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 tested the in vivo suppressive activity of Tregs primed during EAE in pIII + IV−/− → WT, IDO−/− → WT and WT → WT chimeric mice. Purified CD4+CD25+ cells, which are predominantly Foxp3+ for all conditions (Fig. 6A), were transferred into WT hosts one day prior to EAE induction (Fig. 6A and B). Tregs primed in WT mice reduced EAE incidence, delayed disease onset, and conferred significant protection as we have previously reported [23]. In contrast, Tregs primed in mice lacking MHCII expression by pDCs did not exhibit any suppressive activity (Fig. 6B and Table 1). Strikingly, Tregs primed in absence of IDO did not confer any delay in disease onset, nor reduced EAE severity (Fig. 6B and Table 1). Frequencies of donor Tregs primed in WT and IDO−/− mice were similar in dLNs (day 3) and dLNs and SC (day 15) of recipient mice. These results suggest that the absence of EAE protection by Tregs primed in IDO deficient mice does not rely on an impaired migration of donor cells, but rather results from impaired intrinsic suppressive functions (Supplementary Fig. 6A and B). Accordingly, compared to Tregs isolated from WT EAE mice, Tregs primed in IDO deficient EAE mice were significantly less efficient at suppressing MOG35–55 loaded DC induced 2D2 CD4+ T cell proliferation in vitro (Fig. 6C). Furthermore, Tregs isolated from naïve WT and IDO−/− mice did not confer any suppression, further suggesting that Ag-specific priming in IDO+ context is required for the acquisition of Treg suppressive functions (Fig. 6C). In agreement, in vivo, only Tregs activated following EAE immunization in WT mice efficiently inhibited EAE incidence and severity (day 14), whereas neither Tregs isolated from naïve WT, nor Tregs isolated from naïve or immunized IDO−/− mice, conferred any protection upon transfer into EAE mice (Fig. 6D). Therefore, EAE-mediated activation is mandatory to confer IDO-dependent Treg suppressive functions.