Ectopic expression of IL-35Ig by dendritic cells induces IDO-related tolerance in vivo

Dendritic cells (DCs) are specialized APCs playing a central role in activating, but also in regulating, T-cell responses. In previous studies, we demonstrated that their functional plasticity can be exploited to trigger and amplify a tolerogenic state (‘infectious tolerance’), here we focused on the possible modulation of immune responses by ectopic expression of IL-35 in DCs, a potent suppressive cytokine produced by T regulatory cells. We transfected immunogenic CD8– DCs with a single-chain IL-35-Ig gene construct and we found that, after P815AB antigen peptide pulsing, even a minority fraction of transfected cells inhibited the immunogenic presentation of the peptide in a skin test assay, triggering the onset of a long-lasting, specific tolerance subverting the otherwise immunogenic priming to the peptide. This suppressive effect was attributable to changes occurring in vivo in total splenocytes and in their CD11c+ DC fraction after DC/IL-35-Ig/P815AB injection, including induction of tolerogenic indoleamine 2,3-dioxygenase-1 (IDO1) and production of mediators sustaining the mechanism of infectious tolerance (via the IDO1 metabolite kynurenine, TGF-β, and IL-10). On the basis of these findings, we attempted a tolerogenic vaccination protocol to prevent autoimmune diabetes. Interestingly, on treating prediabetic NOD mice with DCs overexpressing IL-35 and presenting the IGRP peptide (one of the most relevant type 1 diabetes self-antigens), we observed delayed and less severe onset of hyperglycemia. In conclusion, DCs overexpressing ectopic IL-35Ig exert an important suppression in vivo, which, on combination with specific antigen, might represent a powerful, selective means of negative vaccination in autoimmune diseases.