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 next evaluated whether, as in steady-state LNs, pDCs remains the major expressors of IDO in LNs in a model of chronic inflammation. IDO has been shown to exert a protective role in EAE using either IDO−/− mice or IDO blocking antibodies [51], [52]. Therefore, we have quantified IDO mRNA expression in different cell types sorted from LNs draining the site of EAE immunization (day 10) in wild type (WT) mice. IDO was predominantly (>5 fold) expressed by pDCs compared to other LN cells (Fig. 3A). Levels of expression were comparable to steady-state LN pDCs (Supplementary Fig. 3A). Next, we analysed IDO expression by MHCII deficient pDCs during EAE. pDCs and cDCs were sorted from draining LNs of WT → WT and pIII + IV−/− → WT mice 10 days after EAE induction. Again, cDCs expressed little IDO mRNA (Fig. 3B). We observed a substantial reduction in IDO expression by pDCs sorted from pIII + IV−/− → WT compared to WT → WT chimeras (Fig. 3B). Differential IDO expression by MHCII competent and MHCII deficient pDCs might be explained by distinct inflammatory cytokinic environments in WT → WT and pIII + IV−/− → WT mice, as the latter developed more severe EAE [23]. Therefore, we performed mixed BM chimeric mice using as before BM cells from Ubi-eGFP WT and pIII + IV−/− mice (ratio 1:1) that were injected into irradiated CD45.1 recipient mice. EAE was induced in mixed BM chimeric mice, and LN cells were sorted 10 days after immunization as pDCs and cDCs from donor BM cells (gated on CD45.2+) and further separated as WT (eGFP+) or pIII + IV−/− (eGFP−) cells. IDO expression was significantly impaired in MHCII deficient compared to MHCII competent pDCs isolated from LN of EAE mice (Fig. 3C). Therefore, decreased IDO expression in absence of MHCII expression by pDCs was not related to different cytokine expression profiles between WT → WT and pIII + IV−/− → WT chimeras during EAE, but rather linked to a defective MHCII expression by pDCs. Altogether, our data suggest that IDO expression by LN pDCs is dependent on their expression of MHCII molecules, independent on the inflammatory status of the mice, and reflects a more general regulation of IDO protein expression at the mRNA level.