Generation of αβ T-cell receptor+ CD4- CD8+ cells in major histocompatibility complex class I-deficient mice upon activation of the aryl hydrocarbon receptor by 2,3,7,8-tetrachlorodibenzo-p-dioxin

Gene-targeted mice lacking the beta(2) microglobulin gene (beta(2)m(-/-) mice), and hence functional major histocompatibility complex (MHC) class I molecules, do not develop CD4(-) CD8(+) cells. We show here that both in vitro and in vivo treatment with 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), a trans-activating ligand of the endogenous aryl hydrocarbon receptor (Ah-R), bypasses the need for MHC class I molecules for selection into the CD4(-) CD8(+) cell pool. When beta(2)m(-/-) dams were given a single dose of 50 mu g of TCDD, approximate to 13% of CD4(-) CD8(+) thymocytes could be detected in their newborn pups. In TCDD-exposed fetal thymus organ cultures of beta(2)m(-/-) mice, approximate to 35% CD4(-) CD8(+) thymocytes were detectable. About 16% of these CD4(-) CD8(+) cells bore the alpha beta T-cell receptor (TCR) and approximate to 33% bore CD3. Only a minority of the CD8(+) cells were heat-shock antigen positive. The cells possessed killing activity as shown using the Cr-51-release assay comprising gamma delta TCR- CD4(-) CD8(+) thymocytes from 3 to 4-day-old beta(2)m(-/-) mice. Thus, TCDD leads to a significant increase of mature CD4(-) CD8(+) thymocytes in relative and absolute numbers. High numbers of CD4(-) CD8(+) thymocytes developed also in organ cultures from thymi, lacking both MHC class I and class II molecules, exposed to TCDD. A 10-fold transient increase of Notch1 mRNA in thymocytes from fetal thymus organ culture, exposed for 4 days to TCDD, was detected in CD4(+) CD8(+) cells compared with controls. We suggest that TCDD affects thymic selection and directs the lineage commitment of CD4(+) CD8(+) thymocytes towards CD4(-) CD8(+) cells, possibly via up-regulation of the Notch1 gene.