Targeted expression of major histocompatibility complex (MHC) class II molecules demonstrates that dendritic cells can induce negative but not positive selection of thymocytes in vivo.
Bottom Line: Using the CD 11c promoter we expressed MHC class II I-E molecules specifically on DC of all tissues, but not on other cell types.In contrast, it only DC expressed I-E in a class II-deficient background, positive selection of CD4+ T cells could not be observed.Thus negative, but not positive, selection events can be induced by DC in vivo.
Affiliation: Basel Institute for Immunology, Switzerland.
It is well established that lymphoid dendritic cells (DC) play an important role in the immune system. Beside their role as potent inducers of primary T cell responses, DC seem to play a crucial part as major histocompatibility complex (MHC) class II+ "interdigitating cells" in the thymus during thymocyte development. Thymic DC have been implicated in tolerance induction and also by some authors in inducing major histocompatibility complex restriction of thymocytes. Most of our knowledge about thymic DC was obtained using highly invasive and manipulatory experimental protocols such as thymus reaggregation cultures, suspension cultures, thymus grafting, and bone marrow reconstitution experiments. The DC used in those studies had to go through extensive isolation procedures or were cultured with recombinant growth factors. Since the functions of DC after these in vitro manipulations have been reported to be not identical to those of DC in vivo, we intended to establish a system that would allow us to investigate DC function avoiding artificial interferences due to handling. Here we present a transgenic mouse model in which we targeted gene expression specifically to DC. Using the CD 11c promoter we expressed MHC class II I-E molecules specifically on DC of all tissues, but not on other cell types. We report that I-E expression on thymic DC is sufficient to negatively select I-E reactive CD4+ T cells, and to a less complete extent, CD8+ T cells. In contrast, it only DC expressed I-E in a class II-deficient background, positive selection of CD4+ T cells could not be observed. Thus negative, but not positive, selection events can be induced by DC in vivo.
Mentions: To further characterize the cells expressing the I-E transgene in the thymus, we isolated the light density cell fraction from collagenase-digested thymi as described earlier (29), and performed flow cytometric analysis. The major cell type obtained in the low buoyant density fraction from thymus were cells with relatively high forward scatter (FS) and intermediate side scatter (SS) signals (data not shown); in contrast, thymocytes (from the high density fraction) had both low FS and SS (data not shown). When the low density fraction was stained for CD11c expression, 80–90% of the cells fulfilling the above described FS/SS criteria were CD11cpositive (see Fig. 5, dendritic cells). While more than 60% of the CD11c-positive cells from C57Bl/6 mice stained brightly for MHC class II I-A molecules, they were all negative for I-E (Fig. 5, B6). In contrast, the CD11c-positive cells from B6-Eαd mice were positive for I-A, but as expected, also showed a remarkably high expression of transgenic I-E molecules (Fig. 5, B6-Eαd ).