Development and Validation of the Antibody-Forming Cell Response as an Immunotoxicological Endpoint in the Guinea Pig

Abstract

The mouse and rat are often the species of choice in standard toxicologj-cal testing. However, due to potential differences in chemical metabolism between these species and humans, they may in some instances be inappropriate models for chemical safety assessment. Because the guinea pig is a well-known model for chemical sensitization and hypersensitivity testing, the objective of the present study was to develop the hemolytic plaque assay in the guinea pig for use as an immunotoxicological endpoint in toxicology studies where metabolic differences between this and other rodent species may affect risk characterization. The present study encompassed six experiments designed to address route of administration, appropriate number of sheep red blood cells (sRBC) for immunization, time course after immunization, and a validation experiment to test the ability to detect suppression produced by a known immunosuppressant. Animals were observed daily for morbidity, mortality, and any signs of distress. Animals were weighed and euthanized and spleens were collected and weighed. The plaque assay was performed and the number of antibody-forming cells (AFQ/spleen, AFC/106 splenocytes, and total spleen cellularity was determined. The data indicate that retroorbital injection of 2 109 sRBC in a volume of 0.5 mL 5 days prior to euthanasia produces the peak AFC response in the guinea pig. In addition, IP injection of cyclophosphamide (30 mg/kg day−9) for 6 days prior to euthanasia produces profound immunosuppression in the absence of any observed overt toxicity. The peak serum anti-sRBC enzyme-linked immunosorbant assay (ELISA) response is delayed in comparison to the AFC response, and occurred on day 8 after retroorbital immunization in the guinea pig.