Mechanism of chimeric vaccine stimulation of indoleamine 2,3-dioxygenase biosynthesis in human dendritic cells is independent of TGF-β signaling

Cholera toxin B subunit fusion to autoantigens such as proinsulin (CTB-INS) down regulate dendritic cell (DC) activation and stimulate synthesis of DC immunosuppressive cytokines. Recent studies of CTB-INS induction of immune tolerance in human DCs indicate that increased biosynthesis of indoleamine 2,3-dioxygenase (IDO1) may play an important role in CTB-INS vaccine suppression of DC activation. Studies in murine models suggest a role for transforming growth factor beta (TGF-β) in the stimulation of IDO1 biosynthesis, for the induction of tolerance in DCs. Here, we investigated the contribution of TGF-β superfamily proteins to CTB-INS induction of IDO1 biosynthesis in human monocyte-derived DCs (moDCs). We show that CTB-INS upregulates the level of TGF-β1, activin-A and the TGF-β activator, integrin αvβ8 in human DCs. However, inhibition of endogenous TGF-β, activin-A or addition of biologically active TGF-β1, and activin-A, did not inhibit or stimulate IDO1 biosynthesis in human DCs treated with CTB-INS. While inhibition with the kinase inhibitor, RepSox, blocked SMAD2/3 phosphorylation and diminished IDO1 biosynthesis in a concentration dependent manner. Specific blocking of the TGF-β type 1 kinase receptor with SB-431542 did not arrest IDO1 biosynthesis, suggesting the involvement of a different kinase pathway other than TGF-β type 1 receptor kinase in CTB-INS induction of IDO1 in human moDCs. Together, our experimental findings identify additional immunoregulatory proteins induced by the CTB-INS fusion protein, suggesting CTB-INS may utilize multiple mechanisms in the induction of tolerance in human moDCs.