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.
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Mentions: The selective expression of I-E on thymic DC, but not on cortical or medullary epithelial cells, B cells, or macrophages, offered an opportunity to investigate whether thymic dendritic cells in vivo would be capable of inducing clonal deletion as reported for DC from spleen (6, 9, 37) or GM-CSF–cultured thymic DC (16) in vitro. On the other hand, we were now able to test if they really could not induce negative selection in the absence of I-E+ thymic B cells as initially postulated by Mazda et al. (9). To assess in vivo clonal deletion, we measured the frequency of I-E reactive Vβ5+ and Vβ11+ T cells in B6, B6-Eαd, and B6CD11c-Eαd mice. T cells expressing Vβ5 and Vβ11 genes were deleted in certain mouse strains that expressed I-E (38, 39) and retrovirally encoded superantigens in the thymus (40). Because B6 mice do not express functional I-E molecules, the frequency among total CD4+ cells of both Vβ5- and Vβ11expressing CD4+ T cells is ∼4%, and ∼15 and 6%, respectively, in the CD8+ population (Fig. 7). In I-E transgenic mice, the efficiency of clonal deletion was reported to be maximal when the transgene was expressed on both thymic epithelium and BM-derived cells (41, 42). This situation was reflected in the case of the B6-Eαd mouse. In comparison with B6 mice, and as reported earlier (43) in this mouse strain, 65% of CD4+Vβ5+ T cells and >95% of Vβ11+ CD4+ T cells were deleted (Fig. 7). In the CD8 compartment, a strong but less complete deletion was also observed (83% deletion of Vβ5+ and 64% deletion of Vβ11+ T cells). In T cells from the B6 CD11c-Eαd strain, we found a similar degree of deletion in the CD4 (63% Vβ5+ deletion, 85% Vβ11+ deletion) and sigificant, but less complete, deletion in the CD8 subset (75% Vβ5+ deletion and 32% Vβ11+ deletion) (Fig. 7). The efficiency of deletion in the CD4 compartment seems to be identical for the two situations studied. It is important to note that the absence of I-E expression on both thymic epithelium and thymic B cells in the B6CD11c-Eαd thymus does not markedly reduce the efficiency of clonal deletion. This correlates with earlier studies using reaggregated organ cultures containing splenic DC as BM-derived cells, showing that professional APC clonally delete T cells much more efficiently than thymic epithelium (6). Also, in BM reconstitution experiments, Burkly et al. (43) showed that when I-E is expressed only on medullary epithelium, negative selection is only partial.