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 this end we crossed the above described mouse strains to the I-A−/− line described earlier (47). When the I-A–deficient background was combined with the natural defect of I-E expression in C57BL/6 mice, this strain showed absence of all MHC class II molecules, and consequently, the absence of CD4+ mature T cells in thymus and periphery (47; Fig. 8, B6I-A−/−). When the I-E gene was reintroduced into these mice under the control of the class II promoter, the reexpression of class II (I-E) on thymic epithelium and BM-derived cells led to the full restoration of the mature CD4+ T cell compartment (Fig. 8, B6-EαdI-A−/−) as it has been described before (46). In contrast, when I-E expression was restricted to thymic DC (Fig. 8, B6CD11c-EαdI-A−/−), restoration of the CD4+ compartment was not observed; the percentages of CD4+ T cells were identical to those of B6I-A−/− mice (see legend to Fig. 8). Cosgrove et al. (46) have described that their ΔX mice, expressing I-E on epithelial and BM-derived cells in the medulla but not in the cortex, showed a slight increase in CD4+ T cells when bred with the I-A−/−–deficient background. The authors interpreted their findings by invoking a leaky expression pattern of the I-E transgene which would eventually be expressed at an albeit low, histochemically undetectable, level on cortical epithelium. Since we do not see such an increase in CD4+ T cells when I-E was expressed exclusively on thymic DC, our findings argue (a) in support of the strictly defined expression pattern of the CD11c-Eαd construct to DC and (b) that medullary epithelium could theoretically be responsible for the weak positive selection effect of CD4+ T cells described by others (46). Furthermore, the complete absence of positively selected CD4+ T cells in our B6CD11c-EαdIA−/− strain confirms previous observations (14, 15) obtained using different in vitro experimental protocols, indicating that thymic dendritic cells in vivo are not able to induce MHC restriction of CD4+ T cells.