Phenotypic analysis of spleen, thymus, and peripheral blood cells in aged C57Bl/6 mice following long-term exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin

A mouse model was used to identify potential biomarkers of exposure to the environmental contaminant 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). Female C57B1/6 mice were treated weekly with 0.2 mu g TCDD/kg body weight or vehicle for 14-15 months. Phenotypic analysis by flow cytometry identified the major cell subpopulations in the spleen, thymus, and peripheral blood as defined by the expression of CD4, CD8, B220, and Mac-1 molecules. These subpopulations were further characterized for the expression of I-A, Pgp-1, CD45RB, and/or T cell receptor antigens (CD3, alpha beta , gamma delta ). A group of young (4 months old) mice was evaluated concurrently to document immunophenotype alterations associated with aging. Results showed several age-related changes in phenotype distribution in the spleen and blood, but not in the thymus, despite significant age-dependent thymic involution. The age-dependent changes in splenic phenotypes included a decreased frequency of CD4 super(+) cells and a major shift in the frequency distribution from naive T cells to effector and memory T cells as defined by Pgp-1 and CD45RB expression. These phenotypic changes in the spleen due to aging correlated with similar changes in the blood, providing preliminary support for the use of spleen cells as surrogates for blood in the development of biomarkers of immunotoxicity. Long-term exposure to a total cumulative dose 12-13 mu g TCDD/kg body weight resulted in no overt toxicity, a 16-fold elevation of hepatic ethoxyresorufin-O-deethylase activity, and residue levels of 1.27 plus or minus 0.16 ng TCDD/g abdominal fat. In comparison to the effects of aging, TCDD treatment produced relatively subtle changes in immunophenotypes. In the TCDD-treated thymus, the proportion of CD4 super(-)CD8 super(-) cells was increased as was the proportion of gamma delta super(+) thymocytes. These effects were very small but of interest in that similar thymic effects have been previously reported following prenatal exposure to TCDD. In the spleen, TCDD exposure did not alter the frequency of CD4 super(+) or CD8 super(+) T cells, B cells, or macrophages but significantly altered functionally discrete subpopulations within the T cell compartment. The most definitive change in TCDD-treated mice was a decrease in the frequency of memory T helper cells, defined as CD4 super(+) Pgp-1 super(hi)CD45RB super(lo), with a concomitant increase in the proportion of naive T helper cells identified as CD4 super(+)Pgp-1