Abstract
Gene-targeted mice lacking the beta2 microglobulin gene (beta2m-/- 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 beta2m-/- dams were given a single dose of 50 microg of TCDD, approximately 13% of CD4- CD8+ thymocytes could be detected in their newborn pups. In TCDD-exposed fetal thymus organ cultures of beta2m-/- mice, approximately 35% CD4- CD8+ thymocytes were detectable. About 16% of these CD4- CD8+ cells bore the alpha beta T-cell receptor (TCR) and approximately 33% bore CD3. Only a minority of the CD8+ cells were heat-shock antigen positive. The cells possessed killing activity as shown using the 51Cr-release assay comprising gamma delta TCR- CD4- CD8+ thymocytes from 3 to 4-day-old b2m-/- 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.