Evaluation of immunotoxicity in a subchronic feeding study of triphenyl phosphate

Triphenyl phosphate (TPP), a potential food contaminant, was fed to weanling Spartan Sprague-Dawley rats at dose levels of 0, 0.25, 0.5, 0.75, and 1.0% for 120 days. The immunotoxicity evaluation, planned as a minimum testing model in a subchronic study design as well as to provide information on TPP, was performed along with the routine testing of a separate group of animals. Traditional measures were made of growth and food consumption, total protein analysis, electrophoretic analyses of serum proteins, lymphoid organ weights in relation to growth, and histopathology, with expanded immunohistochemical evaluation of B- and T- lymphocyte regions in spleen, thymus, and lymph nodes, using immunoperoxidase staining. Assessment was made of the humoral response to a T-lymphocyte-dependent antigen, sheep red blood cells, and was begun at midterm of the feeding period for the primary response followed by secondary and tertiary booster immunizations at 3-week intervals. The kinetics of the responses were measured by hemolysin assay of relative antibody titers at days 3, 4, 5, and 6 postinjection. No significant effects on the responses were noted for either sex at any of the dose levels tested. The only effects noted were a decreased rate of growth at high levels of TPP and increases in the levels of alpha- and beta-globulins suggestive of increased hepatic activity.