Industry experience in the identification of the immunotoxic potential of agrochemicals

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.

Immunotoxic effects of imidacloprid following 28 days of oral exposure in BALB/c mice

Imidacloprid, a neonicotinoid insecticide has been in use worldwide for several years in agriculture and veterinary medicine. It is possible that residue of this compound may be recycled in the food chain and thus information regarding effects from potential exposure to it is warranted. The objective of the present study was to evaluate immunotoxic effects of imidacloprid in female BALB/c mice. Imidacloprid was administered orally daily at 10, 5, or 2.5mg/kg over 28 days. Specific parameters of humoral and cellular immune response including hemagglutinating antibody (HA) titer to sheep red blood cells (SRBC; T-dependent antigen), delayed type hypersensitivity (DTH) response to SRBC, and T-lymphocyte proliferation in response to phytohemagglutinin (PHA) were evaluated. The results showed that imidacloprid at high dose, specifically suppressed cell-mediated immune response as was evident from decreased DTH response and decreased stimulation index of T-lymphocytes to PHA. At this dose, there were also prominent histopathological alterations in spleen and liver. Histopathological analysis of footpad sections of mice revealed dose-related suppression of DTH response. Imidacloprid at low dose of 2.5mg/kg/day did not produce any significant alterations in cellular and humoral immune response and it seemed to be an appropriate dose for assessment of 'no observable adverse effects level' for immunotoxicity in BALB/c mice. The results also indicated that imidacloprid has immunosuppressive effects at doses >5mg/kg, which could potentially be attributed to direct cytotoxic effects of IMD against T cells (particularly TH cells) and that long-term exposure could be detrimental to the immune system.

General aspects of immunotoxicology including validation issues

Histologic examination of lymphoid organs has revealed immunotoxic effects of a broad range of substances. The thymus has proven especially sensitive. The relative lack of sensitivity of mucosa-associated lymphoid cells and tissues may be due to shortcomings in the way they are examined. Validation of ways to examine mucosal lymphoid tissues and cells and development of histopathological tools to flag a compound as a potential inducer of autoimmune disease or allergy are challenges for the future.

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.

Evaluation of immunotoxicity induced by propoxure in C57Bl/6 mice

Insecticides are important candidates for immunotoxicology tests in order to assess the 'No Observable Adverse Effect Level' (NOAEL). Propoxure (PPX), as a carbamate household insecticide, has been used for several decades around the world in agriculture. However, there has been no clear investigation on its immunotoxic potentials. In this study, as Tier I of immunotoxicological screening tests, we examined the effects of subacute exposure to intraperitoneally (i.p.) administered PPX, at doses of 10, 2, and 0.2 mg/kg, on C57Bl/6 female mice. After 28 days administration of PPX, the treated animals were sacrificed and peripheral blood samples were collected. Then spleen (SP), thymus (TM), and bone marrow were collected and weighed. Functional tests including SRBC-hemagglutination (HA), plaque-forming colony assay (PFC), and delayed type hypersensitivity (DTH) response to SRBCs were performed. Furthermore, spleen T-cell phenotype (CD4/CD8) was also determined. Results showed that high doses of PPX could not only produce histopathological changes in TM and SP but also suppress humoral response to antigen (SRBCs). PPX at medium doses (2 mg/kg) did not show histopathological changes in TM or SP but increased the humoral response to SRBCs, as identified by HA and PFC tests. PPX at low doses (0.2 mg/kg) did not produce any significant changes in humoral (PFC and HA) or cellular responses (DTH) of the immune system. It seems that PPX has no adverse effects on mice immune system at low doses of 0.2 mg/kg/day, which is 10 times the PPX allowed daily intake (ADI) limit.