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

We have shown that IDO mRNA was preferentially expressed by pDCs compared to other LN cells both in steady-state and during EAE (Fig. 1, Fig. 3A). However, whether restricted IDO expression by pDCs is sufficient and required to inhibit disease development still need to be clarified. To address this, we used BDCA-2 DTR mice in which endogenous pDCs were depleted following DT injection [54]. It has been demonstrated that active immunization induces DT toxicity, and may therefore confound experiments in DTR transgenic mice for neuroinflammatory models, such as EAE [55]. To avoid this problem, we generated BM chimeric mice by injecting BM cells from BDCA-2 DTR into irradiated WT recipients. In these chimeric mice, no signs of toxicity or lethality in DT-treated mice upon EAE induction were observed. Moreover, efficient depletion of endogenous pDCs in the blood and LNs of BDCA-2 DTR → WT chimeras was achieved after DT injection (not shown). We transferred (i.v.) MOG35–55-loaded WT or IDO−/− pDCs into DT-treated BDCA-2 DTR → WT chimeras one day prior to EAE induction. In these settings, IDO was selectively supplied - or not - by adoptively transferred pDCs. Following EAE induction, only WT, but not IDO−/− pDCs significantly inhibited disease development (Fig. 5A). Foxp3+CD25+ Treg proliferation was increased in LN upon pDC transfer compared to control BDCA-2 DTR → WT EAE mice, whether transferred pDCs expressed IDO or not (Fig. 5B), confirming that IDO expression by pDCs is not involved in MHCII-dependent, pDC-mediated, Treg expansion. In contrast, the frequencies of suppressive Tregs co-expressing CD103 and ICOS (Fig. 5C), expressing high levels of CD25 (Fig. 5D), or upregulating the activation marker CD69 (Fig. 5E), were significantly reduced in LN from BDCA-2 DTR → WT EAE mice transferred with IDO−/− pDCs compared to WT pDCs. These results demonstrate that during EAE, IDO expression by pDCs is required to elicit pDC-mediated suppressive Tregs.