The Development and Application of a Hemolytic Plaque Forming Cell Assay (PFC) and a Cytotoxic T-Lymphocyte Assay (CTL) in Tilapia (Oreochromis niloticus) for Immunotoxicity Risk Assessment of Environmental Contaminants

The prospect of utilizing the cichlid teleost tilapia (Oreochromis niloticus) as an alternative experimental model to mammals for immunotoxicity risk assessment is currently being proposed. As such, the National Toxicology Program's (NTP) standard battery of rodent immunotoxicity assays is being developed for use in this fish species. Included in the testing series are the hemolytic plaque forming cell (PFC) and the cytotoxic T-lymphocyte (CTL) assays, quantitative indicators of antibody production and cell-mediated activity, respectively. The assays were modified in consideration of specific tilapian immune parameters, then tested using fourteen environmental contaminants or drugs, ten of which are classified by the NTP as immunotoxic in rodents. Reduced antibody production via a decrease in plaque number was observed in response to exposure of tilapia to eight of the nine humoral immunotoxicants, and five of the five non-immunotoxicants. Under specific immunization circumstances, immunostimulation (also a response to immunotoxicity) was noted via an increase in plaque number in benzo[a]pyrene (B[a]P) exposed fish using the PFC assay, a result noted in rodents as well. Reduced T-cell recognition and lysis of allogeneic tilapian lymphocytes via a decrease in the percentage of specific 51Chromium (51Cr) release was observed in response to exposure of tilapia to the nine of the ten cell-mediated immunotoxicants, and four of the four non-immunotoxicants. Although the normal teleost immune responsiveness was slightly weaker than seen with mice under comparable conditions (presumably due to differences in antibody structure and decreased cells counts), tilapia were found to exhibit well-defined humoral and cell-mediated immune responses, and responses to immunotoxic and non-immunotoxic chemicals comparable to the rodent model.