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.