Characterization of the Action of Drug-Induced Stress Responses on the Immune System: Evaluation of Biomarkers for Drug-Induced Stress in Rats

Abstract

Toxicological testing of compounds often is conducted at the maximum tolerated dose to identify potential target organs. Toxicities observed at these high doses may result in decreased body weight gain, food consumption and activity. These clinical signs are often associated with a generalized stress response. It has been known that stress may cause increased levels of corticosterone, which causes changes in circulating leukocyte profiles, decreases in thymus and spleen weights and changes in the microscopic structure of lymphoid organs. This makes it difficult to differentiate between stress-related changes and direct toxicity to the immune system in standard non-clinical toxicity testing in rats. In mice, MHC Class II expression was found to be a very sensitive biomarker of stress and maybe useful for the rat. Therefore, the objective of studies presented was to further characterize the effects of corticosterone and stressors on the immune system and identify potential biomarkers of stress in rats. Rats were treated with exogenous corticosterone (20 or 30 mg/kg BID) or ethanol (5 g/kg) for either 1 or 4 days. Restraint stress was also evaluated for a 3-day period. Blood and urine samples were collected during the treatment period for corticosterone measurements. At necropsy, blood samples for leukocyte differentials were collected. Spleen and thymus weights, cellularity, lymphocyte subpopulations and histopathology were also evaluated. Urine corticosterone levels were also investigated as a surrogate to measuring serum corticosterone. The results demonstrate that the pattern of responses to corticosterone or the stressors is different in mice and rats. Although, decreases in MHC Class II were found to be a sensitive indicator of stress in mice, only slight decreases were observed in rats with similar serum corticosterone AUC levels. Decreases in thymus weight were greater than spleen weight with corticosterone or ethanol or restraint stressor. No other single parameter or combination of parameters tested were obvious candidates as sensitive biomarkers of stress in rats. However, the good correlation between urine and serum corticosterone levels suggest that urine corticosterone may be a potential biomarker of stress induced changes to the immune response.