Cholera toxin mediates cAMP-dependent CTLA-2 secretion by dendritic cells to induce regulatory T cells in the EAE model

Therapy of autoimmune diseases by tolerogenic dendritic cells (DCs) represents an interesting option for clinical application. Mostly, immature or semi-mature stages of DCs have been shown to convert nave T cells into FoxP3+ induced regulatory T cells (iTreg). Here we found that murine bone marrow-derived DCs (BM-DCs) matured with cholera toxin (CT) produced cytotoxic T lymphocyte antigen 2 (CTLA-2) via cAMP signaling. CTLA-2 is a cathepsin L inhibitor, however, it exerted cathepsin L-independent tolerogenic functions as shown with cathepsin L knock-out BM-DCs. High or low dose CT (CThi, CTlo) or cAMP treatment of BM-DCs, but not other stimuli such as LPS or TNF, induced mRNA of both isoforms of CTLA-2 and CTLA-2. CThi-, CTlo- or cAMP-DCs display a mature phenotype (MHC IIhi, costimulationhi). Importantly, only CThi DCs secreted IL-1, IL-6 and IL-23. Consequently, CThi DCs instructed Th17 cell polarization while, CTlo- or cAMP-DCs induced TGF--dependent FoxP3+ iTreg conversion but little or no Th17 cells. iTreg conversion could be reduced by blocking of CTLA-2 with siRNAs and in accordance, addition of recombinant CTLA-2 increased the conversion towards iTreg in vitro. Injection of CTlo- or cAMP-DCs exerted MOG peptide-specific protective effects in experimental autoimmune encephalomyelitis (EAE). Together, we identified CTlo- or cAMP-induced CTLA-2 production by BM-DCs as a tool to enhance their iTreg induction in vitro and in vivo. These findings point to CTLA-2 as a novel mediator of immune evasion by cholera bacteria and its potential use to treat autoimmune diseases.

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