The role of conventional pathology and toxicology in evaluating the immunotoxic potential of xenobiotics

ECITTS: An Integrated Approach to the Application of In Vitro Test Systems to the Hazard Assessment of Chemicals,

As a result of a workshop held at Täljöviken, Åkersberga, Sweden, on 27–29 May 1991, a multicentre collaborative research project was established, with the purpose of developing the concept of integrated in vitro toxicity testing. The first priority was the selection of tests within eight appropriate areas: basal cytotoxicity, irritancy, developmental toxicity, hepatotoxicity, nephrotoxicity, immunotoxicity, neurotoxicity and biokinetics. An ideal battery of tests for each area was identified. Since it was realised that it would not be feasible to include the full ideal list of tests in the project, a minimum test list was also agreed. For each area, ten calibration chemicals were selected. From these 80 compounds, 30 were selected for inclusion, together with 20 of the OECD test programme chemicals, in a first test set of chemicals. The toxicity of these 50 test set chemicals will be investigated in the minimum integrated test scheme.

The aim of the project is to employ non-animal methods to assess the toxicological properties of chemicals, and to improve this assessment through the use of knowledge about mechanisms of toxic action. The information produced will contribute to the establishment of a more-scientific and more-efficient toxicological procedure for hazard assessment. Questions concerning which parameters need to be investigated and combined to make hazard assessments, and which parameters relevant to in vivo toxicity can be determined in non-whole animal test systems, will also be addressed.

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.

Report of validation study of assessment of direct immunotoxicity in the rat. The ICICIS Group Investigators. International Collaborative Immunotoxicity Study

The International Collaborative Immunotoxicity Study (ICICIS) was established in 1986 as a joint activity of the International Programme on Chemical Safety (a cooperative programme of the United Nations Environment Programme, the International Labour Office and the World Health Organization), the Commission of the European Union and the United Kingdom Department of Health. The objectives were to examine whether various experimental techniques could be used in the rat to indicate toxic effects on the immune system, and so to suggest their possible value as general indicators of immunotoxicity. For this purpose scientists in a number of laboratories in different countries agreed to do joint studies, first of azathioprine (AZA) and then of cyclosporin A (CYA), as potent immunosuppressive compounds. The general experimental procedures and the detailed techniques employed were selected to explore whether the limited pathological investigations in the conventional 28-day subacute toxicity test in the rat (OECD, 1995), or 'Enhanced Pathology' tests (weight determination and examination of additional lymphoid organs and application of structured assessment and semiquantitative grading of changes in the principal compartments of lymphoid tissues) would suffice to indicate ('flag') the occurrence of immunotoxicity due to a chemical, or whether specific tests of immune function would be required. When the second chemical, CYA, was studied, standardisation of the protocol for the test, and of all the investigations, and prior training of the toxicological pathologists in the structured assessment scheme, were shown to greatly reduce interlaboratory variation in the results. The initial investigation of AZA had shown that the indicative power of the experiments had been diminished by the use of diverse experimental procedures. The studies were done as toxicity tests, in which three dose levels, including the maximum tolerated dose, and a vehicle control group were employed. The prime objective was to detect any immunotoxic effect of the test compound. It was shown that the limited, basic 'Pathology' investigations specified in Guideline 407 (OECD, 1981) were not able to reveal the effects on the immune system of AZA and CYA, but that several of the additional 'Enhanced Pathology' investigations did so. Of the immune function assays, the most reliable and useful was the 'Antibody Forming Cell' technique. Other immunological evaluations, such as examination of proliferation induced by selected mitogens and NK cell assay, showed promise. The methods employed have the potential to reveal an 'immunotoxic' effect, without necessarily indicating its mechanism, although the changes in different lymphoid compartments can afford a valuable guide to the detailed nature of the toxic action on the immune system. Function tests, being inherently quantitative in nature may well suggest the nature or mechanism of any effect seen, in addition to indicating the occurrence of the effect. However, they may be less convenient to do as part of a routine toxicity test. Overall, the work in ICICIS has shown that the immunotoxic actions of two chemicals were detectable within a 28-day subacute oral toxicity test in the rat, provided that the conventional laboratory procedures were extended to include extra investigations. Both additional selected pathology investigations and immune function tests 'flagged' the immunotoxicity.

Stimulatory effects of FK156 in a panel of tests designed to detect changes in immune function

There is a need to evaluate the utility of experimental models in immune function assessment if these are to be accepted in preclinical safety studies. We have evaluated a panel of tests measuring cellularity and functions of the lymphoid system in the Fischer rat in order to determine whether they would detect immunostimulation, rather than suppression. Injection of the peptide immunostimulant FK156 (D-lactyl-L-alanyl-y-D-glutamyl-(L)-meso-diaminopimelyl- (L)-glycine) increased the numbers of macrophages recovered from the peritoneal cavity, and stimulated their activity, as measured by chemiluminescence, adherence, and secretion of interleukin 1. In vitro, T lymphocytes had an increased background incorporation of tritiated thymidine, increased response to sub-optimal concentrations of concanavalin A, and an increase in secretion of interleukin 2 at optimal concentrations of concanavalin A. There was no change in the proliferative responses of B lymphocytes in vitro. Antibody responses to tetanus toxoid in vivo were increased. These changes were not reflected in consistent, statistically significant alterations in the numbers of lymphocytes bearing either lineage markers or the interleukin 2 receptor as a marker of activation.

Techniques for assessing canine mononuclear phagocyte function as part of an immunotoxicologic evaluation

Although the dog is used extensively as a model for toxicologic investigation, few practical techniques for assessing immunotoxicity in this species have been reported. We have adapted techniques for assessing peripheral blood monocyte (PBM) function in the dog. Methods for isolating PBM were compared; a technique employing hypertonic conditioning followed by density gradient centrifugation, which resulted in greater than 90% purity and a greater yield of PBM than that obtained with other previously reported methods is described, along with methods for measuring antibody-dependent cellular cytotoxicity (ADCC) and the production of prostaglandin E2 (PGE2) and superoxide anion (O2-). An assay for the measurement of interleukin-1 (IL-1), based on its ability to induce interleukin-2 (IL-2) production by the EL-4 mouse thymoma line in the presence of the calcium ionophore A23187, was examined. Together, these assays provide the tools to better define drug or chemical effects on the mononuclear phagocyte system (MPS) of this important animal model.