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. F ungicide and Rodenticide (FIFRA) Immunotoxicity test guidelines by the EPA recommended that either the PFC assay or the sheep red blood cell (SRBC) sp ecific serum IgM enzyme-linked immunosorbent assay (ELISA) be used to asses s the primary humoral response to SRBCs. The PFC assay quantifies the numbe r 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 respo nse in B6C3F1 female mice. Female B6C3F1 mice were given a single oral expo sure to different doses of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) or fo ur TCDD-like congeners. One week later, two sets of mice were immunized wit h 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 fou r TCDD-like congeners tested were: 1,2,3,7,8-pentachlorodibenzo-p-dioxin (P eCDD), 1.2,3.4,7-pentachlorodibenzofuran (4PeCDF), 3,3',4.4'.5-pentachlorob iphenyl (PCB126) and 2,3',4,4',5-pentachlrorbiphenyl (PCB118). The results were used to generate dose-response curves for the determination of the ED5 0 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 t he PFC assay to detect immunosuppression. as indicated by lower ED50 values . These results indicate that the SRBC-specific IgM ELISA is a more sensiti ve assay for detecting the T-cell mediated immunotoxicity of dioxin-like ch emicals in this rodent model. (C) 2000 Published by Elsevier Science Irelan d Ltd. All rights reserved.