Early consequences of 2,3,7,8-Tetrachlorodibenzo-p-dioxin exposure on the activation and survival of antigen-specific T cells

TCDD is a potent immunotoxicant that suppresses adaptive immunity by mechanisms that are not well defined. To gain insight at the level of the T cell, we used the DO11.10 transgenic T-cell receptor (TCR) mouse model in an adoptive transfer approach to characterize the influence of TCDD on the responsiveness of antigen-specific CD4⁺ T cells in vivo. Flow cytometry was used to track the response of the OVA-specific transgenic CD4⁺ T cells in syngeneic recipients using an antibody specific for the transgenic TCR (KJ1-26 [KJ]). Consistent with a previous report, exposure of the recipient mice to TCDD (15 μg/kg po) did not alter the initial expansion of the CD4⁺KJ⁺ T cells in the spleen following immunization with OVA but resulted in a significant decline in the number of cells present on and after day 4. The degree of decline was dependent on the dose of TCDD. On day 3 after OVA injection, a higher percentage of the CD4⁺KJ⁺ T cells in the spleens of TCDD-treated mice had down-regulated the expression of CD62L, a phenotype associated with T-cell activation. Also on day 3, an increased number of CD4⁺KJ⁺ T cells were found in the blood of TCDD-treated mice. However, as in the spleen, the number of CD4⁺KJ⁺ T cells in the blood rapidly declined on day 4. CD4⁺KJ⁺ T cells in both the spleen and blood of TCDD-treated mice failed to up-regulate CD11a, an adhesion molecule important for sustained interaction between T cells and DC whereas the up-regulation of the adhesion molecule CD49d was not altered. Based on analysis of cell division history, CD4⁺KJ⁺ T cells in vehicle-treated mice continued to divide through day 4 whereas CD4⁺KJ⁺ T cells in TCDD-treated mice showed no further division after day 3. Increased annexin V staining on CD4⁺KJ⁺ T cells in TCDD-treated mice was also observed but not until days 5 and 6. Fas-deficient CD4⁺KJ⁺ T cells were depleted from the spleen of TCDD-treated mice in a manner similar to wild-type CD4⁺KJ⁺ T cells, suggesting that Fas signaling does not play a critical role in this model. On the other hand, gene array analysis of purified CD4⁺KJ⁺ T cells on day 3 showed that the expression of several genes associated with cell survival/death were altered by TCDD. Taken together, the results are consistent with our hypothesis that TCDD provides an early but inappropriate activation signal to the antigen-specific T cells that allows, and possibly enhances, the initial activation and proliferation of the T cells, yet at the same time, interferes with the vital expression of certain adhesion/costimulatory molecules that serve to enhance the survival of the T cells. These changes result in truncated proliferation, increased T-cell death, and suppression of the adaptive immune response.