Sensitivity of the SRBC PFC assay versus ELISA for detection of immunosuppression by TCDD and TCDD-like congeners.

The splenic antibody plaque forming cell (PFC) assay is a widely used assay in immunotoxicity testing. A recent revision of the Federal Insecticide, Fungicide and Rodenticide (FIFRA) Immunotoxicity test guidelines by the EPA recommended that either the PFC assay or the sheep red blood cell (SRBC) specific serum IgM enzyme-linked immunosorbent assay (ELISA) be used to assess the primary humoral response to SRBCs. The PFC assay quantifies the number of plasma cells in the spleen producing SRBC-specific antibody, while the ELISA measures SRBC-specific IgM antibody in the serum. Because these two assays measure different endpoints, there is a need for comparison of their sensitivity and reliability. The purpose of this project was to determine if these two assays are equally sensitive to suppression of the SRBC response in B6C3F1 female mice. Female B6C3F1 mice were given a single oral exposure to different doses of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) or four TCDD-like congeners. One week later, two sets of mice were immunized with SRBC. The first set was evaluated for the PFC response and the second for the ELISA response, on day 4 or 5 post-immunization, respectively. The four TCDD-like congeners tested were: 1,2,3,7,8-pentachlorodibenzo-p-dioxin (PeCDD), 1,2,3,4,7-pentachlorodibenzofuran (4PeCDF), 3,3',4,4',5-pentachlorobiphenyl (PCB126) and 2,3',4,4',5-pentachlrorbiphenyl (PCB118). The results were used to generate dose-response curves for the determination of the ED(50) for TCDD and each TCDD-like congener. For all chemicals tested, measuring the level of SRBC-specific IgM antibody by ELISA was more sensitive than the PFC assay to detect immunosuppression, as indicated by lower ED(50) values. These results indicate that the SRBC-specific IgM ELISA is a more sensitive assay for detecting the T-cell mediated immunotoxicity of dioxin-like chemicals in this rodent model.