Phenotypic Analysis of Spleen, Thymus, and Peripheral Blood Cells in Aged C57BI/6 Mice Following Long-Term Exposure to 2,3,7,8-Tetrachlorodibenzo-p-Dioxin

A mouse model was used to identify potential biomarkers of exposureto the environmental contaminant 2,3,7,8-tetrachiorodibenzo-p-dioxin(TCDD). Female C57B1/6 mice were treated weekly with 0.2 µgTCDD/kg body weight or vehicle for 14–15 months. Phenotypicanalysis by flow cytometry identified the major cell subpopulationsin the spleen, thymus, and peripheral blood as defined by theexpression of CD4, CD8, B220, and Mac-1 molecules. These subpopulationswere further characterized for the expression of I-A, Pgp-1,CD45RB, and/or T cell receptor antigens (CD3, ß ).A group of young (4 months old) mice was evaluated concurrentlyto document immunophenotype alterations associated with aging.Results showed several age-related changes in phenotype distributionin the spleen and blood, but not in the thymus, despite significantage-dependent thymic involution. The age-dependent changes insplenic phenotypes included a decreased frequency of CD4⁺ cellsand a major shift in the frequency distribution from naive Tcells to effector and memory T cells as defined by Pgp-1 andCD45RB expression. These phenotypic changes in the spleen dueto aging correlated with similar changes in the blood, providingpreliminary support for the use of spleen cells as surrogatesfor blood in the development of biomarkers of immunotoxicity.Long-term exposure to a total cumulative dose 12–13 µgTCDD/kg body weight resulted in no overt toxicity, a 16-foldelevation of hepatic ethoxyresorufin-O-deethylase activity,and residue levels of 1.27 ± 0.16 ng TCDD/g abdominalfat. In comparison to the effects of aging, TCDD treatment producedrelatively subtle changes in immunophenotypes. In the TCDD-treatedthymus, the proportion of CD4^–CD8^– cells was increasedas was the proportion of ⁺ thymocytes. These effects were verysmall but of interest in that similar thymic effects have beenpreviously reported following prenatal exposure to TCDD. Inthe spleen, TCDD exposure did not alter the frequency of CD4⁺or CD8⁺ T cells, B cells, or macrophages but significantly alteredfunctionally discrete subpopulations within the T cell compartment.The most definitive change in TCDD-treated mice was a decreasein the frequency of memory T helper cells, defined as CD4⁺ Pgp-1^hiCD45RB^lo, with a concomitant increase in the proportion ofnaive T helper cells identified as CD4⁺Pgp-1^loCD45RB^hi. Thesechanges are consistent with the known immunosuppressive activityof TCDD. Thus, these results identify Pgp-1 and CD45RB as potentialbiomarkers of TCDD immunotoxicity.