxic antibodies and cytotoxic T cells. ALS, rapamycin, and ALS+rapamycin do not suppress NK activity in Lewis rats, although their use produces a modest prolongation of hamster skin graft survival. The administration of 3.2.3 mAb to Lewis rats results in a marked but transient suppression of NK cell function, which substantially prolongs hamster skin graft survival only when antibody and cytotoxic T-cell production have also been suppressed.