Histopathology of the immune system as a tool to assess immunotoxicity

Immunotoxicology studies the undesired effects of interactions between xenobiotics and the immune system, mainly in toxicity experiments in rodents. The histopathology of the lymphoid organs is a cornerstone in such studies. In this review we describe practical aspects of sampling lymphoid organs and subsequent tissue processing and application of conventional and advanced histologic techniques. Thereafter, some aspects of proper reading and interpretation of histopathology is discussed, in relation to modifying factors such as age, sex, strain of animals, housing conditions, and nutritional status. These factors can substantially confound the outcome and interpretation of experiments, due to the highly dynamic characteristics of the immune system. Immunotoxicity tests are normally performed in a tiered approach. We describe the screening tier in the rat species that has been developed in the National Institute of Public Health and Environmental Protection, Bilthoven, The Netherlands, and illustrate the value of histopathology by an example of immunotoxicity testing of pesticides. Subsequently, the tiered approach in the mouse species followed by the National Toxicology Program in the USA, is described. In the evaluation of chemicals with suspected immunotoxic potential using this approach, histopathology proved to be less sensitive in 'flagging' immunotoxicity. This may be related to the lower doses that are applied in this toxicity design, because at higher doses histopathology is a sensitive indicator of toxicity. A global description of pathologic alterations after toxic insult is given, followed by representative examples taken from immunosuppressive drugs--the cytostatic agent 5-fluorouracil, and drugs interfering with cytokine expression, namely, Cyclosporin A, FK-506, and Rapamycin.