IDO-orchestrated crosstalk between pDCs and Tregs inhibits autoimmunity

Lippens, Carla; Duraes, Fernanda V.; Dubrot, Juan; Brighouse, Dale; Lacroix, Mathilde; Irla, Magali; Aubry-Lachainaye, Jean-Pierre; Reith, Walter; Mandl, Judith N.; Hugues, Stéphanie

This result might be explained by a low TCR self-reactivity for MHCII molecules of the OTII transgenic CD4+ T cells [48]. To test the possibility that distinct TCR self-reactivity would exhibit differential ability to induce IDO expression in pDCs, we isolated pDCs from different CD4+ TCR transgenic Rag1−/− mice that express distinct levels of CD5, which reflects clone-specific strength self-reactivity [48]. Each TCR transgenic T cell population display a specific surface amount of CD5 that covered a range from low (Marilyn, TCR specific for HY), medium (Smarta, TCR specific for LCMV), to high (AND, TCR specific for Pigeon Cytochrome c) (Fig. 1C). IDO mRNA expression was reduced in LN pDCs isolated from all TCR transgenic Rag−/− mice compared to WT mice (Fig. 1C), showing that IDO induction in pDCs is independent of TCR self-reactivity. An alternative hypothesis which might explain why IDO expression in negligible is a potential specific requirement of Tregs in regulating IDO expression in pDCs, this population being almost absent in LNs of the TCR transgenic T cell mice tested (Fig. 1D). Consistently, MOG35–55-loaded Rag2−/− pDCs, exhibit a significant restoration of IDO expression when co-cultured with 2D2 TCR tg CD4+ T cells, which contain a low but significant frequency of Foxp3+ cells (Fig. 1E). Moreover, pDCs isolated from LNs of Scurfy mice, which are devoid of Foxp3+ Tregs [44], presented a significant reduction of IDO mRNA expression compared to WT pDCs (Fig. 1F). Thus, our data suggest that Tregs significantly contribute to induce IDO expression by pDCs in steady-state LN. We next evaluated whether Ag-specific pDC-T cell contacts were required to promote IDO expression in pDCs. To do so, cultures were done using in vitro generated BM-derived pDCs, for which IDO mRNA levels were found to be negligible, reinforcing the idea that a crosstalk with T cells is indeed required for pDCs to competently express IDO (Fig. 1G). When co-cultured with 2D2 TCR tg CD4+ T cells, a modest increase in IDO mRNA expression was observed in BM-derived pDCs (Fig. 1G). IDO expression was further increased by 2 fold when pDCs were previously loaded with MOG35–55 peptide, reaching similar levels of expression as pDCs isolated from WT LNs (Fig. 1G), Therefore, MHCII-restricted Ag-specific interactions with CD4+ T cells significantly contribute to the induction of IDO expression in pDCs. To test a role for Tregs in this process, we repeated co-culture experiments using 2D2 TCR tg CD4+ T cells that were separated into in CD25hi and CD25neg populations. While 2D2 TCR tg CD4+CD25neg cells were incompetent at inducing IDO, 2D2 TCR tg CD4+CD25hi cells significantly enhanced IDO mRNA levels in pDCs loaded with MOG35–55 (Fig. 1H), confirming that Tregs are mandatory at promoting IDO expression in pDCs. pDCs co-cultured with total 2D2 TCR tg CD4+ T cells exhibit a slight, but not significant increase in IDO mRNA levels compared to pDCs co-cultured with 2D2 TCR tg CD4+CD25hi cells, suggesting that although Tregs are required to promote IDO, non-Treg CD4+ T cells might also contribute to this process (Fig. 1H).