PHENOTYPIC ANALYSIS OF SPLEEN, THYMUS, AND PERIPHERAL-BLOOD CELLS IN AGED C57B1 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 (TC DD). Female C57B1/6 mice were treated weekly with 0.2 mu g TCDD/kg bod y weight or vehicle for 14-15 months. Phenotypic analysis by flow cyto metry identified the major cell subpopulations in the spleen, thymus, and peripheral blood as defined by the expression of CD4, CD8, B220, a nd Mac-1 molecules. These subpopulations were further characterized fo r the expression of I-A, Pgp-1, CD45RB, and/or T cell receptor antigen s (CD3, alpha beta, gamma delta). A group of young (4 months old) mice was evaluated concurrently to document immunophenotype alterations as sociated with aging. Results showed several age-related changes in phe notype distribution in the spleen and blood, but not in the thymus, de spite significant age-dependent thymic involution. The age-dependent c hanges in splenic phenotypes included a decreased frequency of CD4(+) cells and a major shift in the frequency distribution from naive T cel ls to effector and memory T cells as defined by Pgp-1 and CD45RB expre ssion. These phenotypic changes in the spleen due to aging correlated with similar changes in the blood, providing preliminary support for t he use of spleen cells as surrogates for blood in the development of b iomarkers of immunotoxicity. Long-term exposure to a total cumulative dose 12-13 mu g TCDD/kg body weight resulted in no overt toxicity, a 1 6-fold elevation of hepatic ethoxyresorufin-O-deethylase activity, and residue levels of 1.27 +/- 0.16 ng TCDD/g abdominal fat. In compariso n to the effects of aging, TCDD treatment produced relatively subtle c hanges in immunophenotypes. In the TCDD-treated thymus, the proportion of CD4(-)CD8(-) cells was increased as was the proportion of gamma de lta(+) thymocytes. These effects were very small but of interest in th at similar thymic effects have been previously reported following pren atal exposure to TCDD. In the spleen, TCDD exposure did not alter the frequency of CD4(+) or CD8(+) T cells, B cells, or macrophages but sig nificantly altered functionally discrete subpopulations within the T c ell compartment. The most definitive change in TCDD-treated mice was a decrease in the frequency of memory T helper cells, defined as CD4(+) Pgp-1(hi)CD45RB(lo), with a concomitant increase in the proportion of naive T helper cells identified as CD4(+)Pgp-1(lo)CD45RB(hi). These c hanges are consistent with the known immunosuppressive activity of TCD D. Thus, these results identify Pgp-1 and CD45RB as potential biomarke rs of TCDD immunotoxicity. (C) 1995 Society of Toxicology.